If you have heard potters talking about grogged clay, it may have left you thinking about a festive alcoholic drink. This is not however what is meant in the world of ceramics. So, what is grog in pottery?
Grog is a granular material that is added to pottery to give the clay useful properties for building, throwing, and firing. Mainly grog adds strength to pottery, reduces shrinkage and cracks, and, adds texture. There is a range of grogs, categorized according to how fine or course they are.
What is Grog In Pottery? – An Overview
Grog is made from a number of materials that have been fired and then ground up. It can be made from ceramic that has been fired specifically to be made into grog. Or it can be made from waste pottery that is broken by accident and recycled by making it into grog.
It can also be made from ground-up brick and other refractory rock. ‘Refractory’ simply means that the material is resistant to melting and can survive high temperatures.
Grog can be added to almost all types of clay. The type of clay that is being used will determine the type of grog that is suitable. Grog can be categorized in a couple of ways.
Soft and Hard Grog
The first way is to distinguish between soft and hard grog. Generally, grog is made of material that has been fired before. If it has been fired to a high temperature, above cone 10, then it is classified as hard grog. However, if it has only been fired to below cone 10 it is considered to be soft grog.
Soft grog is generally used because it is porous and will reduce stickiness in a clay body. However, because it has only been fired up to cone 10, the grog will melt if the piece of pottery is fired above cone 10.
Hard grog is made of ceramic that has been fired above cone 10. This means it is much more robust. Because it is fully mature, it is unlikely to melt before the piece of pottery is completely fired. For this reason, hard grog is more commonly used than soft grog.
Grog and Mesh Size
Another way of categorizing grog is according to the size of its granules. Some fine grogs look like powder. Other grogs with larger particles look like granules of sugar. Grog is graded according to a ‘mesh’ system. This system refers to the number of holes per inch in a piece of mesh. So, for example, ’48 mesh’ refers to a piece of mesh that has 48 holes per inch.
Grog can be given a single mesh number. For example, it may be classified as 150 mesh. This means that there are 150 holes per inch in the mesh. It also means that the grains of grog are small enough to pass through holes this size. Grog that is 150 mesh consists of grains that size and smaller.
Mesh can also be classified according to its range. For example, a mesh might be referred to as a 20/40 mesh. This means that grains of grog would be small enough to fit through mesh with 20 holes per inch. And they would be too large to fit through mesh with 40 holes in it.
The range system is useful because it lets the potter know what the smallest and largest grain will be. This gives them more control over the properties the grog is introducing to the clay.
By contrast, if the grog has a single mesh number, the grains may be anything up to the mesh size specified. It is recommended that 30/80 mesh suits the purposes of most pieces of pottery.
Are All Grogs Equal?
It is not always possible to predict the effect that grog will have on clay. This is because there can be big variations even with the mesh range system. Grog made by different companies, with the same range can be quite different.
This is because one manufacturer may include more particles at the larger end of the scale. Whilst the other manufacturer may include finer particles within the spectrum.
This means two grogs with the same range of grading may affect the pottery differently. Therefore, it is a good idea to experiment with the effect that a particular grog has on your pottery.
What is Grog In Pottery? – The Ingredients
All clays contain alumina and silica in different ratios. Grog contains a high percentage of both. Alumina and silica give grog important properties.
Alumina has a high melting point. It also has a high mechanical strength and can tolerate a lot of wear and tear1.
Silica is a glass-forming component in clay and glazes. But more than that it plays a central role in the structure of clay. It is the ‘skeletal structure’ of the clay. Clay components attach themselves to silica. Silica, therefore, makes clay stronger by improving its structural integrity. Silica also helps to regulate how much pottery shrinks when it dries, and when it is fired.
What is in Grog?
|Alumina||Silica||Iron Oxide||Calcium Oxide|
|Min 40%||Min 30%||Max 4%||Up to 2%|
Data sources here.
Further details in References – item 2
Why is Grog Added To Clay?
Having looked at the question of what is grog in pottery, let’s take a closer look at why it’s used. Grog is a ‘temper’. The term ‘temper’, when used in ceramics, refers to a non-plastic material that is added to clay. You may hear the term ‘grog tempered clay’.
When a non-plastic ‘temper’ like grog is added to clay, it can have the following outcomes:
Adding grog to your clay will…
- Reduce the shrinkage of your clay
- Help prevent cracking
- Make clay more workable and hold its shape
- Add texture to your pottery
Let’s have a look at those items in more detail…
Grog Reduces Shrinkage in Pottery
Grog reduces how much clay shrinks as it dries out and also as it is fired. The more grog added to the clay body, the less it will shrink.
It shrinks less when it dries out before going into the kiln because the grog particles have already been fired. Therefore, they have a lower water content in the first place. So, they lose less volume as the clay dries out.
Also, when ungrogged clay dries out the particles move closer and the distance between them reduces. By contrast, particles of grog produce a porous matrix3 that does not shrink as much.
Grog also enables clay to dry more evenly because of its porosity. The open spaces between the particles allow water to migrate to the surface and evaporate more readily.
When a piece of pottery is fired, it shrinks for a few reasons. One of the reasons it that it loses more water. But it also loses volume because of vitrification. At a certain temperature, the silica in the clay will interact with other properties and form liquid glass.
The glass-forming substances leave the clay particles and fill up the gaps between the particles. This causes a bit of shrinkage. Because the grog has already been fired to maturity, it does not melt.
If the kiln becomes hotter than the temperature that the grog was originally fired at, the grog will melt. The melting point of grog is normally around 3240F (1780c).
Grog Helps Prevent Cracking in Pottery
One of the main reasons that clay cracks is that the clay body is stressed when it shrinks. By reducing the amount that the clay shrinks, the grog also reduces the chance that the pottery will crack.
In addition to this, grog reduces thermal expansion. When pottery heats up in the kiln, it expands. The expansion is only a tiny amount, however, because the ceramic is brittle at this point, a tiny expansion can cause it to crack.
However, grog has already been fired and gone through the process of expanding in heat. Therefore, it reduces the amount the pottery expands and reduces the chance of cracks occurring.
Grog Makes Clay More Workable and Holds its Shape
It is sometimes said that grog makes clay more workable because it improves the clay’s plasticity. However, this is not correct. Plasticity is the clay’s ability to be stretched, worked, and pulled into shape without cracking.
Grog, on the other hand, is a bit like internal scaffolding in the clay that stops it from slumping. It adds structural strength.
One comparison that helps me to visualize the role of grog in clay is that of wattle and daub. Do you remember learning about wattle and daub in high school? From as far back as the Stone Age, buildings have been made from wattle and daub.
Mud is mixed with straw and other debris and then pressed between wooden battens to make walls. The straw is added to the mud to give the mud rigidity and strength and to stop it from slumping.
Although it is not an exact comparison, grog acts a little like wattle and daub. A clay particle is typically 10,000 mesh, whereas a medium-sized grog is 20-48 mesh4. So, grog particles are huge compared to clay particles. If you bear this in mind, it really is like adding a handful of straw and debris to mud.
Grog Reduces Clay Plasticity
Although grog makes clay more workable, it reduces plasticity. Plastic clay is stretchy and bendy. However, if you add grit to a substance, it will make it more granular and less stretchy.
In fact, the finer the grog the more it reduces plasticity. The larger the grains of grog, the more it prevents shrinkage, and the less it reduces plasticity. This suggests that larger-grained grog is better. However, grog does affect the look of pottery. As a result, this will also determine if you want to use a finer grog or a rougher one.
Grog Adds Texture to Pottery
Adding grog to clay can give it a gritty texture. This texture is sometimes called ‘tooth’. Potters refer to grogged clay as having a ‘toothy’ quality.
All clays have grog in them, and some clays naturally have more grog. Earthenware, for example, has more grog than porcelain. This accounts for its gritty surface.
Nonetheless, grog can be added to porcelain. Most grogs have a high iron content and are not used in porcelain. This is because the iron affects the white color of the porcelain.
However, a substance called Molochite can be used as grog in porcelain. Molochite is made of kaolin, a white clay, that has been fired at a high temperature. It is a clean grog with a fine mesh of 120. Also, it is glassy and white when fired, so it does not compromise the texture of porcelain.
You can compare the ingredients of Molochite below, with those of standard grog above.
What is in Molochite?
Data sources here.
Further details in References – item 6
Because most grog gives clay a gritty texture, it can feel rough on the hands when the clay is thrown on a wheel. Whilst this might feel like a chance to get a free exfoliation, it is not good for your skin in the long run. However, if a 20-40 mesh is added to clay that is very plastic, the resulting mix can feel quite smooth5.
It strikes me that grog is versatile. It can give you that rough textured rustic look. However, there is a large range of types of grog. Finer grogs give you the benefit of reduced shrinkage and reducing the chance of cracks without losing your smooth finish.
The answer to the question of what is grog in pottery lies in its versatility. Grog is a way of tempering your clay and giving your ceramics the best chance of surviving the kiln unscathed.
It is worth pointing out that some potters make their own grog. Whilst a homespun approach is not an exact science, it is a money-saving option. Here is a charming video of grog being made DIY style.
4 & 5 https://wiki.glazy.org/t/a-brief-overview-of-grog/697