Sculpture and Raku Clay Bodies - Plainsman
Jim Marshall sculpture made at Medalta by Brian MacArthur in 2014.
Grog is an aggregate of crushed granules of brick (our grogged bodies contain from 5-25% of this). We also use different sizes of grog. Depending on the percentage, the grog lowers drying shrinkage and the particles help to terminate or deflect cracks. That being said, many people doing architectural and sculpture commissions prefer to work with a smooth clay (even porcelain) to get the surfaces they want, and, by employing meticulous working procedures that are successful.
Working Properties of Grogged Clays
Much advice is available from textbooks, websites and practitioners on ways to form, dry and fire sculpture. Because we grind our clays to a coarser particle size than most other manufacturers, some of our sculpture bodies can present a working challenge not as likely to be found in others: During forming, water left on the surface of ware has greater opportunity to penetrate in and begin a split that could cause a piece to collapse. So be mindful of not allowing water (or high water content slip) to sit on wet surfaces too long (especially on stretched or stressed sections). There is also need for caution in drying items: The evenness is much more important than the speed of drying (a faster but even drying is better than a slow uneven one). That being said, slowing down the drying by covering pieces is a way to keep the water content more even across the item.
Some grog bodies are more vitreous that others and overhung shapes can warp during firing. The data sheet for each body will have information about this.
If your ware needs to be able to survive freeze-thaw, please read this page (there is lots of misinformation about this).