Plaster is an industrial product, a high-quality, very fine-grained plaster, with purity greater than 80% in hemihydrated calcium sulfate (CaSO·½HO),[1]​ which is obtained from natural gypsum or aljez "Plaster (mineral)").[2]​.

The term comes from the Italian Scaglióla, diminutive of Scáglia, from the Latin Scaliolae, a soft stone slightly similar to talc.

Until the beginning of the century, plaster was understood as a mixture of plaster with spastic plaster, mixed with glue water. From the traditional point of view, the difference between gypsum and plaster is their aljez purity and different granulometry (plaster is finer). While plaster has purity greater than 70%, plaster must have purity greater than 90%.

From an industrial point of view there is no difference between plaster and plaster: plaster or projecting plaster, finishing plaster or plaster, etc.

Once set, the chemical composition of the plaster is mainly calcium sulfate dihydrate: CaSO·2HO together with traces of industrial gypsum (calcium sulfate hemihydrate, CaSO·½HO) and anhydrite in phase III (CaSO), the greater or lesser proportion of which gives it different characteristics.

Plaster is used for the manufacture of molds in ceramic production, by the procedures of "squeezing", casting, turning and RAM pressing. In this application, the plaster's absorption capacity of the water contained in the clay is taken advantage of, which facilitates molding and gives hardness and solidity to the reproduced piece.

Plaster molds are also used to reproduce sculptures and other volumes in various materials such as resins, glued paper, polyurethanes, latex, plaster, silicones and even low melting point metals.

There are different types of special plasters: beta and alpha types (the latter with additives) suitable for various applications.

Plaster is also widely used today, in the decoration of walls and ceilings through prefabricated pieces in this material. In the past, esparto was the reinforcing component used in this material, but today more modern compounds such as glass fibers or meshes of the latter material with no more than 0.5 mm grid are also used.

The pieces that can be manufactured vary depending on the state of demand of the project to be executed, but prefabricated pieces are also available on the market: moldings, cornices, friezes, eaves, rosettes, soffits, ceilings, sconces, footlights, batons, imposts, balusters, niches, columns, pilasters, capitals or partitions.