by Louis Katz

Calcia O Calcia

Under-appreciated, Overworked, Inexpensive


Glamour is not my forte. Other materials are in vogue. The Clayart list service on the Internet1 has been full of posts on rutile blues, cadmium inclusion pigments are being pushed despite their designer prices, and questions about barium substitutions appear almost daily. So why calcium? I am drawn to plain things. Like pondering individual bricks, flower pots, Styrofoam cups, the appreciation of calcium relies more on the perspective I bring to it than on what it brings to me. It is not exotic.

Like bricks, calcium compounds are ubiquitous yet not banal. We wedge on them, we make molds of them, bind masonry with them, and rely on them to melt celadons and most other stoneware glazes. They crop up in our water flocculating slips, torture us as lime popouts, and scum our most pristine surfaces.

Leach's 1-2-3-4 Celadon is an old stand-by calcium-fluxed glaze for Cone 10. It is a simple recipe, easy to remember, and was formulated by a famous potter. It is a good recipe for variations. Much simpler, not quite as reliable, but easier to remember, is Katz 1-1-1: One silica, one Kaolin, one calcium carbonate. It fires to Cone 10, and with a few percent iron, reminds me of the swirly dull green last seen in the pattern of a linoleum floor from the 50's. The originator of this glaze is not quite as famous.

The real value of Katz 1-1-1 is its graphic demonstration of the alumina calcia-silica eutectic2. The basic principal of eutectics is "that some mixture of any two distinct materials will melt at a temperature lower than either of the two materials will melt at alone." Calcia (CaO), Alumina (Al2O3), and Silica (SiO2) all melt at temperatures above 3200 degrees. When mixed together in the proper proportions (23.25% CaO, 14.75% Al2O3, and 62% SiO2), however, they melt at 2138 degrees F, and make a passable Cone 10 glaze.

Limestone is the source for most of the calcium carbonate3 that goes into glazes, and an understanding of the material and the chemical changes that it goes through is important to understanding its multiple uses in the studio. Just up Last Chance Gulch from the kitchen of our marble (another form of calcium carbonate) countered house in Helena, Montana, are a series of lime kilns. These kilns were used for burning limestone, one step in the process of making lime mortar for masonry. The calcium carbonate limestone was heated with wood to above 1600 degrees F or so, calcium dioxide4 was given off in the process, and calcium oxide or quick lime resulted. Pure quick lime is amazing stuff. It can, under many circumstances, react violently with water. If you add a pound of quick lime to two pounds of water, the mixture may heat to boiling. The reaction with water can send showers of caustic quick lime particles all over a room, causing burns. Getting quick lime in one's eyes will almost certainly cause significant damage, and often results in the loss of sight. This is an amazing transformation for one of the safest materials in our glaze pantries. After the lime has reacted with the water or slaked, it is referred to as slaked lime4.
  Many factors affect the speed of this slaking reaction. Naturally occurring impurities from the limestone generally slow the reaction. Magnesium carbonate intermixed with the calcium carbonate in equal parts can almost completely eliminate slaking. Small additions of clay, or iron oxide, or firing above 2000 degrees F, usually slows the speed of the reaction.
  Slaked lime is mixed with sand to produce lime mortar. Historically, before the development of Portland cement, this mortar held together many masonry structures. The true miracle of slaked lime mortars is that they get stronger with age. Over the span of decades, the mortar slowly reabsorbs carbon dioxide from the atmosphere and again becomes limestone. Slaked lime is the main ingredient in traditional whitewash. On the stucco walls of our house, it repairs cracks, increases water resistance, and kills mold and other growths. It is relatively safe for the environment. It is very cheap.
  Calcium carbonate is occasionally used as wadding to keep pots from sticking to each other. It can be easily removed from fired glaze by slaking. This makes it useful when stacking completely glazed pots on top of one another, or where a runny glaze might flow underneath the wadding and fuse it to the pot. You must remember, however, that the wads come from the kiln as quick lime and might be highly reactive with water-- and dangerous. If you decide to use calcium carbonate wads, you should wear safety glasses when unloading the kiln until you are certain that the wadding reacts slowly.
  Plaster of Paris, like slaked lime, is cementatious. It is made by calcining gypsum5, another calcium mineral. The plaster we get from a bag has an approximate formula of CaSO4 1/2H2O. That is, for each atom of calcium there is 1/2 molecule of water. When we add plaster to water, a chemical reaction takes place. Some of the water combines with the plaster until there are two molecules of water for each molecule of calcium sulfate. As this reaction takes place, the plaster rearranges itself into a more orderly crystalline structure. Heat is given off. The remaining water is incorporated as the pore spaces between the now fully-hydrated calcium sulfate (gypsum). The more excess water there is to be incorporated as pore space, the more porous the plaster becomes. Care must be taken in drying plaster. The reaction that makes plaster harden is reversible. As you heat plaster up, it gives off this chemically-combined water in stages. According to Dick Notkin, the first such dehydration takes place around 145 degrees F. Heating plaster above this point softens it slightly, and can make it soft and powdery. Holding set plaster in about a 350 degree oven and then grinding it returns it to a state where it can be used again. It will not have the exact same qualities as the original plaster did; the additives that the plaster manufacturers add to their plaster may not have such conveniently reversible reactions. As the heating progresses, the plaster continues to lose the chemically-combined water, and eventually releases sulfur dioxide. The calcium, as when you heat limestone, becomes quick lime.
  Plasters, quick lime, and most cements expand slightly when they react with water. This property can be very useful, and the amount of expansion can be controlled through choice of plaster, additives, and water temperature. Quick lime is used in expansion cement, a material used to fill holes and hold bolts into cement slabs. Mold makers working in metal casting are able to use the slight expansion of plaster to compensate for the contraction of the casting on their cooling.
  Unfortunately, the expansion during hydration of calcium cements, so useful to masons and metal workers, often makes life miserable for ceramists. When making molds, this expansion often makes parts of molds not fit each other. Careful attention to the mixing schedule, temperature of the water used, and the freshness of the plaster prevents these problems. Lime blows or plaster popouts in fired work are caused by the expansion of the quick lime, and sometimes plaster that has become trapped in the clay. Since limestone often occurs near seams of clay, lime popouts are here to stay. Interestingly, certain groups of Native Americans used clam shells as grog (temper). Firing temperature in this pottery was critical. The work was apparently fired above the temperature at which clay would no longer slake down, but below the temperature (1600 degrees F) where the formation of quick lime became a problem.
  Gypsum is a major culprit in scumming. Dissolved in water, the gypsum is deposited on the surface of the ware at the water dries. Barium carbonate, also slightly soluble in water, is able to eliminate gypsum scumming very effectively. The dissolved barium carbonate reacts with calicum sulphate becoming calcium carbonate and barium sulfate. Barium sulfate and calcium carbonate are much more relatively insoluble in water, preventing the calcium from migrating with the water to the surface of the clay. After the barium sulfate comes out of solution, more barium carbonate dissolves. Despite its desirable qualities, barium carbonate is poisonous.
  Soap scum is the bane of the soap user with hard water. Soaps are sodium oleate and sodium stearates. These compounds are miscible (mixable) with water. When exposed to gypsum and some other calcium compounds, they become calcium oleate and stearates which are immiscible (unmixable) with water. When washing with water hard with calcium salts, these calcium oleate and stearates form a scum on sinks, clothes, and washing machines. Because they are immiscible with water, these ornery compounds require more than simple rinsing to remove them.
  Mold soap makes use of this reaction. Soap is lathered onto the plastic mold, building up a layer of immiscible calcium soaps on the surface. Excess soap is rinsed away (this prevents the excess soap from weakening the new plaster), and new plaster is poured onto the old. The calcium soaps seal up the old plaster, preventing the new plaster from soaking into the pores of the old and bonding with it.
  Calcium sulfate and calcium chloride are flocculents. A flocculent is a material that increases the amount of water necessary to make a slip fluid. When my friends in Thailand were looking for inexpensive materials for lowfire and midrange glazes, I suggested glass cullet. I talked to Peter Pinnell about it, and he suggested the addition of a few percent plaster to help keep the glaze from settling. This seems to work well. Calcium sulfate is a self-adjusting flocculent. Its limited solubility keeps the amount in solution relatively stable. Used for molds, plaster is indispensable for slipcasting. But small amounts of plaster are enough to cause big problems in deflocculation of the slip. Barium carbonate is added to slips not only to eliminate scumming, but also to eliminate flocculation.
  Calcia is the oxide most commonly in the background of our processes. A metaphor for most potters, sculptors, and educators, Calcia is under-appreciated, overworked, and inexpensive.


(2) The suffix a refers to the oxide, (i.e.) potassi a is potassium oxide.

(3) CaCO3

(4) Slaked lime or calcium hydroxide has the formula CaOH2. Less hazardous than quick lime, slaked lime is still caustic, hard on the hands and dangerous to the eyes. Don't forget your respirator when working with dry quick lime or slaked lime.

(5) CaSO4 2H20

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copyright 1997 Louis Katz