Most potters have experienced the effects of soluble salt migration in a clay body that can occur in the drying, bisque, or glaze firing stages. The industry term is ‘scumming’, and it presents itself as efflorescence, or white crystal powder, forming on the surface of the clay.
The industry term is ‘scumming’, and it presents itself as efflorescence, or white crystal powder, forming on the surface of the clay. It can be observed on random parts of the clay body surface but is most often seen on edges or high points which dry faster than other parts. Faster drying increases the ‘wicking’ action of the soluble salts that travel to the surface of the clay with the evaporating water. Textured surfaces or the ridges on cup handles can reveal a hard-discolored crust on the fired ware.
In the bone dry or bisque state, fingerprints
Why does my clay have a white irregular crystal surface when it is drying?
In the bone dry or bisque state, fingerprints can disturb the soluble salt deposit on the clay surface causing a noticeable flashing or discoloration on the fired clay. High concentrations of soluble salts on bisque clay surfaces can result in fused areas which retard absorption, resulting in uneven glaze deposits. Soluble salt migration can also disrupt the covering glaze surface. Essentially, it can create a mechanical disruption of the clay body/glaze interface which can result in the glaze crawling (the fired glaze rolls back revealing the clay body) or a series of small glaze blisters (sharp edged crater-like holes in the fired glaze) due to the flux. There are several options which can eliminate soluble salt migration.
When developing a clay body or choosing a pre-mixed moist clay, whenever possible select clays low in iron content and/or low in soluble salt content. Often this information can be obtained from ceramics suppliers who mix clay.
The most widely used correction for soluble salts is the addition of barium carbonate,
1/16 to 2% based on the dry weight of the total clay body formula. When thoroughly mixed into the clay body, barium carbonate will precipitate soluble salts from solution yielding barium sulphate, which is insoluble. Brick industries are the largest users of barium carbonate where low-grade, high-iron content shale-type clays are used in the production of bricks.
In the bone dry or bisque state, fingerprints can disturb the soluble salt deposit on the clay surface causing a noticeable flashing or discoloration on the fired clay.
Jeff Zamek walked into a pottery studio in 1967 and started his career as an amateur potter. After completing a degree in business from Monmouth University, W. Long Branch, NJ he obtained B.F.A./M.F.A. degrees in ceramics from Alfred University, College of Ceramics, NY. While there he developed the soda firing system at the college and went on to teach at Simon’s Rock College and Keane College. During this time he earned his living as a professional potter. In 1980 he started Ceramics Consulting Services a ceramics-consulting firm developing clay body and glaze formulas for ceramics supply companies throughout the United States. He works with individual potters, ceramics companies, and industry offering technical advice on clays, glazes, kilns, raw materials, ceramic toxicology, and product development. He is a regular contributor to several ceramics magazines and technical journals. Jeff’s books What Every Potter Should Know and Safety in the Ceramics Studio, featuring the safe handling of ceramic materials, and The Potters Health & Safety Questionnaire are available from Jeff Zamek/Ceramics Consulting Services. His latest book, The Potter’s Studio Clay & Glaze Handbook, was published in June, 2009.