by Jenni Fritzlan
Pottery is an ancient craft, with the earliest historical records of wheel-thrown pottery dating back to 4000 B.C. in Egypt and 3000 B.C. in China. In this article, I’ll talk about how wheel-thrown pottery developed and was refined as a craft and will discuss glazes and types of clay.
Early pots were most likely made using hand-building techniques, particularly coiling, and there are various theories as to how the potter’s wheel developed out of these techniques. One that is fairly reasonable suggests that while using two stone plates, one stacked atop another to make a turntable for coiling, ancient potters realized that if they could spin the top plate faster, they could make smoother pots. Early utilitarian pottery was thick-walled, uneven and typically unglazed. It likely began with people making storage jars for food and water. But whoever could afford to do so, certainly in ancient Egypt, used everyday and ceremonial vessels made of bronze, iron or precious metals. By the middle of the tenth century B.C., though, pottery had developed to the point that there are historical records of a family of potters living in a palace to do the king’s work. In fact, most of the most ancient pottery artifacts in existence today were preserved in king’s tombs.
Although much pottery was made in the Middle East, Oriental cultures contributed most directly to the refinement of pottery as a craft. Some of the earliest glazes the Chinese used were celadons, which were made from the same clay as the pots, combined with wood ash. The glaze was applied to raw pots, which were then fired in a reduction atmosphere, a term I’ll explain shortly. In the Orient, different regions had unique types of celadons, due entirely to differences in the composition of local clays. Clay that contained trace amounts of lime produced celadons that turned blue—the classic Chinese celadon. Classic Korean celadons were pale green, and classic Japanese celadons were olive green. The clay contained a small amount of iron, and the wood ash contributed lime and silica to the glaze. Combined with the right atmosphere in the kiln, the glaze would turn the translucent, pearl-like blue or green color that Oriental potters are famous for.
In addition to celadons, Oriental potters developed red glaze, which, like celadons, depends on a reduction atmosphere in the kiln to turn the right color. In simple terms, reduction is a phase during the firing cycle in which the oxygen level in the kiln is reduced, causing oxygen-starved flames to pull oxygen from the pots. Reduction begins at roughly 1500ºF and goes to about 1900ºF. Only possible in fuel-fired kilns (kilns heated by use of a combustible fuel), reduction does very interesting things to the glazes. Red glazes turn red only under properly balanced reduction conditions. Too little or too much reduction turns the red glaze white, or in some cases, pale green. Celadons are nearly as touchy as reds, in terms of their dependence on the kiln atmosphere, turning pale brown or tan in the wrong atmosphere. For the Chinese to consistently produce their wares they had to be masters of their craft. They developed pottery from its functional but rugged beginnings to a high level of craftmanship, consistently creating beautiful vessels of the finest quality.
Linda Sue Park’s novel, A Single Shard, gives insight into the life of ancient village potters. It tells the story of a twelfth-century Korean boy, who eventually managed to get himself apprenticed to a master potter in a small pottery-making village on the west coast of Korea. Trade secrets and family glaze recipes were carefully guarded in these pottery villages, and pottery was held to a very high standard of excellence.
As the Industrial Revolution arose in France, England, Germany and other parts of Europe, pottery underwent a transition from village craft to mass production. Dinner plates, for example, had been made of pewter or wood prior to the Industrial Revolution. In 1769, Josiah Wedgewood opened the first ceramics factory, located in England, where one person would make a dinner plate, and then factory workers would produce hundreds of replicas by pouring slip (liquid clay) into molds. Whatever benefits may have been derived from the introduction of these and subsequent industrial and technological techniques, much of great value has been lost, including vast treasures of practical knowledge that ancient people had of their craft. For instance, ancient potters all had the skill to go out and collect local rocks and minerals and make them into glazes, but few potters today have this same skill. Our modern knowledge of glazes is mostly limited to the refined powders that we purchase and mix together to achieve our desired results.
Bisque firing is another product of the Industrial Revolution. For thousands of years, pots were baked using a process called single firing. After the pots were made and raw glazed, they were fired very slowly over a couple of days in a wood kiln. Because this process requires much skill and care, single firing, though ideal for the village potter, does not lend itself well to producing large quantities of identical pots. So bisque firing was developed to allow mass production of pottery in factories. Bisque firing is an initial firing that takes the pots through a chemical change, transforming the raw clay into a ceramic material. Fired to about one-third of their maturing point, the pots are still very porous, and will readily absorb the glaze that is applied to them. After bisque firing, the pots are glazed and then fired a second time, which melts the glaze and sets the clay. Bisque firing enables pottery to be produced more uniformly than single firing. It also allows the process of making of clay vessels to be broken down into separate steps, performed by different people, each with limited knowledge of the process as a whole. For example, one person might specialize in making pots, another in firing, yet another in applying glazes, enabling an assembly-line approach to making clay pots on a large scale. Though originally developed to support mass production, bisque firing has become the firing method most commonly used by artisan potters. Yet a number of potters, ourselves included, are taking steps to return to single firing. We know that single firing is advantageous because it requires less energy than bisque firing and saves a considerable amount of the potter’s labor. But we also believe that there may be other benefits to single firing that we won’t see until we have more experience with it.
Sadly, with the rise of industrialism, pottery as a traditional craft began to rapidly disappear, particularly in industrialized nations. By the beginning of the twentieth century, most of the relatively few village craftsman potters who remained were living in China, Japan and various Third World countries. In the early twentieth century, a young Englishman named Bernard Leach began to train under a Japanese master potter. Leach was born in the late nineteenth century in Hong Kong, and spent his young adult years in Japan. After training for many years, he and a young Japanese friend named Shoji Hamada set up a pottery shop in England, where Leach Pottery still exists today. Leach revived the craft of pottery as an art form in England. He also came to the United States and started teaching classes here, as well. In 1935, he taught a class at the Archie Bray Foundation, where he deeply impacted several up-and-coming American potters, such as Warren MacKenzie and Val Cushing. Leach’s influence helped change the course of pottery in America, reviving the art and craft of making and firing earthen vessels.
Types of Clay
Clay is an abundant material resource. While there are many different types of clay, potters only use clays that have a certain range of properties. Some of the most important properties of usable clay include its ability to be shaped without cracking or breaking (this property is known as plasticity); its ability to be fired to the temperatures necessary to transform the raw clay into a ceramic material without distortion; and its ability to be used to produce repeatable results with the same treatment, so the potter can consistently produce the wares he wants.
Clay is formed from decomposed igneous rock, and there are two major categories: primary and secondary. Secondary clay is clay that has been relocated, through land movement or water erosion, away from the site of the parent rock. Primary clay, on the other hand, has not moved from the site of the parent rock. Secondary clay has two major divisions: earthenware and stoneware. As clay moves, it picks up other minerals (iron oxide, for instance) that act as flux. Flux is any substance that lowers the melting point of another substance. The farther clay moves from the site of its parent rock, the more mineral it picks up, lowering the temperature necessary to make fired clay nonporous (called the maturing point of clay). Stoneware clays have moved only a short distance from the parent rock, whereas earthenware clays have moved a long distance. Thus stoneware is fired to a high temperature and earthenware is fired to a low temperature.
Primary clay is composed of kaolin, a name derived from a Chinese word meaning “high ridge.” It was first discovered and used by the Chinese as early as 200 B.C. Kaolin is too heat resistant (refractory) to be used on its own, but it does form an important part of both clay bodies and glazes. Kaolin is the primary ingredient in porcelain, with silica added for strength, along with calcium, in the correct proportion, to flux the kaolin and silica together. Some do not consider porcelain to be a true clay because it never occurs as a natural mixture—it is always man-made.
The most common clay is earthenware. Worldwide, a large percentage of pottery is made from earthenware. It is also commonly used for bricks and for tiles for roofs, sewer pipes and floors. Its name refers to the earthy colors and the common origins of this clay. Earthenware fires to maturity as a semi-soft pottery that can be easily scratched or broken by hard objects. It never becomes completely waterproof, and liquids contained inside unglazed earthenware vessels will be absorbed through the walls of the pot. Earthenware matures between 1700° and 2075°F. One of the reasons earthenware clays are so commonly used is that they can be easily shaped—earthenware is very plastic. It can also be more easily fired than stoneware clays, and there is a wider range of glaze types and colors available that work with it.
There are two types of stoneware clay: midrange stoneware and high-fire stoneware. Midrange stoneware fires to maturity between 2200º and 2283ºF, or cones 5-7. (Cones are small triangles of clay formulated to melt at precise temperatures. Potters place them in the kiln and use them to visually determine the temperature inside the kiln when firing). Midrange stoneware pottery is sometimes referred to as “ovenware” due to the practice of using it to cook in ovens. Midrange stoneware clays are easier to finish out in a home studio. They can be fired at a lower cost in electric kilns, with beautiful results, or, less commonly, they can also be fired in gas or wood kilns. High-fire stoneware is nonporous and very durable because of the high firing temperature. It fires to maturity at 2300°-2400°F (cone range 8-11) and is the most popular clay for functional pottery. High-fired clays are often used for salt-glazed and wood-fired pottery because a higher temperature is required for the salt or wood ash to melt and form a glaze on the surface of the pot. High-fired clays are commonly used for gas-firing as well. Some very striking glazes and effects are possible with these clays—effects that are not possible to achieve in lower temperature firings. Although the results of high-firing are well worth the effort, this type of clay is expensive and time consuming to fire, and kilns used to fire it wear out faster due to the stress of the high temperatures.
Glazes are mixtures of three main ingredients: silica, flux and alumina (the main component of clay). Silica is the part of the glaze that turns to glass, also known as glass former. Because silica requires temperatures above 3000ºF to melt, it is necessary to add flux to lower its melting point. Many different materials can be used as flux, including feldspar, wood ash, calcium carbonate and various other compounds. Alumina is added to thicken the glaze to prevent it from running off the pots during the firing, similar to flour mixed into gravy. Silica, flux and alumina mixed in the proper proportion will make a gloss glaze. Changes in the relative proportions of these components can also produce semi-matte, satin-matte, or a dry stony-matte finish. In mixing a glaze, there has to be enough flux present to absorb the silica/alumina, and there has to be enough silica/alumina present to take in the flux. Firing temperature also influences the melt. A glaze that is formulated to be a matte glaze at cone 6 (2200ºF), when fired to cone 10 (2350ºF), becomes a gloss glaze. 1
This is only a brief overview of the art, craft and history of pottery. Pottery, along with many of the other crafts, is best learned with hands-on experience. Though it takes time to learn the intricacies of the craft, it is rewarding to take a lump of raw clay and transform it into a beautiful and usable vessel, all the way from the forming of the vessel to the finishing of it in the fire.
- Greg Daly, Glazes and Glazing Techniques: A Glaze Journey (East Roseville, Australia: Kangaroo Press, 1999), pp. 7-9. ↩