Wolf Matthes

The Woodburning Technique


The main difference of wood to other conventional fuels used as sources of heat for firing ceramics, is one of the complex compositions of the wood that contains the inflammable organic components (lignin, cellulose, resins, etc.) and inorganic compounds, which with the rise in temperature during firing, increasingly evaporate in the kiln, and can deposit themselves onto the inner walls of the kiln and onto the kiln run. Anything that has not evaporated remains in the ashes.

Through burning wood, a very light, loose ash is created, which usually, mainly consists of calcium oxide and is drawn by the draft of the flue openings (fireplaces, chimney’s) into the combustion chamber, where it can be deposited on the kiln run. This ash, together with the silica and alumina in the ceramic shards above 1280 / I 300 ° C, is able to build a layer of glass. Providing it is adequately thick enough, this molten glass is then a fairly thin fluid and displays a high surface tension, in a way where it contracts itself in decorative trickle-like patterns. If this does not occur in the necessary abundance, some potters who love such surfaces help the process along a little from the outside, by laying pieces of wood directly on top of the goods in the kiln.
The vaporized components of wood are usually carbonates, chlorides, sulphates and phosphate of potassium, sodium or even of other trace elements, but particularly also of iron. They also leave a glaze-like touch. In order for the naturally formed ash glaze and vapour-glazing to be visible, large quantities of wood must be burned, as the content of inorganic substances in the firewood is on average only 0.7 to 3%. Below 1200 °C neither ash nor salt traces react in an obvious way with the shard. But the flue ash is discoloured, dull, mat, crusty, but still an adhering coating and is then rather undesirable. If, an aesthetically pleasing, lively and interesting surface is to be formed simply by using firewood, it must be fired for a long time and at a high temperature, about 1300'C and for perhaps 6-8 days. Whether and how much flue ash is deposited on the kiln run, can be influenced by and depends on the structure of the kiln, the location of vents, the draft conditions and strength in the kiln, as well as the method of firing.

The different woods, but also the different parts of a tree contain very different quantities of ferric oxide (Fe2O3), which can end up with the flue ash, on the product in the kiln. Softwood ashes usually contain more Fe2O3 than hardwood ashes and therefore create more dark-brown to black-brown stains on the goods in the kiln, which often does not look good on a light body. Beech is therefore favoured by potters who don’t like the disturbing result of ash or wood ash glaze. Ash from coal briquettes contain much more silica and iron oxide and already creates a dark brown, shiny glaze starting at 1200 °C if they get onto the goods in the kiln, in sufficient quantity.

The often barely noticeable descent of evaporated salts from the firewood, which because of its small quantity only shows itself clearly after a long burn, can also be deliberately increased in a short burning time. To achieve this sodium salts such as sodium chloride, soda or borax are thrown in at the highest possible temperatures (above 1200 °C), through appropriate openings in the combustion chamber or in the firebox. They then evaporate quickly, spread themselves as salt vapour in the kiln, forming the intentionally produced salt glaze. This allows very thick and uniform layers of glazes to be created.
What is interesting and beautiful is, that the evaporating alkaline salts dye the brown colour of the iron oxide to a red colour, particularly with low amounts of iron oxide and alkaline oxides. This can originate either from the ceramic shards, from deliberately applied slip glazes or out of the combustion gases of the wood. Slip glazes can be composed so that they accept traces of evaporating salts very easily and thus discolour, or so that they don’t react at all to these traces.

Another special feature of the burning wood is the fact that each time logs are placed in the kiln, a surplus of combustible substance causes a lack of oxygen in the kiln (= reducing the kiln atmosphere), after burning and before adding further logs an excess of oxygen is present (= oxidizing kiln atmosphere). Thus, the burning constantly oscillates between reducing and oxidizing conditions. This constant change influences and accelerates the chemical and physical reactions, along with the surface colour development, in the body of the kiln run. It is essential to the look of the fired products, whether or not, at the end of burning over a certain period of time, there is a deficiency of air, or excess air. It is not irrelevant, in which order these conditions are maintained and under what circumstances and how fast the cooling process then takes place. This whole process, along with a sensible kiln construction, allows a cleverly constructed and operated fire box to be target controlled, by the ceramist, in a variety of directions. Iron oxide plays a very important role in the shards or slip-glaze and the carbon in the flue gases.

Unlike other forms of fuel, wood, generates very long flames. It also contains several volatile, highly flammable substances (the ignition temperature of wood is slightly more than 300 ° C). These long flames sweep over the goods being fired, so that a slightly different method of heat transfer onto the goods, follows, in comparison to, for example; electric heating. This must especially be taken into consideration when heating up the fire, but also when inserting the products into the kiln. The kiln should be stocked as close as possible and equally consistently, so that no major gaps, that otherwise act as fireplaces, primarily directing the flame through them, remain. This is how a very uneven temperature distribution arises in the kiln. There, where flames find the shortest and straightest paths amongst the fired goods, it is possible for salt or ash approaches, to make themselves noticeable with the corresponding play of colour, in the "slipstream" of other pots, which may not be touched. The fired result, therefore also depends very much on where and how certain pieces of pottery are placed within the kiln, and how it is operated when burning.

Inadvertently, but also purposely heavy smoke can also cause soot deposits on the goods. In pores of the ceramic warehoused body or the molten glaze carbon, can thus contribute, depending on particle size and quantity to an unexpected play of colour between yellowish-brown – red-brown - dark brown - gray - purple and black.