Plaster of Paris

What is Plaster of Paris?

Plaster of Paris is a popular substance that is utilized 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 sulfate that is usually obtained from calcining gypsum.

Plaster of Paris is also referred to as Gypsum plaster. The chemical formula of plaster of Paris is written as CaSO4·H2O or 2CaSO4·H2O.

Preparation of Plaster of Paris

Plaster of Paris is manufactured by heating gypsum at 423K or 150o C/300o F.

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

On heating gypsum at 423 K, it loses water molecules and becomes calcium sulfate 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), then 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 ita completely anhydrous product.

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 the 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

Plaster of Paris Uses

Usually, 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 utilized to make fine artwork for decoration and beautification of monuments and building. These might be geometric (imitating natural rocks and temples) or inspired from nature (like flowers or forests). These are also frequently used to imitate wood or stone mostly found in ancient buildings and monuments.

In present-day, this material is often utilized for false ceiling. 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 colour and light pattern. Extensive utilization 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 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

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 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 utilized for the building walls reliefs in Egypt and the Near East.  

During Burial Services

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

Medicinal Purposes

In the medical field,  Plaster of Paris is still frequently utilized a mould and casts. Plaster of Paris is generally utilized 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 plaster of Paris. Plaster of Paris is also utilized in radiotherapy for manufacturing individualized immobilization shells for patients. The bandages of plaster are utilized to develop an impression of a patient’s head and neck, and then the paste of plaster is utilized to fill the impression and produce a plaster dummy.

In dentistry, plaster is utilized 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 utilizing a delicate, malleable material that can be expelled from around the teeth and gums with full replication and utilizing the impression to making a wax model of the teeth and gums. The model is utilized 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.

Fireproofing

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

Early forms of these plasters included asbestos strands but at this point have been prohibited in industrialized 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.