What Is Grog In Pottery And Ceramics?
Grog is a type of material added to clay to enhance its properties and make it easier to work with. It gives the clay extra strength, helps prevent shrinking and cracking, and can even add some fun texture! There are different types of grog, all depending on how fine or coarse they are.
For most purposes, a 30/80 mesh size is considered to be the ideal choice. This size gives you the perfect balance between texture and strength for your projects.
What is grog in clay made of?
Grog is made up of two main ingredients: alumina and silica. Alumina is a mixture of aluminum and oxygen, and it has a strong and tightly packed structure. Silica is found naturally, especially in sand and quartz, and is an important building block for clay.
Alumina has a high melting point and is really strong, making it great for use in pottery. Silica helps to build the structure of the clay and make it stronger, and it also helps regulate how much the clay shrinks when it’s drying or being fired.
Soft grog vs hard grog
Soft grog is a type of material that has tiny holes in it. It’s called “soft” because it’s fired at a lower temperature compared to other types of grog. However, because it’s porous, it’s not often used in ceramic making.
On the other hand, hard grog is fired at a higher temperature, which makes it stronger and more suitable for use in standard ceramic art. Because it’s been fired to its full potential, it’s more fitting for making beautiful and long-lasting ceramics.
Grog mesh size
Grog is classified based on the size of its particles. The size is measured in a system called mesh, which is the number of holes per inch in a piece of mesh.
Grog can have a single mesh number, like 150 mesh, or a range, like 20/40 mesh, which means the grains are small enough to fit through mesh with 20 holes and too large to fit through mesh with 40 holes.
The range system is helpful as it gives potters control over the properties the grog is adding to the clay, while a single mesh number may have varying grain sizes. A mesh range of 30/80 is recommended for most pieces of pottery.
Gritty grog clay with fine smooth porcelain. Playing with different clays and forms. #ceramics #clay #throwing #teabowl #porcelain #grit #grog #ceramicstudio https://t.co/rdl9qPLFgC pic.twitter.com/o2G7SGJVVW— Raewyn Harrison (@raewyn_harrison) February 16, 2018
Why is grog added to clay?
There are several reasons why grog is added to clay.
Let’s look at them one at a time.
1. Reduce shrinkage
Grog is added to clay to help reduce shrinkage during the drying and firing process. This is because grog particles have already been fired, which means they have a lower water content and don’t shrink as much as clay without grog.
The grog also creates a porous matrix that allows for more even drying and reduces shrinkage. When a piece of pottery is fired, it loses water and volume due to vitrification, but the grog particles don’t melt and cause shrinkage because they’ve already been fired. If the kiln becomes too hot, the grog will melt, but its melting point is around 3240F.
2. Prevent cracks
Grog helps to prevent clay from cracking during the drying and firing process. When clay dries and fires, it can shrink, which can cause stress on the walls and structure of the piece. This stress can lead to cracks in the pottery.
Adding grog to the clay body slows the shrinking process, making it less likely for the pottery to crack. Grog also helps to stabilize the piece during the firing process, when high temperatures can cause additional stress.
In addition, grog reduces thermal expansion, which is the slight increase in size that occurs when pottery is heated up in the kiln. This reduction in thermal expansion helps to reduce the risk of cracks in the final ceramic work.
3. Add structure
Adding grog to the clay mixture helps to strengthen the pot and make it less likely to fall apart or collapse.
Just like the straw in wattle and daub gives strength and stability to mud walls, the larger grog particles in the clay help support the overall structure of the pot, especially for larger pieces with thinner walls. The grog acts like a reinforcement for the clay, like internal scaffolding, making the pot stronger and less likely to slump.
Handbuilding vs throwing
For handbuilding, a coarser grog is often used, as the large particles help to support the piece as it is built up. When handbuilding you may opt for a grog with a mesh size of 20/40 or 30/60, which will provide both structural stability and a good working surface.
For wheel-thrown pottery, a finer grog is usually preferred, as it will not affect the smoothness and consistency of the clay body. A grog with a mesh size of 60/100 or 80/100 will help to strengthen the piece without affecting its appearance or functionality.
How much grog to add to clay
The amount of grog you need in your clay depends on what you’re making and how you’re making it. For smaller pots, you don’t need much grog, but if you’re handbuilding a large piece, you’ll need a clay with more grog.
If you’re a beginner throwing on the wheel, it’s best to use less grog or no grog at all, as it makes it easier to center and build walls and it’s easier on the hands. But for handbuilding, having more grog is actually helpful. It’s recommended to try different particle sizes to see what works best for your pottery.
How grog affect the glaze
Grog in clay can have an impact on how your glaze turns out. The materials in the grogged clay can sometimes react with the chemicals in the glaze, leading to unexpected or undesired results. Additionally, the texture of the grog can affect the final look of the glaze on the pottery piece. Finally, the firing temperature of the grogged clay can also be a consideration, as it can fire quicker than non-grogged clay, which may affect the outcome of the glaze.
I hope this article helped you become a better potter by explaining what grog is and why it’s used in pottery and ceramics.
If you want to learn more, you can read this article on finding the perfect pottery clay.