Editing Plaster (section)

====Chemistry====
{{see also|Calcium sulfate#Hydration and dehydration reactions}}
Gypsum plaster, gypsum powder, or plaster of Paris, is produced by heating gypsum to about {{convert|120|–|180|C|F}} in a kiln:<ref name=lafarge>{{cite web|url=http://www.lafargeprestia.com/caso4___h2o.html |title=CaSO4, ½ H2O |last=Staff |publisher=[[Lafarge (company)|LaFargePrestia]] |access-date=27 November 2008 |url-status=dead |archive-url=https://web.archive.org/web/20081120163316/http://www.lafargeprestia.com/caso4___h2o.html |archive-date=20 November 2008 }}</ref><ref name="Britannica" />
<chem display="block">CaSO4.2H2O \overset{heat}{{}->{}} {CaSO4.1/2H2O} + 1\!1/2 H2O ^</chem>
(released as steam).

Plaster of Paris has a remarkable property of setting into a hard mass on wetting with water.

<chem display="block">CaSO4.1/2H2O + 1 1/2H2O -> CaSO4.2H2O</chem>

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.<ref name="Singh-2019" />

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, [[toy]]s, and more.<ref name="Singh-2019">{{Cite book|last1=Singh|first1=Lakhmir|title=Science for Class 10 Part-2 Chemistry|last2=Kaur|first2=Manjit|publisher=S. Chand|year=2019|isbn=978-9352837892}}</ref> The initial matrix consists mostly of orthorhombic crystals: the kinetic product. Over the next 72 hours, the rhombic crystals give way to an interlocking mass of monoclinic crystal needles, and the plaster increases in hardness and strength.<ref name="Schmidt">{{cite journal |quote=Hardening stage of plaster of paris is basically a crystalline change in which orthorhombic form of gypsum converts into the monoclinic form of gypsum.
|first1=V. E.
|last1=Schmidt
|first2=J. H.
|last2=Somerset
|first3=R. E.
|last3=Porter
|journal=Journal of Biomechanics
|publisher=Elsevier
|title=Mechanical Properties of Orthopeadic Plaster Bandages
|volume=6
|issue=2
|pages=173–185
|year=1973
|doi=10.1016/0021-9290(73)90086-9
|pmid=4693147
}}</ref> If plaster or gypsum is heated to between {{convert|130|and|180|°C|°F|abbr=on}}, [[hemihydrate]] is formed, which will also re-form as gypsum if mixed with water.<ref>Deer, Howie, & Zussman. ''An Introduction to the Rock Forming Minerals''. Pearson Education Limited, England, 2nd Edition, 1992, Page 614. {{ISBN|0-582-30094-0}}</ref><ref name="Posnjak">{{Citation
| last = Posnjak
| first = E.
| title = The System CaSO<sub>4</sub>–H<sub>2</sub>O
| year = 1938
| pages = 247–
| url = http://earth.geology.yale.edu/~ajs/1938-A/247.pdf
| quote = The long experience in the manufacture of plaster of Paris presents a consistent record that whenever gypsum is heated at not too high a temperature–usually about 130°–the product formed is invariably the hemihydrate; and further, that if the temperature is permitted to go considerably higher, the product becomes 'dead burned', it has then been converted into anhydrite.
}}</ref>

On heating to {{convert|180|°C|°F|abbr=on}}, the nearly water-free form, called γ-anhydrite (CaSO<sub>4</sub>·''n''H<sub>2</sub>O where ''n'' = 0 to 0.05) is produced. γ-anhydrite reacts slowly with water to return to the dihydrate state, a property exploited in some commercial [[desiccant]]s. On heating above {{convert|250|°C|°F|abbr=on}}, the completely anhydrous form called β-anhydrite or dead burned plaster is formed.<ref name="Singh-2019" /><ref name="Posnjak" />