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You may have wondered about mixing different types of clay together. Perhaps you have some clay scraps lying about, or maybe you are just feeling curious. Here is some information that you should know before you get your wedging table out.
It is possible to mix different types of pottery clay together. However, it is important that these clays fire at a similar temperature and have similar shrinkage rates. This helps avoid problems when drying out and firing. It’s also a good idea if you are mixing types of clay that have similar moisture levels.
This article will look at why, what, and how you might go about mixing it up in your pottery studio.
Why Mix Different Types of Clay Together?
When a potter mixes different types of clay together, they may be blending two or more clay bodies to make one. Or they may be joining different types of clay in one piece. An example might be, a mug and its handle being made from different clay bodies.
This article is largely about blending clays rather than joining them. Though there is some cross over and many of the principles are the same.
Reasons For Mixing Clays Might Include…
- Having scraps of clay leftover from other projects.
- To create clay with a particular color.
- Combining the properties of two or more clay bodies.
- Wanting to create a particular effect such as marbling.
It’s worth bearing in mind that a clay body may already have different types of clay in it. The term ‘clay’ actually refers to a kind of ingredient that is used in glazes, clay bodies, and engobes.
Clay is a substance that is made of particular particles. A clay body is a mixture that may contain one or more types of clay. So different clay has already been mixed up in a clay body.
However, clay is manufactured carefully so that the properties of each clay are compatible. If you are considering mixing different types of clay, then you also need to consider their different properties.
What to Remember When Mixing Different Types of Clay
It is often said that it is not a good idea to mix different types of clay. The reason for this is that clay bodies have different properties and different ways of behaving.
For example, porcelain fires at high temperatures and shrinks a lot as it dries out and is fired. By contrast earthenware fires at a low temperature and shrinks much less than porcelain.
If you are considering mixing different types of clay then it is important to consider the following:
Mix Clays That Fire at a Similar Temperature
The most important criterion for mixing different types of clay is that they fire at the same temperature.
If you mix clays that fire at different temperatures, one clay body will fuse at a different time to another. This will create problems when your clay is in the kiln. The pottery will either crack, explode or separate when it is being fired.
For that reason, mixing very different clays like earthenware and porcelain is not recommended. However, some stoneware and porcelain bodies have similar properties. As such you are less likely to encounter problems if you mix these different pottery clays together.
Another related issue is the size and quantity of silica in a clay body. Silica is the glass-forming property in clay that melts when it is fire.
Different clays have silica granules of varying sizes. The size of the silica affects how much of it changes to glass when it is fired.
This affects how porous the clay is and what the clay looks like when fired. But it also affects how much the clay expands and contracts when it is fired (thermal expansion)1.
If you blend two clays that expand and contract at different rates when fired, you may encounter problems when mixing.
Mix Clays That Have a Similar Shrinkage Rate
It is also important that different clay bodies shrink at the same rate. If one clay shrinks more or faster than the other, then they can separate out when drying or being fired.
Potters who mix different clay bodies often say that the best way to find which combinations work is to experiment.
However, if you like to approach things more systematically, you can calculate the shrink rate of different clays. You may want to do this before mixing different types of clay to check if they will blend well together.
Here is a great video by Ceramic Arts Network of how to make a clay shrinkage ruler.
Mix Clays That Have a Similar Moisture Content
Clay with higher moisture content will shrink more than that which is less moist. If you mix clay that has a different moisture content, it’s more likely to separate and crack when it dries.
However, this does depend on how you are mixing your clay. If you are blending it thoroughly, then the mixing process will distribute the moisture evenly.
However, if you are part blending to create a marbling effect, then uneven moisture content can be problematic.
To Mix or Not to Mix – That is The Question….
Some potters are quite relaxed about mixing different types of clay. Experienced potters often have a moto that the best way to find out what works is through experimentation.
I’ve read accounts by seasoned potters who say that they often mix clay that fires at different temperatures without difficulties. Likewise, they seem to have success in mixing clay with different shrink rates.
However, this approach seems to work best for potters who have many years of experience under their belts. We mere mortals seem to do better with a more careful approach.
How to Mix Different Types of Clay Together
The easiest way to mix two different types of pottery clay together is by using the stack and slam wire wedging method.
The process for stack and slam is as follows:
- Shape your clay into two roughly equal-sized balls or blocks and stack them on top of one another.
- Slam the stack onto your wedging table.
- Slice through the clay and stack them again on top of each other. Slam the clay down on the table again.
- Repeat this process.
- You will gradually see the layers of each clay become thinner until each layer of clay is indistinguishable. It will take around 30 slams to fully blend two pieces of clay.
Wedging together two pieces of clay until they are completely mixed by hand would be difficult and time-consuming. Not to mention, tough on the wrists.
If two pieces of clay are of different consistencies, wedging them together can be tricky. Stack and slamming them first and then wedging them by hand after will mix them more thoroughly. And it will be much easier.
However, if you are opposed to wire wedging, you can simply wedge by hand until the clays are mixed together.
Creating Different Effects by Mixing Different Types of Clay
Agateware
When you are mixing your clay, you can blend them completely. Or, you can stop short of completely mixing them together.
By doing this, you will have a piece of clay with visible layers of contrasting colors.
This can create a beautiful marbling effect in your pottery. This is the principle behind agateware pottery.
In agateware, the different layers of color are a distinguishing feature of the piece.
Ball and Smush (as I like to call it)
By simply balling the clay together and smushing it with your hands you can get a more random swirly effect.
Chequered Ball
Alternatively, select two clays, one dark, and one light. Make a medium-sized ball of clay out of each separate clay body. Then cut them into quarters using a wire cutter.
Swap two sections in each ball of clay. In each batch, you should then have two from the light clay and two from the dark clay.
Arrange them so they are in a chequered pattern. Press the sections together and throw them on the wheel. This can also give an interesting marble effect.
Oxides and Stains
If you would rather not mix different clay bodies, you can add oxides and stains. Divide your clay into two pieces. Add a stain to one piece and mix the clays together using one of the above techniques.
Tips for Throwing Mixed Pottery Clays on the Wheel
- If you want to achieve a marble effect you need to throw your piece quite quickly. The longer you spend centering a piece and finessing it, the more the clays will mix together. This can result in a sludgy blurred looking mixture, rather than clean beautiful strata in the clay.
- Once you have thrown your piece, use a rubber kidney to scrape the slurry off the surface of the clay. By removing the slip, you will allow the marbling effect to pop.
- Alternatively, you can remove the slip when you are trimming the piece when it is leather hard.
- If you just wedge a few times you get a striped spiral rather than marbling – you can experiment with patterns.
Final Thoughts
The wonderful thing about pottery is that you are often surprised by the results that you get. Mixing different types of clay together can be a bit of an experiment. It is important to be prepared for things to not work out quite how you intended them to be.
If you are in doubt about whether two clay bodies will be compatible, you can ask your regular pottery supplier.
Also, remember that you can buy hybrid clays from many suppliers. For example, you can buy a hybrid of porcelain and stoneware clay which has a blend of their best properties. If you’re mixing to get a mixture of properties, a more reliable way may be to buy a hybrid clay.
References