About Wood Firing
A Brief History
Wood fired ceramics is at the very root of civilization. Its beginnings were probably in the use of grass baskets lined with clay to carry water.
Set by an open fire long enough, water evaporated from the heat, and eventually the clay became sintered to low bisque. (Clay contains two types of water: water of plasticity and chemical water. Water of plasticity is the water that allows clay to be pliable, and its evaporation is what initially makes clay hard. But the clay can still be re-hydrated at this point to a soft state. Chemical water is removed at temperatures above 1000°F. Once the chemical water is gone, the clay cannot be softened again by adding water. At the lowest temperatures where this happens (called bisque here) the clay is generally very porous. As the temperature the clay is heated to increases, the clay becomes less porous, or equivalently, more vitreous).
The discovery that the fired clay held water, which remained clean of mud, was attractive. Another possibility might be that early people used clay to describe fertility - clay mother effigies have been found dating back 30,000 years.
The actual beginning of fired ceramics isn’t important, however, without its discovery civilization might never have evolved as it has. Wood firing is the history of civilization recorded in clay. As our knowledge increased so did our uses for clay. From simple figures, bowls, storage jars, and even as a medium for writing on, all were wood fired. It wasn’t until the discoveries of fossil fuels (coal, oil, later propane) and electricity, that other ways of firing clay were found.
As we now know, the invention of wood fired clay was universal throughout the world. However, it didn’t develop equally everywhere, but rather evolved to meet the regional needs of various peoples. In Africa and the Americas, the firing techniques were simple but sufficient for their needs. Still, open firing (on the ground without a kiln) evolved to covering the simple bonfires with potshards and even to concern about how pots were placed in and among the wood and grasses used as fuel. Readily accessible clay was used (mostly earthenware which has a low melting point). Although the clay had been fired to hard bisque, water would still leach through it. Still, the ware functioned for its intended use. There is some evidence that the Aztecs and Mayans, who were less nomadic people, used simple kilns of a more permanent design.
Mediterraneans evolved their fired clay work with kilns, of permanent structures built using clay, which hold in the heat better. These were up draft bottle kilns that were used to fire pithos jars from Knossos off the coast of Italy, and the red and black figure vases of the Romans. Moving up the European coastline to England, all these cultures used this method to fire their earthenware clays. With the move to fire stoneware clay things changed. The Germans moved on to salt fired stoneware clay at about 2280°F. (In salt firing, when the kiln is at a high temperature salt is thrown in where it breaks down and the sodium reacts with the clay to form a durable glaze.) England moved to a bourry box style kiln. (This kiln has a firebox where the flame is first pulled down and then into the kiln.) These kilns are contemporary in the scope of ceramics' long history.
The Western Pacific Rim region developed kilns by simply digging a hole into the hillside, hollowing out a chamber to put the work in, and adding a hole for the flame to move out through. These cross draft kilns filled the need to fire clay to higher temperatures. This region included China, Japan and Korea, to name a few countries. All these cultures were firing stoneware and porcelain centuries before the rest of the world due primarily to the easily accessible clays they had.
Without going into depth here, it follows that different styles of kilns were built and used to fit the needs of each region's sensibility as to both style and functional aesthetic. For example, the early Romans used earthenware clays to make red and black figure vases. The up draft kilns they used were fired fairly quickly - probably less than 15 hours - in what would be a neutral to very little reduction atmosphere. (Atmosphere is described as oxidizing if it contains excess oxygen to fuel, reducing if less oxygen is available than the fuel needs, or neutral if the two are in balance.) The kiln design was effective for their needs. The work was protected from the wood being thrown in by having a barrier made of clay that separated the firebox from the ware chamber. The temperature could be brought up slowly at first to protect the work from too quick of a climb. They used different colored slips to force color change, which eventually became lead glazes for majolica ware.
This differed a great deal from the African and American Indian methods of open firings. They put the work in and among wood and grass allowing a direct interaction between the pots, wood and flame. This type of firing was short in duration - less than 5 hours in most cases.
The Western Pacific Rim kilns had longer firings due mostly to firing to higher temperatures. Originally, the work put into these cross draft kilns had no barrier from the wood and fire. Pots in front near the firebox would be hit with wood and wood coals allowing interaction with the clay. These people discovered that the ash would melt on the pieces in this area to form a glaze making the clay more water retentive. As cultural regions developed aesthetically, people started to put pots inside other pots to protect them from the onslaught of ash and flame. The quick burst of heat following a stoke could shatter the work. Later, with the development of glazes, they started making saggers. These specially made vessels were designed to protect the glazes and clay bodies from fluctuations in temperature. Plus the neutral atmosphere inside them allowed greater control to the outcome of the glazes (pots would have the same look all around ).