Plaster of Paris

Plaster of Paris is a very popular chemical substance that is mostly used for sculpting and also in gauze bandages. We all know that the plaster of Paris is used by doctors to support fractured bones, it is also used in the construction industry, in making casts for statues and making designs for ceilings.

There are many uses of Plaster of Paris in our day to day life. Plaster of Paris is a white color powder that is nothing but hydrated Calcium Sulfate and it is obtained from Calcining Gypsum. In order to obtain Plaster of Paris, Calcining Gypsum is heated at a very high temperature. 

Plaster of Paris is a hemihydrate of Calcium Sulfate. It is obtained when Gypsum $(CaSO. 2H_{2}O)$ is heated to 393 K.

The following reaction occurs

$2CaSO_{4}. 2H_{2}O \rightarrow  (\text{heated at 393 K}) \rightarrow 2 (CaSO_{4}). \frac{1}{2} H_{2}O + 3 H_{2}O + Heat$

              Gypsum                                      Plaster of Paris         Water

When heated above 393 K, no water of crystallization is left, and we get an anhydrous calcium sulfate, $CaSO_{4}$. This is known as ‘dead burnt plaster’.

The plaster of Paris has a remarkable property of setting with water. On mixing it with an adequate amount of water, it forms a plastic mass that gets into a hard solid in 5 to 15 minutes. The Plaster of Paris name has been used since ancient times because of the abundant preparation of Gypsum found near Paris. 

Properties of Plaster of Paris

There are two main properties of Plaster of Paris: 

• Non-flammable: 

• Non-combustible

Plaster of Paris is non-flammable as well as non-combustible as it has a low chemical reactivity. In extreme conditions, it also acts as an oxidizing agent. Plaster of Paris has fire resistance quality and its material is ideal for heat insulation. The material does not shrink when it sets. Hence, there are no cracks on the surface of the dry Plaster of Paris. At high temperatures, the plaster of Paris also decomposes and becomes toxic oxides.

Characteristics of Plaster of Paris

• Plaster of Paris is easy to spread

• It is non-flammable

• It forms a very thick surface and can resist normal knocks after drying. 

• It does not expand much on setting 

Plaster of Paris is a dry white powder and can be effectively worked with metal or even sheets and it can be molded as desired. It is applied in the form to enable quick setting using water. Water when added, Gypsum crystals are formed in order to achieve the solid-state.

Types of Plaster of Paris

1. Gypsum Plaster

This plaster is made by heating Gypsum at 120 to 180℃. Gypsum Plaster is used for setting fractured bones by doctors in hospitals. It is also used by dentists for preparing casts. There is a variety of toys and decorative items available made out of Gypsum Plaster. It can be used as a fire-proof material as well due to its fire-resistant quality. 

2. Clay Plaster 

The Clay Plaster is made by mixing clay, sand and water along with plant fibers. Clay Plaster is also used in making the interiors of a house. The use of Clay Plaster is not as much as it was in the early Nineteenth century in Utopian villages. 

3. Lime Plaster

The Lime Plaster is made by mixing Calcium Hydroxide and Sand. When the plaster comes in contact with the Carbon dioxide in the air it transforms into Calcium Carbonate. This plaster is made by heating limestone or Calcium carbonate above the temperature of $850^{\circ}C$. 

4. Cement Plaster

The Cement Plaster is made by mixing suitable plaster, sand, water and Portland cement. The Cement Plaster was also known as Adamant Plaster. This was mainly used to obtain a smooth surface while building construction. Cement Plaster is used for its properties of strength and durability. In construction, there is a layer of Gypsum Plaster added on top of the Cement Plaster. 

5. Heat Resistant Plaster

The Heat Resistant Plaster is most commonly used for coating walls and chimneys. It is used to resist fire. The Gypsum plasters have been replaced with Heat Resistant Plaster as it can stand very high temperatures in comparison with Gypsum Plasters. 

Now, let’s understand the preparation of the plaster of Paris.

Preparation of Plaster of Paris

The plaster of Paris is prepared by heating Gypsum at 393 K.

For the preparation of plaster of Paris, the following conditions are necessary:

1. The temperature should not be allowed to go up 393 K as, above this temperature, the whole water of crystallization is lost.

The resulting anhydrous $CaSO_{4}$  is known as the dead burnt plaster because it loses its property of setting with water.

2. The Gypsum should not be allowed to come into contact with carbon-containing fuel. This is necessary because $CaSO_{4}$ will get reduced to Calcium Sulfite $(CaSO_{3})$, and we won’t get our desired product.

3. It has the remarkable attribute of setting with water. On mixing with one-third its weight of water, it forms a plastic that sets into a hard mass of interlocking crystals of gypsum within 5 to 15 minutes. Because of this reason, it’s called plaster.

The addition of common salt (NaCl) accelerates the rate of setting, while a little borax or alum reduces it.

The setting of the plaster of Paris is because of rehydration, and its reconversion into Gypsum.

$2 (CaSO_{4}). \frac{1}{2} H_{2}O + 3 H_{2}O \rightarrow 2CaSO_{4}. 2H_{2}O$ 

Plaster of Paris                          Gypsum

During the process of setting, a slight expansion (~ 1%) in volume occurs. As a result, it can take up the shape and impression of the mold in which it is put.

This means we can set the plaster of Paris in any shape we want to.

How is the Plaster of Paris Prepared?

Following are the steps to preparing the plaster of Paris:

1. Gypsum is a hard rock that is converted into gypsum plaster by diving off some chemically combined water.

2. Gypsum is heated at $120  - 130^{\circ}C$ for one hour.

After one hour of heating,  we get the product as calcium hemihydrate with the release of three-quarters of water. So, our desired product, i.e The plaster of Paris formula, is $CaSO_{4}. \frac{1}{2} H_{2}O$.

The plaster of Paris is also known as Gypsum hemihydrate.

Do you know that we have an alternative way of making the plaster of Paris?

Let’s discuss how to make it in another way:

Making Plaster of Paris with Flour

Materials required:

1. All-purpose flour

2. Lukewarm water

3. Mixing bowl

4. Wooden stick for stirring

Let’s prepare the plaster of Paris step-by-step:

Step 1: Mix 3 cups of all-purpose flour with 2 cups of lukewarm water.

Step 2: Stir the mixture well to avoid the formation of lumps. 

Step 3: Blend it with your hands to form a smooth dough.

Step 4: If necessary, add more water to get a smooth or firm dough.

Step 5: Knead the dough for around 5 to 7 minutes to get a homogenous batter of thick plaster of Paris.

This is how we can make plaster of Paris with all-purpose flour.

(Image will be Uploaded soon)

Plaster of Paris Uses

The uses of the plaster of Paris are:

1. It is used in medicine for making plaster casts for fixing the affected part organ where there is a bone fracture or sprain.

2. It is utilized in dentistry.

3. Used as the cement in ornamental casting.

4. Used for producing molds for pottery and ceramics.

5. For making casts of statues and busts.

6. Used for making models and decorative materials, such as designs on ceilings.

7. Used for preparing blackboard chalks.

8. Used as fireproofing material.

9. Used to fill in small gaps on roofs and walls.

10. Used in making toys.

11. Used in smoothening the walls before painting them.

Rules for POP Casts

The following are the rules for POP (Plaster of Paris) casts:

1. 8 inch for thigh, 6 inches for leg, 4 inches for the forearm.

2. Mold the POP with palm to avoid indentation. 

3. Joints should be pinned in a functional position.

4. Not too tight or loose (for adequate paddling).

5. Dip POP vertically in water till the air bubble ceases to come.

6. The uniform thickness of the plaster of Paris is preferred.