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Step-By-Step: How To Stop Plaster From Sticking To Mold

Plaster of Paris can be traced back to the Egyptian empire, where they used it to mold trinkets, jewelry, and ornaments. Despite its origins, the name Plaster of Paris originates after a large deposit of gypsum in Montmartre near Paris.

The best way to stop plaster from sticking to a mold is to first cover the mold with a thin layer of a soft slipper substance such as dishwashing liquid. As the plaster cures, the dishwashing liquid will prevent the plaster from attaching to the mold’s friction surface, which will make it simple to remove.

There are just as many opinions about the most effective way to remove plaster from a mold as there are uses for plaster of Paris itself. The solution is straightforward and is found in your household supply cabinet.

The Best Way To Extract Plaster From A Mold

The mold has been carefully prepared and incorporates all of the excellent decorative details you have formed on the original item to be used as the mold “positive.”

The paste is poured over the “positive” item and left for 24 hours. When you extract it, pieces of the new “negative” mold keep breaking off. It sticks because the viscose plaster of Paris mix flows into all of the friction material that makes up the “positive.”

This problem is straightforward to overcome without purchasing any available “mold barrier” products.

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If you carefully smear a suitable barrier over the “positive” mold and leave it for up to 48 hours, the new “negative” will quickly disappear without leaving anything behind.

If the barrier material is too thick and stiff (Vaseline for example), it will be challenging to prevent brush marks or fingerprints from being transferred. It’s also not easy to treat the mold’s tighter spots.

The barrier liquid needs to be very slippery and can flow easily to ensure it goes into all of the small areas of the item you want to mold.

What Are The Best Mould Barriers?

The most influential materials which act as a suitable barrier release agent include:

  1. Liquid dishwasher soap. It is the best substance; not only does it flow into hard-to-reach areas and maintains its “slipperiness” while the plaster of Paris is curing, but it also is not absorbed by the plaster of Paris
  2. Vegetable Oil is just as effective as liquid dishwasher soap but is much messier to work with
  3. Cooking spray works; however, if the plaster of Paris is not poured in quickly enough, it does evaporate
  4. Commercial mold release agents are available and work and dishwashing liquid,  but at an increased price

Why Does Plaster Of Paris Stick To The Mold?

When water is mixed into Plaster of Paris, it hydrates and converts it from a powder into a viscous (thick, sticky consistency between solid and liquid) paste.

You can pour it into a cast as a viscous mixture where it undergoes an exothermic (it releases heat to the surrounding air) reaction.

Because it is viscous and therefore able to seep into the tiniest cavities, it attaches to the high friction surface of the mold and so sticks when it activates and hardens.

Using very smooth surfaces such as rubber and glass as the mold material makes it easier to remove than other more conventional molds.

What Is Plaster Of Paris?

The steps in which Plaster Of Paris is manufactured and then used are as follows:

  1. Calcium sulfate dihydrate is the base material of Plaster of Paris, known as gypsum
  2. It is called dihydrate because there are two water molecules for every molecule of calcium sulfate
  3. Gypsum (powdered calcium sulfate) is first dried in a kiln
  4. The material is ground down and then heated to a temperature of 150 C (300F) which evaporates between 1½ molecules of water as steam
  5. The material is now called calcium sulfate hemihydrate because the water content has been reduced to ½ molecule for every molecule of calcium sulfate
  6. The drying causes an endothermic reaction where the material absorbs heat
  7. The processed material is very reactive, and when water is added, it releases heat and crystallizes
  8. When the powder is cast into the mold, water is added
  9. The water is absorbed back into the crystal structure, and the material converts into a viscous state
  10. The now pasty Plaster Of Paris is poured into the mold
  11. The plaster of Paris now undergoes the opposite heat reaction (exothermic), which releases heat back into the atmosphere
  12. As it heats up, it crystallizes, hardens, and slightly increases in volume

There are three ways in which the material can be cured:

  1. It will generally cure for 24-48 hours
  2. You can speed up the curing process if a catalyst (sodium chloride) is applied or hot water is used to mix it
  3. You can impede the curing process by adding a retardant such as alum or borax. Another even easier way to slow the curing time is to use cold water

What Are The Characteristics Of Plaster Of Paris?

When cured, plaster of Paris is rated as a level 2 hardness material (talcum being  Level 1 and diamond being level 10).

It means it can be handled without losing its shape and can be easily sanded, trimmed, and cut. It also means that it can be easily damaged.

When plaster of Paris has cured, you cannot reverse the process by adding water, and in its cured state, it is not considered a water-soluble material.

Plaster of Paris has a low rate of expansion, about 0.1 %. Because of its properties and ease of use, it has applications across many industries and projects.

These include the manufacture of orthotic limbs, molding ornaments, use in the building industry, across many industries where it is used as a mold).

When used to cast human parts (such as a broken arm), the technician must cover the arm with a protective bandage because the heat generated by the exothermic reaction could cause injury.


The best way to stop plaster from sticking to a mold is to use a very slippery substance such as dishwashing liquid, which will ensure that when the plaster of Paris has cured, you can easily remove it from the mold.

While several commercially available products perform the task adequately, dishwashing liquid is as effective.