Grog is clay that has already been fired, ground down and mixed back into a raw body. Because it has been through its shrinkage once, it cannot shrink again, so a grogged body moves less as it dries and fires. That single property is why large work, sculpture and hand building are almost always done in a grogged clay.
Why grog reduces cracking
Cracks come from differential shrinkage: one part of a piece contracting against another that is not ready to. Grog particles are inert, so a body carrying twenty percent grog has twenty percent less material that wants to shrink, and the network of hard particles also gives water an easier path out. The result is a body that tolerates thick sections, uneven drying and big joins far better than a smooth one.
What it costs you in the hand
Grit. A heavily grogged body is abrasive and a long throwing session in one will take skin off fingertips, which is why production throwers use smooth or lightly grogged clays and hand builders use coarse ones. It also holds less fine detail: a fingerprint or a crisp edge reads softer in a grogged body than in a smooth one.
Coarse, medium and fine grog
Suppliers grade grog by particle size and it changes the character completely. Fine grog gives most of the structural benefit with far less abrasion and is the sensible compromise for anyone who both throws and builds. Coarse grog is for large sculptural work and gives the toasted, gritty surface many potters buy the body for in the first place.
Grogged and grog clay on a price list
Suppliers list grogged versions of standard bodies alongside the smooth ones, usually under the same name with a G in the code, and the price difference is small. Bath Potters and CTM both list grogged and ungrogged versions of the same bodies, which makes it easy to try both without changing anything else about how a piece is made.
Questions people ask about grog clay
What is grog in clay?
Fired clay, ground and mixed back into a raw body. It has already shrunk once, so it reduces the shrinkage and cracking of the body it is in.
Is grogged clay harder to throw?
It is more abrasive on the hands and holds less fine detail. Lightly grogged bodies give most of the benefit with much less of the cost.
When should I use a grogged body?
Large work, sculpture, hand building, slabs, and anything with thick sections or big joins. For fine thrown tableware a smooth body is usually better.