grog

Grog is ceramic material that has already been fired once, then ground into particles and screened to a target size. It is mixed back into wet clay bodies before forming and firing again. The particle size typically ranges from 200 mesh (75 microns) for fine grog to 20 mesh (850 microns) for coarse grog, depending on the intended application and the desired surface texture or structural properties of the final piece.

The primary function of grog is to reduce shrinkage and warping during drying and firing. When clay dries, it shrinks significantly as water is expelled; grog particles do not shrink because they are already fired and inert. By occupying space in the clay body without shrinking themselves, they act as a kind of spacer, reducing the net shrinkage of the entire piece and the stresses that come with it. A typical grog addition might be 10 to 40 percent by weight, though sculptural work may use higher percentages.

Sources and production: Grog is made from scrap clay: unfired waste (trimmings, failed greenware), fired kiln stilts and supports, or broken fired ware. This waste is fired at cone temperature if not already, then crushed in a ball mill or jaw crusher and passed through screens. Using scrap in this way reduces material waste and the cost of clay purchases, making grog economically valuable as well as functionally important. Some potters and industrial producers make their own grog; others buy it in standardized grades.

Grog also improves workability and reduces plasticity, making clay stiffer and easier to handle in large forms or when hand-building. The particle size distribution matters greatly: fine grog produces a smoother surface finish and works well for tableware or casting, while coarse grog is preferred for sculptural work, raku, and other applications where texture and open structure are desired. Calcined clay (clay that has been heated but not fully fired) is sometimes called grog or used as a grog substitute, though it differs chemically.

Problems arise when grog is poorly screened or mixed unevenly into the clay body. Oversized particles can cause lamination defects or surface pitting during firing if they crack from thermal shock. Very fine grog particles can actually increase plasticity and water absorption if present in excess, defeating the original purpose. The color and firing characteristics of grog must also match or complement the base clay; mixing grog from different clay bodies or firing temperatures can introduce incompatibilities.