2Gypsum Materials Gypsum is a mineral that is widely found in nature. Gypsum is ideal for making dental casts.The production of gypsum casts requires meticulous attention to detail.Inaccurate or incomplete casts are of little use to the dentist.
3PropertiesChemically, the mineral gypsum is a dihydrate of calcium sulfate.Gypsum is mined in a solid mass and then is ground into a powder called a hemihydrate.When a hemihydrate is mixed with water, it becomes a flowable mixture.An exothermic chemical reaction occurs, and the material hardens.
4Properties (cont’d)Even though gypsum is always basically the same, different dental plasters are formed according to how the material is processed.If the material is heated in open vats at approximately 115°C, the material will be porous and irregular in shape.This is called model plaster of β hemihydrate.
5Properties (cont’d)If the heating process is done under pressure and at a higher temperature, a more uniformly shaped and less porous form is produced.This is called a dental stone or a hemihydrate.A further increase in pressure and heat will produce a material called die stone.
6Strength and HardnessTwo factors contribute to the strength and abrasion resistance of the final product: shape of the particles and porosity.The strength of gypsum products is related to the amount of water used to produce the study or working cast.Factors that affect the strength of gypsum products also affect their hardness.Increased porosity of the particles makes it necessary to use more water to convert the hemihydrate particles back to dihydrate particles.
7Physical PropertiesPhysically, gypsum products are manufactured as plaster, stone, high-strength stone, and gypsum-bonded investment.Differences in these types of gypsum depend on the size and shape of the particles, the porosity of the powder, and the way it was processed.
8Physical Properties (cont’d) Larger, irregular particles make a weaker stone:Break easilyAbrade easilySmaller, more uniform particles are stronger:Do not break easilyDo not abrade
9Dimensional AccuracySetting expansion occurs with all gypsum products.Plaster has the highest rate of expansion.Stones have the lowest rate of expansion.Setting expansion is the result of growth of crystals as they join together.
10Dimensional Accuracy (cont’d) It is important to control setting expansion.Some expansion is acceptable, but large amounts of expansion will keep the end result from fitting.With accurate following of the manufacturer’s instructions for mixing (powder-to-water ratio), expansion can be limited.
11Reproduction of Detail The greater the porosity of the final gypsum product, the less surface detail it has.Even products that have the least amount of porosity have surface irregularities.
12Solubility Set gypsum products are not highly soluble in water. Solubility is directly related to the porosity of the material; therefore plaster is much more soluble than stone.Exposing models to water for prolonged periods should be avoided.
13Classification of Gypsum Products The desired physical properties and the behavior necessary for a particular use determine the criteria for selection of a gypsum product.If you are pouring up diagnostic models, you use plaster.If you are making a crown or a bridge, you use die stone.
14Classification of Gypsum Products (cont’d) Impression Plaster (type I)Rarely used todayModel Plaster (type II)Frequently used for diagnostic castsDurable but relatively weak materialDental Stone (type III)Ideal for making full or partial denture models, orthodontic models, and casts
15Classification of Gypsum Products (cont’d) Dental Stone (type IV)Often referred to as die stoneUsed to fabricate crowns and bridgesHigh-Strength, High-Expansion Dental Stone (type V)Good to use with the newer base metals because of casting shrinkage
16ManipulationSelection of a gypsum product should be based on the desired properties of the material.If a diagnostic cast were being fabricated, dental plaster would be an appropriate choice because of the lowered needs of physical properties.A working cast would require good strength and accuracy and may be formed from dental stone. The dimensional accuracy and strength required for a die would make high-strength stone the best choice.
17Powder-to-Liquid Ratio It is important that the mix have sufficient flow to reproduce details of the impression.Water should be measured with a graduated cylinder, and powder should be weighed on a scale.Too much water will make the mix weak.Too little water, and the mix will not flow.
18MixingMost commonly, plaster and stone are mixed in a flexible rubber bowl with a broad metal spatula.Mechanical devices are used when control of spatulation is critical.
19Mixing (cont’d) Add the water to the bowl first. Sift the powder into the water, and mix with a broad spatula.Spatulate the material until smooth.It takes about 1 minute to mix the material thoroughly.After mixing, about 5 minutes of working time is available.
20Initial Setting Time–Working Time After mixing for 1 minute, the working time begins. During this time, the semifluid mixture is poured into the impression with the help of a mechanical vibrator.As the viscosity of the mixture increases, the flow characteristics will be decreased and the product loses its glossy appearance.This loss of gloss indicates that the gypsum has reached its initial set.
21Final SetThe final set is reached once the material is hard and the exothermic reaction cools completely.Most manufacturers recommend that about 1 hour should pass before the impression is separated from the cast.If the impression is removed too soon, the model or teeth will break.If left too long, the impression will dry out and will possibly snap off the teeth.
22Control of Setting Times It is impossible to accelerate the final set of a mixture without also accelerating the initial set, thereby reducing the working time.If it is necessary to alter the setting time, this can be accomplished by several methods:Altering water-to-powder ratioPerforming spatulationChanging temperatureUsing accelerators or retarders
23Fabricating and Trimming Diagnostic/Working Casts Diagnostic and working casts have two parts: the anatomic portion, which replicates the hard and soft structures, and the art portion or base.The anatomic portion is poured by vibrating small increments of flowable gypsum into the duplicated oral imprints.The mixture should be poured slowly in small increments under vibration and should be allowed to flow from tooth to tooth, pushing out air as it moves, thus eliminating air voids.
24Double-Pour MethodThis technique involves two separate mixes. The anatomic portion of one or both arches is poured and is left in the upright position.Approximately 10 minutes after the loss of gloss, a second mix is produced for the art portion(s).This mixture is placed on a glass tile or in a base former.The filled impression is inverted onto the base, and the peripheries of the two portions are joined.
25Single-Step MethodOne mix of gypsum is produced to pour the anatomic and art portions of the cast.After the impression is poured, the remaining material is used for the base.This material is placed on a glass tile or in a base former, the impression is inverted onto it, and the peripheries of the two portions are joined.
26Boxing Method A strip of wax is used to surround the impression, forming a wall into which the gypsum is poured.The wax should not distort the impression. It should extend at least 0.5 inch higher than the highest point of the impression and should create a base that is parallel to the occlusal plane.
27Storage Gypsum products can absorb water from the environment. Humidity and close proximity to water sources will adversely affect the powder.Gypsum should be stored in airtight, moisture-proof containers.
28CleanupGypsum mixing and handling equipment must be kept meticulously clean.As was previously mentioned, set gypsum will accelerate the setting times of a mixture.Bowls, spatulas, mechanical vibrators, and mixing devices should be cleaned of all traces of gypsum as soon as possible after manipulation.
29Separating the Impression From the Cast Upon setting of the gypsum, the impression tray and the cast must be separated.When the impression is poured, care should be taken to make sure the gypsum does not flow onto the tray, locking it into the set gypsum.Gently ease a lab knife under the tray, and lift the tray slightly in several areas.
30TrimmingModels are trimmed with a model trimmer to produce an attractive, symmetric model with easy access to all anatomic portions of the model and a base of sufficient bulk for stability.Bases made from dental stones should be soaked in water for 5 to 10 minutes before trimming to saturate the stone, making it easier to trim.The cast should be trimmed so that proportionately the base makes up one third and the anatomic portion two thirds of the total depth.
32Metal-Plated and Epoxy Dies and Resin-Reinforced Die Stone Type IV and V gypsum products are commonly used die materials.These materials are very hard, but they are susceptible to abrasion during carving of wax patterns.Silver or copper plating can create metal-plated dies that are highly resistant to abrasion.
33Investment MaterialsInvestment materials are used to form metal castings through the lost wax technique.These materials, which combine gypsum and silica, are able to produce models sufficiently strong to allow molten metal to be poured into them.
34SummaryGypsum products are used to produce diagnostic and working models of the patient’s hard and soft tissues.The properties of strength and hardness, setting expansion, and solubility are directly related to the amounts of water used in construction.