Fire, as I have said, is probably the most important technology to be mastered by humans. But in order to proceed in the use of fire, we need containers that can withstand the heat. Wood containers burn; metal and glass containers melt. Ceramic materials are the ones best suited to high-temperature work. Ceramics predated and were used in the development of metal and glass.
We have already discussed the silicate minerals. Another very common family of minerals are the aluminosilicates. There are hundreds of aluminosilicate minerals including the micas which form flat sheets and are quite common in central Virginia. A local variety of mica is biotite, K3AlSi3O10(OH)2. Another aluminosilicate mineral, kyanite, Al2SiO5, forms long, fibrous crystals and is mined locally at Willis Mountain. Feldspars are a broad class of aluminosilicates, of which microcline, K[AlSi3O8], is a local variety. Finally, Kaolinite, Al2Si2O5(OH)4, is an aluminosilicate mineral which, when powdered and mixed with water, form one variety of clay.
At this point, it is not so important that you memorize the formulas of all these minerals as it is to remember that they have formulas. That is, that like all minerals, these aluminosilicates are characterized by a definite proportion of elements, in this case aluminum, silicon, oxygen, and, optionally, hydrogen and potassium. By contrast, aluminosilicate rocks are composed of a mixture of different minerals. Granite is a class of igneous rock consisting of a mixture of quartzite, feldspar, and mica. Granite is commonly used as a building material and is quarried extensively in central Virginia for use in roads. Chances are that any rock you pick up around Hampden-Sydney will be a granite with embedded mica fragments. Another local aluminosilicate rock is slate, which is composed of mica and other aluminosilicate minerals.
When quarts and quartzite weather, the result is sand. When granite, mica, and other aluminosilicates weather, the result is clay. Clay minerals accumulate at the bottom of lakebeds. As the layers build the clay is compressed and the organic material decays. If the lake dries up, the dried clay becomes a soft stone. Eventually, someone may quarry this mineral, pulverize it, and sell it to potters, either as a dry powder, or mixed with water. The exact formula of the clay may vary considerably from the rock from which it originated but all clays contain aluminum, silicon, and oxygen. One of the most common clays, kaolinite, has, as mentioned earlier, the formula Al2Si2O5(OH)4. This may seem a little complicated but the formula simply tells you the relative proportions of each element in the compound. The (OH)4 part just means there are four OH groups, so there is a total of 5+4=9 parts oxygen per formula unit.
Now, pure kaolinite is a soft, white stone. The presence of iron may give it an orange color. Other impurities may render it yellow or gray. When pulverized and mixed with water it becomes plastic, a term used to describe the ability to be molded into various shapes. Clay, then, is a mixture of a clay mineral like kaolinite and water. A mixture of different clay minerals, for example, kaolinite and kyanite, with water is called a clay body. Clay, like wood, may contain variable amounts of water. A little water will produce a very stiff clay, more water will produce a more plastic clay, and lots of water will produce a very runny clay.
When a clay object is left out to dry, most, but not all, of this variable water is released to the atmosphere. This is analogous to the drying of wood. The result is an object which appears dry and very similar to the original kaolinite but which still contains a variable amount of water. Such an object is quite fragile and which reverts to clay if it is re-exposed to water. These observations tell us that this water is not incorporated into the chemical formula of kaolinite, for, if it were, it would always be the same proportion of water.
When an apparently dry clay object is heated above about 100°C, the remaining variable water is evaporated leaving truly dry clay. If the object is not sufficiently dry to begin with, pockets of steam may develop which will crack, and possibly explode, the clay object. As the clay is heated further, (above about 800°C for kaolinite) additional water is eliminated from the clay, but unlike the previous water, this water is part of the chemical formula and so the proportion, for a given clay, is constant. For kaolinite the reaction is:
3 Al2Si2O5(OH)4(s) -----> Al6Si2O13(s) + 4 SiO2(s) + 6 H2O(g)
3 kaolinite -----> mullite + 4 silica + 6 water
The solid products of the reaction, mullite and silica, form interlocking crystals which render the fired object much stronger than the original kaolinite. Furthermore, mullite and silica do not absorb water as readily as kaolinite and so after firing, pottery is impervious to water. This process, called "sintering" is the pottery equivalent of turning wood into charcoal. For our purposes, the primary test of whether sintering has occured will be to place the pottery in water to see whether it reverts to clay.
In order to vitrify, pottery must be heated to extreme temperatures. Even low-fire pottery must be heated until it glows red. Potters have traditionally used clay cones to measure kiln temperature. Made of different clays, these cones begin to melt or deform at temperatures characteristic of the clays from which they are made. The cones have numbers and the firing temperature is usually referred to by cone value rather than by degrees Centigrade or Fahrenheit. Here are the approximate temperatures for a range of cones:
| Color at which pottery glows | Cone range | Fahrenheit | Centigrade |
|---|---|---|---|
| Lowest visible red to dark red | 022 to 019 | 885 to 1200°F | 470-650°C |
| Dark red to cherry red | 018 to 016 | 1200 to 1380°F | 650-750°C |
| Cherry red to bright cherry red | 015 to 014 | 1380 to 1500°F | 750-800°C |
| Bright cherry red to orange | 013 to 010 | 1500 to 1650°F | 800-900°C |
| Orange to yellow | 09 to 03 | 1650 to 2000°F | 900-1100°C |
| Yellow to light yellow | 02 to 10 | 2000 to 2400°F | 1100-1300°C |
The temperature to which a piece of pottery must be fired depends on the clays present. Ordinary kitchen stoves only get as hot as 260°C (500°F) which is not nearly hot enough to vitrify even low-fire clay. The low-fire clay we will use fires between cone 06 and 04 (980°C to 1050°C ,1800°F to 1950°F). The coalbed of a campfire may reach cone 013, which will only paritally vitrify this clay. To have any chance of success, pottery fired in an open fire must reach the same temperature as the coal bed--glowing red hot. If this is to happen, the coal bed must be deep and the pottery must be completely covered by the coals.
One way to accomplish this is to dig a fire pit and build a large fire in this pit. Covering the pit with broken pottery (or sheet metal) prevents the loss of heat, but openings must be left so that oxygen can reach the coals.
---->exhaust gases
^^^^^^^^^^^^^^^^^^^^^^ / __________________________
^^ooooooooooooooooooo /
^^ooooooooooooooooooooo /
air --> ooooooooooooooooooooooo /
________ ooooUooUooUoUoooUoooooooo/
\________________________/
o = coals
U = pots
^ = broken pots or sheet metal
More reliable firing can be acheived in a special structure called a kiln. A primitive bee-hive kiln can be built from clay consisting of a double firemouth and an exhaust stack. The firemouths are simply clay tubes about 12 inches in diameter and three feet long. They are attached to a clay stack which is shaped like an upside-down bowl (or bee-hive). The stack is 3 feet in diameter at the base, 1 foot in diameter at the top, and about 3 feet tall. Pottery is inserted through the top of the stack before the fire is lit, and is removed through the top of the stack after the kiln has cooled. A typical firing in this kiln takes 1 hour to slowly build up the fire, 2 hours with the fire at full blast, and overnight to cool.
exhaust gases
^
|
__ | __
/ \
/ \
_______________/ \_______________
ooooooooooo U ooooooooooo
air---> ooooooooooooo U U U U ooooooooooooo <---air
ooooooooooooooo U U U U U U U ooooooooooooooo
______________________________________________________________________
o = coals
U = pots
_ = clay
Even more control can be had from an electric kiln. It may seem odd to include and electric kiln, considering that our discussion of electrical power will come very late in the semester. But our focus here is on the clay. You can use any source of heat which is sufficient to vitrify the clay. You can try firing in an open campfire, a pit fire, a bee-hive kiln, or an electric kiln.
We have already used the term oxidation to describe the burning of wood. Oxidation requires a supply of air (or more specifically, oxygen). We have also seen that the production of charcoal takes place in the absence of air. This process is described as reduction. The appearance of your pottery depends on the conditions within the kiln. If white clay is fired in an oxidizing atmosphere (with more than enough oxygen for the fire), it will come out white. If white pottery is fired in a reducing atmosphere (with only enough oxygen for the fire), the pottery will come out black. An oxidizing fire is characterized by flames coming out the stack. A reducing fire is characterized by smoke coming out the stack. In primitive kilns, some areas may be oxidizing and others reducing. Under these conditions, pottery may come out white in some areas and black in others.
Oxidation and reduction are concepts we will use over and over. When we discuss glass and glazes, the color of the glaze will depend on the oxidizing condition of the fire. The production of metals will require reducing conditions. Gunpowder, batteries, and electrochemistry will require a more careful consideration of these concepts.
The pottery quiz will consist of three questions on the following topics.
You have great lattitude in creating your pottery project. You may prospect for your own local clay or you may use clay which I will provide. You may make any object you wish: a bowl, a dish, a pipe... You may choose to make a crucible and blowpipe for use in a subsequent metal smelting project. Whatever you make, scratch your initials somewhere to identify it.
After you have crafted your pottery, be sure to let it dry completely. This may take several days or a week. You can accellerate the drying by placing it in a sunny spot.
If you choose to fire in a campfire, you must maintain a hot fire for 3-4 hours. The temperatures required for firing pottery are not easily achieved in a small fire. You will want a fairly large fire and you will want to fan it so that it gets as hot as possible for as long as possible. You must have a 5 gallon pail of water available for controlling the fire! Be wary of sparks that may set fire to nearby brush or vegetation.
If you choose to fire in the kiln, further instructions will appear in this spot. Watch for them.
When the firing is complete you can test your pottery by placing it in a bowl of water for 15 minutes. If it falls apart, or if the surface becomes soft then the firing was incomplete. You may dry your piece and fire it again or start over from scratch. The only real requirements are that your pottery survive the firing intact and that it not revert to clay when it is placed in water.
When you have completed and fired your pottery, you should take the pottery quiz. Turn it in along with your pottery to be graded. Your pot should show evidence of careful construction, be free of cracks, and remain hard when exposed to water.
If you miss any of the quiz questions, you fail. If your pottery softens in water, you fail. If, in my opinion, a neolithic teenager would be embarrassed to show your pot to his parents, you fail. You may, of course, try again another day. Can you tell which of these pots would pass? Click on each pot for the answer.