Grog is a fired ceramic that is pulverized and added to various clay bodies during the mixing process. There are two large categories of grog: soft grog and hard grog.
Soft grog is rarely used in the ceramic arts because it is fired at a lower temperature than Cone 10 and it is porous. In a biscuit firing, the soft grog might be softer-fired than the peak biscuit temperature. This means that the grog will melt within the clay. The advantage of soft grog is that due to its porosity, it can temper sticky clays, and aid in quick drying. Hard grog, on the other hand, is fired over Cone 10 and has gone through the entire clay firing phase, making it impossible to melt inside an artist’s clay body. The characteristics and advantages of hard grog make most potters use hard grog.
Almost all types of clay contain grog them, including some porcelains. Grog is added to a clay body for various reasons. In handbuilding, grog is added to minimize building and drying problems. In throwing, it also minimizes building and drying problems, but also provide a “toothy” quality to the clay. Toothy refers to clay that feels rustic, gritty, and rough, which are one of the qualities that are desired in a stoneware. In slip casting, grog is never added because the advantages of adding grog do not outweigh the disadvantages and problems it will create, such as rough texture.
Common misconceptions that exist about grog is that grog is the reason why a clay body is more “plastic” than clay that does not have any grog added in. According to recent studies, this is not exactly true. Plasticity is how much clay can be worked, stretched, and/or pulled without cracking. What grog does is creating refractory networks throughout the body, increasing the percentage of non-plastics in a clay body, and minimize building and drying problems.
Another misconception is that grog in a clay body decreases the shrinkage rate of the body. This is also not really true. A clay body is split into two parts. The clay molecules, and the non-plastics. The clay aspect melts into glass and mullite when fired, the non-plastics do not.
Let’s do a small thought experiment:
There is Clay body A and B.
Clay A is 50% clay 50% non-plastics (no grog)
Clay B is 25% clay and 75% non-plastics (25% is grog)
Both clays are fired, and Clay B turns out to have half of the shrinkage rate of Clay A. Does that mean the shrinkage rate has been reduced? No. There is only half the amount of material that can melt in the firing. Therefore, it can be concluded that grog does not decrease shrinkage rates the way that most people think.
It is also not required to add grog into clay bodies like stonewares. Although grog can increase the Green Packing Density, it is important to realize the level of size that we are working with as potters. (Higher green density in a clay body means higher plasticity, higher building strength, and dry strength.)
A clay particle is 10,000 Mesh.
Flint, a glass former, is 325 Mesh.
Medium sized grog is 20-48 Mesh.
Even the highest commercial mesh size is around 325. This shows the big difference between clay particles and grog. Therefore, if the clay body itself has a high Green Packing Density, there is no need to include grog from a greenware performance standpoint.
But, if grog is going to be added to a clay body, 4% fine grog will be good enough to have the toothy quality, and maybe aid in some GPD/particle size distribution. It is always good to have a wide variety of particle sizes in a clay body. But sometimes large grog sizes may cause problems in a clay body due to its’ immense size compared to all other particle sizes within the clay body.
There are various companies that currently manufacture/supply grog.
Axner Pottery Supply, Craneyard Clay, Mulcoa, Imerys Refractory Minerals, ICCS Refractories, C-E Minerals Inc., Christy Minerals Co., Sial, Etc…
Each company has its own products that are unique and have different mineral compositions.
The Mulcoa company has various grogs that are on the market. One of their products is called 20 Mulcoa. This means that all the particles will pass through a 20-mesh sieve. This does not mean that all the particles are 20-mesh sized. There will still be smaller mesh-sized particles within, all from 30 mesh to 200 mesh. There are also other grogs that are named like 20x80 Mulcoa. This means that the grogs will pass through a 20 mesh, but not through an 80 mesh. The chemical make-up of 20 Mulcoa is:
46.8% Alumina, 50.00% Silica, 1.89% Titanium, 0.95% Iron, and 0.36% Trace elements.
The 20 Mulcoa is a light grey grog which has been calcined or fired at high temperatures, then ground. This grog is mostly Silica and Alumina, which is normal for most grogs that artist’s use. The percentages also show that the grog is contaminated with Titanium and Iron. This is also common in grogs. In a stoneware body, contamination levels like that of 20 Mulcoa are perfectly fine.
Another example of grog is the 200 Mulcoa, which is a much smaller particle sized grog than 20 Mulcoa. The 200 Mulcoa looks a lot lighter than the 20 Mulcoa. This is because the 200 is a lot finer, which makes it look lighter as shown in the picture above. Both grogs come from the same mine source, so they will have very similar chemical compositions. But, it is still important to remember that the chemical compositions that the manufactures provide are the average percentages for the entire mine. As clay is a natural source, every batch of grog will be slightly different, even though it might not affect the clay body in a significant way.
Another grog type is called Plainsman Grog. This grog is created from pulverizing already fired construction bricks that have 5% porosity by weight. It is very important to know not only the source of the grog but also the chemical composition of the grog. This is because different grogs have different chemical compositions, fired temperatures, and even porosity levels depending on the source. As grog is switched from one to another, the clay itself be altered to have consistent results.
In a white porcelain body, most grogs will not be desirable due to their high levels of Iron and Titanium, which are basically colorants/contaminators. Therefore, Molochite is an alternative to grog that is added in. Molochite is basically white kaolin that is fired until vitrified (at or past Cone 10) and pulverized. The downside to Molochite is if large particles of them are added to clay bodies, it might cause firing failures such as micro-cracking and large-scale cracks.
200 Molochite is the second-finest mesh that you can find in the market, 325 being the highest. Unlike the other grogs that are included in this article, Molochite is very clean. The chemical composition of Molochite is:
42% Alumina, 54.50% Silica, 1% Iron, 2% Potassium, 0.07% Titanium, 0.06% Calcium, and .31% Magnesium
In contrast to the Mulcoa and CE Minerals grogs, Molochite is chemically cleaner and contain fewer contaminants. When fired, Molochite will be white and glassy, like porcelain. Therefore, most potters and manufacturers add Molochite to their porcelain bodies if need to.
Usually, if there is no grog manufacturer nearby, the local pottery supply store is a good alternative. Not only do they have a selection of grogs in stock, but they also have the connections with suppliers to make special orders. Nowadays, due to the internet, it is possible to order grog from virtually all the manufacturers in the world. As an artist, having the knowledge of grog and the ability to create personal batches of grog to suit specific clay bodies is a great idea. This opens up possibilities like colored grog, advantages of soft/hard grog, identical chemical make-up to a clay body, and more. In conclusion, grog is only as good as its manufacturer and user. The more knowledge you have about grog, it will become a useful tool to aid in your artistic practice.