Plaster of Paris

Plaster of Paris is a well-known chemical compound that is widely used in sculpting materials and gauze bandages. While we have seen numerous applications of this substance in our daily lives, Plaster of Paris is a white powdered chemical compound that is hydrated calcium sulphate that is typically produced by calcining gypsum. In other words, Plaster of Paris is often produced using heated gypsum at a high temperature.

What is Plaster of Paris?

Plaster of Paris is a quick-setting gypsum plaster made of fine white powder (calcium sulphate hemihydrate) that hardens when wet and left to dry. Plaster of Paris, known since ancient times, is so named due to its production from the plentiful gypsum found in Paris. 

When dry, the plaster of Paris does not shrink or fracture, making it an excellent material for casting molds. It is often used to create and hold decorative plasterwork on ceilings and cornices. It’s also used in medicine to produce plaster casts to keep broken bones immobilized while they recover, however many current orthopedic casts are composed of fiberglass or thermoplastics.

Plaster of Paris Formula

The chemical formula of Plaster of Paris is CaSO₄.1/2H₂O known as Calcium Sulphate Hemihydrate. Plaster of Paris is a chemical compound in which an atom of calcium is bonded with a combination of an atom of Sulphur along with four atoms of oxygen to form Sulphate. It is then bonded with two molecules of water to form Calcium Sulphate Dihydrate. Hence, the structure of Plaster of Paris is given as,

How To Make Plaster of Paris?

Plaster of Paris is prepared from the chemical compound, calcium sulfate dihydrate, which is also known as, gypsum. Gypsum is represented by the chemical formula CaSO₄.2H₂O. PoP is manufactured by heating the element gypsum at a very high temperature of about 373K. When this happens at such a high-temperature value of 373K, approximately three-fourths of its water of crystallization is lost. forming Pop.

CaSO₄ · 2H₂O + Heat ⇢ CaSO₄ · 0.5 H₂O + 1.5 H₂O (Discharged as steam)

Since the presence of moisture may slow down the setting of plaster by bringing about the hydration process. Therefore, it is stored in moisture-proof containers.

Types of Plaster of Paris

Plaster of Paris is primarily of three types, that are widely used,

1. Gypsum Plaster – Gypsum plaster, also referred to as Plaster of Paris, is made by heating gypsum to a temperature of 300 °F. Additionally, Anhydrite is produced when gypsum is heated over 392 °F. Dry gypsum plaster powder or Anhydrite transforms into gypsum when combined with water. By adding water, Plaster of Paris hardens very quickly.
2. Cement Plaster – Cement plaster is a compound made of suitable plaster, Portland cement, sand, and water. The interiors and exteriors are plastered with cement to provide a smooth surface. Over the cement plaster, a final coat of gypsum plaster is frequently applied.
3. Lime Plaster – Lime Plaster is a compound made of sand, calcium hydroxide, and various inert fillers. Quick lime is created by heating limestone, while slaked lime is created by mixing water into the quick lime. Wet putty or white powder are two common names for it.
4. Clay Plaster – Clay Plaster is simply the mixture of sand, water and clay along with the addition of plant fibers for more strength. This plaster had been in great use since ancient times for making the interiors of houses. 
5. Heat Resistant Plaster – This kind of plaster is mostly used as a building material for coating walls, ceilings, chimneys, etc. An example of heat-resistant plaster is Portland cement.

Properties of Plaster of Paris

Plaster of Paris exhibits the following properties:

• Plaster of Paris is a white-colored powder, that forms crystals of gypsum when mixed with water.
• However, when it is heated at 473 K forms an anhydrous calcium sulphate. 
• It expands slowly and slightly upon setting. So. it is highly fire-resistant.
• It results in the formation of a thick surface to resist regular knocks after drying.
• It is easy to spread on any surface. 
• It is easy to level.
• It does not cause cracking of surfaces.
• It gives a decorative interior finish.

Uses of Plaster of Paris

Plaster of Paris founds its usage in the following areas:

1. 3D Printing: Gypsum plaster can be used for 3D Printing. The water is applied by the inkjet head.
2. Architecture and Decorations: Plaster of Paris is used to produce fine artwork to decorate and beautify monuments and buildings. It is also used to imitate wood or stone which is found in ancient buildings and monuments.
3. During Burial Services: Plaster of Paris is used by executives of funeral houses in order to remake the damaged tissues and fill up the wounds.
4. Medicines: It is used as a mold and cast. It is used to heal broken bones and cast into a supportive coating known as an orthopedic cast.
5. Fireproofing and fire protection systems

Difference Between Gypsum and Plaster of Paris

Here are some important key differences between Gypsum and Plaster of Paris,

Gypsum	Plaster of Paris
Gypsum is a hydrated calcium sulphate mineral with the molecular formula CaSO4·2H2O.	Plaster of Paris is a commonly used mineral obtained by Gypsum and has a chemical formula CaSO4·1/2H2O.
It is a Dihydrate compound.	However, this is a hemihydrate compound.
Gypsum is a soft crystal.	While Plaster of Paris (PoP) is a white powder.
Gypsum cannot be molded into different shapes.	Plaster of Paris can be molded into different shapes when moistened.
Gypsum can be found naturally.	However, PoP is a man-made compound.

Plaster of Paris: Advantages and Disadvantages

Here are some important key Advantages and Disadvantages of Plaster of Paris:

Advantages of Plaster of Paris	Disadvantages of Plaster of Paris
Forms a dense layer resistant to impacts.	More expensive compared to standard cement or lime cement.
Does not crack as it does not shrink during hardening.	Not suitable for use in damp or humid conditions due to low water solubility.
Exhibits thermal conductivity, helping to block heat transfer into structures.	Requires skilled application, increasing labor costs.
Can be used for insulation and fireproofing.

Plaster of Paris – FAQs

What is Plaster of Paris (POP)?

Plaster of Paris, commonly abbreviated as POP, is a white powdery substance derived from gypsum, a rocklike mineral. When gypsum is heated to about 160-170°C, it loses water and transforms into the hemihydrate form known as POP.

What makes Plaster of Paris hard?

Plaster of Paris Hardens by adding water to it. When PoP is mixed with water it takes on the unique property of hardening into a solid mass. 

How Plaster of Paris is prepared?

Plaster of Paris is prepared from the chemical compound, calcium sulfate dihydrate, which is also known as, gypsum. Gypsum is represented by the chemical formula CaSO₄.2H₂O.

Why is Calcium Sulphate Hemihydrate called Plaster of Paris?

Calcium Sulphate Hemihydrate is called Plaster of Paris because of the fact that it is prepared from the chemical compound called gypsum which is found to be most abundant near Paris.

Is the Plaster of Paris considered to be eco-friendly?

No, Plaster of Paris (POP) is considered to be hazardous for the environment. It gets dissolved in the water and soil thus making it harmful. 

What happens when Pop is burnt above 250 °C?

When Plaster of Paris is burnt above 250 °C, a compound called β-anhydrite or dead burnt plaster is formed and it is a completely anhydrous product.

What is dead burnt plaster?

When the chemical compound of plaster of Paris CaSO₄ . 0.5 H₂O is heated above the temperature of 393 K, its water of crystallisation is lost and anhydrous Calcium sulphate is left as a residue which is known as a dead burnt plaster.

What are the main uses of Plaster of Paris?

Plaster of Paris is versatile, used in the medical field for making casts and molds, and in construction for decorative ceilings and fireproofing. Its ability to form a smooth, hard surface when mixed with water makes it ideal for creating intricate artistic and architectural designs​.

Is Plaster of Paris environmentally friendly?

While Plaster of Paris itself is derived from a natural mineral, the environmental impact largely depends on the mining and processing methods used. It is generally considered non-toxic but should be handled with care to avoid dust inhalation​.

Can Plaster of Paris be used outdoors?

Plaster of Paris is primarily recommended for indoor use as it is not water-resistant. Exposure to moisture can degrade its structure, leading to damage. For outdoor projects, other materials that withstand weather conditions are preferable​​.

How long does Plaster of Paris take to set?

Typically, Plaster of Paris sets within 30 to 45 minutes, and it is advisable to work quickly once it is mixed with water. It achieves a very hard finish upon drying, which can take up to 72 hours depending on the thickness and ambient conditions​.

Can Plaster of Paris be painted and sanded?

Yes, once fully cured, Plaster of Paris can be sanded to smooth out any imperfections and painted with most types of paint. This makes it a popular choice for craft and repair projects where a specific finish is desired​.