Clay slip, or liquefied clay, can be used in varying ways. When working with earthenware, the clay bodies are higher in iron content and darker in color, ranging from red, to brown, to black. A lighter colored slip applied over darker clay can be a nice way to highlight surface changes, provide a background for pattern, texture, or design, or to encourage flashing.

Slips formulated for the cone 1–4 range might include a lower temperature frit or flux to aid in cohesion with the clay body or to improve soda-vapor surfaces. The thickness of the slip should be considered for highlighting or obscuring the dark clay surface.

Katie Fee’s rocks cups (see right) illustrate how to use a line blend to incrementally test materials. The gradual variation of ingredients allows us to see the changes as colorants are added (or subtracted) to a base slip (or glaze) mixture. The finished test pieces, which have each been dipped into one incremental test of slip or glaze, can easily be evaluated against the recipes to determine which variant is best suited for future work .

Katie Fee’s rocks cups, 3 in. (8 cm) in diameter, wheel-thrown, wire-cut and stretched earthenware, dipped in flashing slips to represent a line-blend test of slip made from 6 Tile Kaolin and Newman Red clay, fired in a cross-draft soda kiln in oxidation to cone 3, 2017. Left to right, the cups correspond with recipes 1–4 in the chart below.



Excerpted from Low-Fire Soda by Justin Rothshank. Published by The American Ceramic Society, 2020. Available in the Ceramic Arts Network Shop at

Topics: Glaze Chemistry