1. Impression Materials
Introduction
Impression materials are used to make replicas of oral structures. All impression materials must be in a plastic or fluid state while the replica is being made. Physical change, chemical reaction, or polymerization convert these fluid materials into either elastic or nonelastic (ie, plastic or brittle) negative replicas of the soft and/or hard tissues of the mouth. A model or cast material (eg, high-strength stone) is poured into the impression and, upon setting, produces a positive impression of the tissues of interest.

2. Requirements
The requirements of impression materials can be conveniently discussed under four main headings:
(1) Factors which affect the accuracy of the impression.
(2) Factors which affect the dimensional stability of the impression, that is, the way in which the accuracy varies with time after recording the impression.
(3) Manipulative variables such as ease of handling, setting characteristics, etc.
(4) Additional factors such as cost, taste, colour etc.

3. Classification:

4. Nonelastic Materials
Impression plaster
Plaster of Paris is seldom used as impression material now that elastomeric materials are available, but it can be used as a "wash" (a thin lining material placed over a stiffer base material or tray) material for edentulous impressions. The main component of impression plaster is calcium sulfate hemihydrate, which reacts with water to form calcium sulfate dihydrate. Manufacturers incorporate additives to adjust
the setting time and setting expansion.

5. The water/powder (W/P) ratio recommended by the manufacturer should be measured out carefully. The powder should be sprinkled into the water, allowed to sit for 30 seconds to wet the powder, and then mixed for the minimum time necessary to obtain a homogeneous mix. Impression plaster is rigid and will break rather than bend. The plaster must be stored in an airtight container because it will absorb water from the air and its setting time will be
adversely affected.

6. There are two types of dental compound as defined by the ADA
There are two types of dental compound as defined by the ADA. Type I is used for impression taking, and Type II is used for tray preparation. Although dental compound has fallen into disuse, it can be used for full-crown impressions (Type I), impressions of partially or completely edentulous jaws (Type I), and impression trays in which a final impression is taken with another material (Type II). Compound
cannot be used to record undercuts since it is not elastic.

7. Impression compound is available in either cakes or sticks in various colors.
Dental compound is thermoplastic; it is used warm (45C) and then cooled to oral temperature (37C), at which it is fairly rigid. The setting mechanism is therefore a reversible physical process rather than a chemical reaction.

8. Composition
Dental compound contains several ingredients. Natural resins, which comprise about 40% of the formulation, make the compound thermoplastic. Shellac is often used. Waxes (about 7%) also produce thermoplastic properties. Stearic acid (about 3%) acts
as a lubricant and plasticizer. Fillers and inorganic pigments account for the remaining 50% of the formulation. Diatomaceous earth, soapstone, and talc are examples of commonly used fillers.

9. thermal conductivity of dental impression compounds is very low
thermal conductivity of dental impression compounds is very low. These materials do not conduct heat very well and therefore require heat soaking to attain a uniform temperature throughout the mass. When heated or cooled, they soften or harden quickly on the outside, but time is needed for the temperature to become uniform throughout the entire mass. If the impression is removed from the mouth before it has cooled completely, severe distortion may occur.

10. Advantages
Dental impression compound is compatible with die and model materials
Disadvantages
The handling of dental impression compound is very technique sensitive. If it is not prepared properly, volatiles can be lost on heating, or low-molecular-weight ingredients can be lost during immersion in a water bath. Also, excessive wet kneading can incorporate water into the mix and change the flow properties of the compound. Due to a high coefficient of thermal expansion, the dimensions of the impression are not likely to be the same as the dimensions in the mouth. These materials are nonelastic and may distort on removal from the mouth. The casts should be poured within 1 hour.

11. Troubleshooting
1. Distortion. If the material is not completely cooled, the inner portions of the impression will still be soft when the impression is removed, resulting in distortion. Also, if water has been incorporated as the result of wet kneading, the material could have excessive flow at mouth temperature, producing distortion during removal from the mouth. If the tray used to carry the compound to the mouth is too flexible, distortion can result. It is important to select a tray that is strong and rigid. A delay in preparing the stone cast also may cause distortion. The cast should be poured as soon as possible after the impression has been removed from the mouth.
2. Compound is too brittle or grainy. Prolonged immersion in the water bath willcause low-molecular- weight components to leach out.

12. Zinc oxide-eugenol
Zinc oxide-eugenol's main use as an impression material is for dentures on edentulous ridges with minor or no undercuts. It can also be used as a wash impression over compound in a tray or in a custom acrylic tray.

13. Composition This material is commercially available in a powder and liquid form and as two pastes. One paste, called the base or catalyst paste, contains zinc oxide (ZnO), oil, and hydrogenated rosin. The second paste, the accelerator, contains about 12% to 15% eugenol, oils, rosin, and a filler such as talc or Kaolin. These two pastes have contrasting colors so it can be determined when the pastes are thoroughly mixed.

14. Note:
The shrinkage of these materials during the hardening process is approximately 0.1%. Subsequently, no additional dimensional change should occur.
Advantages
The advantages of zinc oxide-eugenol include high accuracy of soft tissue impressions due to its low viscosity. The material is stable after setting, has good surface detail reproduction, and is inexpensive. It also adheres well to dental impression compound.

15. Disadvantages
The Disadvantages of this material are messiness and a variable setting time due to temperature and humidity. Eugenol is irritating to soft tissues. This material is nonelastic and may fracture if undercuts are present.