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Cotton and plaster casting material (plaster cast), 4ply. Plaster casts consist of a cotton bandage that has been combined with plaster of paris, which hardens after it has been made wet. Plaster of Paris is calcined gypsum (roasted gypsum), ground to a fine powder by milling.
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]
With judicious heating, gypsum converts to the partially dehydrated mineral called bassanite or plaster of Paris. This material has the formula CaSO 4 ·( n H 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.
A common casting medium, plaster is also a popular pick for making molds. Using this substance, which is manufactured in powder form, is ideal when you want a rigid structure to make casts as an ...
When the dry plaster powder is mixed with water, it rehydrates over time into gypsum. The setting of plaster slurry starts about 10 minutes after mixing and is complete in about 45 minutes. The setting of plaster of Paris is accompanied by a slight expansion of volume. It is used in making casts for statues, toys, and more. [19]
In this case the wax "pattern" is melted out from the mould cavity which is normally a rigid plaster like material rather than sand, so the wax "pattern" can only be used once. [5] Plaster of Paris is usually used in making master dies and molds, as it gains hardness quickly, with a lot of flexibility when in the setting stage. [5]
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 ...
Fiberglass casts were introduced in the 1980s or 1990s. The curing time of a fiberglass cast is far shorter than plaster of Paris, letting the patient walk with an outer boot within an hour of application. Because casts made of fiberglass have lower breakdown rate and do not impede patient mobility, this material has become the choice for TCC.