What is clay and why does it do the things it does? Clay is the product of the erosion of feldspar and granite. Clay found close to the source rocks is called primary source clay. This is porcelain, and for a very long time, you could only get it if you lived in the right part of China. It is very white, but challenging to work with. This clay is the most pure clay available and, as a result, is not very plastic (malleable) compared to other clays. Clay which has washed further down river (secondary source clay) has picked up additional minerals, which tend to darken it, sand (which reduces the shrinkage and improves the clay’s ability to hold its shape), and organic matter (which improves the clay’s plasticity).
This picture is my own amateurish depiction of a clay platelet, edge on. From the top, it would be rounded and much more boring. It is platelets that make clay more than simple ground-up rock, as sand is. Platelets slide across one another while still hanging together. They account for the plasticity (ability to be shaped and to retain that shape) of clay. As you can see, the platelet has internal holes or caverns. In these, water hides, keeping the clay workable and soft until the water dries away. If it weren’t for these spongy holes, clay would drain as sand does (well, maybe not quite as fast) and couldn’t be manipulated nearly as well as the clay we’re used to. You can’t see platelets with an ordinary microscope. You need an electron microscope to see them.
You can find clay in many places–along old river beds, in your garden, at a building site where the surface dirt has been pulled back . . . I once dug a hole for a firing pit and made a little ocarina of the clay I found there. It has tiny silver sparkles from the mica that was in the clay. You can make things from native clay, but you’ll need to study and experiment to see how to treat your clay. I’m not going to go into that here, as I don’t use much native clay.
Most potters think clay comes from the ceramic supplier’s warehouse, and this is not as wrong as you might think. Pre-packaged clays come in 25 pound bags, which come two to a box. They’re carefully formulated to fire hard and glass-like (vitreous) at a given temperature. They’re made of clays of different types, feldspar, silica, and other ingredients deemed necessary to give the clay the appearance and working qualities desired. It’s rare to find a native clay with the working and firing properties you desire already built in. Sometimes you will, but more often you’ll have to strain out debris, add sand, or any of a number of other things to bring the clay up to your standards. Good ceramic suppliers have a collection of clay formulations, and they work hard to make sure their clays maintain consistent working qualities. Since the clays are made of materials mined from the ground, ingredients can vary quite a lot, so this requires constant monitoring.
Notice the differing colors in the photo above. The chalice is bone dry and its color has lightened. When clay reaches the bone-dry stage, it can no longer be easily worked with. You may sand or rasp it, but bone dry pottery is hard and therefore difficult to manipulate. The platter, in contrast, was barely dry enough to move. It flexed as I put it on the photography table. See how much darker it is? The base of the platter hasn’t even been trimmed yet. It’s too soft. It’s difficult to tell from appearances alone whether a pot is still soft, or has reached the leather hard or cheese hard stage. It is during the leather hard stage that carved and apliqued decorations (such as the pine cones and branch on the chalice) are added. This is also the best time to carve pottery and to trim the bases of wheel-thrown pots if necessary. You can tell a pot is leather hard because it no longer flexes when you move it, and when carving or incising the pot, fragments from carving fall away in strips. When it reaches the leather hard stage, a high-mid-fire pot will have shrunk half its total shrinkage (which total amounts to about 12-13% in most mid-fire stoneware clays) If the pot is approaching the bone dry stage, carving waste will crumble and quickly dull your carving tool. At the bone dry stage, the pot will no longer feel cool to the touch.
I threw out a couple of terms just now that not everyone will be familiar with. Clays come in a number of varieties. Most (not all) secondary source native clays are low-fire. This means they mature at a temperature of around about 1800 degrees fahrenheit. Potters call this cone 06. Mid-fire clays (often called high-fire by people who have previously been familiar only with low-fire ceramics) mature at around cone 6, which is plus or minus 2200 degrees fahrenheit. High fire ceramics are most often fired to cone 10, or around 2400 degrees fahrenheit. There are clays (and pottery techniques) which require firing to lower temperatures, and there are clays which need to be fired much higher than ^10, but for the most part, commercial clays fall into these temperature ranges.
Clay has been fired to maturity when it has become as dense as it can become without beginning to bloat and melt. Low-fire clays don’t get as dense as higher firing clays. Even when fired to the maximum temperature the clay can tolerate, low-fire (or earthenware) pieces retain absorbency. They never become perfectly water-tight unless completely encased in a well-fitted glaze. Clays that mature at ^4 or higher can usually be fired to a degree of vitrification more or less impermeable to liquids. Vitrification means the clay is approaching glass status–it never quite becomes a glass, though porcelain can come close. If you see a clay advertised as ^6-10 clay, that clay does not mature at ^6. It can be glaze fired to ^6, but it will remain permeable to moisture and ^6 glazes will fit poorly and will tend to craze or crackle. A ^6-10 clay should mature at ^10, though this depends on the reliability of the manufacturer’s testing practices. Cone 6 is the most popular firing temperature in the “high fire genre,” and is the temperature I’ll be focusing on in this blog. I always buy ^6 (not ^6-10) clay, which most manufacturers will lable ^5-6 or ^4-6.
Beyond firing temperature, clays have other characteristics which you might want to consider before buying a large amount of any type. The most obvious is color. Iron oxide is the most common coloring agent in clays, and is inseperable from most native clays. Iron oxide in the clay produces reds, browns, and buffs. Dark clay tends to be more forgiving of careless drying practices because of its chemical structure. White clay is more picky, with porcelain the most persnickety of all clays, requiring long and slow drying under plastic to avoid warping and cracking. Porcelain aside, white and light colored clays aren’t difficult to work with–they just typically require a slightly slower drying process. They’re good for working with children because they don’t stain clothing as readily as darker clays and the kids love seeing the bright colors they can get when glazing light colored clay.
Besides iron, dark clays may be colored with a number of other minerals, particularly manganese dioxide. Manganese is a common mineral and present in many water supplies as well as free in the dirt in many locales, but it is toxic when the fumes are inhaled, as during the firing process. You’ll have to decide for yourself about manganese. Clay suppliers sometimes add it to dark clays to produce speckling or mottling. So long as your kiln is properly vented and you keep your clay working area clean, manganese speckling in your clay is probably not a problem, but I buy my clay sans-manganese, especially for working with children. Dark clays will stain clothing and mute the bright colors of glazes. Some glazes look better on light clays, some on dark, and a great part of this consists in what you, personally, like.
There are many other variations in clay; clays specifically for sculptural use, raku clays (for the unique needs of the raku firing method), clays designed for hand building or wheel throwing, etc. I purchase a general purpose mid-colored clay body for working with children, and in my studio, I use a dark brown all-purpose clay, as you can see in the photo above. It fires to a nice toasty color. I buy it in 5000 lb lots, but you really need to go through a lot of clay for this to work for you (it’s cheaper, though). Once you decide on a clay body, buy no more clay than you expect to use in six months to a year, then store it in a nice damp spot. It will dry out over time, and while it’s possible to remoisten it, why put yourself through the trouble?
Clay may look like one of the simpler things in this world God made for us, but as always seems to be the way–the longer and closer you look at the simplest of God’s creations, the more complex and amazing they become. The idea of taking clay, the product of the decomposition of solid rocks, forming it into a shape of my choosing, then heating it up (as happens naturally with metamorphic rock) until it fuses permanently into that shape, absolutely fascinates me. I feel as if I’m making cup-shaped, bear-shaped, mushroom and frog-shaped rocks. Obviously, I made them. They needed someone to make them–you won’t find rocks of that sort coming up in the field when you go to do your spring planting. How much more does the world require a maker?