This document discusses the classification and properties of alginate, a material used for dental impressions. It belongs to the category of elastic printing materials, which harden from a fluid to a solid state. Alginate is derived from brown seaweed and is composed of sodium salts that allow for easy handling. The document outlines the advantages and disadvantages of alginate, including its low cost but also dimensional changes. It also lists several brand names of alginate and provides references for further information.