What is a Hydrostone?

You might be wondering what hydrostone is and if it is a good material for your project. If you are looking for a plaster that you can use for your craft and art project, you will be surprised to know that there is a wide array of options that you can choose from. Aside from the popular plaster of Paris, there are different types of plaster that are used for various applications. This article will talk about a specific type of plaster called hydrostone. Check out the rest of the article below for more tips and ideas.

What is a hydrostone? Hydrostone is considered the hardest and strongest plaster available. It is mainly used in tooling and art object applications. Hydrostone is a wear-resistant material and is designed for casting detail and strength. This material is 40 to 50 percent harder as compared to other casting plasters. Moreover, hydrocal works like plaster with regard to mixing, pouring and setting.

With the wide array of plasters that you can choose from, hydrostone is your best choice if you are working on a project that requires strong and hard plaster. This plaster works best for certain applications like tooling and prototyping and for art and statuary. Learn more about hydrostone and its characteristics below.

What is a Hydrostone
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What to Know About Hydrostone?

Hydrostone is a Gypsum product that is harder as compared to plaster of Paris. Aside from that, you can work on hydrostone directly because it does not have the period of plasticity in its setting curve. You need to do surface cleanup immediately once you start demolding because hydrostone will continue to harden. The good thing about hydrostone is that you can add filler to it. You can also check out this article that we have shared that discusses how to use hydrostone. We have shared tips and ideas in using hydrostone that you can refer to.

What are the Characteristics of a Hydrostone?

Hydrostone is a type of plaster that is hard, strong, and has a high water absorption resistance. Its characteristics allow it to works well in plaster as well as in most flexible molding compounds. Hydrostone is usually used in making high-quality art projects and statuary castings. The slurry produced by this material has low viscosity and can be used for filling multiple molds at a time.

How to Use Hydrostone

To use hydrostone, you need to follow steps like preparing the ingredients, measuring, soaking, mixing, pouring, and drying. The steps are the same as when you work with other types of plaster. However, there are differences in the measurement as well as in the handling of the material. For instance, you cannot mix hydrostone by hand as you would when mixing plaster of Paris. Let us learn more about this below.

Step 1 – Preparation

When preparing hydrostone, you need to use potable water that is between 70 and 100 °Fahrenheit. Take note that the variations in the temperature of the slurry lead to variations in the set time. Because of this, it is vital to keep hydrostone and water at a stable temperature before you use them.

Step 2 – Measure

The next step is to measure the materials. Weigh the hydrostone and water according to the recommended use consistency for the mixture. Make sure to read the label on the packaging for the instructions. Moreover, the ratio between water and hydrostone is vital because it affects the physical properties of the cast.

Step 3 – Soak

The next step is to soak the hydrostone into the water. To do this, sift hydrostone evenly and slowly into the water. Next, let it soak for 1 to 2 minutes. Make sure that the hydrostone is completely dispersed in the water before you mix it.

Step 4 – Mix

Proper mixing of hydrostone is a vital step to create casts that are strong and hard. As compared to other plasters and gypsum cement, hydrostone should not be mixed by hand and is recommended to be mixed mechanically. Furthermore, this type of plaster can only be mixed using a batch process. Having the right mixing equipment and mixing speeds are vital when mixing hydrostone.

Step 5 – Pour

The next step is to pour the hydrostone into the molds. Careful pouring of the hydrostone slurry into the mold will avoid air entrainment. It will also make sure that the casts have a smooth and even surface. When pouring the slurry, carefully pour in the deepest area. Doing this allows the flurry to evenly flow across the surface of the mold. If you directly pour a large amount of slurry on the mold, it can cause slight densification in the casts. 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.

Step 6 – Dry

Like any other plaster, hydrocal casts should be dried as quickly as possible so that they can develop maximum physical properties. Choose a drying room that offers uniform and fast air circulation. Aside from that, there should be equal temperatures in the room.

If the room has high humidity, it inhibits the efficiency of the drying process. The reason for this is that air that is pulled into the room cannot pick up much moisture from the cast pieces. Moreover, you need to take note that the maximum temperature that hydrostone casts are safe from calcination is 120 °Fahrenheit.

How to Cast Hydrostone

There are different methods to cast hydrostone. It will have to depend on the type of project that you are working on. Aside from making sure that you are doing the casting properly, you also need to consider the mixing and pouring of the slurry. If it is your first time using hydrostone for casting, read the instructions on the packaging on how to mix the hydrostone with water properly. How you mix the slurry will affect how your project will turn out in the end.

✅ Video – How to Cast Hydrostone Plaster into a Mold / Tips & Techniques

Jon Burns Fine Art shared the video below on YouTube. It shows how to cast hydrostone plaster into a mold. In the video, Jon discusses the methods for casting as well as explains the nature of both hydrostone and plaster. Moreover, he shared tips and techniques on how to remove air bubbles. Check out the video for more tips and ideas.

Is Hydrostone Dangerous?

If not properly used, hydrostone can be dangerous. Mixing hydrostone with water causes the material to become hard and very hot. It is advised that you do not attempt to make a cast that encloses any part of the body using hydrostone. Otherwise, it can cause severe burns that may need surgery.

Where to Use Hydrostone?

Hydrostone is a plaster that is especially suited for applications where high strength and resistance to water absorption are required. It is ideal for applications in both the tooling and prototyping and art and statuary categories. Let us learn more about this below.

For Tooling and Prototyping

Hydrostone is an excellent material that you can use for applications that require extreme surface hardness as stretch-press dies. It provides greater expansion as compared to hydrocal or other gypsum cement. Moreover, hydrostone has a heavy and syrupy consistency that is ideal for pouring into solid models or patterns. The good thing about hydrostone is that it has a high absorption resistance so it can adapt to most flexible molding compounds.

For Art and Statuary

Hydrocal is a hard and strong material that is used in making high-quality art novelty as well as statuary castings. It provides excellent paintability and can also duplicate fine details. Hydrocal works well in plaster and also in most molding compounds.

What is the Difference between Hydrocal and Hydrostone?

The difference between hydrocal and hydrostone is that hydrocal is used for casting and making pattern molds. On the other hand, hydrostone is a hard and durable material that is used for making extremely hard dense castings. You can also check out this article that we have shared that discusses what hydrocal is. We have shared an in-depth discussion about the topic.

Is Hydrostone a Plaster?

Hydrostone is a type of plaster. It is a gypsum product that is harder as compared to the regular plaster of Paris. Moreover, it does not have a period of plasticity in its setting curve. As a result, it enables the plaster to be worked directly. When working with hydrostone, surface cleanup should be done immediately during demolding. 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.

Where Can I Purchase Hydrostone?

You can purchase hydrostone at your local hardware store or even online. This type of plaster is widely available. You can choose online for the best choices that you can have. Ordering online proved to be convenient and easy especially if you do not have the time to visit your local hardware store. You will need to pay an additional shipping fee, but the package will be delivered right at your doorsteps. We have also shared this article that discusses if hydrostone is waterproof. Check out the article for an in-depth discussion about the topic.


You have reached the end of this article that discusses what a hydrostone is. We have found out that hydrostone is a hard and strong plaster that is used for specific applications. It is widely available and is easy to use. We hope that this article that we have shared has helped you differentiate hydrostone from other types of plasters. Thank you for reading!

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