5.2.

The aluminosilicates are based on two compounds, alumina and silica. You must not confuse aluminum, the low-melting metal, with alumina, its high-melting oxide, Al2O3. Silica, as we have seen, is the traditional name for silicon dioxide, SiO2, the compound which makes up the mineral, quartz. An aluminosilicate mineral contains both alumina and silica. For example kyanite has the formula Al2O3SiO2, or Al2SiO5. The oxide formula emphasizes the relative proportions of the oxides, alumina and silica, while the empirical formula emphasizes the relative proportions of the elements, aluminum, silicon and oxygen. Both formulae have the same number of each kind of atom and either one may be used to denote kyanite.

The surface of the Earth is approximately 59% silica and 15% alumina by weight, and the aluminosilicates are second only to the silicates in abundance. They are amazingly diverse as well. In kyanite the ratio of alumina to silica was 1:1. Altering the ratio to 3:1 gives mullite, which has the formula 3Al2O32 SiO2, or Al6Si2O13. The feldspars contain oxides in addition to alumina and silica. Anorthite, for example, has formula CaOAl2O32 SiO2 and orthoclase is K2OAl2O36SiO2. Muscovite, KAl3Si3O10(OH)2, and biotite, K4Mg10Fe2Al4Si12O40(OH)7F are micas, which form thin, flat sheets. The complexity of these formulae gives you some indication of the almost infinite variety of the aluminosilicates.

When aluminosilicates are weathered by the action of wind and water, an enormous variety of clay minerals are produced. From the viewpoint of the potter, the most important of these is kaolinite, Al2Si2O5(OH)4, or Al2O32SiO22H2O, indicating that for every alumina, there are two silica and two waters. It is important to realize that no matter which way we write the formula, kaolinite is a pure substance, not a mixture or solution. The "H2O" in the formula indicates only the relative proportions of hydrogen and oxygen and does not imply the presence of liquid water. Similarly, the formula of cellulose, CH2O, did not indicate the presence of liquid water. No, both pure cellulose and pure kaolinite are bone dry.

In practice, however, pure kaolinite is seldom found in nature. Just as wood contains compounds in addition to cellulose and obsidian contains compounds in addition to silica, natural clays may contain compounds in addition to kaolinite: other clay minerals, sand, iron oxide, and decayed vegetable matter. Different clays may be blended to produce clay bodies, and water can be added to render the clay plastic, that is, to allow it to be shaped. The plasticity of clay is what makes it possible to mold it into almost any conceivable shape. The clay retains this shape when it dries out, but the addition of more water will bring it back to a plastic state. An amazing transformation takes place, however, when clay objects are fired.

The firing of pottery takes place in three stages, each occurring over a range of temperatures. At temperatures up to 100C, the clay simply dries out. The liquid water, which was added to make the clay plastic, evaporates. In this water-smoking stage, no chemical reaction takes place, the water simply boils off. Were the kiln never to go above this temperature and the dried clay object placed into a bowl of water, it would absorb the water and become plastic again. The variable composition of the clay body hearkens back to Lucifer's description of mixtures; the clay body may contain a lot of water, a little water, or no water at all. Once the liquid water has been driven off, the temperature continues to rise.

Equation 5-1. The Calcination of Kaolinite

If we continue heating the clay from 350C to 500C, that is, if we heat the bejeezus out of it, the kaolinite in the clay undergoes an irreversible chemical reaction, as shown in Equation 5-1. The process of heating the bejeezus out of the clay is called calcination. Here we see that six waters, the bejeezus, are literally driven from the kaolinite by the fire over the equal sign. We have seen the same kind of reaction in the production of charcoal from cellulose. If you add water to charcoal, you get wet charcoal, not wood. If you add water to mullite and silica, you get wet pottery, not clay. Notice that unlike the drying stage, there is a definite, fixed amount of water driven off. As Lucifer told you, this is what marks kaolinite as a compound, rather than a mixture. With the bejeezus gone, the solid products, mullite and silica, have changed from clay to stone and are impervious to water, as they were before the weathering process began.

Clay heated to 500C will no longer revert to plastic clay when wet, but it has not yet become pottery. For this, a third stage, vitrification, is required and this stage depends more on the impurities in the clay than on the kaolinite. Both mullite and silica have extremely high melting points, which is what makes pottery useful as a refractory material. But impurities in the clay, notably iron oxide, melt at lower temperatures. The temperature at which this happens will depend on the impurities which happen to be present in the clay body. Low-fire clays may vitrify at 900C, while porcelain clays may require temperatures as high as 1300C. As the impurities melt, the liquid soaks into the pottery, coating the crystals of mullite and silica. When the pottery cools, the melted impurities solidify, in effect gluing the crystals together. The resulting structure is now very strong and impervious to both fire and water. It has become pottery.

I have not yet explained how this firing may be accomplished. No mere campfire will do the job. No, the fire must be bejeezus-hot if it is to heat the bejeezus from the clay. A common cooking stove will not rise above 500F, or 260C, sufficient for the water-smoke stage only. The coals of a common campfire will seldom rise above 800C, enough to convert kaolinite to mullite and silica, but not enough to vitrify it. To vitrify even low-fire clay wares, we need a fire at least 100C, and preferably 200C hotter than the hottest campfire; we need a kiln. But to understand the design and operation of the kiln, you must first understand the relationship between heat and temperature.

Heat and temperature are not the same thing. To see this, imagine two pots on the stove, one containing a quart of water, the other, a cup of water. Turn the burners to the same setting and measure the time it takes for each pot to come to a boil. The cup will boil before the quart. They reach the same temperature, but more heat is required to boil a quart of water than a cup. Repeat the experiment, but this time leave the burners on for only two minutes and measure the temperature of the two pots. The same amount of heat was delivered to each, but the cup will have a higher temperature than the quart. One more test will further clarify it for you. Pass your finger through the flame of a cigarette lighter. Then hold your finger in the same flame until you get the point. The flame is the same temperature no matter how long your finger is there, but your finger absorbs more heat in a second than it does in an instant. Heat is what causes temperature to rise.

Table 5-1. Three Temperature Scales

To vitrify clay, to change it from clay to stone, it must be heated to red heat, that is, to incandescence. Different clay bodies, however, vitrify at different temperatures. Long before the advent of electronic kiln controls, potters learned to judge the temperature of the kiln by placing into it small numbered cones made from different clays. The clays were chosen to deform, each one at a different characteristic temperature so that the temperature of the kiln could be judged by noting which of several pyrometric cones had deformed during a firing. The cone numbered 022, for example, is made from a clay that deforms at the lowest temperature for which incandescence is visible, a dull red heat. Cone 021 deforms at a slightly higher temperature and the scale proceeds from cone 020 to cone 019, for which the incandescence is dark red. As the temperature climbs further we progress from cone 018 to cone 010, from dark red to orange incandescence. Clays which vitrify in the range from cone 09 to cone 03 are called earthenware clays; those which vitrify at higher temperatures are called stoneware clays. The scale proceeds from cone 02 to cone 01, but from that point the scale moves to cone 1 to cone 2, all the way up to cone 10. Porcelain clays vitrify at the upper end of the scale, from cone 6 to cone 10. At such temperatures pottery is not so much red-hot as yellow-hot. The deformation of a cone depends on time as well as temperature, so there is no simple conversion from the cone scale to the more familiar centigrade and Fahrenheit scales. Nevertheless, approximate temperature ranges are given in Table 5-1. Modern kilns can be programmed in centigrade and Fahrenheit but if you are going to communicate with potters, you should be conversant in the cone scale, as well. It is important not to confuse, for example, cone 10 with cone 010, as they differ by 400C!

A kiln must satisfy two competing demands simultaneously. Being Athanor, you are well aware that the fire needs air, or more specifically, oxygen to breath. Blow on a coal and the fuel burns faster, producing heat at a greater rate. More heat causes the temperature to rise and the coal becomes brighter. Coals buried deep in a coal-bed are starved for oxygen, so we should spread the coals out over a large area to provide them with access to fresh air. But the rate at which a hot object loses heat is proportional to its surface area. You know instinctively that when you need to conserve body heat, you should curl up into a ball. To conserve the heat of the fire, we should rake our coals together into a deep, compact coal-bed. A traditional kiln works by providing a continuous supply of fresh air to a deep coal-bed.

Figure 5-1. Athanor at the Furnace

Figure 5-1 shows an assaying furnace from De Re Metallica, ca. 1556 AD.[1] It is essentially a deep, insulated coal-bed with an opening at the bottom. As the hot waste gases (chiefly carbon dioxide) exit from the flue at the top of the furnace, a draft is created, drawing fresh air in through the fire-mouth at the bottom. The fresh air causes the charcoal to burn hotter, which heats the air even more, increasing the draft. The ultimate temperature of the furnace is determined by the size of the furnace, the insulating value of its walls, and the relative sizes of the fire-mouth and flue. The temperature may be decreased by closing off the fire-mouth and it may be increased by forcing air into the furnace with a bellows or even by fanning the flames, as shown in the figure.

Conditions within the kiln will affect the color of the finished pottery. In areas where there is plenty of oxygen, we say the conditions are oxidizing. The fired pottery will be light in color, white or red, depending on the original color of the clay. In areas where the coals were starved of oxygen, a reducing atmosphere, the fired pottery will be dark in color, brown or black. In a primitive kiln or campfire, where there is little control over the flow of air within the coal-bed, a single pot may show areas of both oxidation and reduction.

A bonfire may be sufficient to vitrify low-fire clay, but will not attain the temperatures needed for calcining limestone and smelting bronze. For economy and convenience, the electric kiln is better suited to these applications. We shall discuss the principles of electricity later in the book, but for the time being I recommend the electric kiln in the same spirit of convenience that Samson used the 2-liter soft-drink bottle. Used kilns may be purchased for a few hundred dollars and a small one may be built from scratch for about a hundred. Alternatively, you may make the acquaintance of a potter, who, being Athanor, will be disposed to help a brother or sister in need.

WarningMaterial Safety
 

If anything is safe and natural it must be clay. Would it surprise you to learn that this material is described in an MSDS?[2] Your ceramics supply will be able to furnish you with a copy. Alternately you search the Internet using the keyword "MSDS" and the CAS number for kaolinite (CAS 1332-58-7). Summarize the hazardous properties of kaolinite in your notebook, including the identity of the company which produced the MSDS and the potential health effects for eye contact, skin contact, inhalation, and ingestion.

Any hazards in this project are likely to be more physical than chemical. If you are firing in an electric kiln, be sure to follow the manufacturer's instructions. If you are firing in a bonfire, be careful to build it away from flammable structures. Have a source of water on hand should the fire get out of control and be careful to avoid smoke inhalation. Do not leave a fire unattended and use gloves to handle hot pottery or burning wood. If you are unwilling to accept responsibility for the safe use of fire, you should give your matches to a grown-up and skip to the next chapter.

NoteResearch and Development
 

You should not remain ignorant if you are to proceed in the Work.

  • Know the meanings of those words from this chapter worthy of inclusion in the index or glossary.

  • You should have mastered the Research and Development items of Chapter 1 and Chapter 2.

  • Know how clays are formed from their parent rocks and minerals. Be familiar with the hazardous properties of clay.

  • Be familiar with the cone, Fahrenheit, and centigrade temperature scales.

  • Know formulae for alumina, kaolinite, mullite, water, oxygen, and silica.

  • Know the equation for the calcination of kaolinite.

  • Know how a kiln achieves higher temperatures than an ordinary campfire.

  • Know what has driven the history of humankind.

Notes

[1]

Reference [1], p. 223.

[2]

The MSDS was introduced in Section 3.2.