Water, Water, Everywhere,

All the Same I think?

Dear doctor,

Every fall I have trouble with all of my glazes crawling. My bisque is free of dust and my fingers are clean. I have even tried wearing rubber gloves. Could this crawling be caused by a diet rich in cottage cheese?


Sore Knees

Dear Ms. Knees,

Ingredients that we use in our clay and glazes come from nature and are variable in composition. Unfortunately this can also be true of our water. Soluble salts from our water (and clay) can bring about crawling in at least two ways. As greenware dries these soluble compounds are deposited on the surface of the clay. The resultant scum that appears is most frequently whitish. It can cause bad adhesion between the dry glaze coat and the ware. This poor adhesion allows the melting glaze to bead up like water does falling through the air.

Salts in glaze suspensions can also cause crawling from flocculation. Flocculated glazes require more water to have the same degree of fluidity in application as unflocculated glazes. More water usually means more shrinkage that can cause cracking in the unfired glaze coat. More shrinkage also causes a weaker adhesion between the bisqued clay body and the dry glaze coat. Alone or together the cracks and the poor adhesion allow the glaze to pull itself in beads or just crack and fall from the pot.

If you live in places where the tap water is hard, and are having problems with scumming, crawling, or can’t get terra sigs or casting slips to deflocculate properly you might want to consider other sources of water. The graduate studios at Arizona State University use rain water collected from the roof downspouts to breed mosquitos and to mix clay. Clay mixed with the rain water is much superior to clay mixed with Tempe tap water. Here at Texas A&University -Corpus Campus we use the condensate water from a small room size dehumidifier for slips, sigellattas, and troublesome glazes. This is the same distilled but dirty water extracted from the air by air conditioners. It is nearly free of soluble impurities. At large universities this water is often available from the physical plant or a tap in each building. Ask your heating and cooling person.

Our dehumidifier is also used to dry and heat the air for a closet sized hot box, used for drying slipcasting molds. Since we use the water taken from the molds for the casting slip this arrangement begins to approach a closed system.

Barium Carbonate is often added to clay to reduce scumming. It removes it because it is slightly soluble in water and reacts with calcium sulfate to produce two insoluble compounds. Being insoluble prevents the new compounds from being scumming agents. Since Barium Carbonate is poisonous (used to be used as rat poison) many people have stopped adding it to their clay and slip recipes and decided to live with scum. I like scum,particularly in cone 6 soda kilns but I have come up with this list of suggestions to help keep scumming to a minimum without the use of barium Carbonate:

  1. Keep your clay barrels, slip buckets, and slop bins covered to reduce evaporation. Every bit of water you dry into your clay leaves its soluble salts.
  2. If you recycle, use an excess of water but take the water off the top of the slop bucket before you begin to dry it out. Do not transfer excess water from one slop bucket into another, just send it down the drain. This actually has the potential to carry some soluble material away from the clay, and keeps the effects of salts in your water to a minimum.
  3. Use distilled water in spritz bottles, and keep their use to a minimum.
  4. Avoid the use of Gertsley’s borate in slips.
  5. Be kind to younger siblings, especially those newly elected to the NCECA board.