underslip

In ceramic production, an underslip is a thin layer of liquid clay slip applied to the surface of a piece before the final glaze coat. It serves as an adhesion layer, helping the glaze bond securely to the ceramic body and preventing crawling, where glaze pulls back into beads and leaves bare patches. The underslip is typically thinner and more fluid than a decorative slip, and it fills the porous surface of bisque-fired ware, creating a more uniform substrate for the glaze to cling to.

The composition of an underslip differs from both the body and the glaze. It is usually formulated from clay with a specific gravity and plasticity that allows it to dry without cracking while still maintaining enough fluidity to apply evenly by dipping, spraying, or brushing. Common underslips for stoneware or earthenware contain ball clay and kaolin in proportions that balance adhesion with drying speed. The thickness applied is typically 0.5 to 1.5 millimetres; too much creates crawling; too little fails to solve adhesion problems.

Why underslips matter in production

Glazes and ceramic bodies contract at different rates during cooling. If glaze contacts a dry, porous bisque surface directly, the glaze layer can separate during the final firing. An underslip reduces this mismatch by providing a layer with intermediate porosity and firing shrinkage. This is especially critical on vertical surfaces and on pieces where glaze application is thick or where the glaze composition has high shrinkage.

Underslips are not the same as engobe, which is applied before bisque firing for decorative or slip-casting purposes. An underslip goes on bare bisque and stays beneath the glaze throughout firing. They also differ from glaze suspenders or deflocculants added to glaze itself; underslips are a separate layer with their own recipe and function. On tableware and sanitary ware, manufacturers often use proprietary underslip formulas to reduce defect rates and improve firing consistency across batches.