Presentation on theme: "Yarmouk University Department of Conservation and Management of Cultural Resources."— Presentation transcript:
Yarmouk University Department of Conservation and Management of Cultural Resources
Our cultural heritage is made with almost all type of materials produced by the nature and used by men to realize several types of artefacts from very simple mono-components to complex structures integrating inorganic and organic matters. These cultural heritage objects, even if made with the more “resistant” stones and metals, are influenced by the environmental parameters, which can modify their structure and composition
What Governs Deterioration of Cultural materials The deterioration and preservation of materials depends on two things: The nature of the material Environment surrounding the material.
Nature of the Material
What kinds of materials will I find in a museum collection? Museum objects are often divided into three material-type categories: organic, inorganic, and composite. You must understand the properties of each of the materials in each of these categories.
Organic Objects: Organic objects are derived from things that were once living — plants or animals. Based on C. Contains O, N, H Materials are processed in a multitude of ways to produce the objects that come into your collections.
Organic Objects: Various material types include wood, paper, textiles, leather and skins, horn, bone and ivory, grasses and bark, lacquers and waxes, plastics, some pigments, shell, and biological natural history specimens.
All organic materials share some common characteristics They: Contain the element carbon Are combustible Are made of complicated molecular structures that are susceptible to deterioration from extremes and changes in relative humidity and temperature Absorb water from and emit water to the surrounding air in an ongoing attempt to reach an equilibrium (hygroscopic) Are sensitive to light Are a source of food for mold, insects, and vermin
Inorganic Objects Inorganic objects have a geological origin. Just like organic objects, the materials are processed in a variety of ways to produce objects found in your collections.
Inorganic Materials Material types include: metals, ceramics, glass, stone, minerals, and some pigments.
All inorganic objects share some common characteristics. They: · have undergone extreme pressure or heat · are usually not combustible at normal temperature · can react with the environment to change their chemical structure (for example, corrosion or dissolution of constituents) · may be porous (ceramics and stone) and will absorb contaminants (for example, water, salts, pollution, and acids) · are not sensitive to light, except for certain types of glass and pigments
Composite Objects: Composite or mixed media objects are made up of two or more materials. For example, a painting may be made of a wood frame and stretcher, a canvas support, a variety of pigments of organic and inorganic origin, and a coating over the paint. A book is composed of several materials such as paper, ink, leather, thread, and glue.
Composite Objects: Depending on their materials, composite objects may have characteristics of both organic and inorganic objects. The individual materials in the object will react with the environment in different ways. Also, different materials may react in opposition to each other, setting up physical stress and causing chemical interactions that cause deterioration.
What is inherent vice? In addition to deterioration caused by the agents of deterioration, certain types of objects will deteriorate because of their internal characteristics. This mechanism of deterioration is often called inherent vice or inherent fault. It occurs either because of the incompatibility of different materials or because of poor quality or unstable materials.
In nature, materials often possess characteristics that protect them from natural degradation. Their structure and composition may include features such as protective layers, insect and mold resistant chemicals, and photochemical protection. Processing during object manufacture can remove these natural safeguards
Short-lived materials: Short-lived materials are often the result of manufacturing processes that do not consider the long-term stability of the items that were produced. Examples of impermanent materials with inherent vice include: · cellulose nitrate and cellulose ester film · wood pulp paper · many 20th century plastics · magnetic media, including electronic records
Structural nature: Inherent vice can also be related to the structure of an object. Poor design, poor construction, or poor application of materials may cause structural failure. Examples of such damage include: · drying cracks in paint improperly applied · broken or lost attachments · loose joints
History: The way an object was used or where it was stored or deposited before it comes into your collection may lead to inherent vice. Here, damage and deterioration is caused by the original function of the object, its maintenance, or its environment. Examples of inherent vice caused by history include: · accumulation of dissimilar paint layers, such as oil and latex · saturation in a wooden bowl that had been used as a container for oil · deposits of soluble salts in an archeological ceramic during burial
You may have trouble identifying deterioration caused by inherent vice because often there is little or no information on the selection and processing of materials, manufacturing details, and previous use of an object. Train your critical eye by reviewing similar objects and by developing knowledge of object technology. Over time, you will become more proficient at identifying inherent vice.
What makes archeological objects different from other materials commonly found in museum collections? The condition of these objects depends entirely on their reaction with the environmental conditions to which they have been exposed through time. Underground the object reaches a kind of equilibrium with the surrounding soil. Then, when the object is excavated, it must adjust to a new and radically different environment. Reactions can involve both physical and chemical changes.
Regardless of the condition of the object before excavation, the moment it becomes exposed it is vulnerable to rapid deterioration. Figure I.1 illustrates the deterioration rate of archeological objects
Deterioration of Museum Objects
What is deterioration? Deterioration is any physical or chemical change in the condition of an object. Deterioration is inevitable. It is a natural process by which an object reaches a state of physical and chemical equilibrium with its immediate environment.
Types of Deterioration The types of deterioration can be divided into two broad categories: physical deterioration and chemical deterioration. Both types often occur simultaneously.
What is chemical deterioration? Chemical deterioration is any change in an object that involves an alteration of its chemical composition. It is a change at the atomic and molecular level. Chemical change usually occurs because of reaction with another chemical substance (pollution, water, pest waste) or radiation (light and heat).
Examples of chemical change include: · oxidation of metals (rusting) · corrosion of metals and stone caused by air pollution · damage to pigments by air pollution or reaction with other pigments · staining of paper documents by adjacent acidic materials · fading of dyes and pigments
Examples of chemical change include: darkening of resins darkening and embrittlement of pulp papers burning or scorching of material in a fire embrittlement of textile fibers bleaching of many organic materials cross-linking (development of additional chemical bonds) of plastics rotting of wood by growing fungus
What is physical deterioration? Physical deterioration is a change in the physical structure of an object. It is any change in an object that does not involve a change in the chemical composition. Physical deterioration is often caused by variation in improper levels of temperature and relative humidity or interaction with some mechanical force.
Examples of physical deterioration include: melting or softening of plastics, waxes, and resins caused by high temperature cracking or buckling of wood caused by fluctuations in relative humidity warping of organic materials caused by high relative humidity warping or checking of organic materials caused by low relative humidity ·
shattering, cracking, or chipping caused by impact stone cracking and scaling structural failure (for example, metal fatigue, tears in paper, rips in textiles) loss of organic material due to feeding by insects and/or their larvae ·staining of textiles and paper by mold
Physical deterioration and chemical deterioration are interrelated. For example, chemical changes in textiles caused by interaction with light also weaken the fabric so that physical damage such as rips and tears may occur.
Why is it important to understand the environmental agents of deterioration and how to monitor them? If you understand basic information about the chemistry and physics of temperature, relative humidity, light, and pollution, you will be better able to interpret how they are affecting your museum collections. This chapter gives you a basic overview of these agents and describes how to monitor them. You will be able to tell how good or bad the conditions in your museum are and whether or not the decisions you make to improve the environment are working the way you expect.
In the past, simplified standards such as 50% RH and 65°F were promoted. With research and experience, it is now understood that different materials require different environments. You must understand the needs of your collection for the long-term in order to make thoughtful decisions about proper care.
Microenvironments You will want to develop microenvironments for storage of particularly fragile objects. A microenvironment (microclimate) is a smaller area (box, cabinet, or separate room) where temperature and/or humidity are controlled to a different level than the general storage area. Common microenvironments include: · freezer storage for cellulose nitrate film · dry environments for archeological metals · humidity-buffered exhibit cases for fragile organic materials · temperature-controlled vaults for manuscript collections
Agents of Deterioration
Temperature What is temperature? Temperature is a measure of the motion of molecules in a material. Molecules are the basic building blocks of everything. When the temperature increases, molecules in an object move faster and spread out; the material then expands. When the temperature decreases, molecules slow down and come closer together; materials then contract. Temperature and temperature variations can directly affect the preservation of collections in several ways.
How does temperature affect museum collections? Temperature affects museum collections in a variety of ways. · At higher temperatures, chemical reactions increase. For example, high temperature leads to the increased deterioration of cellulose nitrate film. If this deterioration is not detected, it can lead to a fire. As a rule of thumb, most chemical reactions double in rate with each increase of 10°C (18°F). · Biological activity also increases at warmer temperatures. Insects will eat more and breed faster, and mold will grow faster within certain temperature ranges. ·
How does temperature affect museum collections? At high temperatures materials can soften. Wax may sag or collect dust more easily on soft surfaces, adhesives can fail, lacquers and magnetic tape may become sticky. In exhibit, storage and research spaces, where comfort of people is a factor, the recommended temperature level is 18-20° C (64-68° F). Temperature should not exceed 24° C (75° F). Try to keep temperatures as level as possible.
Avoid Fluctuating Temperatures Avoid abrupt changes in temperature. It is often quick variations that cause more problems than the specific level. Fluctuating temperatures can cause materials to expand and contract rapidly, setting up destructive stresses in the object. If objects are stored outside, repeated freezing and thawing can cause damage. Temperature is also a primary factor in determining relative humidity levels. When temperature varies, RH will vary.
Relative Humidity What is relative humidity (RH)? Relative humidity is a relationship between the volume of air and the amount of water vapor it holds at a given temperature. Relative humidity is important because water plays a role in various chemical and physical forms of deterioration. There are many sources for excess water in a museum: exterior humidity levels, rain, nearby bodies of water, wet ground, broken gutters, leaking pipes, moisture in walls, human respiration and perspiration, wet mopping, flooding, and cycles of condensation and evaporation.
Relative Humidity All organic materials and some inorganic materials absorb and give off water depending on the relative humidity of the surrounding air. Metal objects will corrode faster at higher relative humidity. Pests are more active at higher relative humidities. We use relative humidity to describe how saturated the air is with water vapor. “50% RH” means that the air being measured has 50% of the total amount of water vapor it could hold at a specific temperature.
Relative Humidity It is important to understand that the temperature of the air determines how much moisture the air can hold. Warmer air can hold more water vapor. This is because an increase in the temperature causes the air molecules to move faster and spread out, creating space for more water molecules. For example, warm air at 25°C (77°F) can hold a maximum of about 24 grams/cubic meter (g/m3), whereas cooler air at 10°C (50°F) can hold only about 9 g/m3.
Relative Humidity Relative humidity is directly related to temperature. In a closed volume of air (such as a storage cabinet or exhibit case) where the amount of moisture is constant, a rise in temperature results in a decrease in relative humidity and a drop in temperature results in an increase in relative humidity. For example, turning up the heat when you come into work in the morning will decrease the RH; turning it down at night will increase the RH. Relative humidity is inversely related to temperature. In a closed system, when the temperature goes up, the RH goes down; when temperature goes down, the RH goes up.
What is the psychrometric chart? The relationships between relative humidity, temperature, and other factors such as absolute humidity and dew point can be graphically displayed on a psychrometric chart
The following definitions will help you understand the factors displayed on the chart and how they affect the environment in your museum. · Absolute humidity (AH) is the quantity of moisture present in a given volume of air. It is not temperature dependent. It can be expressed as grams of water per cubic meter of air (g/m3). A cubic meter of air in a storage case might hold 10 g of water. The AH would be 10 g/m3. · Dew point (or saturation temperature) is the temperature at which the water vapor present saturates the air. If the temperature is lowered the water will begin to condense forming dew. In a building, the water vapor may condense on colder surfaces in a room, for example, walls or window panes. If a shipping crate is allowed to stand outside on a hot day, the air inside the box will heat up, and water will and condense on the cooler objects.
· Relative humidity relates the moisture content of the air you are measuring (AH) to the amount of water vapor the air could hold at saturation at a certain temperature. Relative humidity is expressed as a percentage at a certain temperature. This can be expressed as the equation: RH = Absolute Humidity of Sampled Air/ Absolute Humidity of Saturated Air at Same Temperature x 100
Example In many buildings it is common to turn the temperature down in the evenings when people are not present. If you do this in your storage space, you will be causing daily swings in the RH. Suppose you keep the air at 20°C (68°F) while people are working in the building. A cubic meter of air in a closed space at 20°C (68°F) can hold a maximum of 17 grams of water vapor. If there are only 8.5 grams of water in this air, you can calculate the relative humidity. The AH of the air = 8.5 grams The AH of saturated air at 20°C = 17.0 grams Using the equation above RH = 8.5 x 100%/17 = 50%
Example 50% RH may be a reasonable RH for your storage areas. But, if you turn down the heat when you leave the building at night, the RH of the air in the building will rise rapidly. You can figure out how much by using the same equation. If the temperature is decreased to 15°C (59°F), the same cubic meter of air can hold only about 13 grams of water vapor. Using the same equation The AH of the air = 8.5 grams The AH of saturated air at 15°C (59°F) = 13.0 grams RH = 8.5 x 100%/13 = 65% By turning down the heat each night and turning it up in the morning you will cause a 15% daily rise and fall in RH.
How do organic objects react with relative humidity? Organic materials are hygroscopic. Hygroscopic materials absorb and release moisture to the air. The RH of the surrounding air determines the amount of water in organic materials. When RH increases they absorb more water; when it decreases they release moisture to reach an equilibrium with the surrounding environment. The amount of moisture in a material at a certain RH is called the Equilibrium Moisture Content (EMC).
What deterioration is caused by relative humidity? Deterioration can occur when RH is too high, variable, or too low. · Too high: When relative humidity is high, chemical reactions may increase, just as when temperature is elevated. Many chemical reactions require water; if there is lots of it available, then chemical deterioration can proceed more quickly. Examples include metal corrosion or fading of dyes. High RH levels cause swelling and warping of wood and ivory. High RH can make adhesives or sizing softer or sticky. Paper may cockle, or buckle; stretched canvas paintings may become too slack. High humidity also supports biological activity. Mold growth is more likely as RH rises above 65%. Insect activity may increase. ·
What deterioration is caused by relative humidity? Too low: Very low RH levels cause shrinkage, warping, and cracking of wood and ivory; shrinkage, stiffening, cracking, and flaking of photographic emulsions and leather; desiccation of paper and adhesives; and dessication of basketry fibers.
What deterioration is caused by relative humidity? Variable: Changes in the surrounding RH can affect the water content of objects, which can result in dimensional changes in hygroscopic materials. They swell or contract, constantly adjusting to the environment until the rate or magnitude of change is too great and deterioration occurs. Deterioration may occur in imperceptible increments, and therefore go unnoticed for a long time (for example, cracking paint layers). The damage may also occur suddenly (for example, cracking of wood). Materials particularly at high risk due to fluctuations are laminate and composite materials such as photographs, magnetic media, veneered furniture, paintings, and other similar objects.
Light Light causes fading, darkening, yellowing, embrittlement, stiffening, and a host of other chemical and physical changes. Be aware of the types of objects that are particularly sensitive to light damage including: book covers, inks, feathers, furs, leather and skins, paper, photographs, textiles, watercolors, and wooden furniture.
What is light? Light is a form of energy that stimulates our sense of vision. This energy has both electrical and magnetic properties, so it known as electromagnetic radiation. To help visualize this energy, imagine a stone dropped in a pond. The energy from that stone causes the water to flow out in waves. Light acts the same way. We can measure the “wavelength” (the length from the top of each wave to the next) to measure the energy of the light.
The energy in light reacts with the molecules in objects causing physical and chemical changes. Because humans only need the visible portion of the spectrum to see, you can limit the amount of energy that contacts objects by excluding UV and IR radiation that reaches objects from light sources. All types of lighting in museums (daylight, fluorescent lamps, incandescent (tungsten), and tungsten-halogen lamps) emit varying degrees of UV radiation.
The unit of measurement is the nanometer (1 nanometer (nm) equals 1 thousand millionth of a meter). We can divide the spectrum of electromagnetic radiation into parts based on the wavelength. The ultraviolet (UV) has very short wavelengths ( nm) and high energy. We cannot perceive UV light. The visible portion of the spectrum has longer wavelengths ( nm) and our eyes can see this light. Infrared (IR) wavelengths start at about 760 nm. We perceive IR as heat.
The energy in light reacts with the molecules in objects causing physical and chemical changes. Because humans only need the visible portion of the spectrum to see, you can limit the amount of energy that contacts objects by excluding UV and IR radiation that reaches objects from light sources. All types of lighting in museums (daylight, fluorescent lamps, incandescent (tungsten), and tungsten-halogen lamps) emit varying degrees of UV radiation.
This radiation (which has the most energy) is the most damaging to museum objects. Equipment, materials, and techniques now exist to block all UV. No UV should be allowed in museum exhibit and storage spaces. The strength of visible light is referred to as the illumination level or illuminance. You measure illuminance in lux, the amount of light flowing out from a source that reaches and falls on one square meter.
Reciprocity law When considering light levels in your museum you should keep in mind the “reciprocity law.” The reciprocity law states, “Low light levels for extended periods cause as much damage as high light levels for brief periods.”
The rate of damage is directly proportional to the illumination level multiplied by the time of exposure. A 200-watt light bulb causes twice as much damage as a 100-watt bulb in the same amount of time. A dyed textile on exhibit for six months will fade about half as much as it would if left on exhibit for one year.
So if you want to limit damage from light you have two options: · reduce the amount of light · reduce the exposure time
What are the standards for visible light levels? You can protect your exhibits from damage caused by lighting by keeping the artificial light levels low. The human eye can adapt to a wide variety of lighting levels, so a low light level should pose no visibility problems. However, the eye requires time to adjust when moving from a bright area to a more dimly lighted space. This is particularly apparent when moving from daylight into a darker exhibit area. When developing exhibit spaces, gradually decrease lighting from the entrance so visitors’ eyes have time to adjust. Do not display objects that are sensitive to light near windows or outside doors.
Basic standards5 for exhibit light levels are: · 50 lux maximum for especially light-sensitive materials including: - dyed organic materials - textiles - watercolors - photographs and blueprints - tapestries - prints and drawings - manuscripts - leather - wallpapers - biological specimens - fur - feathers
· 200 lux maximum for less light-sensitive objects including: - undyed organic materials - oil and tempera paintings - finished wooden surfaces · 300 lux for other materials that are not light-sensitive including: - metals - stone - ceramics - some glass
In general don't use levels above 300 lux in your exhibit space so that light level variation between exhibit spaces is not too great.
In order for collections to be seen and used in various ways (for example, long-term exhibit, short-term exhibit, research, teaching) you should take into account a variety of factors: · light sensitivity of the object · time of exposure · light level · type of use · color and contrast of object
Dust and Gaseous Air Pollution Air pollution comes from contaminants produced outside and inside museums. Common pollutants include: dirt, which includes sharp silica crystals; grease, ash, and soot from industrial smoke; sulfur dioxide, hydrogen sulfide, and nitrogen dioxide from industrial pollution; formaldehyde, and formic and acetic acid from a wide variety of construction materials; ozone from photocopy machines and printers; and a wide variety of other materials that can damage museum collections.
pollutants are divided into two types: · particulate pollutants (for example, dirt, dust, soot, ash, molds, and fibers) · gaseous pollutants (for example, sulphur dioxide, hydrogen sulphide, nitrogen dioxide, formaldehyde, ozone, formic and acetic acids)
What are particulate air pollutants? Particulate pollutants are solid particles suspended in the air. Particulate matter comes both from outdoor and indoor sources. These particles are mainly dirt, dust, mold, pollen, and skin cells, though a variety of other materials are mixed in smaller amounts. The diameter of these pollutants is measured in microns (1/1,000,000 of a meter). Knowing the particulate size is important when you are determining the size of air filters to use in a building. Some particles, such as silica, are abrasive. Pollen, mold and skin cells can be attractive to pests. Particulates are particularly dangerous because they can attract moisture and gaseous pollutants.
Three forms of Damage Mechansims · A source for sulfates and nitrates (These particles readily become acidic on contact with moisture.) · A catalyst for chemical formation of acids from gases · An attractant for moisture and gaseous pollutants
What are gaseous air pollutants? Gaseous pollutants are reactive chemicals that can attack museum objects. These pollutants come from both indoor and outdoor sources.
Outdoor pollutants Outdoor pollutants are brought indoors through a structure’s HVAC system or open windows. There are three main types of outdoor pollution: · sulfur dioxide (SO2), and hydrogen sulphide (H2SO) produced by burning fossil fuels, sulfur bearing coal, and other organic materials · nitrogen oxide (NO) and nitrogen dioxide (NO2), produced by any kind of combustion, such as car exhaust as well as deteriorating nitrocellulose film, negatives, and objects · ozone (O3), produced by sunlight reacting with pollutants in the upper atmosphere and indoors by electric or light equipment, such as photocopy machines, printers, some air filtering equipment
When sulfur and nitrogen compounds combine with moisture and other contaminants in the air, sulfuric acid or nitric acid is produced. This acid then causes deterioration in a wide variety of objects. Ozone reacts directly with the objects causing deterioration.
The main sources of indoor air pollution · wood, which can release acids · plywood and particle board, which give off acids from wood and formaldehyde and acids from glues · unsealed concrete, which releases minute alkaline particles · some paints and varnishes, which release organic acids, peroxides, and organic solvents · fabrics and carpeting with finishes, such as urea-formaldehyde, and wool fabrics that release sulfur compounds. · glues, used to attach carpets, that can release formaldehyde · plastics that release plasticizers and harmful degradation products such as phthalates and acids
Museum objects themselves may also contribute to indoor air pollution. Examples of sources of pollutants from museum objects include: · celluloid and other unstable plastics used to produce many 20th-century objects · cellulose nitrate and diacetate plastic, used for film · pyroxylin impregnated cloth used for book bindings · residual fumigants, such as ethylene oxide
Object Materials Deterioration Primary by Air Pollutants
Metal corrosion/tarnishing sulfur oxides
stone surface erosion and discoloration
Soluble Salts and Deterioration of Archeological Materials Porous archeological artifacts such as ceramics, stone, bone, and ivory often contain soluble salts. Ground water and seawater can carry these salts into the pores of the artifact during burial leaving them behind when the water evaporates. After excavation, these salts can crystallize at or just below the surface of the artifact causing damage.
A variety of descriptive terms are used for this damage including spalling, flaking, powdering, and sugaring. The force of growing crystals can break apart the surface of bone, stone, ceramics and other porous materials so that detail is lost. In bad cases it can remove the entire surface of an artifact. In the worst cases, it can destroy an artifact.
Soluble Salts and Insoluble Salts Conservators divide the salts that are deposited in and on an artifact during burial into two groups: insoluble salts and soluble salts.
Soluble salts will dissolve in moisture in the air. This property is known as deliquescence. The salts can move through the porous structure of an artifact as moisture is drawn out through evaporation. As the salts reach the surface of the artifact they may crystallize as white, often furry growths on the surface. If the surface is less porous than the underlying structure they can crystallize just below the surface. These crystals exert immense pressure and may cause the surface layer to spall off.
Salt Damage to Porous Materials Salt damage is largely attributable to two mechanisms: crystallization of salts from solution hydration of salts, that can exist in more than one hydration state. The growth of salt crystals within pores can cause stresses, which are sufficient to overcome the stone's tensile strength When the migration of the salt to the surface of the stone is faster than the rate of drying, the crystals deposit on the top of the external surface and form visible efflorescences, which do not damage the stone. When the migration is slower than the drying rate, the solute crystallizes within the pores, at varying depth, causing crumbling and powdering of the stone.
Why does salt speed up corrosion? Water is required for corrosion and salt speeds up the process. Corrosion is the transfer of electrons from one substance to the other so salt present in water improves the capability of water to carry electron through redox reactions. Rusting in metals is the oxidizing of metal to metal oxide. Water acts as the medium to transfer the electrons and salt helps the corrosion process to speed up the process.
Insoluble salts “Insoluble” salts are not truly insoluble but will take days or weeks to dissolve in water. They are not deliquescent and so will not cause further damage after excavation. Insoluble salts can, however, be quite disfiguring, and may require removal for identification or reconstruction of an artifact.
Identifying Salts In order to identify the salts conservators use analytical methods such as spot tests or x-ray diffraction. Soluble salts are visible as a white growth on the surface of an artifact. In newly excavated material, they often form first along cracks or abraded areas of a surface. Often they can look like a white bloom or haze on the surface. As the crystals continue to grow and form they will extend further from the surface and appear as a white powder or even look somewhat like table salt. They may have a soft, fuzzy feel if touched.
Deterioration of Archaeological Materials
Deterioration of Ceramics, Glass, and Stone
Physical Forces The agents of deterioration that can have the most profound effect on ceramics, glass, and stone in museum collections are direct physical forces. If ceramic or glass objects are dropped, they usually break. Most stone will chip, crack, or break if dropped. Cumulative damage can occur with improper handling––pieces can be chipped off and residues left from handling. Some ceramic, glass, and stone objects also have flaws, either inherent or from their previous use, that make them vulnerable to heat or moisture.
Deterioration of Pottery and Ceramics
How ceramics were made? Ceramic objects are made up of a mixture of natural materials that are combined, formed into shape by a variety of processes, and transformed by heat to create a solid, brittle substance not found in nature. Different firing temperatures produce objects with a vast range of hardness and porosity.
Most clay objects are a mixture of materials: Clay is a fine-grained mineral--the smallest particles produced by the weathering of certain rocks. When heated to a high temperature it chemically and physically changes to a hard, brittle material. Adding fluxes such as soda, mica, potash, magnesia, or lime lowers the firing temperature of clay. These fluxes may also be found in natural clay deposits. Non-plastic additives (temper) are added to clay to reduce shrinkage and cracking during firing and drying. Temper also increases porosity in the finished object. These basic materials are mixed together by the potter to produce a heterogeneous plastic mass that is then formed into the ceramic object.
Ceramics are loosely divided into four groups. These groups are based on their firing temperature, clay type, and physical characteristics: Adobe or mudbrick is an unfired clay mixture. This material is often used for building, but mudbrick objects, such as cuneiform tablets and sculpture, are often found in museum collections.
Earthenware Earthenware is a low-fired clay mixture. These objects are fired between about ºC. At this low temperature sintering occurs but not vitrification. Earthenware is generally soft and scratches easily. It is often red in color from naturally occurring iron in the clay; brown, black, and yellow are also common colors. Earthenware has the following characteristics: − It is porous and will readily absorb water unless glazed. − The structure is often granular in appearance with numerous coarse particles. − There is a clear distinction between the ceramic body and any glaze layer.
Stoneware Stoneware is fired between ºC. Stoneware objects are partially vitrified. Common colors for stoneware are buff, brown, and gray. Stoneware has the following characteristics: − It is partially vitrified and less porous than earthenware. − It is harder and denser than earthenware and does not scratch easily. − If tapped lightly, the body will give a distinctive ring. − The glaze and body are tightly adhered.
Porcelain is fired at very high temperatures, usually above 1300ºC. Porcelain is made of a special clay called kaolin. This clay is difficult to work and must be fired under precise conditions. Porcelain can be formed into objects with thin, complex structures. Porcelain has the following characteristics: − The body is completely vitrified and impervious to water (nonporous). − The clay body is white and translucent and extremely hard and brittle. − When tapped lightly, the object rings with a higher tone than stoneware. − In cross-section, glaze and body are nearly indistinguishable.
A general rule of thumb is that lower-fired ceramics will easily absorb water, while higher- fired ceramics will absorb little or no water. To test this, you can use a small paintbrush to apply a little water to an unglazed area of ceramic, and watch to see if it is drawn in. Because high-fired ceramics are less likely to absorb water, they have fewer salt problems
Ceramics may have different surface finishes, coloration, or impressed designs. A glaze is a thin layer of clear or colored glass on the ceramic surface. A slip is usually more like a thin layer of clay and has a matte appearance and is a different color than the clay body. Ceramics may be coated with other materials as well, including paints and inks.
Ceramics are decorated most commonly with a slip or glaze that is fired on, or melted onto the surface when it is fired. Ceramics with a fired-on overall glaze or other decorations are impervious to normal variations in temperature below several hundred degrees. These fired-on decorations also help protect ceramics from humidity. In recent decades, some ceramics have been initially fired but later decorated with paint or some other decoration that is never fired. These unfired decorations are very fragile and are easily damaged by exposure to water, heat, or light.
Repaired ceramics may suffer damage from temperature and humidity extremes. Broken ceramics reassembled with adhesive have weaknesses. Most adhesives soften and give way at elevated temperatures. Ceramic pots with repairs may sag, collapse, or fall apart if they are stored in a hot area, such as an attic or a building that does not have air conditioning.
Salts can damage or destroy ceramics. The clay may have originally contained a significant amount of salt, and other types of earth added to adjust the properties of the clay may include salt. Water or foods stored in ceramic vessels often leave salts behind. Contact with seawater or burial below ground can also introduce salts.
Fluctuating humidity levels aggravate the harmful effects of salts in ceramics. Above 60 percent relative humidity, the salts dissolve and move around inside the ceramics. When the ceramics dry, the salts migrate to the surface and are left behind when the water evaporates. This is called salt efflorescence. Efflorescence generates tremendous forces, pushing off areas of glaze or decoration and even breaking up entire ceramics.
Generally, mold will not grow on ceramics, and insects will not attack them. In very wet conditions, however, mold or lichens may grow on ceramic surfaces, although the mold will not digest the ceramic itself. Insects will eat food residues left on ceramics and will eat materials applied after firing. However, proper environmental conditions prevent mold, lichens, and insects.
Light is not harmful to ceramics as such, but pigments used in surface decoration could be damaged by over exposure.
Some old repair methods have caused damage in the long term. Very strong adhesives were used in the past, but in ageing they have been found to discolour and shrink, and in shrinking a layer of the ceramic can be pulled away from the body of the pot. Today's conservators have a wide range of adhesives from which to choose. Those used with ceramics will usually be weaker than the ceramic body to prevent too strong a join from causing further damage.
What flaws might I find in ceramic objects? It is important to recognize the flaws that may occur during the manufacturing process so you can separate flaws from damage or active deterioration.
Deterioration of glass
The Nature of Glass Objects Glass has been used for personal adornment, containers, construction materials, and a host of other purposes throughout the last four millennia. In order to understand how to preserve glass objects, you must understand how they are produced.
What materials make up the structure of glass objects? The basic materials of glass are silica and alkaline oxide (also known as flux). Silica generally comes from sand or crushed flint. The flux interacts with the silica and lowers the melting temperature. Typical fluxes include lead, calcium, potassium, and sodium oxides. Other oxides (iron, copper, cobalt, manganese, chromium and nickel) are added as colorants. When melted, this mix of materials flows readily to form various shapes.
Glass component Relative amount (%) Silica (glass former) 60–75 Soda (modifier) 30–15 Lime (stabilizer) 15– 8 Iron oxides (generally added unintentionally) 10–less than 1
GLASSMAKING The basic process for making glass, although not the actual technology, has changed little since antiquity. Over the centuries the technology has advanced, being continuously improved and refined beyond recognition Six main manufacturing stages are involved in the glassmaking process: 1. Selecting the raw materials 2. Comminuting and mixing the raw materials 3. Heating and melting the mixture 4. Fabricating, that is, forming and shaping objects 5. Annealing the objects 6. Finishing
Glass is a unique material––a rigid liquid. A liquid is an amorphous material that does not have an organized, crystalline structure. Most materials, such as metals, form a crystalline lattice as they cool from a liquid to a solid state. Molten glass, however, cools too quickly for this structure to form. The structure is "frozen" into a random network of molecules. Glass is rigid and brittle at room temperature. Depending on the materials included in the mix, it can be transparent, translucent, or opaque.
Lattice VS Amorphous
Glazes and Enamels Glazes and enamels are also glasses with small differences in composition from bulk glass. Glazes are applied to ceramics; enamels are usually applied to a metal support. Glazes and enamels are generally opaque and fired at lower temperatures than glass.
What flaws might I find in glass objects? Flaws can be introduced during the manufacturing process. Learn to distinguish these flaws from active deterioration problems. Look for:
Bubbles They may also be added intentionally for decorative effect. A few isolated bubbles will not weaken a glass object, however, a cluster of bubbles might. The shape of the bubbles gives clues to the direction that the object was worked in the molten state.
Inclusions Inclusions or foreign bodies: These are more noticeable in translucent glass. Often these flecks come from contamination in the crucible or impurities in the raw materials. Small inclusions may disrupt the surface and look of an object, but they will not affect its strength. .
Compositional flaws Compositional flaws: Sometimes these are not apparent for many years.
How does glass deteriorate? Most damage to glass is mechanical. It is easily broken and chipped. Water is the major chemical agent of deterioration for glass and the susceptibility of glass to deterioration depends greatly on its original chemical structure.
THE DECAY OF GLASS Glass, a supercooled liquid, is in a metastable state, that is, an apparently stable condition that may be perturbed by external conditions and undergo unpredictable changes, so that the supercooled liquid may be converted to a solid. When glass is made from a well-balanced mixture of former, modifier, and stabilizer, it is remarkably stable.
Environmental changes may, however, cause the glass to crystallize, or, as the condition is known, to devitrify, that is, to lose its vitreous (glassy) properties.
Glass exposed to the environment or buried in the soil under dry conditions, even for long periods of time, is usually stable and undergoes very little devitrification or decay. The more humid the environment or burial site, the more easily glass decays and the more extensively it devitrifies. Extended periods of alternating dry and wet conditions may result in periodic decay effects and the formation of devitrified layers, first on the outer faces and then throughout the bulk of the glass
The chemical decay of glass often starts when its alkaline components, soda, potash, or lime, are leached by water from the surrounding of the glass (leaching is the process of extraction of the soluble components of a solid by their dissolution, usually in water but also in mild acids).
Decay of Glass The tendency to, and the extent of decay of glass are determined mainly by its composition, the environmental temperature and humidity, and/or the surrounding water conditions at the location sit of the glass. Salts in, and the pH of groundwater, and even microorganisms with which glass is in contact, alter its rate of decay.
Stages of Glass Decay Various stages in the decay of glass have been defined: dulling, which entails the loss of clarity and transparency, is the simplest; frosting, the formation of a network of small cracks on the surface follows;
Stages of Glass Decay Strain cracking, the occurrence of small cracks running in all directions, is a more advanced form of decay that may result in the partial or total disintegration of glass
Stages of Glass Decay Frosting and strain cracking Take place particularly when water is abundant: the water leaches from the glass most of the soda and potash and part of the lime, leaving behind only thin layers of hydrated silica.
Stages of Glass Decay The final stages of such a decay process may result in the glass becoming just a residue of generally separate, flaky, highly porous, layers of hydrated silica displaying a sugarlike appearance that eventually totally disintegrates.
Crusts of weathered glass. The deterioration of glass and the formation of weathered layers under humid or wet conditions is a rather complex process. Seasonal variations of temperature and of the amount of environmental moisture may provide the trigger that initiates the glass weathering process. Still, the final product of the process are opaque layers composed mainly of hydrated silica (silica is the main component of glass).
Partial leaching and devitrification , Although basically damaging, often enhance the appearance of old glass; they give origin to iridescence, the display of rainbow-like variegated colors when illuminated old glass is moved or turned. Iridescence generally occurs on ancient glass that has been recurrently exposed to seasonal variations, in some climates in yearly cycles, of temperature and humidity. Such cyclic processes result in the formation, on the outer, exposed faces of the glass, of very thin layers of decay products (composed mainly of hydrated silica); the thickness of single layers has been measured and found to vary in the range 0.3–15 microns. Light incident on these layers causes interference between beams reflected from their front and back surfaces and gives rise to the variety of colors often seen on ancient glass objects
Crizzling Glass : Crizzling is a fine network of surface cracks that turn glass translucent. Moisture in the air reacts with unstable glass containing too little lime (calcium oxide). The moisture causes potassium and sodium in the glass structure to leach out. As the structure weakens, small cracks appear.
Weeping Glass Weeping is caused by leaching sodium or potassium absorbing water on the surface of deteriorating glass to form sodium or potassium hydroxide. These compounds accumulate on the surface of the glass and may give it a greasy feeling. The hydroxides may also react with carbon dioxide in the atmosphere to form carbonates, which can absorb even more water.
Crusty Crusty or waxy deposits on the surface, which may have a white crystalline appearance, are typically seen on ethnographic beadwork and may be a reaction of the glass deterioration products to oils in adjacent leather.
Iridescence Iridescence is a rainbow-like effect on the glass surface and is an indication of deterioration. The colors are visible when light is diffracted between the air-filled layers of deteriorated glass.
Devitrification Devitrification is the production of small areas of crystal growth in the otherwise amorphous glass structure. These crystals may be intentionally produced during production as they give glass good thermal shock resistance. Unintentional devitrification is caused by unstable glass with too much alumina or too much calcium.
DETERIORATION OF BUILDING STONE
The factors considered to be among the leading causes of building stone deterioration include: salt crystallization aqueous dissolution frost damage, microbiological growth human contact original construction
Salt Crystallization Crystallization of salts within the pores of stones can generate sufficient stresses to cause the cracking of stone, often into powder fragments. This process is considered to be the major cause of stone deterioration
Closely related to the crystallization of salt is damage caused by salt hydration and by differential thermal expansion of salts
The resistance of stone to salt damage is dependent on the pore size distribution and decreases as the proportion of fine pores increases Crystallization damage caused by highly soluble salts, such as sodium chloride and sodium sulfate, is usually manifested by powdering and crumbling of the stone's surface
Less soluble salts such as calcium sulfate form glassy, adherent films which cause spalling of a stone's surface
A major source of salts in urban environments is the reaction between air pollutants and stone. For example, limestone can react with sulfur dioxide to ultimately produce calcium sulfate. Other sources of salts include ground water airborne salts sea spray and chemical cleaners
Aqueous Dissolution (Pollution) Carbonate sedimentary stones e.g., limestone), carbonate-cemented sandstone, and marbles are types of stone that are susceptible to dissolution by water acidified with dissolved carbon dioxide, sulfur dioxide, and nitrogen oxides problem
It has been reported that the rainwaters in many urban areas in the United States and Europe are sufficiently acidic to accelerate the weathering of exposed building stone. In areas where the rainwater is relatively free from pollutants, the dissolution of most common building stones is usually not a serious problem
Frost Damage Certain stones which are exposed to freezing temperatures and wet conditions may undergo frost damage. The frost susceptibility of a stone is largely controlled by its porosity and pore size distribution ].
Of stones with a given porosity, those with the smallest mean pore size will generally be the most susceptible to frost damage. Frost resistance also generally decreases with increased available porosity pore volume which is accessible to water.
Some European stone conservators believe that in their countries frost damage is not an important process in the deterioration of stone. They regard frost damage as a secondary process, e.g., frost damage may be responsible for the final fragmentation of stone damaged by other processes, such as salt crystallization. However, because of the use of possibly more frost- susceptible stone and more severe climates, frost damage may be an important factor in the southern and eastern part of Jordan.
Microbiological Growth The attack of stone by a variety of plants and animals has been reported including roots of plants, ivy vines, microorganisms, boring animals, and birds. Of these, microorganisms appear to be the most destructive.
Microorganisms Some types of bacteria, fungi, algae, and lichens produce acids and other chemicals which can attack carbonate and silicate minerals . It appears that under certain environmental conditions attack by microorganisms can be a serious problem . However, it seems that many conservators feel that such instances are uncommon and that microorganism growth usually takes place in stone which had been partially deteriorated by other processes.
Human Contact Because of an increasing interest by the public in historic structures, the effects of human contact upon the condition of stone, as well as all other building materials, is of growing concern. For example, stone floors are gradually worn by foot traffic, stones are damaged by people either collecting souvenirs or poking into soft stone, and graffiti removal has become an important maintenance problem. It is conceivable that human contact may become a major problem challenging the ingenuity of both stone conservators and maintenance specialists.
Tourism and Urban development
Original Construction The durability of stone structures also depends on factors encountered during their original construction including proper design, good construction practices, and proper selection of materials. Unfortunately, these are factors over which the preservation scientist has no control. However, the same mistakes should not be repeated in repairing or restoring historic structures. For example, normal steel and cast iron anchors, dowels, reinforcing rods, etc., were often used in the construction or repair of stone structures. Certain ferrous metals are susceptible to corrosion which can lead to the cracking and spalling of stonework . Therefore, noncorroding material should be selected, e.g., epoxy-coated steel ], certain types of stainless steel ], or non- corroding non-ferrous alloys.
A large portion of stone durability problems are the consequence of using poor quality stone in the original construction.
Corrosion of Metals
3.1-What happens to ancient metal as it ages? Metals in nature, the way they are found in the ground, are generally fairly stable. Malachite, the gemstone, is for instance, a stable form of copper found in nature. It has naturally combined with things in the environment to create a substance that looks almost nothing like the metal copper, yet it is made of more than 70% copper and can be refined to create metallic copper. When the metal copper, which is not stable, is returned to the earth, it will unrefine itself, slowly, recombining naturally with elements in the soil, and the result, within a few hundred years will be a layer of malachite and other related minerals on the surface of the metal. This is the type of deterioration known as verdigris.
3.1-What happens to ancient metal as it ages? Another example of this is the black or gray tarnish that you see on silver items. This is silver combining with sulfur in the environment, and copper alloyed into the silver, combining with oxygen, both returning to a stable natural condition, and at the same time, becoming less attractive and useful.
3.1-What happens to ancient metal as it ages? So, to clarify, metal ores are, through heat, refined and purified into pure metals that must eventually, at normal temperatures, combine with elements in their environment and return to their more stable natural states. This process can take hundreds or even thousands of years, and is what we know as patination, verdigris, corrosion, and the other properties of aged metal.
3.1-What happens to ancient metal as it ages? The second important thing that happens as metals age is that, those which are alloyed, or made of a combination of two or more metals, may separate slowly into their components. An example of this is ancient silver coins which become brittle. Silver used in coins is almost always a combination of silver with about 1.5 to 15% copper. Adding a little bit of copper to silverlowers it's melting point and makes the normally soft silver harder, and more resistant to wearing down. Silver and copper don't really mix all that well, however, and over time ( years or more), at normal temperatures, the copper will sometimes begin to separate itself from the silver.
3.1-What happens to ancient metal as it ages? The technical name for this is the precipitation of copper at the grain boundaries, which means copper coming out of the alloy at the edges of the natural crystals of the metal. This is known as crystallization of the metal, to coin collectors, all though it is really just the crystals of the metal becoming visible as the copper comes out of the alloy and begins to corrode, thus weakening the metal. To clarify this point, some alloys are not stable, and, over hundreds or thousands of years, they will begin to separate back into more stable natural states.
3.2-Corrosion of metals The overall driving forces of nature work to return metals to their stable oxidised states, that is, combined with oxygen, sulphates, carbonates, sulphides and chlorides. Unoxidised or native metallic element is produced when metals are unbound from their compounds with oxygen, sulphate, carbonate, sulphide and chloride.
3.2-Corrosion of metals For this to happen there must be a sufficient driving force available through a high energy intervention. This intervention can be a carbon reduction or smelting. When metal ores are processed to produce metals, they start to corrode.
3.3-A simple overview of corrosion The corrosion of metals consists of two separate reactions: an oxidation reaction; and a reduction reaction.
Oxidation-reduction To explain these reactions, it is necessary to give a simple overview of the structure of atoms. Atoms are made up of a nucleus which contains neutral particles called neutrons and positively charged particles called protons. Electrons, which are negatively charged particles, orbit around the nucleus of the atom. The number and activity of the electrons will determine how readily the atoms will react with other atoms. Many metals, because of the way their molecules are structured, can readily lose electrons. When they do this, they are no longer atoms. They are positively charged and are called ions. Because of the charge, ions are not stable and combine readily to achieve a stable, electrically neutral state.
Oxidation An oxidation reaction is one in which an atom loses electrons. This can be represented very simply by the equation: M Mn+ +ne- where 'n' represents the number of electrons lost For example, copper—Cu—can be put into this equation. In an oxidation reaction: Cu Cu++ e- It can be oxidised further: Cu Cu2+ + e-
Copper is described as polyvalent, that is, it has different combining powers: a Cu+ ion needs one negative ion to achieve a stable state, while a Cu2+ ion needs two negative ions to form neutral compounds. Once these ions combine with other substances, they produce cuprous and cupric compounds respectively. For example, Cu2O is cuprous oxide or copper (I) oxide and CuO is cupric oxide or copper (II) oxide.
These electrolytic reactions are used to produce solid metals from their ionic solutions. The negative ions can be supplied by a range of materials. For example, if the metal object is in a seaside location, chloride ions—Cl-—will combine readily with the metal ions. They will also combine with: sulphides—SO3-—sulphates—SO4-2—nitrates— NO32-—from atmospheric pollutants; and oxygen.
If the metal combines with oxygen, it forms a metal oxide on the surface of the metal. If this metal oxide is continuous, then the overall corrosion rate of the underlying metal will slow down and it will become passivated or protected.
3.4-Corrosion cells Corrosion cells are small areas on metal objects where electrical differences are set up. Electrons flow between the charged areas, just as an electrical current flows between the positively and negatively charged electrodes of a battery.
A corrosion cell is an electrochemical cell which acts very much like a battery. The corrosion of metals consists of two separate reactions: oxidation. The oxidation reactions are called anodic reactions; and reduction. The reduction reactions are called cathodic reactions.
In an electrochemical cell the anodic, oxidation, half of the cell produces electrons as the metal is oxidized, while at the cathodic half of the cell, reduction occurs. The electrons are taken and held by the oxidising agent, which in aerated environments is oxygen. In a corrosion cell, these reactions can continue in a cycle. The localised corrosion activity causes pitting in the metal. The rate at which the electrons move out of the metal and across into the oxygen molecules is the principal factor controlling the overall corrosion rate.
3.5-Fats, oils and sweat Organic acids—formed by the oxidation of oils and fats—are capable of attacking metals which rely on a protective oxide coating to produce a good corrosion resistance. To prevent this type of damage, avoid direct contact between the object and the source of the organic material. Some examples of this type of damage are leather objects with copper fittings. The gradual deterioration of old candle wax in leather-lubricating oils leads to organic acids penetrating the protective copper oxide film, and reacting with the underlying metal— to form outgrowths of bright green organic copper compounds.
Human sweat on metal objects causes corrosion. Bacterial reactions with sweat can produce sulphides as metabolic by-products, and convert inherently inert sulphate ions into reactive sulphide ions. Uneven coatings of oil—from sweaty hands for instance—can alter the ease of access of oxygen to metal surfaces. This has two major effects. It hinders the formation of passivating layers of corrosion. It also alters the relative reactivities of areas of the metals; and so it causes one part of the metal to corrode at the expense of another.
3.6-Acids Inorganic acids such as hydrochloric acid—derived from the decay of plastics like polyvinyl chloride—and nitric and sulphuric acids—derived from air pollution—will attack metals which are either in the same storage environment as the plastic or in the open air.
3.6-Acids Anything that prevents direct contact between the metal surface and acidic solutions helps to prolong the life of the object. Therefore, vapour phase inhibitors, lacquers, waxes and other coatings minimize the damage from air pollution. The filtering of external air also greatly helps to minimize corrosion damage.
3.6-Acids Normally unreactive metals such as copper and silver can suffer significant corrosion in the presence of sulphide ions. Common sources of sulphide ions are: hydrogen sulphide—H2S—from the anaerobic decay of plant material; and carbonyl sulphide—COS—from the degradation of sulphur-containing proteins, such as those found in wool.
3.7-Forms of corrosion There are eight forms of corrosion (based on visual characteristics. These are: 1) Uniform corrosion: most common form of corrosion, characterized by a chemical- electrochemical reaction over the entire exposed surface;
Galvanic Corrosion 2) Galvanic corrosion or two- metal corrosion: driving force for current flow and metal corrosion is the potential developed between the two metals;
Crevice Corrosion 3) Crevice corrosion: intense localized corrosion that occurs frequently within crevices on metal surfaces exposed to corrosives;
Pit corrosion 4) Pit corrosion: localized attack that results in holes in the metal. This is one of the most destructive forms of corrosion;
Intergranular corrosion 5) Intergranular corrosion: localized attack at and adjacent to grain boundaries, with relatively little corrosion of the grains. It can be caused by impurities at the grain boundaries causing the alloy to disintegrate and/or lose its strength;
Selective leaching 6) Selective leaching: removal of one element from a solid alloy by corrosion processes;
Errosion corrosion 7) Erosion corrosion: acceleration or increase in rate of deterioration or attack on a metal because of relative movement between a corrosive fluid and the metal surface
Stress corrosion 8) Stress corrosion: caused by the simultaneous presence of tensile stress and a specific corrosive medium.
Recent studies have expanded the corrosion categories and redefined them by mechanisms rather then by visual appearance. Overlap between the mechanisms may exist. Factors affecting Corrosion can be accelerated by differential temperature cells or by the presence of mechanical forces in conjunction with chemical forces.
There are six major factors that affect the rate of corrosion of alloys in an aqueous environment: 1) acidity; 2) presence or absence of oxidizing agents; 3) presence or absence of films on the alloy; 4) temperature; 5) velocity of moving aqueous solution; 6) heterogeneity both in the solution and in the alloy.
3.8-Effect of environment on metal corrosion:
3.8.1-Atmospheric Environments Specific factors influencing the corrosivity of atmospheres are dust content, gases in the atmosphere, and moisture (critical humidity). Atmospheres are often classified as rural, industrial, or marine in nature, but this is an over simplification. There are locations along the seacoast that have heavy industrial pollution in the atmosphere and so are both marine and industrial. Two decidedly rural environments can differ widely in average yearly rainfall and temperature and therefore can have considerably different corrosive tendencies.
3.8.1-Atmospheric Environments Industrial expansion into formerly rural areas can easily change the aggressiveness of a particular location. Finally, long-term trends in the environment, such as changes in rainfall pattern, mean temperature, and acidity of the rainfall, can make extrapolations from past behavior much less reliable. Other factors that limit the usefulness of atmospheric exposure data are the general nonlinearity of weight loss due to corrosion over time and the fact that most atmospheric corrosion data are presented as an average over the entire test panel surface. Most atmospheric exposure data for steels show a decrease in the rate of attack with duration of exposure so that extrapolations of such data to times longer than those covered by the exposure data can lead to significant errors.
3.8.1-Atmospheric Environments Atmospheric corrosion proceeds in a relatively complicated system consisting of surface electrolyte, atmosphere, metal, and corrosion products. Analyses of the corrosion products give the following general characteristics. Nearly all nitrates and acetates are soluble and these anions are not found in corrosion films. An exception to this is with copper where basic nitrates have been detected. Simple chlorides and sulfates are soluble and generally are not found in corrosion films.
3.8.1-Atmospheric Environments All hydroxides are insoluble as are many mixed salts that include hydroxide as one of the constituents. Both hydroxides and mixed salts are common corrosion products. Normal and hydroxy carbonates are common constituents of corrosion films. Atmospheric corrosion is an electrochemical process with the electrolyte being a thin layer of moisture on the metal surface. The composition of the electrolyte depends primarily on the deposition rates of the air pollutants and varies with the wetting conditions.
3.8.1-Atmospheric Environments Environmental factors can cause the median thickness loss to vary by as much as 50% or more in a few extreme cases. Those environmental factors that tend to accelerate metal loss include high humidity, high temperature (either ambient or due to solar radiation), proximity to the ocean, extended periods of wetness, and the presence of pollutants in the atmosphere, such as sulfur oxides (SOx), nitrogen oxides (NOx), hydrogen sulfide, ammonia, and carbonyl sulfide (COS).
3.8.1-Atmospheric Environments The most important corrosive constituent of industrial atmospheres is sulfur dioxide, which originates predominately from the burning of coal, oil, and gasoline. The small amount of carbon dioxide normally present in the air, neither initiates nor accelerates corrosion. Metallurgical factors can also affect metal loss. Within a given alloy family, those with a higher alloy content tend to corrode at a lower rate. Surface finish also plays a role in that a highly polished metal will corrode slower than one with a rougher surface.
3.8.1-Atmospheric Environments The atmospheric contaminants most often responsible for the rusting of structural stainless steels are chlorides and metallic iron dust. Chloride contamination may originate from the calcium chloride used to make concrete, from exposure in marine or industrial locations, or from the use of road salts. Rural atmospheres, uncontaminated by industrial fumes or coastal salt, are extremely mild in terms of corrosivity for stainless steel, even in areas of high humidity. Industrial or marine environments can be considerably more severe.
3.8.1-Atmospheric Environments Copper and copper alloys are suitable for atmospheric exposure. They resist corrosion by industrial, marine, and rural atmospheres, except atmospheres containing ammonia, sulfur dioxide, or oxides of nitrogen. The severity of the corrosion attack in marine atmospheres is somewhat less than that in industrial atmospheres, but greater than that in rural atmospheres. However, these rates decrease with time due mainly to the formation of a protective film (e.g., copper chloride or copper sulfate) that develops on the surface of the copper or copper alloy.
3.8.2-Soil and Groundwater Environments Soils are defined as unconsolidated rock material over bedrock and/or freely divided rock-derived material containing a mixture of organic matter and capable of supporting vegetation. Worldwide, corrosion of metals in soil is responsible for a large percentage of corrosion and corrosion failures. While several individual characteristics of soils have been used to indicate the corrosivity of soils, currently no method describes the synergistic effects of these characteristics.
In particular, the corrosivity of soil is based largely upon the interaction of electrical resistivity, dissolved salts, moisture content, total acidity, bacterial activity, and concentration of oxygen. Other secondary factors are also important but are more difficult to define. Thus, simply testing metals and alloys in a variable pH solution or in aerated or deaerated solutions will not accurately describe the conditions in soil. In addition, soil environments are generally stationary electrolyte exposure conditions. Therefore, depleting and/or concentrating effects can occur at the surface of alloys.
Factors controlling soil corrosion However, some factors associated with the soil environment which can have an impact on the corrosion rates of metal alloys include: soil texture, internal drainage, resistivity, redox potential, moisture content, permeability, chloride ion content, sulfide and sulfate ion content, presence of corrosion-activating bacteria, oxygen content, pH, total hardness and hardness as calcium carbonate of soil moisture, and stray direct currents (dc).
Soil texture Soil texture is determined by the proportions of sand, silt, and clay that make up a soil. Clay, having the finest particle size and minimum pore volume between particles, tends to reduce the movement of air and water and can develop conditions of poor aeration when wet. Sand has the largest particle size and promotes increased aeration and distribution of moisture. Soil texture thus has as important influence on the diffusivity of soluble salts and gases.
Internal drainage Internal drainage is that property of soil that describes the water retention properties of a soil and is related to soil texture. Internal drainage is also affected by the height of the water table. Thus, a sandy soil which would normally have good permeation to moisture is considered to have poor internal drainage if the water table is high and keeps the soil in a saturated condition.
Soil resistivity Soil resistivity is a measure of how easily a soil will allow an electric current to flow through it. This is also a measure of how effective the soil is as an electrolyte. The lower the resistivity of a soil, the better it will behave as an electrolyte and the more likely it is to promote corrosion. A soil with a resistivity below 500 ohm-cm is considered to be corrosive. Above 2000 ohm- cm the relation of soil resistivity to soil corrosivity is less reliable.
Temperature The temperature of the soil is an important factor in the corrosion process. The resistivity of soil is inversely proportional to temperature and therefore an increase in soil temperature would be expected to increase the rate of the corrosion reaction. However, an increase in temperature also reduces the solubility of oxygen, which tends to reduce the rate of reaction. The net result is that soil temperature does not have as large an effect on underground corrosion as would be expected.
pH Soil pH is the acidity or alkalinity of the soil media. Most soils and all loams are fairly well buffered, resulting in a soil pH that is not affected by rainfall. Sand, because of its high moisture diffusivity, can have its soluble salts leached out or diluted to the point that its pH will change during a heavy rain. This can cause the corrosion rate to either increase or decrease.
Redox potential The redox potential or oxidation- reduction potential of a soil gives an indication of the proportions of oxidized and reduced species in that soil. Very high corrosion rates can occur in poorly aerated (reducing) soils where anerobic bacteria often thrive.
The presence of increasing concentrations of chloride ions lowers the resistivity of soil and water and will cause an increase in the corrosion rate. The presence of sulfides and sulfates is often an indicator of sulfate reducing bacteria (SRB’s). These bacteria can shift the pH in the acidic direction, causing an increase in corrosion. The higher the water hardness, i.e., the higher the concentration of calcium carbonate in the soil, the lower the corrosion rate will be.
Seawater or Marine Environments Seawater is a biologically active medium that contains a large number of microscopic and macroscopic organisms. Many of these organisms are commonly observed in association with solid surfaces in seawater, where they form biofouling films.
Immersion of any solid surface in seawater initiates a continuous and dynamic process, beginning with adsorption of nonliving, dissolved organic material and continuing through the formation of bacterial and algae slime films and the settlement and growth of various macroscopic plants and animals. This process, by which the surfaces of all structural materials immersed in seawater become colonized, adds to the variability of the ocean environment in which corrosion occurs.
The amount of oxygen and other gases dissolved in seawater depends on the temperature and the salinity of the seawater and the depth. In some seawater compositions, hydrogen sulfide is also present. Hydrogen sulfide is formed in seawater by the action of sulfate-reducing bacteria (SRB), usually under deposits where oxygen is depleted or when the seawater is stagnant or polluted and becomes anaerobic, even in large volumes. Silt deposits in estuarial waters are also contributory. Mineral and organic materials are also carried in suspension by the seawater, particularly near the mouths of rivers.
Since seawater is a complex, delicately balanced solution of many salts containing living matter, suspended silt, dissolved gases, and decaying organic material, the individual effect of each of the factors affecting the corrosion behavior is not readily separated. Because of the interrelation between many of the variables in the seawater environment, an alteration in one variable may affect the relative magnitude of the other variables. The factors which effect the amount or rate of corrosion may be divided into chemical, physical, and biological.