How Do You Use Hydrostone?

In this article, we will discuss how to use hydrostone. This type of plaster is considered one of the hardest and the strongest available. It is used in a wide array of applications. If it is your first time working with hydrostone, you need to familiarize yourself with the proper usage of this type of plaster. Check out the article below for useful tips and information that you can refer to.

How do you use hydrostone? Hydrostone is a U.S. Gypsum product that is harder as compared to plaster of Paris. It does not have a period of plasticity in its setting so you can work on it directly. The steps in mixing and using hydrostone are similar to using any type of plaster. Make sure to do surface cleanup immediately when demolding because it will continue to harden thereafter. Moreover, considerable amounts of filler can be added to hydrostone.

What are the tips for using hydrostone for casting? How do you use hydrostone plaster? Is hydrostone hard? How long does it take for hydrostone to set? These are some of the questions that we will discuss. Continue reading below to find out more about hydrostone and how to use it.

How do you use hydrostone
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How to Use Hydrostone Plaster

To use hydrostone, there are a few steps that you need to follow which include preparation, measuring, mixing, and pouring. Hydrostone is a hard and strong plaster that produces a heavy and syrupy consistency that is easy to pour when properly mixed. This type of plaster is ideal for making high-quality statuary castings that need hard surfaces. Let us learn more about this below.

#1. Prepare

The first thing that you need to do is to prepare the materials that you need. When using hydrocal, make sure to use water at temperatures between 70 and 100 degrees Fahrenheit. Bear in mind that difference in the slurry temperature leads to differences in the setting time. Hence, it is vital to store water and hydrostone at a stable temperature before use. You can also check out this article titled, what is hydrostone? We have shared an in-depth discussion about the topic.

#2. Measure

The next step is to measure hydrostone and water. Weigh out both materials at the required use consistency for mixing. The ratio between hydrostone and water is important because it affects the physical properties of the cast piece.

#3. Soak

Sift hydrostone slowly and evenly into the water. Let it soak for about one to two minutes. Make sure that it is completely dispersed in the water before mixing. Moreover, small batches need less soaking as compared to large batches.

#4. Mix

The next step is to mix hydrostone. Mixing hydrostone properly is vital to create casts with maximum strength. As compared to other plasters and gypsum cement, hydrostone should not be mixed by hand. Moreover, it can only be mixed by a batch process and not by a process mixer.

#5. Pour

You must pour the hydrocal slurry carefully into the mold to create a uniform and smooth surface. This will also prevent air bubbles. Pour the slurry into the deepest area of the mold so that it flows evenly across the surface. Avoid pouring a large amount of slurry on the face of the mold because it can result in the densification of the cast.

#6. Dry

Hydrocal casts should be dried as quickly as possible to achieve maximum physical properties. Dry the casts in drying rooms or ovens with uniform and fast air circulation and equal temperatures. Do not place the casts in a room with high humidity because it cannot pick up much moisture from the casts. Moreover, the maximum temperature at which hydrostone casts are safe from calcination is 120 degrees Fahrenheit.

Tips on Using Hydrostone for Casting

Hydrostone castings should be mixed properly and poured slowly into the mold to prevent air bubbles. Proper mixing of hydrostone helps avoid water separation that can damage the surface of the casts. Check the hydrostone plaster before using it. If it has hard lumps, this means that it has become hydrated and should not be used. Make sure to wear protective gear like a dust mask and gloves when working with hydrocal.

Mix Properly

When using hydrocal, you might end up with casts that are smooth and hard or, uneven and crumbly. The quality of the cast differs. There are instances wherein the plaster will settle and have a thick layer of water which can affect the height of the cast. If this happens, it means that you need to mix more. The longer you mix hydrocal and water, it will increase the more strength and decrease the set time. You can use an electric mixer and thoroughly mix for about four to five minutes.

Remove Air Bubbles

To remove air bubbles, mix a little dishwashing detergent and some water. Fill up a large container with water and squirt a tablespoon or two of dishwashing detergent. Next, place the hydrocal casts into the water and rub your finger to release the air bubbles.

✅ Video – Removing Bubbles: How to Remove Air Bubbles in Plaster Castings

ThePlasterMaster shared the video below on YouTube. It shows how to remove air bubbles in plaster castings. In the video, tips on how to prevent air bubbles from forming when pouring plaster castings are discussed. This is an easy method that you can do to remove air bubbles. Check out the video below.

Work Quickly

When using hydrostone, it is important that you work quickly. Measure and mix hydrostone properly. Take note that hydrostone hardens quickly, so you need to work on it as soon as you pour it into the molds. If you are not fast enough, it will start to harden and it will be hard to scrape. Make sure to scrape the molds immediately.

Measure Properly

Another tip when working with hydrostone is to measure properly. It is recommended that you weigh the materials. Measure three parts of hydrocal plaster and one part of water.

Tap the Molds

Tapping the molds will remove air bubbles. You can pound the table or you can also use a vibrating table. Moreover, it is best to use a 12 inches ceramic floor square for your work area. After filling the molds with hydrocal slurry, raise and drop one end of the tile about eight times to help release the air bubbles. On the other hand, tapping the molds will not work to remove air bubbles if the plaster gets too hard. You can also check out this article that we have shared that discusses how humidity stones work. We have shared tips and ideas on how to maintain moisture for your indoor plants.

How to Properly Mix Hydrostone?

To properly mix hydrostone, you first need to sift the plaster slowly and evenly into the water. Next, let it soak for about two to four minutes. After that, mix the plaster and the water. Stir carefully to avoid entrapping air which can cause air bubbles. Mix for two to five minutes until you create a nice consistency of slurry. We have also shared this article titled, is hydrostone waterproof? Check out the article for an in-depth discussion about the topic.

Using Mechanical Mixer

An effective and easy way to mix hydrostone is by using a mechanical mixer. Mixing mechanically is superior as compared to hand mixing. To get the best result, you need to use a high-speed propeller mixer. The mixing shaft should be angled at 15 degrees vertically. Make sure that the shaft is halfway between the center and the side of the container and two inches from the bottom. Moreover, the propeller rotation of the mixer should be downwards. You can use a propeller. However, there are instances where it does not offer the right turbulence. If this is the case, you can use two propellers. Aside from this, you can also use three-blade mixers and flat disc mixers.

How Long Does it Take for Plaster to Set?

How long it takes for the plaster to set will have to depend on the thickness of the plaster, the temperature, and how it is mixed. Take note that the setting time refers to the period between mixing plaster with water and the time where it is ready to use. Generally, the setting time varies depending on a few factors. In the first portion of the setting time, the plaster mix is still fluid and easy to pour. Later, it transforms into a plastic state which makes it possible to be shaped and used for various applications. Once the plaster has been set, it cannot be altered. On the other hand, the surface can still be altered through sanding, carving, and adding more plaster layers into it. You can also check out this article that we have shred that discusses how long hydrostone lasts. We have shared an in-depth discussion about the topic.

Is Hydrostone Hard?

When mixed with water and allowed to set, hydrostone becomes a hard material. It is 40 to 50 percent harder as compared to casting plaster. Because of this, it can be used for various applications.


In this article, we have discussed how to use hydrostone. The methods in using this material are the same as when any type of plaster. On the other hand, there are specific methods and instructions that you need to take note of to make sure that it works perfectly. Thank you for reading!

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