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Ask the Doctor: Preventative Medicine, "Taking Stock" (written late 1990's)

Dear Doctor,

My tenmoku used to be opaque black, breaking nicely on edges to a reddish brown. Now it breaks to a translucent cola-like color. Also my rutile blues have lost their intensity. The size of my seconds pile is giving me the blues.What gives?

Sincerely,

Rutile Blue in Memphis

Dear Rutile,

Working in a small ceramic supply house, I had the experience of packaging and selling quantities of ceramic chemicals in 1lb., 5lb., 10lb., 25lb. and full 50lb. bags. I handled many customer questions about glaze problems and tried to help. The most difficult problems came from people who used large quantities of materials but bought in small quantities. It was frustrating to hear, "the five pounds of rutile that I bought this month is not as dark as the five pounds I got two months ago and my rutile blue no longer works."

A favorite customer of mine purchased materials in quantity: full pallets of feldspar, full bags of rutile, five pounds of stain. He is rumored to have clay left over from the sixties. I have always recognized the wisdom in this practice of stockpiling important materials. Now, the only ceramics instructor at a small university, I make purchasing decisions and balance consistent material supply, with space, budget, and safety concerns. Here are my thoughts on what materials to purchase in quantity and what materials to purchase as needed:

Some materials are easily available in very high purities. Purchasing large quantities of them would serve no purpose except perhaps receiving a quantity price break. Precipitated magnesium carbonate, barium carbonate, lithium carbonate, silica, zinc oxide and strontium carbonate, are examples

Soda ash, borax, and other water soluble materials clump when exposed to humid air and become much harder to handle. Anhydrous borax picks up water from the air and changes chemically. Zinc oxide clumps over the long haul and sometimes, so does wollastonite(although I don't know why). With these materials I would limit my stash to about a one year supply.

Space and budget play a big role in the decisions of what to buy. Some materials are too expensive to stock. Others, like a clay body feldspar, might use up too much space.

I plan to buy these in at least five year supplies:

    Iron oxide (unless you only use the more expensive 99% pure variety which can be purchased as needed)
    Rutile
    Whiting, (unless you have a steady source of whiting with a guaranteed purity). In the limestone industries the word whiting applies to compositions that vary from pure calcium carbonate to dolomite. It would do us good to refer to calcium carbonate as calcium carbonate and leave the term whiting for people to use in industries that don't care about the magnesium content.. I think that a good deal of variation in artist's glazes is attributable to variations in whiting.
    Dolomite (like in whiting, the magnesium content varies)
    Spodumene
    Bone Ash
    Soda feldspar
    Potash feldspar for glazes
    Gerstley Borate

Clays that do not have easy substitutions such as:

    Albany Slip,
    Ochmulgee
    Avery Kaolin
    Jordan

    (too late, these are all ready unavailable or very expensive),

If these clays are important to your production, you should always have some on hand. Even a buffer of a three month supply might give you the cushion needed to develop a substitute.

    Barnard Slip
    Redart
    Helmar Kaolin.

Wood Ash. Most people don't purchase woodash. But, if you do, don't expect consistency. If you collect it and consistency is an issue, collect a big supply and blend it all together.

    Lets look at the costs and quantities of materials on my list: [not included due to outdated information. ]