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SURFACING & FIRING INFORMATION
Most of these notes are unresearched and are in no way a definitive statement on any topic. They may be used as a starting point for your own research in both the practical and theoretical sense. By the end of Semester further information on these topics and others should be included in your journals. Check the library and the folders in the grey plan drawers in the ceramics studio for articles on these topics. T.N.
Bonfiring has been the firing technique used by many previous civilizations and by tribal groups. The characteristic surface is often flashed by the burning wood. It simply involves the construction of a fire over the clay pieces, then feeding the fire for 10-20 minutes, instant ceramic!! However, there are some things to consider to avoid disaster:
*A very open clay body (i.e. a raku body) clay with additions of grog or temper in the form of crushed brick, sand, stones, sawdust etc. It is best to include as much of this material as possible to the base body as you can without shortening (i.e. losing plasticity) the clay so much that you lose the workability of the clay.
*The piece must be dry before you attempt the process. One way to help ensure this is to warm the piece in a kiln or beside a small fire beforehand. The most reliable method to avoid firing breakage is to bisque-fire the piece to approx. 900 deg. C It would be interesting to compare the quality of surface of bisqued and raw-fired pieces since there is a theory floating around that for many of these techniques which rely on the natural surfacing afforded by the firing (cf. pit firing, saggar firing, anagama or long wood firing, and salt glazing) acquire more interesting surfaces when they are fired raw.
*Generally, rounded forms tend to fare better in this type of firing than angular forms. The explanation for this is that as the piece heats up unevenly in the fire, the uneven temperature rise in the structure of the body can more easily transfer itself around curves than around corners where heat tends to accumulate, setting up heat differentials between sections of the piece. Because the silica in the clay expands as it is heated, these differentials mean that one part expands faster than another causing cracking
*A bonfire is a fairly torrid environment for fragile clay elements, (there are large lumps of wood floating around in there) so it is probably advisable to make the pieces reasonably robust with a minimum of geegaws hanging from them!
A pit kiln is simply a hole in the ground into which the clay pieces are placed, and over which a bonfire is built. Usually they are a bit more sophisticated than this, with the firers using sheets of corrugated iron over the pit opening to control the flame and the amount of reduction (oxygen-starvation) occurring in the kiln. Much of the information for bon-firing is relevant here, though temperatures reached in the pit are probably higher and the firing takes longer The kiln may be quite small in scale (say 1 metre cubed) or smaller. We once had one at UNENR that was 5 metres long X 2 metres wide X 2 metres deep - unfortunately it filled with water! A large pit is required for big pieces to ensure that they heat up evenly to avoid dunting (cracking). An artificial pit can be made by building a structure with bricks above ground - but the hole seems to work better.
Clay: Even bisque-fired porcelain can sometimes survive this type above ground - but the hole seems to work better. treatment- however, a raku style body and rounded forms tend to have a better success rate.
Surfacing: A large range of surfacing techniques can be experimented with using this kiln type: *slips can be used on the clay body (see separate notes on slips) *oxides and underglaze colours can be painted on to the piece.
*frits (alone or in combination with oxides, u/glaze colours, clays etc.) can be daubed or sprayed
*lithium can be used as for frits above- an old favourite is a 50/50 mix) can be daubed or sprayed of copper carbonate and lithium mixed with water and applied in different thicknesses.
*much of the surfacing can result directly from the impingement of the fire on the work.
There is yet another theory in circulation that many of the interesting surfaces that come from the process oriented surfacing techniques (mentioned in bonfiring above) are enhanced by natural salts being induced out of the ground by the intense heat. These salts volatilize and flash the surface of the pieces in the kiln. There is an argument then, for digging, rather than building a pit (maybe more kilns of every style should be built directly on the ground rather than on slabs of concrete?) However, there is a tendency amongst pit-firers (even those who dig holes) to assist the salt flashing by introducing salt into the kiln when it is packed. Common table salt, lithium(also a salt) epsom salts, copper- carbonate, zinc granules, seaweed, banana skins (commercial bananas are dipped in copper oxychloride to inhibit the growth of fungus) eggshells, -in fact the whole compost bin (as long as it is dried first) can be used to some effect in the pit fire. See also in saggar firing
Packing the kiln
The usual procedure (though it is good to experiment) is to lay about 15-20cm. of sawdust over the entire floor of the pit, the clay pieces are placed on the sawdust -often upside down or on their sides, Salts and oxides etc. are then put in small heaps or in bisqued containers beside the work- zinc granules, seaweed, compost etc. is placed on the work. Small pieces of wood are packed between and over the work. Gradually the wood size increases until quite large lumps of wood are placed on top to ground level It is advisable not to put raw salt directly on the work. This is because during the firing it is absorbed into the matrix of the clay; salt is hydroscopic (absorbs water) and some time after the firing(say during the wet-season) the piece may absorb water from the atmosphere, the salt under the surface may expand, and sheets of clay and surfacing may peel off the work.
Small fires are set on top of the pile and as the larger wood catches, the fire is gradually dampened down by placing a partial lid over the kiln- perhaps sheets of corrugated iron The fire will usually continue for some hours (3-9 ? depending on the size of the pit, the quantity of wood, how much extra stoking is done, the amount of damping done and probably many other variables) This is a direct hands on experience and different firing procedures will probably give a variety of results- Experiment and enjoy. After the firing It will take at least 12 hours for the pieces to cool down enough for you to be able to pick them up. Be careful of hot embers. Give the pieces a good wash with a stiff brush and water to brighten them. Some people use wax (bees; Johnsons-floor; etc.) to reproduce the brightness you get when you wet the fired pieces.
This is a simple firing technique which puts a finishing surface on already-bisqued pieces. Bisque temperature should be about 850 deg. C. If bisqued higher than 900 deg. C, any burnishing done on the piece is likely to be lost; also the higher the temperature, the less likely the blackened smoke markings are to be found on the final piece Since the temperature in a sawdust firing seldom reaches any more than about 500 deg. C at which stage not all of the chemically - combined water has been burnt off, pieces fired raw in this technique do not have the permanence of bisqued pieces- they will often decompose in water.
Burnishing is a technique often used here and in other firings such as black- firing and saggar firing. It involves the rubbing of the piece with a smooth tool (back of a spoon; car tail light bulb; polished river stone) to make the surface of the piece smooth and shiny. When fired in the sawdust, the surface becomes a very beautiful bright shiny black.(refer to Maria Martinez) Burnishing can be done two ways:
1. Allow the smooth piece to dry and sand off any unevenness, apply a slip and burnish before completely dry.
2. Begin burnishing from the leather-hard stage through to dry. It is slip and burnish before completely dry. important to dry the piece slowly as fast drying causes dullness.
When the piece has reached the dry stage, use margarine, fat, or lard to lubricate for further burnishing. Polish in one direction,usually best suggested by the shape of the piece. Make nylon gloves by cutting the feet out of stockings to avoid fingerprints on burnish as well as to rub over the piece to bring up the final gloss. Thin plastic stretched over the index finger also serves well as a final polisher.
Many of the surfacing techniques suggested in pitfiring, blackfiring, bonfire, and saggar firing are useful here. eg Spraying with iron chloride before bisque firing gives the work a pleasant flashed appearance. Some slips that you might like to try include:-
1. 90 Ball Clay (Parts by weight), 26 Silica, 4 Frit 4508
2. 80 Clay of pot, 8 Silica, 3 Bentonite
3. 60 Ball Clay, 20 Kaolin, 20 Feldspar
Oxides can be added to give a range of coloured slips. Also slips high in flux may
be less receptive to smoking producing an interesting contrast to the smoked areas.
4. 30 Ball Clay, 10 Kaolin, 10 feldspar ,15 Colemanite
Two teaspoons of Calgon (Water-softener) added to a 1/2 litre of slip acts as a deflocculant and produces an even smoother slip. Try sprinkling salt, lithium, or low melting fluxes such as colemanite borax, or frits, around the pieces as you pack the kiln. Again, it is advisable to use bisqued-ware.
Sawdust kilns are usually built with common house bricks. The size depends on the number and size of the works you wish to fire. A common size is a box about 80 cm cubed., with four brick walls, a brick floor and sheet metal for a lid. It is probably as well to mortar the bricks together in construction because too much air entering the kiln tends not only to burn away(oxidize) the carbon effects (smoking) but can also cause the pieces to dunt (crack). Metal drums function quite well as sawdust kilns- at least for a few firings.
Packing the kiln
Begin with 10-15 cms of sawdust on the floor. Place the pieces in the kiln,s. remembering that often the strongest surfaces occur where the piece has been sitting on the last remaining smouldering sawdust, which is underneath the piece.. To avoid the strong black marking on the underside of the packed piece, place it on a brick in the kiln with no sawdust between them. Most artists in this field tend to place pieces on their sides or upside down to take advantage of this fact. Pour sawdust around the pieces, ensuring that the topmost work is covered by at least 10 cm of sawdust. When firing large numbers or heavy pieces it is a good idea to place a sheet of chicken wire between layers Some people like to fire only one piece at a time, building the kiln to size each time, and leaving 5-10 cm between the piece and the walls of the kiln. This tends to give more specific smoking rather than the more generalized smoking obtained in community firings. Pouring motor oil over the sawdust pile leaves interesting markings on the pieces. Sump oil tends to leave dull marks, whilst high grade motor oil gives a shiny black metallic flashing as it burns out.
WARNING: OIL contains lead which is very poisonous- the fumes from this type of firing can be injurious to health- not only your own but that of other people in the general area. For this reason you are requested only to light this type of firing late on a Friday afternoon to lessen the health risks. Since this is a very low-tech method it is preferred if you can do it away from the university altogether.
The finer the sawdust, the blacker the firing is likely to be. Fresh sawdust is often more resinous and may leave a resinous, sticky deposit on the surface of the piece. The aesthetics of this depend on individual taste and the type of work being fired. Slower firing tends to produce blacker pieces. Therefore, the slower burning hardwood sawdust is preferred by some over softwood
Simply light a small fire on the top of the sawdust and leave it until the sawdust is smouldering well. Place the sheet metal over the kiln to slow the burning. Only small whisks of smoke should emerge from the kiln. Excessive smoke indicates that there is too much air and that the sawdust is burning too quickly- place bricks on the metal lid and/or bog up any cracks in the brickwork of the kiln and/or throw some sawdust to smother any flames. The sawdust should smoulder for anything from 24 to 96 hours. any less than 24 indicates too quick a firing. Leave the kiln to cool naturally before removing the work.
There is a lot of documented info on this topic in the folders in the grey file drawers in the ceramics studio. Saggars are containers in which ceramic pieces are sometimes placed in the file drawers in the ceramics studio. kiln. Saggars may have been in use in China as early as the 3rd Century. Originally, they were thrown cylinders of clay with lids and bases. Pots - often made of delicate porcelain- with fine glazes were placed in them to protect the pieces from the impingement of the flame and from the less refined speckling and dribbling created on clay surface created by the melting fly-ash in wood-fired kilns.. The saggars had other advantages- they also functioned as a bag-wall or deflector to redirect the flame in kilns to ensure a more even temperature throughout the kiln; in the period before the introduction of sophisticated kiln shelves they also allowed pieces to be stacked higher in the kiln allowing for improvements in kiln design. In modern times with the use of gas, oil, and electric kilns, (plus the fact that most wood-firers prefer the process-indicator of fly-ash marks on their work) traditional saggars are seldom used. However, some studio artists are currently using saggars extensively -but in a perverse way. Many artists are interested in having their work reflect the process of making (including the flashing and burn-scarring of the firing ) So, instead of placing the work in the saggar to protect it from the marks of the burning organic matter of the fire-they include the organic matter in the saggar to heighten those effects. Materials such as wood, sawdust,seaweed,& other organic materials are used. Most commonly this is done at low temperature (app. 1000 deg. C.) However, there seems no reason why this experimentation couldn't be undertaken at higher temperatures(up to 1300 deg C)-with and without traditional glazes. Saggars can be made from a variety of materials-
* 20 litre drums work OK but sometimes the iron contained in them, which oxidizes with the heat, can cause the pieces to become too dark.
*kiln-shelves work if you can find a way of supporting them in place warning: kiln shelves are expensive items - please get some advice from staff before you attempt to use them *bricks can be used but they tend to insulate the inside of the saggar too much, they also tend to take up too much kiln space. *ceramic fibre works but is very expensive
*work can be placed in cardboard boxes which are then wrapped in slip-dipped Hessian. The box burns away in the firing but the slip filled Hessian maintains it's shape and functions as an effective saggar. Done carefully, this is an effective technique.
*by far the best solution seems to be the use of purpose-built saggars made from a mixture of fireclay,grog,and petalite or zircon. We have some of these -ask
Raw clay from the ground or from the plastic bag can give very interesting surfaces in this type of firing- depending on the organic materials placed in the saggar. Generally, the early-vitrifying clays (eg. Feeneys red raku) are not as successful as those clays which can be fired to a higher temperature. Its a good idea to discuss clay types with staff or with students using particular firing techniques before you begin.
Anything goes!! Refer also to pit firing in these notes Slips of all kinds are worth experimentation. Especially good is terra -sigillata. We have two different terra sigs in the glaze room. Oxides,sulphates,carbonates,underglazes Lithium or lithium/copper mixes washed on to pieces can give good results Salts and oxides in small crucibles can be placed beside pieces in the kiln. Glazes of all kinds (even glazes that won't mature at intended temp.) Very strong results can be obtained by the use of seaweed or brine -soaked straw, sawdust etc. There is one seaweed type that works best -ASK!! For best results these need to be placed on the pieces so that they won't fall off during the firing -often quite a challenge! Some people have wired the material on. Others, (refer to John Dermer ) have used ceramic fibre wrapped around the work to contain the organic material. Eggshells, tea bags, banana peel, leather, zinc granules, pine needles, grass, all have their effects - experiment!!
Packing the kiln
This is best done with some supervision the first time. Saggars are placed in the kiln, organic material is placed on the floor,grass, all have their effects - experiment!! the work is then placed on the stuff on the floor, often on its side or upside down - more material - more work- stacked on top of one another with as much surface contact as possible,often pieces of shard are used to assist in this regard. It is easier to get good surfacing on flat pieces than on full rounded or complicated forms.This is because more surface contact with the seaweed etc. is possible. Best results seem to come from using smaller rather than larger saggars. i.e. rather than placing all your work in one large saggar it seems better to construct a number of smaller saggars and place just a few pieces in each. This may have something to do with the fact that you can pack the smaller saggars tighter and the work is closer to the walls floor and roof of the saggar. Volatilized materials seem to blow-back their surface-enhancing properties onto the pieces if they are within 5 or 6 cms of the walls. The saggars are usually well-sealed with slops clay to prevent the carbon and fumes escaping too easily. Some people prefer the more subtle results obtained from leaving gaps in the walls or roof of the saggar!!
Any kiln except electric kilns are satisfactory for this process. The burning organic matter tends to destroy the elements in electric kilns very quickly. However, gas, oil, or woodfired kilns seem to be able to cope quite well with this kind of abuse- even the more fragile ceramic-fibre lined kilns.
-- Tony Nankervis
Phone: +61 66 203000
University of New England, Northern Rivers (Lismore) NSW Australia
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