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Gypsum plaster, [12] also known as plaster of Paris, [13] is a white powder consisting of calcium sulfate hemihydrate. The natural form of the compound is the mineral bassanite . [ 14 ] [ 15 ]
With judicious heating, gypsum converts to the partially dehydrated mineral called bassanite or plaster of Paris. This material has the formula CaSO 4 ·(nH 2 O), where 0.5 ≤ n ≤ 0.8. [8] Temperatures between 100 and 150 °C (212–302 °F) are required to drive off the water within its structure.
The plaster is not pure plaster of Paris, but rather has additives to improve green strength, dry strength, permeability, and castability.For instance, talc or magnesium oxide are added to prevent cracking and reduce setting time; lime and cement limit expansion during baking; glass fibers increase strength; sand can be used as a filler. [1]
The word gypsum is derived from the Greek word γύψος (gypsos), "plaster". [9] Because the quarries of the Montmartre district of Paris have long furnished burnt gypsum (calcined gypsum) used for various purposes, this dehydrated gypsum became known as plaster of Paris. Upon adding water, after a few dozen minutes, plaster of Paris becomes ...
Gypsum mortar, also called plaster of Paris, was used in the construction of many ancient structures. It is made from gypsum, which requires a lower firing temperature. It is therefore easier to make than lime mortar and sets up much faster, which may be a reason it was used as the typical mortar in ancient, brick arch and vault construction.
A slip is a suspension of fine raw materials powder in a liquid such as water or alcohol with small amounts of secondary materials such as dispersants, surfactants and binders. Pottery slip casting techniques employ a plaster block or flask mould. The plaster mould draws water from the poured slip to compact and form the casting at the mould ...
It is chiefly made of plaster of Paris (powdered gypsum), with a little cement, glycerin, and dextrin, mixed with water until it is about as thick as molasses. When staff is cast in molds, it can form any shape. To strengthen it, coarse cloth or bagging, or fibers of hemp or jute, are put into the molds before casting. It becomes hard enough in ...
Depending on the setting time of the plaster. once the moisture of the plaster starts to be drawn by the board a second pass is made. this is called knocking down. it is much like applying paint with a roller in wrist action and purpose. to smooth out any lines and fill in any major voids that will make extra work once the plaster starts to ...