Impression materials are used to record the shape of the teeth and alveolar ridges. Most of them are the same as the ones used for humans. There are elastic and non elastic materials. There is a wide variety of impression materials, each with their own properties, advantages and disadvantages.

There are two parts in an impression procedure: the negative impression (mold), and the positive model (in general made of plaster).

In animals, most of the impressions are used as a step in the construction of a prosthetic crown (metal or non metal), to replace a lost crown. Also it is used in orthodontics, to record the shape of the teeth and later develop an advice to correct malpositioning. Ideally, the impression materials should be non toxic and non irritant, and be acceptable to the patient; however, in vet dentistry, all of the impressions are done under general anesthesia.

For containing the impression material, it is necessary to use an impression tray, which is different for animals. Rare companies produce special trays in metal; however, there are plastic ones in different sizes for different breeds.

The non-elastic materials are rigid and therefore exhibit little or no elasticity. Any significant deformation produces a permanent deformation. They are used where there are no undercuts and are mainly used for edentulous patient cases. Rarely, they are used in animals. The main non-elastic impression materials are impression compound, impression plaster, zinc oxide eugenol, and waxes.

The elastic impression materials can be stretched and bent to a fairly large degree without suffering deformation. These are used for recording the patient's mouth where undercuts are present. In humans, they are usually used for partial dentures, overdentures, implants and crown and bridge work. In dogs and cats, they are used mainly for prosthodontics and prosthodontics crown construction. The elastic impression materials are hydrocolloids (agar and alginates) and elastomers.

A colloid is a state of matter in which individual particles of one substance are uniformly distributed in a dispersion medium of another substance. When the dispersion medium is water, it is termed a hydrocolloid. The colloid becomes viscous and jelly like, and is called a gel. Some colloids have the ability to change reversibly from the sol state to the gel state. A sol can be converted into a gel in one of two ways:

1.  Reduction in temperature, reversible because sol is formed again on heating (e.g., agar)

2.  Chemical reaction which is irreversible (e.g., alginates)

The most import features of alginate are syneresis (losing water, and then shrinks) and imbibition (acquitting of moisture causing expansion). Due to this all impressions done with alginate must be done taking serious consideration of the procedure timing. An impression tray with the mold can not be put into the table for over minutes, otherwise it can absorb and lose moisture, which will alter the shape of the impression. Therefore it needs to be casted as soon as possible, in general with plaster poured into the tray.

The alginates are made of a powder mixed to the water. The powder contains alginate salt (e.g., sodium alginate), calcium salt (e.g., calcium sulphate), trisodium phosphate. The powder quantity should be measured according to the manufacturer's instructions. Room and water temperature should be taken into consideration, as well as the ability of mixing, with energy, and time thinking. The tray needs to have perforations in order to keep the alginate attached to it. Once removed from the mouth, the impression should be rinsed with cold water to remove any saliva or blood. It should then be covered in a damp gauze/napkin to prevent syneresis (not placed in water which would cause imbibition-expansion). The impression should be cast as soon as possible, then, to obtain the positive model, in general made of plaster.

The elastomers are used where a high degree of accuracy is needed, especially in crown work. They have two main advantages over the hydrocolloids - good tear resistance and dimensional stability. They are mainly hydrophobic rubber-based materials. All of these materials come in different viscosities ranging from low to high viscosity. The light-bodied material maybe used as a wash impression over a medium-or heavy-bodied material in one-stage or two-stage procedure impression. The main types of elastomers are polysulphides (mercaptans), silicones (addition and condensation), and polyethers.

Due to the dimensionally stable properties, most of these materials can be cast over 24 hours after impression is taken, some even days after. This is very comfortable for vet dentists once in most of the cases all is done in one of two episodes of anesthesia.

To sum up, into steps. When a positive model of the teeth is necessary, such as in plaster, the first step is to mold the teeth with a tray with impression material. After the impression, a cast is done, pouring the plaster into the negative areas of the impression, and waiting for the plaster to harden, in general 30 to 40 minutes. The plaster is removed from the tray-impression set, clean, and shaped accordingly. On this positive model, the work is done, such as a metal crown, or orthodontic advice.