In ancient times potters did not have clay suppliers to provide them with raw materials for their glazes and clay bodies. They had to find, collect and process all the materials that they needed to produce their pottery from their local environment. This may have meant digging clay from the river bank, grinding up feldspar from an outcropping or collecting ashes from the fire pit. The result was that potters in one area had distinctly different clay and glazes from potters only a hundred miles away.
Today, because of private property laws, it is more difficult to run out and start mining local clay from your neighbor’s yard. But there are other sources of ceramic raw materials available to the modern potters that are both locally available and already processed. They are in the products we use everyday, which are sold in hardware, grocery or discount super stores.
These materials are similar to the ones that our ancestors used for their clay and glazes, but they are processed for other uses that are more relevant to our daily needs, like antacids, cat litter, sunscreen, toothpaste, etc. The trade off is that, although we may not be getting the same regionally unique materials our ancestors did, we do have a readily available source of uniformly processed raw materials with little or no effort. We just have to have our credit cards available. Using products intended for other uses to create glazes, a potter can still come up with a unique and interesting range of colors and surfaces and at the same time learn about material properties and their interactions in glazes.
This 8” x 7” x 7” vase, wheel thrown, porcelain, cone 10 gas fired reduction, Carmel Glaze with Iron Yellow
To create glazes the traditional way you need some basic building blocks: clay, feldspar, silica, whiting, dolomite, talc, colorants, etc. The same is true when you are creating glazes from store bought products. A simple reading of the ingredients list will reveal that many of the products we use contain high quantities of clay, silica, feldspar, etc. The difficulty can be finding sources of pure materials because one product may contain several ingredients. But that is no different from the problem our ancestors faced when they dug materials from the ground. It simply requires a series of tests to determine what combinations will make suitable glazes.
First we must find sources of the basic building blocks of a glaze. Clay is available in many products, a major one being cat litter which, in America, is usually coarsely ground Missouri fire clay. This is a very consistent and inexpensive source of clay for your glazes and it comes in handy 20 pound bags. You could also select the clumping type of kitty litter that contains bentonite. Bentonite is a type of clay that, when wet, swells up to fifty times its volume. This is added to kitty litter so that after the cat has used the litter box the owner can easily identify the obvious swelling clumps that need to be removed. This type of kitty litter is great to use in glazes because it helps keep the glazes well suspended. Clay is also present in many other products including makeup and mud masks, but using these products to make glazes would be too expensive.
Feldspars are available in the hardware or paint store in the form of a sanding medium called rottenstone or pumice stone. These are very finely ground, 325 mesh, iron bearing feldspars. The pumice stone can be used to make a wonderful green celadon glaze while the rottenstone, which has more iron, makes great tenmokus and caramel colored kakis. A nice thing about the rottenstone is that it contains magnesium oxide, which produces wonderful satin surfaces. If you cannot find the ground pumice stone it is also available in a block form that is used to remove corns and calluses from the feet. This can be broken up by hand to provide a coarser mesh feldspar.
Silica is a little difficult to find in its pure state in products sold for uses other than glazes. It is used in many products such as, toothpaste, cleanser, gel bags in electronic products and insecticides. I used cleanser to supply silica and the soap provides the sodium oxide. Toothpaste also contains silica, which is listed on the label as hydrated silica. Silica is also available as diatomaceous earth, which is sold in garden stores as an insecticide. It is a rock mined in coastal California composed of the shells of dead microscopic organisms called diatoms. When the one celled creatures die, their sharp silica shells remain. These sharp shells are useful as an insecticide because they scratch the exoskeleton of insects causing them to get infections and die. It is also used in abrasives, cement, roofing, paint and plastics.
Portland cement is a source of silica but also contains alumina and calcium oxides. It can be used in glazes but the difficulty is that it has a very short shelf life because it hardens very quickly. In the same aisle as cement are the cement stains or colorants. These are a great source of iron, manganese and other coloring oxides that we use in ceramics. Actually my favorite source of iron is a cement pigment made by Bayer, called synthetic red and yellow iron oxide. Synthetic iron is the strongest form of iron oxide available, because of its purity and extremely small particle size. I use it in many of my standard iron glazes.
Calcium carbonate has been used for centuries by humans for a variety of processes and products. Our ancestors used plant and wood ash as a source of calcium carbonate as well as sea shells and limestone. Today, one of the most well known uses of calcium carbonate is in antacids. Rolaids and other generic antacids are primarily calcium carbonate but some also contain magnesium carbonate. Calcium carbonate and magnesium carbonate together form dolomite. Calcium carbonate is also sold in hardware stores as window cleaner and as the powder used to make lines on football fields.
A short walk down the aisle of any drug store will provide a multitude of other possibilities for glaze materials. Alka-Seltzer is an excellent source of sodium bicarbonate, otherwise known as baking soda. In addition to sodium bicarbonate, Alka-Seltzer Gold contains potassium bicarbonate or what is commonly referred to as pearl ash. Sodium bicarbonate is also used in manufacturing textiles and glass, neutralizing acids, purifying air, treating paper and in the production of fire extinguishers.
Magnesium carbonate is readily available from several products, the tastiest of which is Milk of Magnesia. Baby powder is another excellent source of magnesium oxide in the form of talc, which means that you also get silica and a wonderful fragrance. Baby powder is treated to resist water, so it is somewhat difficult to mix into a glaze batch. I used a bit of liquid soap to break the surface tension, allowing the powder to become wet. Be sure to check the ingredient list of the “Baby Powder” as some now contain corn starch instead of talc.
Historically, magnesium oxide was used as flash powder by early photographers and is still used for special effects at rock concerts. Magnesium carbonate, along with calcium carbonate, acts as a buffer in aspirin to reduce stomach upset. Magnesium, in the sulfate form, is also present in Epsom salts which potters commonly use for flocculation in glaze buckets.
Zinc oxide is available in the drug store in the form of antiseptic ointment or powder called Gold Bond and as a white nose cream to prevent sunburn. In industry, it is mainly used as a protective coating for galvanizing metals, but is also used as a paint pigment, in storage batteries, as a filler in tires, in television tubes and fluorescent coatings. Zinc oxide is also used in make-up, and for the treatment of acne, poison ivy in calamine lotion, and eczema.
I found Neutrogena sunscreen for sensitive skin to be a great source of titanium dioxide. Regular sunscreen resists sun chemically but this product reflects sun by putting a thin layer of white titanium dioxide particles on the skin. Titanium dioxide is used in the ink to print the label on each M & M candy. It is also used extensively in paint and liquid paper as a whitener. In about 1940, titanium dioxide replaced white lead as a whitening agent in paint. Because of its lightweight property as a metal, it is used in the construction of aircraft, bike frames and eyeglass frames. It is used in joint implants and for rods and pins in orthopedic work because bone will grow and attach to it.
One difficulty I encountered was finding non-iron colorants. Although make-up was a great source for both kaolin and colorants, it turned out to be terribly expensive. A variety of colorants can be found in concentrated form as grout and cement stains. Generally, these colorants are iron-based blacks, grays and reds, but you can occasionally find blues and some other non-iron colors.
As I tested the materials, I found that the iron impurities in pumice stone and rottenstone were sufficient to produce a nice range of colors including light blue, green, yellow, caramel and browns from light to a rich dark brown. Iron has a great range of color response in glazes depending on firing atmosphere and other oxides present. Calcium has a bleaching effect on iron, and can turn an olive green celadon to a light green, and with sufficient reduction and some added silica, will produce a very nice blue color. I added Crest extra-whitening toothpaste to my green celadon and, in heavy reduction, got a nice light blue color. In the presence of magnesium and calcium, small amounts of iron will produce a nice iron yellow.
This experiment in glazing has provided me with both a beautiful glaze palate produced solely from locally available products and an expanded understanding of the uses of ceramic materials in everyday life. The following glaze recipes were achieved solely by conducting multiple series of line blend tests.
I do caution against using these glazes for pottery that will be eaten on or food stored in, as all the ingredients as well as the subsequent compounds formed during the firing process are not known. Experimentation should be pursued only on non-functional pieces in well-ventilated outdoor kilns.
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