Plaster of Paris is stored in moisture-proof containers, because the presence of moisture can cause slow setting of plaster of Paris by bringing about its hydration, which will make it useless after some time. When the dry plaster powder is mixed with water, it rehydrates over time into gypsum.
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.  Temperatures between 100 and 150 °C (212–302 °F) are required to drive off the water within its structure.
- 136.14 g/mol (anhydrous), 145.15 g/mol (hemihydrate), 172.172 g/mol (dihydrate)
- 1,460 °C (2,660 °F; 1,730 K) (anhydrous)
- 0.21g/100ml at 20 °C (anhydrous), 0.24 g/100ml at 20 °C (dihydrate)
The word gypsum is derived from the Greek word γύψος (gypsos), "plaster". 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 tens of minutes, plaster of Paris becomes regular gypsum (dihydrate) again, causing the material to harden or "set" in ways that are useful for casting and construction.
Additionally, plaster of Paris casts break down if patients get them wet. Due to the limitations of plaster of Paris, surgeons have also experimented with other types of materials for use as splints. An early plastic like material was gutta-percha obtained from the latex of trees found in Malaya. It resembled rubber, but contained more resins.
- Body casts, plaster cast, surgical cast
Feb 27, 2020 · Noun plaster of Paris (uncountable) A hemihydrate of calcium sulfate, made by calcining gypsum, that hardens when moistened and allowed to dry; used to make casts, molds and sculpture.
- Advantages and disadvantages
Plaster mold casting is a metalworking casting process similar to sand casting except the molding material is plaster of Paris instead of sand. Like sand casting, plaster mold casting is an expendable mold process, however it can only be used with non-ferrous materials. It is used for castings as small as 30 g to as large as 7–10 kg. Generally, the form takes less than a week to prepare. Production rates of 1–10 units/hr can be achieved with plaster molds. Parts that are typically made...
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. The ratio of ingredients is 70–80% gypsum and 20–30% additives. The pattern is usually made from metal, however rubber molds may be used for complex ...
Plaster mold casting is used when an excellent surface finish and good dimensional accuracy is required. Because the plaster has a low thermal conductivity and heat capacity, the metal cools more slowly than in a sand mold, which allows the metal to fill thin cross-sections; the minimum possible cross-section is 0.6 mm. This results in a near net shape casting, which can be a cost advantage on complex parts. It also produces minimal scrap material. The major disadvantage of the process is that i
Plaster of paris, quick-setting gypsum plaster consisting of a fine white powder (calcium sulfate hemihydrate), which hardens when moistened and allowed to dry. Known since ancient times, plaster of paris is so called because of its preparation from the abundant gypsum found near Paris.
plaster of Paris A white powder of dried calcium sulphate dihydrate which, mixed with water, gives off heat and hardens. Reinforced with loose bandage it forms a strong and useful support (CAST) or dental mould. Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005
plaster of Paris. n. 1. a white powder that sets to a hard solid when mixed with water, used for making sculptures and casts, as an additive for lime plasters, and for making casts for setting broken limbs. It is usually the hemihydrate of calcium sulphate, 2CaSO4.H2O.
Mar 02, 2019 · For thousands of years, plaster of Paris has been a popular material for artists, decorators and even medical professionals. Named for the gypsum reserves found near Paris in the 17th century, this cheap, easy-to-use medium can be found in everything from leg casts to ancient statues.
- related to: plaster of paris wikipedia
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