1. Impression materials (requirement and classification)
Dr. Waseem Bahjat Mushtaha
Specialized in prosthodontics

2. Definition :
* The function of an impression material is to accurately record the dimensions of the oral tissues and their related structures.
* The impression gives a negative reproduction of these tissues.
* The positive reproduction is called a model or cast.

3. Requirements : 1) Good dimensional accuracy.
2) Adequate flow properties.
3) Sufficient mechanical strength not to tear or permanently deform during removal.
4) Suitable setting time
5) Ease of manipulation and reasonable cost.
6) Acceptability to the patient.
7) Safety “non toxic or irritating”
8) Compatibility with die and cast materials

4. Classification : * According to uses: I- Single tooth impression:
1) Rubber ) copper band + compound
II- Partial denture:
1) Primary Alginate
2) Secondary : a) Alginate b) Rubber c)2 pieces impression
III- complete denture:
1) Primary : a) compound b) Alginate
2) Secondary : a) Rubber b) Zinc oxide Eugenol c) plaster of parise

5. * According to behavior of material after setting:
I- Non – Elastic Impression:
1) Plaster of Paris 2) compound 3) Zinc oxide eugenol 4) waxes
2) Elastic Impression :
1) Alginat 2) Agar, Agar 3) Rubber

6. Hydrocolloid impression materials
A colloid must be distinguished from a solution and suspension. A solution is a homogenous mixture. For example, in an aqueous solution the solute exists as small molecules or ions in the solvent. In contrast to this, a suspension is heterogeneous, consisting of particles of at least sufficient size to be seen microscopically, dispersed in a medium. Thus a suspension is two-phase system.

7. Colloids fall between these two extremes
Colloids fall between these two extremes. They are heterogeneous, ( 2 phase systems), like suspensions, but the particle size of the dispersed phase is smaller, usually in the range nm. However, it is not always possible to distinguish between a colloid and a solution on the one hand and a colloid and suspension on the other.
When the dispersion medium of a colloid is water, it is termed a hydrocolloid.

8. Colloid may exist in the sol and gel state
Colloid may exist in the sol and gel state. In the sol state, the material is a viscous liquid. A sol can be converted into a gel- a material of gelatin-like consistency, due to agglomeration of the molecules of the dispersed phase, to form fibrils, or chains of molecules, in a network pattern. Theses fibrils enclose the dispersion medium, for example water.

9. A sol may be converted into a gel in one of two ways:
1) By a reduction in temperature: such processes are reversible, since on heating, a sol is formed again; an example of this is agar. In such a gel the fibrils are held together by Van Der Waals forces.
2) Other materials can form a gel by a chemical reaction, which is irreversible (for example, alginates)

10. The strength or toughness of gel depends on:
1) The concentration of fibrils – the greater the concentration, the stronger the material
2) The concentration of fillers – inert powders can be added to a gel to render it less flexible.
A gel can lose or take up water or other fluids. Loss of water can occur by evaporation. Syneresis can also occur; this happened when the gel molecules are drawn closer together, for example by continuation of an setting reaction. As a result a fluid exudate appears on the surface of the gel. Uptake of water is called imbibition.

11. In the use of hydrocolloids for dental impression, the material is inserted in the mouth in sol state, when it is sufficiency fluid to record detail. No gelation should have occurred at this stage. It is removed from the tissues after the gel is formed, when it exhibits elastic properties.
Clearly evaporation form, and imbibition by the gel should be avoided, as the former is associated with shrinkage and the latter results in expansion.

12. Agar ( Composition ) Constituent Approximate percentage Function
Agar colloid
Borax strengthens the gel, but retards the setting of dental stone model materials
Potassium sulphate To accelerate the setting of stone
Water Dispersion medium

13. Manipulation
a) The material is supplied in sealed containers to prevent evaporation of water.
b) It is brought to a fluid state by heating the tube in boiling water in a processing unit for minutes.
c) After boiling, the material can be stored at 65ºC for up to 8 hours
d) It is important that the entire tube of material is softened
e) The material is extruded onto metal trays with mechanical interlocking.
f) The filled tray is placed in a tempering bath at 45ºC for 2 minutes, before insertion into patient’s mouth.
g) With water-cooled trays, the agar material set quickly.
A higher temperature is required for the conversion of gel to sol than for reverse reaction.

14. Properties 1) Accuracy :
* The material can be suffiently fluid to record fine details if it has been correctly manipulated.
* The first material to set is that which is in contact with the tray (contrast with alginates) since this is cooler than the tissues. Thus the material in contact with the tissues stays liquid for the longest time, and can flow to compensate for any inaccuracy due to dimensional changes, or to inadvertent movement of the tray.

15. The set material can be withdrawn over undercuts
* The set material can be withdrawn over undercuts. The adhesion of agar to metal is poor, so perforated trays are used.
* Models should be cast up immediately from agar impressions, to avoid the possibility of the evaporation or imbibition. If this is not possible, it has been claimed that stability is achieved by storage in 2% K2SO4 solution or humidor.

16. * The compatibility with cast materials depends on the chemicals in the impression material. Without an accelerator for the setting of stone (e.g. K2SO4) a soft surface may be obtained.
2) These materials are non-toxic and non-irritant.
3) Their setting time is rather slow, unless efficient cooling is achieved.
4) Tear resistance is poor.
5) Their shelf-life is adequate. The material can be reused and can be sterilized. Loss of water, with an increase in viscosity of the sol, may occur. Water can be added if required.
6) Can be sterilized by immersion for 10 minutes in dilute aqueous solutions of sodium hypochlorite or glutaraldehyde.

17. Applications
These materials may be used to some extend for prosthetic impressions, and in crown and bridge work. Agar can be employed in the laboratory, for model duplication, since they can be re-used many times, due to the reversible nature of the reaction.

18. Alginates (composition and setting)
Constituent approx.% function
Sodium potassium react with ca2+ to give calcium gel
Slowly soluble calcium salt caso releases ca2+ to react with alginate
Trisodium phosphaste reacts with ca2+ to give ca3(po4)2 to delayed gel formation
Filler (diatomaceous earth) increases cohesion of mix and strengths gel
Silicon fluorides small quantity improves surface of stone model
Flavouring agents small quantity makes materials more acceptable to patient
Chemical indicator small quantity changes color with PH change, to indicate different stages in manipulation, e.g. violet color during spatulation, pink when ready to load the tray, white when ready for insertion into the mouth.

19. On mixing the powder with water a sol is formed, and the alginate, the calcium salt and the phosphate begin to dissolve. The following reaction occurs to form an elastic gel of calcium alginate:
NaAlg +CaSO Na2So4 + CaAlg………..1
Only the outer layer of each particle of sodium alginate dissolves and reacts.
However, the mixing and tray loading procedures. This is obviously undesirable, since the material should deform plastically, not elastically, on insertion into the mouth. Gel formation is delayed by trisodium phosphate, which reacts with calcium sulphate to give a precipitate of calcium phosphate, as following:
2Na3Po4 + 3CaSo Ca3(Po4)2 + 3Na2So4…………2
This latter reaction does not contribute any elastic properties to the material.
Reaction 2 occurs in preference to 1, no substantial quantity of calcium alginate gel is formed until the trisodium phosphate is used up. The manufacture can therefore control the setting time of this product by adjusting the quantity of this constituent.

20. Manipulation
The following points should be observed in order to obtain the best results:
1) The container of powder should be shaken before use to get an even distribution of constituents.
2) The powder and water should be measured, as directed by the manufacture. On brand of powder has been supplied in water soluble sachets, which help to ensure a uniform consistency of mix.

21. 3) Room temperature water is usually used, slower or faster setting times can be achieved, if required, by using cooler or warmer water respectively.
4) Retention to the tray is achieved by one or both of two means:
a) Perforated tray
b) An adhesive such as molten sticky wax or methyl cellulose.

22. 5) There should be vigorous mixing by spreading the material against the side of the bowel (one minute)
6) An alginate impression should be displaced sharply from the tissue- this sudden displacement ensure the best elastic behavior. The impression is removed about 2 minutes after set.
7) On removal from the mouth, the impression should be:
a) wash with cold water to remove saliva.
b) Converted with a damp napkin to prevent syneresis.
c) Cast up as soon as possible, preferably not more than 15 minutes after taking the impression.

23. Properties
1) Alginate are sufficiently fluid to record fine detail in the mouth.
2) During setting of the material it is important that the impression should not be removed. The reaction is faster at higher temperatures and so the material in contact with the tissues set first any pressure on the gel due to movement of the tray will set up stresses within the material, which will distort the alginate after its removal from the mouth.

24. 3) The material is sufficiently elastic to be withdrawn over undercuts, tearing of the impression material may occur with sever undercuts.
4) Alginate are not dimensionally stable on storage, because of evaporation.
5) Compatibility with plaster and stone can be good.
6)The materials are non toxic and non irritant, their taste and odor are usually acceptable

25. 7) The setting time depends on the composition and temperature of mixing
8) The alginate powder is not stable on storage in the presence of moisture.
9) Difficult to sterilize, spray disinfection diminish the sharpness of surface detail, while immersion in solutions adversely affects dimensional accuracy.

26. Application
These materials are not generally used for impression for inlay, crown and bridge work, but are applied with great success for prosthetic and orthodontic purposes. Alginates are dimensionally less stable than the elastomer.

27. Elastomeric Impression Materials
Chemically there are 4 kinds:
1) polysulphid
2) Condensation polymerizing silicone
3) Additional polymerizing silicone
4) Polyether

28. 1- Polysulphide rubber impression materials:
1) The first rubber impression material.
2) Didn’t have major changes during storage that agar and alginate have.
3)Impression was much stronger and more resistant to tearing than agar and alginate.
4) Rubber could be electroplated therefore metal die as well as gypsum models could be prepaired.

29. Chemistry of the polysulphides
Alternative names: rubber-base, mercaptan, Thiokol.
Composition :
The materials supplied as two past:
1) The base past
2) The reactor past

30. 1) past base contains:
* 80% low molecular weight organic polymer containing reactive mercaptan gp (-SH)
* 20% reinforcing agents (titanium oxides- zinc sulphate, copper carbonate, or silica.
2) The reactor past ( accelerator or catalyst past)
* Lead dioxide (PbO2) this causes (polymerization and cross-linking), dioxide using it result in paste being dark brown to dark gray.

31. * Sulphur
* An inert oil (dibutyle or dicotyle phthalate)
To make past.
* Other catalyst system use copper hydroxide, organic peroxides (not used since they are volatile).
N.B. the viscosity of the material is controlled by the molecular weight of the mercaptan and by the selection of the reinforcing agents.

32. Setting
The –SH groups can be oxidised by PbO2, giving S-S linkages, as follows:
R-SH+PbO2+HS-R R-S-S-R+PbO+H2O
This linear polymer contains approximately 1mol% of branches to provide enough pendant mercaptan groups as chain cross-linking sites. This polymer is usually cross-linked with an oxidizing agent such as lead dioxide. It is the lead dioxide that gives polysulfide its characteristic brown color. During the condensation reaction of the lead dioxide with the SH groups of the polysulphide polymer.

33. Two phenomena occur:
1) Chain- lengthening polymerization from the reaction with the terminal-SH groups
2) Cross-linking from the reaction with the pendant-SH groups.
Because the pendant groups compose only a small percentage of the available –SH groups, initially, the polymerization reactions result in chain lengthening, which causes the viscosity to increase. The subsequent cross-linking reactions tie the chains together, forming a three-dimensional network that confers elastic properties to the material.

34. The polymerization reaction of polysulfide polymer is exothermic, the amount of heat generated depends on the amount of total material and the concentration of initiators. Moisture and temperature exert a significant effect setting of polysulfide impression material. The condensation reaction by-product is water. Loss of even this small molecule from the set material has a significant effect on the dimensional stability of the impression.

35. Silicon rubber impression materials
Were developed to over come the disadvantages of polysulphides which are :
1) Objectionable odor.
2) Staining clothes by the lead dioxides.
3) Long setting times.
4) Fairly high permanent deformation.

36. Composition
1) Base :
- Past contains low molecular weight silicon liquid\ dimethyl siloxane which has terminal reactive “ OH” gp.
- Reinforcing agents “ silica”
1) Proper consistency to the mix.
2) Stiffness to the set rubber.

37. 2) Accelerator :
Usually supplied as a liquid but sometimes as a past by the use of thickening agent tin octate and an alkyl silicate such as artho -ethyl silicate.
3) Silicone pastes supplied:
Consistency controlled by a) concentration of reinforcing agents.
b) Molecular weight of dimethyle siloxan
*light…………………… % reinforcing agent
* regular………………….
*heavy-bodies………mol.w
*putty………………mol.w……..75%

38. Setting reaction
1) Dimethyl siloxan + ortho-ethyl silicate + tin octacle silicone rubber + ethylac.
2) The multifunctional ethyl silicate produces a cross-linked structure that partly accounts for the low value of permanent deformation and flow of silicon rubber.
3) Ethylac (by product) evaporates gradually reasonable for shrinkage during 24 hrs after setting.
4) Setting reaction is more sensitive than polysulphid to moisture and heat-increase either of them the setting and working time( normally the silicon setting and working times are shorter than polysulphides)

39. Addition type
1) Silane – containing siloxane + vinyl-terminal siloxane + chloroplatinic acid silicone rubber (by volatile byproducts) minimal dimensional changes during polymerize action
2) In Temp the rate of reaction and shorten the setting time.
3) If hydroxyl gp. Present in the addition silicon
Side reaction occurs with the result of H2 gradually released from the set. Impression bubbles in the gypsum models prepared after less than 1hr often impression taken. Overcome by :
1) Control the presence of hydroxyl gp.
2) Put H2 absorber such as palladium

40. Polyether rubber impression materials
It offers mechanical properties than polysulphides and dimensional changes the condensation silicone. However it possesses limitations such short working time and high stiffness.

41. Composition and setting reaction
1) Base:
1) Lower molecular weight polyether contain ethyleneneamin terminal gp.
2) Plasticizers
3) Filler
2) Catalyst:
Aromatic sulphonate acid ester these terminal gps reacted together by the action of catalyst to form cross-linked mol.w.rubber.
2) Plasticizer
3)Filler
Polyether + sulphonic ester cross linked rubber

42. Visible light-cure impression material
1) Polyether urethanedimethacrylate resin which visible light-cure photo initiator and photoaccelerater added.
2) The silicon dioxide filler has a refractive index close to that of the resin in order to provide the translucency for maximum depth of cure.

43. Manipulation
a) Uniform mixing is required. Some products are now produced with an automatic mixing system, in which the two pastes are mixed as they are extruded together through a spiral in the nozzle of a syringe.
b) Addition silicones are very sensitive to contamination, especially by contact with traces of:
1- other silicon products
2- sulphur compounds from disposable latex gloves.

45. c) Retention of the material on the tray is achieved by use of an adhesive such as a rubber solution.
d) Ideally there should be a uniform thickness (2-3 mm) of the impression materials.
e) Numerous techniques of impression taking have been reported. Two step impression (use of a fluid past in a putty impression) can lead to dimensional distortion and problems of the adhesion between the two materials. A single-step technique is usually favored, either with two mixes of material of different viscosity, or with a single mix of elastomer.
f) Set elastomers should be displaced sharply from the tissues to ensure elastic behavior.

47. Properties a) Accuracy :
1- the fluidity of these materials depends largely on their composition. Some polysulphides are supplied in a range of viscosities, for example, light bodes for injection by a syringe, and medium and heavy bodies for use on a tray. Some materials are supplied with a diluent to enable the operator to alter the viscosity of the mixed material a required. Silicones are also supplied with a range of consistencies including putty type and lower viscosity materials. In general elastomers can record fine detail. Unmixed elastomers are usually psudoplastic in nature.

48. 2- there is a small contraction on setting of these materials, because of polymerization shrinkage. Contraction also occurs on cooling the impression from the mouth to room temperature. The coefficient of thermal expansion of these materials is in the order:
polyether > silicon > polysulphide
The magnitude of the thermal shrinkage is reduced by the adhesion of the material to the tray.

49. 3- theses materials are sufficiently elastic to be withdrawn over undercuts, and are usually tougher and less likely to tear than the alginates. Polyether materials are stiffer than the other elastomers, therefore are the most difficult to withdraw over undercuts.
4- on storage, contraction can result from further polymerization of the material. Evaporation of volatile by products from a condensation reaction (.e.g. alcohol) is another source of shrinkage. The dimensional stability of polyethers may be poor in the presence of moisture. In terms of dimensional stability, addition silicones are the best, followed by polyether, then polysulphide

50. 5- these materials are in general compatible with model and die materials, though they can cause a small degree of softening of a dental stone surface. In early silicones evolution of hydrogen from materials containing an organo -hydrogen siloxane caused pitting of stone surfaces. Present day silicones either do not use this system, or contain a compound to react with hydrogen as soon as it is generated. Theses impression materials can be electroplated to give metal coated dies

51. Other properties:
1- in general these materials are non-toxic and non-irritant as set. However, unmixed pastes can be irritant and produce tissue reactions. The odor and taste of some paste containing lead dioxide is not pleasant.
2- the setting time depends on the composition of the material (for example, the quantity of reactors) also, the presence of water and high temperatures accelerate the setting of polysulphides.

52. 3- in term of tear energy:
A- Higher viscosity materials are often better than lower viscosity products.
B- polysulphides are better than silicones, but distortion can occur instead of tearing.
C –polyethers are similar to regular grade silicones.
4-Stability on storage of the unmixed materials is not always ideal, some of the reactors are unstable over a two-year period, but keep better if stored in a refrigerator.
5- Elastomers can be sterilized by immersion in aqueous hydrochloride or glutaraldehyde solutions, short-term immersion does not have any significant effect on dimensional stability.

53. Applications
The chief use of elastomers is in impressions for inlays, crowns and bridges, or for partial dentures when the undercuts are so sever that alginate would tear on removal from the tissues. Because of their expense, these materials are not frequently used impressions requiring large quantities of material.

54. Non-elastic impression materials
1) Plaster of Paris
2) compound
3) Zinc oxide eugenol
4) waxes

55. 1) Plaster of Paris Chemistry:
The main constituent of the impression plaster is calcined calcium sulphat hemihydrate. On mixing with water this reacts to form a rigid mass of calcium sulphate dihydrate.
During the setting an expansion may be demonstrated, this is substantially reduced by the addition of potassium sulphate.

56. As this accelerates the setting too much, borax is added to slow down the process. Another additive is alizarin red, this imparts a pink color to the impression, enabling it to be distinguished from the cast that is prepaired from it. A flavoring agent may also be present.
The additives may be blended with the plaster powder, or may be supplied as an aqueous solution to the mixed with plaster. In the later case a suitable solution contains: 4% potassium sulphate, 0.4-1% borax ( the precise value being chosen to give a desirable setting times; this may vary to give a desirable setting time, this may vary for different batches of plaster) and 0.04% alizarin red. This solution is called “AE” or ‘anti-expansion’ solution.

57. Some impression plasters may also contain:
*Additive such as gum tragacanth, to improve the cohesive properties of the mixed plaster.
*Starch, incorporated so that the set plaster will disintegrate due to the swelling of the starch when boiling water is poured over it this facilitates removal of the impression from the cast model. Materials containing starch are called “soluble plasters’

58. Manipulation
The plaster should be mixed with water or an AE solution in the ratio of 100gm to 50-60ml. Care must be taken to ensure that the mix is free of air bubbles, since they may appear on the surface of the impression leading to inaccuracy.

59. Properties a) Accuracy :
1- Plaster is excellent at recording fine detail since the mixed material is very fluid when insertion into the mouth.
2- The dimensional changes on setting are small, due to anti-expansion additives.
3- If there are undercuts present, the plaster impression will fracture on removal from the mouth.

60. 4) On storage of the plaster impression, the dimensional changes are small, though a small degree of drying shrinkage may occur
5) Before preparing a cast in plaster or dental stone, the plaster impression must be treated with a separating agent. An alginate mould seal, a varnish or waterglass or soap solution may be used.
6) Impression plasters are non-toxic. However they may be unpleasant for the patient, because they produce a dry sensation in the mouth.
7) The setting time can be precisely controlled by use of the appropriate quantities of additives.
8) Plaster is stable on storage over a long time, provide it is kept in a sealed container.

61. Impression composition (compound)
Constituents and applications:
These materials are generally composed of a mixture of natural resins (e.g. colophony and shellac and\or waxes), filler (soapstone or talc) and lubricants (stearic acid or stearin). They are thermoplastic ( they soften when heated and harden when cooled, without the occurrence of a chemical reaction.

62. The available materials may be classified into types:
a) Type 1. lower fusing materials:
1- For recording prosthetic impressions, such as preliminary impressions of edentulous patients, supplied in sheets about 4-5 mm thick
2- Peripheral seal materials
3- Supplied in stick from for cobber band impression for inlays and crowns and marginal additions to special tray
b) Type II. Higher fusing materials, used as tray materials, which are sufficiently rigid to support other impression materials

65. Manipulation
a) For prosthetic impressions the composition is heated in a water bath at 55-60C. Since the material has a low thermal conductivity it must be immersed in the water bath for sufficient time to ensure complete softening. However, if it is left too long, some of the constituents may be leach out into the water bath, so altering the properties of the material. If the composition is kneaded in the water bath, water will become incorporated in the material, and act as a plasticizer.

66. If the composition is too cool, it will not flow properly in the mouth, if its too hot, the material becomes sticky and can burn the patient’s mouth. In all cases the water bath should be lined with a napkin, otherwise the material will adhere to the bath.
b) For copper band impressions, e.g. for inlays and crowns, the stick of composition is heated in a flame. If overheating occurs, some of the constituents may be volatilized, with a consequent alteration of properties of the material.

67. Properties a) Accuracy :
1- In general, this material, though plastic on insertion into the mouth, is not sufficiently fluid to record all the fine detail of the mouth.
2- Impression composition has a high coefficient of thermal expansion, thus, on cooling during setting, there is considerable shrinkage. This can be minimized to some extent in prosthetic impressions by heating the surface of the set material in a flam and retaking the impression. Since only a small quantity of composition is contracting, the actual magnitude of the contraction is small. Shrinkage also occurs on cooling from mouth temperature to room temperature (about 1.5% by volume).

68. 3- Impression composition will distort on removal over undercut areas.
4- Dimensional changes can occur on storing the impression in the laboratory. Stress can be set up with the material, particularly if is manipulated or deformed when it is not fully softened. Subsequently distortion can occur due to relief of these stresses, particularly if the impression is left for some time, in a warm atmosphere, before casting up the model.
5- These materials are compatible with cast and die materials.

69. b) Other properties:
1- Non-toxic and non irritant
2- Hardness in a reasonably acceptable time in the mouth.
3- shelf-life is vary adequate, but changes in shellac may cause deterioration over a long period.

70. Zinc oxide- eugenol and similar pastes
a) The zinc oxide is supplied in paste form. This is achieved by the addition of an oil ( e.g. olive oil, light mineral oil or linseed oil). The oil acts as a plasticizer for the material. Hydrogenated resin can also be incorporated, it quickens the setting and makes the impression paste more cohesive.

71. b) The eugenol contains talc or kaolin as a filler, to form a paste.
c) Either or both pastes may contain accelerators, such as zinc acetate.
d) At least one proprietary paste contains a substitute for eugenol. This is a carboxylic acid, which can react with zinc hydroxide (possibly formed by hydrolysis of zinc oxide) to form a salt as follows:
Zn(OH) +2RCOOH (RCOO)2Zn + 2H2O

72. Manipulation
The two paste are provided in contrasting colors. The correct proportions ( usually equal lengths of the two) are mixed together on a slab or mixing pad with a flexible spatula until a homogeneous color is obtained.

75. Properties a) Accuracy :
1-these impression materials are sufficiently fluid to record the fine detail in the mouth.
2- there is probably little or no dimensional change associated with the setting process.
3- the set material is not elastic, so will not record undercuts.

76. 4- the set material appears to be stable on storage in the laboratory.
5- impression pates are compatible with dental stone cast materials. The past can be removed from the stone by softening it in water at 60C
Other properties:
1- these materials are non-toxic, but those containing eugenol can be irritant, giving a tingling or burning sensation to the patient and leaving a persistent taste, which some people may regard as unpleasant.

77. 2- The paste can adhere to tissues, so the lips of the patient are usually coated with petroleum jelly.
3- The setting time is usually satisfactory if the manufacture’s instructions are follow and the correct ratio of paste used. The presence of water, and an increase in temperature, both reduce the setting time.
4- The shelf-life of these materials is satisfactory.

78. Application
This material is generally used in thin sections (2-3mm) as a wash impression. A zinc oxide eugenol impression can be taken using a close-fitting special tray, or in an existing denture, particularly one that is to be relined.

79. Impression waxes
Waxes, sometimes in combination with resin of low melting point, can be used as impression materials. Theses materials differ from impression composition, in that they sometimes flow at the mouth temperature. In contrast to the zinc oxide-eugenol paste, they don’t set by chemical reaction. Difference combinations of waxes and resins can be blended to give a range of the materials for different techniques. However, theses materials are not used frequently. A cast should be prepared immediately from such an impression to avoid distortion.