3     Raku Techniques
         and Problems

Glazing and Application

It is advisable, and a practice which I prefer in order to be on the safe side, to glaze all work the day before you raku, although it is possible to dry the work by sitting it on or near the edge of the hot kiln on the day. This is really only acceptable if you are glazing only small items; wet or damp glazed work will crack or even shatter in the kiln and this can destroy the kiln and make a terrible mess of its shelves.

Glazes may be applied in any manner that you find fits the piece and that you are comfortable using. Pouring, dipping, sponging, painting and spraying are all suitable for a raku glaze. As in all glazed surfaces, ensure that you have sufficient glaze on the work; for instance, a crackle glaze will not crackle if it is too thin, and a matt copper will not present you with the iridescent colours you are seeking if insufficient glaze has been applied (but this glaze in particular often gives you better results if it has only a thin layer, a complex and frustrating 'Catch-22' situation, that, after many trial and error efforts, can be overcome with experience).

Crazing

Crazing looks like a network of cracks in the glaze. In a normal glaze this would be a defect but in a raku glaze this is often the effect you are looking for. Crazing is caused by a different expansion/contraction between the body and the glaze; in other words, the glaze is actually too small for the body and, when cooling, contracts more than the body, thus causing the craze effect. When a glaze has been created to obtain this effect it is called a crackle glaze. Depending on the composition of the glaze, you can obtain large or small crackles and often you do not see them until you have warmed the piece again and placed ink over it, which then goes into the crackles; after washing off the piece the cracks are shown up by the ink.

'Water Nymph' (white crackle, multi-fired).

Pinholing

Pinholing looks as it suggests, like many small pinpricks on the surface of the glaze after the firing. This can occur when a glaze is applied too thinly: insufficient glaze is available to heal over the pinholes in the firing. But if a glaze is too thick you get a much larger bubble and larger pinholes. These holes appears during the fusion process of the firing, the gas bubbles burst during the firing and do not 'heal' over, creating this effect.

Bubbles are created in large numbers in all glazes during the fusion process. Sometimes it is purely contamination of a glaze that generates this fault, therefore always make sure that you practise good housekeeping in your studio and ensure that your bulk liquid glazes are well covered from dust, dirt and any other contamination and that no dried out glaze falls into the mix since this can also contain air bubbles.

Crawling and Beading

Bare patches on your glazed pieces are often the fault of 'crawling'. This happens when the bisqued work has had dirt, dust, oil or grease inadvertently applied to it. The glaze then rolls away from these areas, acting like a resist. Always keep bisque ware away from any of these contaminants. However, some glazes are prone to crawling due to their high surface tension.

Shivering or Peeling

The opposite of crazing is 'shivering' which is caused by the glaze being too big for the body and thus on cooling the glaze shivers or peels off the body, most often on the edge or rims. What is happening is that the body of the piece is squeezing through the glaze; this is not a very common problem because a glaze can withstand much compression before it will start to shiver.

Starved Glaze

You sometimes get a glossy glaze that comes out looking dull and lifeless; this is often called a 'starved glaze'. The cause is quite frequently that of underfiring, and in such a case you can refire the piece to obtain a better result. Too thin an application of glaze may also give the same dull appearance; this may be caused by the use of a spray gun. Sprays can be difficult to use since you can over-glaze one area, while not giving another area enough. A glaze that is mixed with too much water and is thus too thin will give you the same effect. A good rule of thumb for a glaze is that it should be as thick as cream.

Dunting

If a crack has penetrated completely through the wall of the piece this is called 'dunting' and can be caused by factors such as thermal shock (see below) in cooling or heating. In the cooling-down phase a dunt can be identified by its sharp, jagged edges, while a heating dunt will have smooth, rounded edges.

Slumping and Warping

Overfiring a piece of work will often result in a form's slumping or warping. This is caused by the body being subjected to heat beyond its normal firing range and this in turn causes the fluxes to soften, which results in the body's collapsing. Often this is caused by placing a large piece of work too close to the heat source, thus it is better to put a tall piece in the centre of the kiln. Badly designed work can also lead to your work's collapsing. If the base of your work is thicker than the walls this can cause uneven heating, which, in turn, causes stress and ultimately the slumping and warping of the piece.

Thermal Shock

Thermal shock is caused by stress within a ceramic object resulting from temperature changes and is the biggest cause of cracks and faults in firings; this can happen with a sudden change in temperature from hot to cold, or the placing of a piece so that one area of it gets a full blast of heat from the kiln while another is shielded by other work or a kiln prop. The structure of work may also add to thermal shock: having a thick bottom and thinner sides will cause parts of the work to react differently to heating and cooling, often resulting in fractures.

Glaze Fit

Glaze fit is the suitability of a glaze to fit to the underlying clay body with regard to its thermal expansion and contraction. The body of the work and the glaze need to have the ability to expand and contract together with reasonable harmony. However, an exact match is not possible and fortunately not necessary since there is a degree of latitude before a defect occurs. This is a predicament that can often be fixed by first analysing the problem.

'Le Promenade' (slip and raku hat). PHOTO: VICTOR FRANCE

'Temple Bells' (bronze/beads/copper red glazes).

'Artist and Le Promenade' (hand-built slip and raku figures).

PHOTO: VICTOR FRANCE

Glaze Storage

Raku glazes are usually made up as needed since they do not store well and will often settle at the bottom of a container if left for any length of time. If you have made up a large amount of glaze and need to store it, make sure that its container has a tightly fitting lid. I usually re-sieve the whole lot and give it all a good stir to make sure that all the ingredients are thoroughly mixed again. Copper compounds are well-known for settling.

Experiment with other oxides and colourants if you find a base glaze you like. Raku firing is built on unpredictability and change, so take full advantage of this. Do keep a good record of what you have done since there is nothing worse than getting a fantastic result and then not knowing exactly how you managed to get it. The methods used to apply the glaze can affect the outcome, too. Dipping, pouring, brushing on or spraying are all used and can present you with different colours and effects; the thickness of the glaze is also a factor that can alter the colours.

    TIPS AND TRICKS IN RAKU

• Over-glaze spray: spray copper oxide thinly over a white crackle then fire and reduce.
• Lustre effects: place copper carbonate in a muslin bag and shake it over a damp piece, causing it to stick on the surface; you can also paint a thin wash of either copper carbonate or copper oxide on to a bisque piece, under a glaze.
• Frit 4108 can often be used in a glaze recipe as a substitute for Gerstley borate, but always test first.
• To a little warm water add 1g of silver nitrate and a pinch of salt; let it dissolve and use over a glaze, for instance, copper red, and then fire as usual.
• When using borax I buy a household brand since it is cheaper and finer; to use it I place the borax and the oxides needed in a separate bowl of hot water to dissolve them; this is then added to the general recipe; by this means the borax does not go hard and lumpy.
• To highlight the cracks in a crackle glaze: warm the piece in the oven after firing, then...