Gypsum Plaster / Plaster of Paris

Derived from the Greek “Enplastron” and Roman “Emplastrum”, Plaster is a building material used for the protective and/or decorative coating of walls and ceilings, and for moulding and casting decorative elements.

gypsum powder

Image source: http://www.donnamoderna.com/casa/fai-da-te/gessetti-profumati/photo/lavorare-gesso

 

Types of Plaster

The most common types of plaster mainly contain either gypsum, lime, or cement, but all work in a similar way. The plaster is manufactured as a dry powder and is mixed with water to form a stiff but workable paste immediately before it is applied to the surface. The reaction with water liberates heat through crystallization and the hydrated plaster then hardens. Gypsum plaster, or Plaster of Paris, is the most recognized name for casting plaster, but the two terms are used interchangeably. There are several different types of plaster that can be used for a variety of casting.

Info source: https://en.wikipedia.org/wiki/Plaster

Origins of Gypsum Plaster

Plaster of Paris got its name do to a large gypsum deposit at Montmartre in Paris. In the 1700s, the king of France ordered that every wooden building be coated in plaster to provide fire deterrent. Gypsum plaster was used as early as ancient Egypt, according to “A Brief History of Plaster and Gypsum.”

Info source: https://sciencing.com/difference-casting-plaster-plaster-paris-6557743.html

Elaborate plaster reliefs decorating the Chateau de Fontainebleau were hugely influential in Northern Mannerism. There is a plaster low-relief decorative frieze above.

Image source: https://en.wikipedia.org/wiki/Plaster

How is it Composed ?

Plaster of Paris is made up from gypsum, It is a naturally occurring mineral. It is a hydrated calcium sulfate mineral with the molecular formula CaSO4·2H2O. This is the most common sulfate mineral. Usually, the color of the crystal is white or colorless, but can have other shades of colors as grey, red or yellow too. Crystals, also can be transparent or translucent. Gypsum is a soft crystal, which can even be scratched by a fingernail. Further, it is flexible, and its thermal conductivity is low.

This material is made from calcium sulfate that is derived, as we said, from gypsum. Extreme heat evaporates the water from the gypsum and create a fine powder that, when mixed with water, produces a cement-like material.

Gypsum plaster is produced by heating gypsum to about 150 °C:

CaSO4·2H2O + heat → CaSO4·0.5H2O + 1.5H2O (released as steam).

The chemical reaction that occurs when plaster is mixed with water is exothermic. When the plaster sets, it can reach temperatures of more than 60 °C. When the dry plaster powder is mixed with water, it re-forms into gypsum. The setting of unmodified plaster starts about 10 minutes after mixing and is complete in about 45 minutes; but not fully set untill 72 hours.

Info source: http://www.differencebetween.com/difference-between-gypsum-and-vs-plaster-of-paris/

Info source: https://en.wikipedia.org/wiki/Plaster

gypsum crystal

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How is it Used?

Gypsum plasters have been used for centuries in the construction area, thanks to its excellent and unique properties. It provides excellent thermal and acoustic properties while providing wavy lines and leveled walls with superior finish.

It is commonly used for prefabricating and preserving hold parts of ornamental plasterwork placed on ceilings and frames. It is also used in medicine to make plaster casts to immobilize broken bones while they heal, though many modern orthopedic casts are made of fibreglass or thermoplastics. Some sculptors work directly in plaster of paris, as the speed at which the plaster sets gives the work a sense of immediacy and enables the sculptor to achieve the original idea quickly. In medieval and Renaissance times, gesso (usually made of plaster of paris mixed with glue) was applied to wood panels, plaster, stone, or canvas to provide the ground for tempera and oil painting.

Info source: https://www.britannica.com/technology/plaster-of-paris

a worker puts the first layer of plaster on a wall, to give the building a better thermal insulation
Teeth modeled on plaster of paris

 

 

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Image source: https://www.britannica.com/technology/plaster-of-paris