Plaster of Paris

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What is Plaster of Paris?

We all know that the plaster of Paris is a substance that doctors use as support for fractured bones, also used in the construction industry, in making casts for statues and making designs for ceilings.

Plaster of Paris is a hemihydrate of Calcium Sulfate. It is obtained when Gypsum (CaSO. 2H2O) is heated to 393 K.

The following reaction occurs:

2CaSO4. 2H2O → (heated at 393 K) → 2 (CaSO4). ½ H2O + 3 H2O + Heat

              Gypsum                                      Plaster of Paris         Water

When heated above 393 K, no water of crystallization is left, and we get an anhydrous calcium sulfate, CaSO4. 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.

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 CaSO4  is known as the dead burnt plaster because it loses its property of setting with water.

  1. The Gypsum should not be allowed to come into contact with carbon-containing fuel. This is necessary because CaSO4 will get reduced to Calcium Sulfite (CaSO3), and we won’t get our desired product.

  2. It has a 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 (CaSO4). ½ H2O + 3 H2O → 2CaSO4. 2H2

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°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., plaster of Paris formula, is CaSO4. ½ H2O.

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.

FAQ (Frequently Asked Questions)

Q1: Write the Advantages of the Plaster of Paris.

Ans: The advantages of the plaster of Paris are:

  1. POP is light and durable.

  2. It doesn't shrink while setting.

  3. Has a low thermal conductivity.

  4. It is a heat-insulating material.

  5. Forms a thick surface to obstruct knocks after drying.

Q2: Write the Disadvantages of the Plaster of Paris.

Ans: The disadvantages of POP are:

  1. We cannot use POP in a moist environment.

  2. It is more expensive than cement and Gypsum.

  3. We cannot mix POP with cement.

  4. POP is not so solid, so if something is hanged to the ceiling, it can damage the ceiling and the object.

Q3: What Happens if we Eat Plaster of Paris?

Ans: On eating the plaster of Paris, we may face health issues like abdominal pain, nausea, vomiting, and epigastric distress.

Q4: Write the Difference Between POP and Putty.

Ans: The table below shows the difference between POP and putty:





How is it made?

Made of gypsum

White cement, polymers, and fibers.


Derived in Paris and invented in Egypt 

It is a generic term.


The Dutch military surgeon Anthonius Mathijsen.

The engineer James Wright.


Gypsum, lime plaster, cement plaster.

Coarse putty and fine putty.


Less water-resistant.

Highly water-resistant.