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Plaster of Paris

What Is Plaster of Paris?

Plaster of Paris is a popular chemical substance that is used most commonly for sculpting materials and in gauze bandages. While we have seen many applications of this material in our everyday lives, if we try to understand its chemistry, plaster of Paris is a white powdery chemical compound, which is hydrated calcium sulphate that is usually obtained from calcining gypsum. In other words, we can say that the plaster of Paris is usually made up of heated gypsum at a high temperature. 

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Plaster of Paris is also referred to as gypsum plaster. The chemical formula of plaster of Paris is written as CaSO4·1/2H2O.

Properties of Plaster of Paris

Some of the important properties of the plaster of Paris are as follows:

  • Usually white in colour and available in powder form.
  • When water is added, gypsum crystals are formed, leading it to achieve a solid state.
  • The exothermic setting process can be catalysed by sodium chloride. The plaster of Paris is retarded by alum or borax.
  • Plaster of Paris forms anhydrous calcium sulphate at 473 K. This is sometimes known as the dead burnt plaster of Paris.

Other Characteristics of Gypsum Plaster

Plaster of Paris is usually a white dry plaster powder. It can be effectively worked with metal apparatuses or even abrasive sheets and can be shaped as per requirements. The strength of plaster of Paris is not as strong as other compounds, and it often requires external support when a large amount is used. It is often applied in the form of a quick-setting paste with water.

Types of Plaster of Paris

Let us look at some of the different types of Plaster of Paris.

Gypsum Plaster

Gypsum plaster mainly comprises white powder of calcium sulphate hemihydrate. This plaster is mostly produced by heating gypsum at 120–180 °C (248–356 °F). If the temperature goes beyond 392 degrees Fahrenheit, anhydrite is formed. The natural form of gypsum plaster is the mineral bassanite. However, when the dry plaster powder is mixed with water, it rehydrates back into gypsum.

Some of the uses of gypsum plaster are listed below:

  • Used for setting fractured bones in hospitals.
  • Used in dentistry to make casts.
  • Making toys, decorative materials, cheap ornaments, cosmetics, blackboard, chalk, and casts for statues.
  • Used for protection as fire-proofing material.
  • Used in sealing air gaps in apparatus in labs.

Clay Plaster

Clay plaster was used extensively during ancient times and in early Nineteenth-Century Utopian villages. This is a mixture of clay, sand and water with the addition of plant fibres for tensile strength over wood lath. Clay plaster was used in making the interiors of the houses.

Lime Plaster

Lime plaster is basically a plaster consisting of a mixture of calcium hydroxide and sand. When the plaster comes in contact with carbon dioxide in the atmosphere, it starts to set as calcium hydroxide and is transformed into calcium carbonate.

In order to prepare lime plaster, calcium carbonate or limestone is heated at temperatures above 850 °C (1600°F). This results in the production of quicklime (calcium oxide). Then, we can add water to produce slaked lime (calcium hydroxide). Now, this is commonly sold as a wet putty or a white powder. Some more water can be added to form a paste that is stored in airtight containers. On exposure to air, calcium hydroxide turns back into calcium carbonate as it reacts with atmospheric carbon dioxide.

This form of plaster is used in building materials.  It is also used for frescoes or mural paintings.

Cement Plaster

The mixture of suitable plaster, Portland cement, sand, and water makes up the cement plaster. This type of plaster was first introduced in America around 1909. During that time, it was known by a generic name, “adamant plaster.” Cement plaster is mainly applied in masonry to obtain a smooth surface while constructing a building. Often an added layer of gypsum plaster is added over the cement plaster. Cement plaster is known for its strength, hardness, quick setting time and durability.

Heat Resistant Plaster

It is a type of plaster that is used extensively for coating walls and chimney breasts. It is also used as a fire barrier in ceilings. Today, these plasters are used as a replacement for conventional gypsum plasters to withstand very high temperatures.

How to Make Plaster of Paris

Plaster of Paris is manufactured by heating gypsum at 373 – 393 K or 150° C/300° F.

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

On heating gypsum at 373 – 393 K, it loses water molecules and becomes calcium sulphate hemihydrate. This product is known as the plaster of Paris. However, when water is mixed with dry plaster of Paris, it re-structures into gypsum. As for the process of hardening and setting, it starts around 10 minutes subsequent to blending and is completed in about 45 minutes. It is not completely set for 70-75  hours.

Also Read:  Calcination

On the other hand, when plaster or gypsum is heated at temperatures higher than 266 °F (130 °C), we obtain hemihydrate. Other compounds are also formed when gypsum is formed at different temperatures.

For example,

When it is heated to about 180 °C, γ-anhydrite is formed. Similarly, when it is heated above 250 °C, β-anhydrite or dead burned plaster is formed, and it is a completely anhydrous product.

Plaster of Paris – Video Lesson

Plaster of Paris

Plaster of Paris Uses

There are different categories of plaster of Paris, and they have different applications. We will further look at some of the most common uses below.

In Architecture and Decorations

Plaster of Paris is used to make fine artwork for decoration and beautification of monuments and buildings. They might be geometric (imitating natural rocks and temples) or inspired by nature (like flowers or forests). It is also frequently used to imitate wood or stone mostly found in ancient buildings and monuments.

In the present day, this material is often utilised for false ceilings. In this, the powder of plaster of Paris is changed into a sheet structure, and then these sheets are fixed to roofs.  It provides large scope for creativity in various structures containing different colours and light patterns. Extensive utilisation of this plaster can be found in the construction sector.  The walls of the buildings are white-washed, which (in certain nations) is only calcium carbonate. Subsequent to the application of a coating of the calcium carbonate, it turns white after drying, and afterwards, the walls are fit to for colours and paints.

In Art

The majority of the great classical wall painting works of art in Europe, similar to Michelangelo’s Sistine Chapel ceiling, are executed in fresco, which means they are painted on a fine layer of wet plaster called intonaco; the colours sink into this layer in such a way that the plaster itself turns into the medium holding them, which results in the brilliant strength of fresco. Extra work might be included in a secco top of the dry plaster; however, this is commonly less tough.

Plaster (also called stucco in this situation) is a far simpler material for making reliefs than stone or wood and was generally used for building wall reliefs in Egypt and the Near East.  

During Burial Services

Plaster is utilised by numerous morticians and executives of funeral houses to remake the damaged tissue, rejoin cut-off parts of dead bodies, and fill wounds that occurred.

Medicinal Purposes

In the medical field,  plaster of Paris is frequently used as moulds and casts. Plaster of Paris is generally utilised as a plaster to join broken bones; a bandage soaked with plaster is added to water and afterwards folded over the broken part of the body, setting into a protective and supportive coating, known as an orthopaedic cast.

Different kinds of moulds and prototypes are made with the help of the plaster of Paris. It is also used in radiotherapy for manufacturing individualised immobilisation shells for patients. The bandages of plaster are utilised to develop an impression of a patient’s head and neck, and then the paste of plaster is utilised to fill the impression and produce a plaster dummy.

In dentistry, plaster is utilised for making a replica of oral tissues and teeth. Plaster is used to make wax false teeth. The wax is removed afterwards and filled with a mouldable dental base material. The regular acrylic dental replacement base replaces the plaster.

Moreover, false teeth (dentures) are made by first taking a dental impression using a delicate, malleable material that can be expelled from around the teeth and gums with full replication and using the impression to make a wax model of the teeth and gums. The model is used to make a plaster form (which is heated, so the wax melts and streams out), and the dental replacement materials are infused into the shape. After some time, the mould is opened, and the false teeth are finished.


Many fireproofing products and fire protection systems make use of the plaster of Paris. The plaster coating discharges water vapours when the building catches fire, and thus helps to retard the spread of the fire. It also gives some protection to slow down the heat circulation into steel and concrete components, which would lose their strength and break down in a fire. 

Early forms of these plasters included asbestos strands, but at this point, they have been prohibited in industrialised countries. Present-day plasters fall into different types, such as fibrous, cement blend, and gypsum plasters.

3D printing

Gypsum plaster is also used in 3D printing nowadays, where the water is specifically applied by the inkjet head. 

Some Important Questions

Frequently Asked Questions (FAQs)


What is the chemical composition of gypsum?

Gypsum is made of hydrated calcium sulfate. Its chemical formula is CaSO4.2H2O.

How is gypsum different from the plaster of Paris?

Gypsum is different from the plaster of Paris in terms of its water of crystallisation. Gypsum has two moles of water of crystallisation, whereas plaster of Paris has half a mole of water of crystallisation.

How is the plaster of Paris formed?

The plaster of Paris, abbreviated as POP, is formed by heating gypsum at 373 K. Gypsum loses its water molecules and forms calcium sulfate hemihydrate, also called POP.

What is the composition of lime plaster?

Lime plaster is made of sand and calcium hydroxide.

What is the use of POP?

POP has many uses. One of the main uses is that it is used to make casts for statues and moulds. It is also used in ornamental casting.
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