Making Pottery Glazes and Slips
( Originally Published 1937 )
THE POTTER should make his own glazes. True, he can buy them ready-made, just as he can also buy the pot ready-made, ready-glazed and ready-fired; and if all he cares about is to get a perfect pot as soon as possible let him go to the department store and buy one, and forget that ever he wanted to learn the potter's art. Let us never make the mistake of regarding the product before the producer—of regarding what is made more highly than him who makes it—for the great value of any art is not what is made but what happens to him who makes it during its making. For there is no satisfaction like that of mastering a craft from beginning to end, of having command at your finger tips of all materials and processes. All short-cuts and artificial aids are to be put aside as a healthy man would put aside crutches. We should walk every step of the way on our own feet.
CHEMISTRY OF GLAZES
The modern method of calculating glazes from the "empirical formula" (which might be considered a blue-print for a glaze) replaces the older custom of handing down recipes in potters' families. They did not understand the exact action of each ingredient, nor visualize the molecular patterns, but surrounded the mixing with an aura of mystery close to superstition. Not that they did not produce fine results. For so true is it that pottery is an art rather than a science, that not the whole of our vast army of ceramic technologists, supervising the production of immense factories (a single machine in one factory makes 14,000 dozens of pieces in a day) has produced anything finer than some of the pieces made in small, woodfired kilns hundreds of years ago. The scientific approach, nevertheless, is extremely valuable. It puts into our hands a method of designing glazes on paper; of manipulating molecular relationships in a very fascinating way, so that experimenting with glazes is no longer a blind stabbing in the dark, but the projecting of an intellectual concept into physical-reality, accompanied by an exciting curiosity as to the outcome.
When you start to design glazes you will need to know something about the characteristics of the various ingredients.
First of all, a glaze is, in effect, a coating of silica or flint (Si02) . over the pot. Everyone knows that flint is a hard glossy stone, and many people know also that flint and quartz and pure white sand are chemically the same. Flint having an extremely high melting point, we dissolve it with basic oxides usually classified into three groups:
1. The alkalies: Potash (K20), Soda (Na2O).
2. The oxides of the base metals: Lead oxide (PbO), Zinc oxide (ZnO).
3. The alkaline earths: Calcium oxide (lime) (CaO), Barium oxide (BaO).
Alumina (Al203) or oxide of aluminum is added in relatively small amounts to extend the firing range: With it the glaze remains fluid but sufficiently viscous not to run off over a wide range of temperature.
Lead oxide is the most powerful flux and the easiest to use, but the alkalis, potash (K20) and soda (Na20) are more interesting and also more difficult to use since they are soluble in water. They do not harmfully affect the colors as lead sometimes does. K20 is brought in by orthoclase or potash feldspar (K20, Al203, 6SiO2) ; otherwise the carbonate or nitrate is usually used. Sodium carbonate or soda ash (Na2CO3) is used to bring in Na2O. This is also brought in by borax. Where borax (Na2O, 2B203) is used, the boric oxide (B2O3), being acid, replaces some of the silica and requires a further calculation. Boric oxide has the effect of lowering the melting point and reducing the thermal expansion. You may remember that the 200-inch telescope lens that was cast at Corning, N. Y., a couple of years ago was made of "Borosilicate glass." Boric oxide was used here for these specific properties.
Lime (CaO) is usually added in the form of whiting or ground limestone (CaCO3)—calcium carbonate—a very convenient material because it is not soluble in water.
Zinc oxide is used somewhat to replace lead oxide, also to increase the opacifying effect of tin oxide in opaque glazes and enamels. It is not so strong a flux as lead but is non-poisonous; and having a low co-efficient of expansion it is sometimes used to overcome crazing.
Other metallic oxides are used to give color. The typical color is exhibited when it is applied over a pure white body and fired in an "oxidizing atmosphere"—i.e., when the fire has more than sufficient air. to burn the fuel with which it is supplied. In a "reducing atmosphere"—when there is insufficient air for the fuel—the carbon monoxide steals oxygen from the metallic oxides in the glaze and "reduces" them, altering their coloring effect.
The composition of a glaze is expressed in chemical terms by the "empirical formula." This indicates the quantities of the different oxides, the sum of the bases always being unity, and the alumina and flint represented by the decimal fractions of this sum.
These figures represent numerical proportions of the molecules of each oxide. Before you could make such a glaze, you would have to work out a "batch" by translating these numerical proportions into weight proportions of available substances, which would introduce these oxides into the mixture. Set off in a row the fractions and multiply each fraction by the equivalent weight of the substance used to introduce it.
Notice that the feldspar, calculated for its potash content also brings in sufficient alumina and a large portion of the silica. If the feldspar did not bring in enough alumina, clay (usually kaolin) would bring in the remainder, and it also would bring in silica with it. This addition, or any other, should be set down just in the fashion shown, always referring to the chemical formula of each substance to see what it contains.
Mix a batch weighed out in grams with about a cupful of water, or sufficient to make a thick creamy mixture. While it is best to grind this in a ball-mill, or a mortar and pestle, these operations are not absolutely necessary unless—in the case of colored glazes—one wishes an absolutely even color. I have had good results, with merely giving the mixture a vigorous stirring with a rotary egg-beater.
MAKING A BALL-MILL
You may, however, easily improvise a ball-mill in this way: make a wooden cover for a stoneware crock and provide it with means for securing the cover firmly to it with a soft rubber washer between. Arrange a frame for the crock that will permit it to be revolved at about 55 r. p. m. for a gallon crock, or a peripheral speed of about 130 feet per minute for any size. Half fill the crock with smooth flint or quartz pebbles about the size of a walnut, and have enough glaze slip just to cover the pebbles. Start your mill turning, and when the time is up (about three-quarters of an hour for a raw glaze), pour out the glaze into a sieve made by fastening 120-mesh bolting cloth, or bronze screen across a wooden frame. Rub it through with a soft brush, rinse the pebbles and mill with water, which is then to be put through the sieve after the first, to wash it out. Then let the glaze settle for a while and syphon or decant the surplus water off the top.
You may also use a wooden keg or churn in the place of the crock. Very little wood is ground off, and it will do no harm to the glaze, since it practically disappears in the intense heat of the firing.
The glaze may be applied by dipping the biscuit pot into it, or by pouring the glaze over the pot, by painting it on with a brush—preferably a soft, squirrel or camel hair mop—or by spraying. If spraying is done, have a spray booth with provision for carrying off the exhaust air. Otherwise there is danger to the workers from breathing the suspended lead and silica.
The pot to be glazed must be freshly dusted and quite clean. If the absorption of the biscuit clay is too rapid, dip the pot for an instant in water before applying the glaze. Make the glaze coating as smooth and even as possible; and if you do not make it smooth enough the first time, wash it off in a bowl of water and save the glaze, allowing it to settle as before. Bright (shiny) glazes correct faults in application to some extent, by melting and flowing together in the firing, but matte glazes do not flow at all and show up every defect. The best way of applying bright glaze is to pour the pot full and immediately dump it out, shaking it upside down a moment, and then, holding it by the bottom with the tips of the fingers, to plunge it into the glaze until it comes just to the bottom of the pot, immediately withdrawing it with further shaking. The pot is stood right side up and the fingers marks touched up with the brush. Matte glazes are thickened with gum tragacanth mucilage: approximately one tablespoonful to the pint.
Here are formulae and batches you may use as the basis of experiments. When you alter them, work from the formula. Do not, for instance, simply add 100 grams flint to the batch. Keep a clear mental picture of the molecular relationships by increasing the equivalent of SiO2 in the formula and calculating from that how much additional flint will be required in the batch. This way every alteration will add to your knowledge of the characteristic reaction of the oxides, whereas adding to or taking from the batch is feeling in the dark from which you can profit little.
Any of the above glazes may be colored with the addition of from one to eight parts of the coloring oxides given in the list. The coloring oxides may be mixed together to obtain different shades but care should be taken not to use too much. The cobalt oxide is the strongest in its effect, one part of it being equal to several of most of the other oxides. Under-glaze colors and glaze stains sold by ceramic supply houses may also be ground with the glaze to color it. If you wish to get a mottled effect do not add the coloring oxide until after the batch has been ground. Then merely stir it in. This does not apply equally to all colors, nickel oxide, for instance being too coarse to be added in this fashion.
USING CRUDE MATERIALS
Interesting results are obtained by experimenting with the crude materials: iron rust or scales from the anvil; copper scale from annealing; wood ashes; common blue clay; lead oxide made by melting old pipe, skimming it clean and keeping the metal red hot until it turns to yellowish powder; tin oxide made the same way from old tooth paste and paint tubes, and tinfoil from cheese.
USING SOLUBLE MATERIALS
Soda, borax and potash are sometimes "fritted" (melted with other glaze ingredients) to render them insoluble in water. The mixture is poured into cold water while red hot, to facilitate grinding later.
It is possible to use these substances without fritting by dissolving them in the correct amount of boiling water and thickening the water with laundry starch before adding the non-soluble ingredients of the glaze. It requires experimenting to determine the exact amount of starch and the proper handling. The idea is worth investigating, since it is much easier than fritting and grinding. Also by using the alkalis and borax it is possible to do without the poisonous lead. Not that lead is so very dangerous, but pre-cautions should always be taken; keep cut fingers out of it; do not let it get into the air nor on the floor where it can get stirred into the air by trampling. The lead is not dangerous if fritted first.
Dissolve the sodium carbonate in one cup of boiling water and stir in one-half teaspoonful of the ordinary laundry mixture of starch and cold water. Then stir the whiting, feldspar and flint into it and continue stirring vigorously. The last three ingredients should have been well mixed with each other by dry grinding, or by shaking them in a box or bottle. No water can be taken away from this type of glaze, as it can from a raw glaze, and your handling technique should be regulated accordingly. 4 gram of black cobalt oxide added to a batch of the above will make a sapphire blue, and 8 grams of black manganese oxide will make an egg plant color. The colors are sensitive to a great many slight influences and you must not expect perfect results at once nor always, but even the failures are interesting because they are all characterized by the typical softness of texture, which distinguishes an alkaline glaze from any other. The full value of the colors is only developed over a pure white body or engobe.
As a clear glaze it is usually quite successful. At a lower temperature than cone 0r the glaze is less shiny and sometimes looks "frosty."
A HANDY LOW-FIRING GLAZE (Cone 012 or Lower)
This is very easy to make and use, and is applicable to low-firing local clays that are used in the crude state, without additions of other material to increase their firing range. It is also good for putting on over underglaze painting.
You are likely to have difficulties in the application and firing of glazes, and these you will have to learn to deal with. Here are some of them.
CRAWLING: The glaze seems to roll up in mounds or peel right off the body. This is caused sometimes by having an excess of raw kaolin in the mixture. Replace part of it with calcined kaolin (calcine it by putting some finely sifted raw kaolin in a pot and firing it with other pottery in the kiln). The crawling may be caused by dust on the pot before glazing, or a body being fired too high, or by the glaze being ground too fine or being cracked on the pot before firing.
BUBBLING: Where this is a purely local condition affecting only one part of the kiln, it is due to a defect in the firing. All raw glazes bubble during the time that they mature and if the heat is suddenly withdrawn the bubbles harden before they have time to break. A more even firing and slower cooling are indicated here. Over-firing may cause bubbling, the vitrification of the clay closing the pores and forcing air out through the glaze. Where either of these conditions is accompanied by strongly reducing gases in the kiln there is likely to be a dirty, scummy appearance on what should have been a bright glaze. Bubbling or blistering may also be caused by a sulphate scum on the surface of the pot from traces of gypsum in the clay. This may be cured by the addition of about i % of barium carbonate to the clay body. The affinity of barium for sulphur compounds is well-known and is applied in many industries. Do not use clay so treated in plaster moulds.
CRAZING: Fine cracks spread over the surface of the glaze. This is usually cured by adding more flint to either the clay or the glaze. As this addition to the glaze is liable to raise its maturing point it may then be necessary to further alter the glaze by manipulating the relative proportions of the bases. For this reason many ceramists consider it easier to fit a clay to a glaze than to fit a glaze to a clay.
GLAZES AND SLIPS FOR SPECIFIC PURPOSES .
Instead of starting with one formula and making small alterations to it one after another, until you get the desired result, it is simpler to start with two formulae, each distinctly different from what you want. It is important that they be quite definitely separated in their characteristics. To fit a glaze to a certain body make two batches—one of a glaze too small for the body, and crazed, another of a glaze too large for the body and tending to shiver off. Then make tests on small tiles or fragments of clay with mixtures of these two glazes: 1 part of A to 9 of B, 2 of A to 8 of B, 3 of A to 7 of B, and so on, measuring the glaze slips out with a tablespoon or measuring cup and keeping the stock glazes thoroughly stirred while you work. Out of these tests select one or two and work on them further. Use the same system in working out a clay body. Calculate the correct formula of the best sample from the proportions of the two contributing slips.