Plaster of Paris

Plaster of Paris (POP; Calcium sulphate hemihydrate) is a whitish powder obtained from minerals found in rocks and used in the production of casts. It is made by heating gypsum or calcium sulphate dihydrate at a high temperature to create a pasty composition that becomes dry in the air at a normal temperature. POP is mainly used in architecture to coat walls, partitions, and ceilings that provide fire protection. This is the primitive process to make the coating of the wall stronger and more lasting. In ancient times, POP was used to create architectural sculptures in the walls of temples that have lasted for thousands of years. Those plasters were generally made with gypsum and provided cleanliness to the walls. It is also used for medicinal purposes.

Chemical Properties:

Widely usable POP is a well-known chemical compound with unique properties that make it more universally adaptable.

• It is available in a powdered form and dries while hydrated.
• It has abundant gypsum that is mainly found in Paris.
• As the gypsum sets and heat evolves, it is called an ‘exothermic process’.
• At a temperature of 473K, it forms an ‘anhydrous calcium sulphate’, called ‘dead burnt POP.
• POP is usually fire or heat resistant.
• It is easy to spread and level, providing a smooth and clean finish.
• Helps in preventing the surface from cracking.

Main types of POP:

• Gypsum plaster is the most used, abundant, and primitive type of plaster, the powder of ‘calcium sulphate hemihydrate’ or the gypsum found in Paris. 
• Clay plaster is a mixture of clay water and sand suitable for interior decorations. 
• Lime plaster is composed of ‘hydraulic hydrated lime’, sand, and water. The powdered form of limestones is heated and broken down to make them usable.
• Cement Plaster or ‘adamant plaster’ is a mixture of ‘Portland cement’, sand, and water.

Preparation of gypsum plaster: 

POP is white gypsum or chemically termed as ‘Calcium Sulphate with half a water molecule per salt molecule’.

It is prepared by heating calcium sulphate dihydrate or gypsum at a high temperature of 373K, where it breaks down and loses 3/4th of its water content to form a crystalline POP structure. The reaction can be expressed as:

CaSO4. 2H2O + 373k heat → CaSO4.1/2H2O + 3/2H2O

During usage, it retains its gypsum structure or property when rehydrated with water and starts drying after 10 minutes.

Uses of POP: 

Architecture: POP has been used to form various art forms and construction work since ancient times, owing to its complex properties and pleasing looks. It is used to cast walls and ceilings to give them a clean and finished look, providing a clean white look appropriate for further painting. It is also valuable for making sculptures to decorate buildings and temples. POP can protect walls from cracking. 

Medicine: it is used to cast bandages to fix or support broken bones as it limits mobility and helps the affected area heal faster. 

Burials: It is used at funeral houses to re-join the broken tissues of dead bodies and remake the injured or damaged portions of the body by filling in the wounds.

Fireproofing: POP possesses fire-resistant properties, helping structures against heat and fire. Plaster coatings also help prevent the spread of fire and reduce heat transfer to the steel or iron rods used to support the framework of the building.

Moulding: POP’s showpieces and decorative artefacts are both attractive and elegant.

Radiotherapy: POP is used to fix and develop impressions of the neck and head by using it as a plaster bandage.

3D Printing: POP is now being used in 3D printing as an ‘inkjet head’.

Conclusion:

POP or calcium sulphate hemihydrate is a powdered form of gypsum that serves multiple purposes, including architecture and medicine applications. It is produced by heating gypsum at a high temperature and mixing the resultant powder with water to make a white paste composition that becomes hard after drying, giving the product a finished white look. It possesses fire and heat proofing properties and helps prevent cracks in walls. Gypsum is also known as calcium sulphate dihydrate and is mainly found in Paris.