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.
Etymology and history. 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.
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- Hydration states and crystallographic structures
- Production and occurrence
- Retrograde solubility
- On planet Mars
Calcium sulfate is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite, it is used as a desiccant. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water.
The compound exists in three levels of hydration corresponding to different crystallographic structures and to different minerals in nature: 1. CaSO4: anhydrous state. 2. CaSO4 · 2 H2O: dihydrate. 3. CaSO4 · 1⁄2 H2O: hemihydrate, also known as plaster of Paris. Specific hemihydrates are sometimes distinguished: α-hemihydrate and β-hemihydrate.
The main use of calcium sulfate is to produce plaster of Paris and stucco. These applications exploit the fact that calcium sulfate which has been powdered and calcined forms a moldable paste upon hydration and hardens as crystalline calcium sulfate dihydrate. It is also convenient that calcium sulfate is poorly soluble in water and does not readily dissolve in contact with water after its solidification.
The main sources of calcium sulfate are naturally occurring gypsum and anhydrite, which occur at many locations worldwide as evaporites. These may be extracted by open-cast quarrying or by deep mining. World production of natural gypsum is around 127 million tonnes per annum. In addition to natural sources, calcium sulfate is produced as a by-product in a number of processes: 1. In flue-gas desulfurization, exhaust gases from fossil-fuel power stations and other processes are scrubbed to reduce
The dissolution of the different crystalline phases of calcium sulfate in water is exothermic and releases heat. As an immediate consequence, to proceed, the dissolution reaction needs to evacuate this heat that can be considered as a product of reaction. If the system is cooled, the dissolution equilibrium will evolve towards the right according to the Le Chatelier principle and calcium sulfate will dissolve more easily. Thus the solubility of calcium sulfate increases as the temperature decrea
2011 findings by the Opportunity rover on the planet Mars show a form of calcium sulfate in a vein on the surface. Images suggest the mineral is gypsum.
- 7778-18-9, (hemihydrate): 10034-76-1, (dihydrate): 10101-41-4
- 1,460 °C (2,660 °F; 1,730 K) (anhydrous)
- 0.26 g/100ml at 25 °C (dihydrate)
Feb 27, 2020 · 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, which hardens when moistened and allowed to dry. Given that it does not generally shrink or crack when dry, it is an excellent medium for casting molds. Learn more about how plaster of paris is prepared, its uses, and history.
Jun 25, 2021 · To make plaster of paris with glue instead of flour, heat 1 cup (240 mL) of water to 100°F (38°C). In a mixing bowl, combine the water with 2 cups (470 mL) of white school glue. The resulting plaster will have a soupy consistency. Use this plaster within 15 minutes and let it set 3 days to completely dry.
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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.
Aug 10, 2019 · The origins of plaster of Paris can be traced to Montmartre in North Paris where it was extensively mined from. It is also known as POP. As described by ‘S.C. Rangwala’ (Author of Engineering Materials book), Plaster of Paris is prepared by heating gypsum crystals at the temperature of 160–170 °C (248–356 °F).