alt firing is an old firing technique that originated in Europe many hundreds of years ago. Germany is well known for it's salt fired gray and blue stoneware, e.g. it's beer steins. The term 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 so-called 'salt peel' 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 small quantities of chlorine gas. The chlorine combines with moisture to form hydrochloric acid which 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. Often a typical 'orange peel' effect occurs. As the salt creates the glaze, pre-application of glazes is unnecessary, although underglaze decoration may be applied to great effect. Sufficient space should be left between individual pieces, so the salt vapor can circulate freely, reaching as much of the work as possible.
Due to the escaping small amount of 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.
The salt kiln should be made from dense, high alumina bricks to resist deterioration from the salt as long as possible. After many uses, a thick layer of salt glaze will form on the surface, virtually fusing the inside of the kiln. This is the natural course of the salt kiln.
A typical salt firing may start in the afternoon or early evening. If in a secure environment, the kiln can then be left on overnight to get to mid range temperature, then fired to stoneware temperature the next day. When the kiln reaches cone 9 or 10, salt is introduced into the kiln in increments of about half a pound into each available port, while the firing continues. Typically a salt-kiln will have two or more salt ports, where the salt, packed in small paper sachets, can be thrown in. Alternatively a long piece of angle iron serves well to dip the salt deep into the ports. Caution! The salt may splatter out from the port. Thick leather gloves, goggles and possibly a good gas mask should be worn. If the kiln doesn't have special salting ports, the burner ports will have to do. White smoke will billow out from the flue. This smoke contains toxic acid, from the conversion of sodium chloride, silica and water vapor to sodium silicate and hydrochloric acid. The amount of salt thrown into the kiln will depend on the kiln's size, but about 10-14 pounds fine salt should be enough for a medium sized kiln. Less is required if it is an older salt kiln, as salt residue will help to get the desired effect. Moisture added to the salt will also help the conversion, but also increases the amount of smoke.
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