Ceramic Firing Techniques

There are many ceramic firing techniques, some which most people are familiar with, and others that the layman will never have heard of. This article is an attempt to create an overview of those various techniques of firing (the process of hardening and/or vitrifying clay through heat), that are available to the studio potter and ceramic artist. This week we start with anagama, black firing, pit firing and raku. In the next feature we will look at wood firing, salt & soda/vapor firings and electric firings.

The Anagama kiln will usually consist of one long firing chamber with a firebox at one end and a flue at the other. Often there will also be smaller stacking ports on the side of the kiln. Traditional Anagama kilns are built on a slope, so that a better updraught can be achieved. Firing time can vary from one day to several weeks. The Anagama kiln is the oldest style of kiln in Japan and has been around since medieval times. The beauty of Anagama style firing lies in the natural ash glazes that can be achieved, and in the excitement of the long firing itself, appreciated by many potters all over the world. There are many different designs for the Anagama kiln, not only in Japan, but on other continents. Not only are there different designs, but also different methods of firing and stacking. No two firings are ever exactly alike, in contrast to let’s say, the electric kiln.

Black Firing
Amongst the ‘primitive’ firing techniques you will find the technique of Black Firing. This method involves heating a primitive gas-brick kiln to about 1000oC (these days with gas) and then adding copious amounts of sugar, which then volatilize and impregnate the clay with carbon, giving it a matt black surface. Glazes may also be used with this method, which can result in some interesting effects. The required temperature is usually reached in about five hours. The gas is then shut off and flue and any cracks sealed. Sugar is pushed into the burner port and volatilizes. Then the burner port is quickly sealed with bricks and fire clay. The kiln cools slowly and can be opened the next day.

Pit Firing 
Early cultures found clay in the ground and must have discovered its plastic and fired qualities by accident, probably discovering some burnt clay in a camp fire. This very basic firing then evolved into the pit-firing. Pit-firing involves placing unfired or bisque fired pottery in a pit in the ground, then covering the pottery with suitable burning materials, e.g. dried grasses and branches. Depending on the amount of work to be fired, a pit of the appropriate size is dug. A bed of dry leaves and twigs and possibly coal, which will burn slowly, is placed at the bottom of the pit and the pottery placed on top of this. The work is then covered with more leaves and twigs and dung, if available, building up a mound over the pieces. Once the stacking process is finished, the pile can be lit around the edges and left to smolder for several hours, if not until the next day. Towards the end of the burning process, it is possible to bury the pit in earth or sand, which will cut off the oxygen supply and create a strong reducing atmosphere inside the mound. Not all clays are suitable for such a firing, especially the more refined types available from suppliers. Additions of grog ‘open up’ the clay and make it more resistant to heat shock. Clays dug directly from the earth may be suitable ‘as is’, or might profit from additions of grog or volcanic ash, which also resists severe temperature differences. If using a commercial clay, get a clay suitable for raku firings. The best color results can be achieved with iron bearing, or red clays.

Raku originated in Japan in the 16th century, where raku vessels were and still are used in the traditional tea ceremony. It is a low-fire technique, where bisqued work is quickly heated to red hot temperature and then taken out of the kiln and reduced in wood shavings, newspaper or a similar combustible material. Raku ware is decorated with low-fire glazes, which usually contain a lot of frit. The clays used for this firing technique contain a high percentage of grog, so the work will be able to withstand the high temperature fluctuations, although this doesn’t mean that other clays can’t be used. Raku, with its battle with the elements of fire and smoke is an exciting technique, one that is suited to communal firings, as many dedicated workshops can testify. While it is an age-old tradition from Japan, it has found many devout followers in western ceramic communities.

Wood Firing
Woodfiring has an age-old tradition in ceramics. The very first ceramics ever fired were done (probably accidentally) in camp fires and rudimentary kilns thousands of years ago. Under woodfiring we now understand firing a brick kiln with wood as fuel. This necessitates a a certain kiln construction type, sometimes with several burning chambers and stoking ports for feeding in the wood. As the wood burns, ash is created which deposits itself on pots in the kiln, creating a natural ‘wood ash’ glaze. These natural glazes are made from silica, potash, calcia and other various ingredients that form naturally in the burnt wood. Special knowledge of woodfiring is necessary -which wood burns best? What different types of wood will result in special ash glaze effects? Where in the kiln should pots be placed for certain effects? (This is an individual matter for each and every kiln and firing cycle). How long will a firing take? (It may actually be a matter of days or even weeks.) What firing cycles are necessary? If you want to know more, check out these tips on how to build a woodfiring kiln.

Salt Firing
Salt firing refers not so much to the fuel used to fire the kiln, but to the introduction of salt towards the end of a firing to get a salt glaze effect. Usually done in large wood or gas kilns, salt is introduced into the mature kiln chamber by the pound at the end of a firing. Due to the intense heat, the salt volatilizes and the sodium chloride splits into sodium and chlorine gas. The chlorine combines with moisture to form hydrochloric acid, escapes into the kiln atmosphere and exits via the flue, while the sodium combines with aluminum oxide and silica oxide in the clay, forming a glaze on any exposed surface of the work. Due to the escaping hydrochloric acid, which is highly toxic, the utmost care must be taken and a good mask with a gas filter (a dust filter is not good enough!) must be worn. Of course such firings must be done in the open, and not in residential areas. Due to the environmentally unfriendly nature of the salt firing, some people prefer soda firing.

Soda Firing
The soda or vapor firing is similar to the salt firing, except that the salt is substituted with sodium carbonate (soda ash) or sodium bicarbonate (baking soda). The sodium (bi)carbonate delivers the necessary sodium for glazing, but the effect is much weaker. Often the interiors of high walled pots do not glaze well, so extra glaze may need to be applied on the inside. But as carbon is additionally generated, instead of hydrochloric acid, this is an environmentally friendly alternative.

Electric Firing
The previous firings we have looked at have all been combustion firings. That is, they require a fuel, such as wood or gas to build up the necessary heat in the kiln chamber. The heat in electric kilns is generated electrically with the use of special coils built into the walls of the kiln. Electric kilns are most suitable to earthenware or midrange firings. Stoneware temperatures are possible, but the coils will deteriorate much faster. Mind you, they can be replaced (at a cost). Also, because there is no combustion in the electric firing, these are limited to oxidation firings. Reduction can be achieved by introducing combustible materials, e.g. twigs, into the kiln chamber towards the end of a firing (usually through the spy hole), but this will be at the expense of coil life.

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