How Long Does it take Plaster of Paris to Dry

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Last updated: April 7, 2023

Whether you are using it for molds, art projects, or basic castings, knowing how much time is required for plaster of paris to dry is very useful. The good news is that this material is straightforward and simple to use and can be set in just a few minutes.

Continue reading below to learn more about plaster of Paris and its drying time.

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How long does it take plaster of paris to dry

Removing the plaster of Paris from the mold may take at least one hour. In addition, it may also take approximately 24 to 48 hours to fully cure a plaster of Paris casting.

How to make plaster of paris dry faster

I can suggest several things you can do and steps you can take to help the plaster of paris dry faster.

The first method here is to use a heat treatment machine, and paper towels may also come in handy to speed up the drying process.

You can also put the plaster in a warm room and use a heat gun to heat the cast until it becomes almost dry. Consider a vacuum cleaner to suck out the air from the plaster.

Some people also suggest adding salt to the mix of plaster of Paris to help it set faster. However, the only downside to this method is that it may ultimately lead to forming a somewhat softer cast.

What is plaster of paris

Plaster of paris is a quick-setting gypsum made of calcium sulphate hemihydrate, a fine white powder that turns hard when wet and left to dry. Known and used since time immemorial, the plaster of Paris is named as such because of its production from the abundant gypsum located in Paris.

Once it dries, plaster of paris doesn’t fracture or shrink, making it an excellent material for casting molds. This is also typically used for creating and holding decorative plasterwork on cornices and ceilings. Medicine also uses this to produce plaster casts to immobilize broken bones as they recover.

A widely available material, plaster of paris is also popular for making crafts. Even more interesting here is that you can prepare your very own plaster of Paris in the comforts of your home using water, glue, and flour.

Does plaster of paris turn hard

Yes, plaster of paris is a type of hard substance with white color. This is made by adding water to partly dehydrated and powdered gypsum. The hardness of the plaster of paris is similar to that of cement or concrete but does not have the same level of strength. This is why it is not recommended to use this as a material for extremely thin castings.

Does plaster of paris shrink when it dries

In general, the plaster of paris doesn’t crack or shrink when it dries out, which is why this is an excellent medium for casting molds. This is also typically used to hold and precast the parts of ornamental plasterwork pieces installed on cornices and ceilings.

male contractor applying plastic of paris to a wall interrior

How do you know when plastic of paris is dry

First, gently touch the cast’s top portion using the tip of your finger once it looks dry enough. Just take extra care and avoid pressing too hard, as you could damage the plaster if it turns out that it is not dry yet. This happened once to my friend doing his DIY (do it yourself) project work at home a while back.

If the plaster still feels soft and wet, allow it to sit for 5 more minutes before checking again. Continue to check until the plaster feels completely dry. Also, it should produce a ceramic sound if you try tapping on it.

What happens when there is too much water in the plaster of paris

When mixed with water, the plaster of Paris turns into a stiff paste due to the recombination of the water molecules that makes the gypsum hard again.

The plaster of Paris can also absorb water and form dihydrate, and it will set and form a hard and unified mass. Since it is not water-soluble, the plaster of paris can also retain its form once exposed to water.

However, if you end up putting too much water in the plaster of Paris, it may take a relatively long time for the mix to reach its creamy stage, and once it does, it might set extremely fast all of a sudden. Even if the piece still has a good homogeneity, the set plaster will still be softer compared to how it would have been if you used the desired or correct ratio.

You can get a better feel for plaster as you become more familiar with the material and gain more experience using it. It will make you more aware of imbalances in the mix early on in the mixing process and allow you to better combine ingredients to achieve the right balance.

how much should a 10 x 10 kitchen remodel cost interior-prepared-for-plastering


Does plaster of Paris break easily

Even though the plaster of Paris is hard and strong, it is still prone to breaking. You need to be extra careful when handling the material because it can incur damage if you drop it to the ground. It can also slough off, and chip and is not a suitable material to use if you want to make some miniature pieces.

How do you keep plaster of paris from cracking

The first and most important thing you can do to keep plaster of Paris from cracking is to prevent it from drying out too fast. It means you should never plaster and render under direct sunlight or in an area with strong wind. The ideal time to plaster or render is early morning during warmer months because of the lower humidity.

It is also necessary to coat the material plastered with watered-down PVA glue to ensure the surface doesn’t absorb the plaster’s water content.

But most importantly, you need to remember that any cracking on plaster of paris indicates something wrong with the materials you chose to use. This is why material selection is critical in this case. Avoid using low-quality materials to prevent unwanted cracks on your finished project.

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