The materials used in alginate generally consist of alginic acid, which comes from the cell walls of phaeophyceae (brown algae). Brown algae is more commonly known as seaweed. Fumed silica is also found in alginate impression material. Fumed silica is generally used to increase viscosity or as a thickener. Cristobalite, a crystalline form of silica, is also an ingredient in alginate impression material. The Materials Safety Data Sheet (MSDS) for alginate lists cristobalite as a hazardous material. The State of California recognizes cristobalite as a carcinogen.
Alginate impression material begins as a powder. When mixed with water, a thick pasty material, similar to cake frosting, appears. The dental impression tray is filled with the viscous alginate impression material and placed over the teeth, one arch at a time. The alginate impression material sets after 30 to 60 seconds in the patient's mouth. Cold water prolongs the setting time of alginate, while warm water shortens the set time considerably. Once the alginate impression material has set, it becomes solid, with a rubbery texture and feel.
The fully set alginate impression is stored in a moist environment until the operator is ready to create the final stone models. Set alginate impression material tends to dry out quickly when exposed to air, therefore distorting the impression of mouth. However, there are long-lasting alginate impression materials available that can withstand longer periods without losing accuracy.
When ready to create the final study models, the operator will fill the set alginate impression material with a stone mixture. This process produces a stone diagnostic cast of the patients maxillarymandibular arches.