POTTERY Being a simple account of the history of pottery and a description of some of the processes employed in its manufacture By Charles J Noke and Harold J Plant 1927

Extract Chapter 12 ENAMEL KILNS HARDENING-ON and enamel kilns are very similar in their construction. These kilns are of the muffle type, i.e. the flames are not allowed to come in direct contact with the ware, but run underneath and round the sides of the kilns from the fire mouths along flues so arranged that they are equally distributed over the whole portion of the kiln as near as is practicable. The muffle may measure anything from 3 ft. high to even 10 ft. or 12 ft., and the same in length, with a width of about half the length. The mouths are arranged at the side, and level with the bottom of the kiln chamber itself, and flues arranged as previously mentioned carry the heat into a tall cone, finishing in what might very simply be called a bottle-shaped chimney stack. The opening to the muffle is closed by iron doors having two spy holes, which enable the fireman to inspect the inside of the kiln from time to time. The iron doors are protected from the heat by a firebrick lining. All crevices are stopped up with china clay softened down to the consistency of a paste, but this stopping up must never be carried out until the firing has been in progress sufficiently long to expel all fumes from the kiln. Coal is used for the raising of the necessary temperature, and the firing usually takes 10 to 12 hours. Great care has to be exercised in order that sulphurous fumes may not penetrate through any cracks or crevices in the side of the kiln, or disastrous results would ensue. The muffle of a new kiln is always thoroughly lime-washed before use. Afterwards it may be found necessary to stop up any cracks which have developed in the brickwork of the kiln with the aid of china clay, again washing over with lime wash. In the case of china, these kilns usually contain from 400 to 500 dozens of ware. In spite of every effort to construct the kilns so as to obtain uniform temperature throughout the muffle, it has been found in practice that this is almost impossible. Experience has shown that in the actual placing of the ware, great care must be taken. It is, therefore, necessary to put only those articles that will not suffer from any

excess heat in the danger places, owing to the fact that certain colours allow for a very slight margin of temperature, 20 degrees in excess of the correct heat being sufficient to destroy the tint. As previously mentioned, the temperature required for the firing of enamel colours varies from 800 to 900 degrees centigrade. This heat is sufficient to cause the glaze to soften slightly on the surface, and methods have had to be invented for the placing of ware in the kilns in such a way that no two pieces are allowed to touch. In order to prevent this contact, and to enable the manufacturer to fill the kilns to the greatest advantage, various articles made of a hard semi-vitreous body, and known in the trade by such names as butterflies,'' dumps," thimbles," and stilts " are used. They are made in many shapes, and in such form that while separating the actual ware, only the finest point comes in contact with the glaze. For better classes of ware, and expensive pieces, it is usual to place the ware in cranks, which are really skeleton cases, each small case containing one piece, and one may even go so far as totally to enclose the piece in a small case or box crank of its own. Thimbles are also used much in the same way as already explained for placing ware in the glost oven. Naturally, the use of these articles is quite an expensive part of the manufacturing process. Modern research has resulted in the introduction of various types of continuous kilns for the firing of enamel colours. The Dressier" and Climax" may be cited as types of these kilns, which have proved very successful under the right conditions, but there are in use simpler types which only cost a few hundred pounds, and are capable of an output of 6,000 dozen of ware per week, resulting in a saving of 50 to 75 per cent of fuel as compared with the use of ordinary kilns. One other great advantage in the continuous enamel kiln is that there is great freedom from dust and kiln dirt. Probably, however, the greatest improvement effected by this method of firing is in the colours. This is due to the fact that the colours are brought up to the required temperature in a shorter period of time. This gives the desired brilliancy, and avoids to a large extent an excess of volatilization from the colours. A pro¬longed firing in nearly every case tends to make an enamel colour become very dry, due to the reason explained above. No great space is required for a simple kiln of this description, a room 15 yards square being quite adequate for the purpose, and it can be highly recommended to any potter who is only satisfied with the best possible. In cooling a kiln the stoppings from round about the doors may be removed a few hours after the fires are out. Sometimes it is necessary to cool very quickly, but this results in dunting, and is thus a source of increased loss. Especially where very large articles are placed in a kiln as much time as possible must be allowed for the cooling process. The kiln records, with particulars of the count, the percentage of loss, and the fuel used, is required. All the ware that has been paid for and placed in the kiln should be accounted for. The value of a record of this description increases as the years go by. Kiln losses are caused by a number of faults which are common to the trade—

1. OVERFIRED. The tint of the colours in this case will sometimes be spoiled hopelessly, and may even be crazed or blistered. 2. UNDERFIRED. Here the colours will be short of the necessary brilliance, and in the case of gold, although there may be the brilliance, it would be quite easy to rub it off with the finger. 3. SPIT-OUT. This type of fault is very difficult to control, the reason being that it is due to so many causes. It is often put down to dampness in the ware itself, this assumption being due to the fact that old white ware that has been decorated often develops this fault during the kiln firing. Personally, we have doubts as to this assumption being correct, but one thing is certain, it is always better to use ware for decorating that has not been lying about for any length of time. 4. SULPHURED WARE. This may be due to bad stopping of the muffle, or insufficient lime-washing, or even to some fault developing in the muffle during the firing which was unobserved when the kiln was being prepared. It may also be due to a faulty method of bating (stoking up). It is always necessary in the firing of pottery ovens and kilns to allow sufficient air space over the top of the fuel. Otherwise sulphurous fumes in excess are almost capable of penetrating through the most perfect type of muffle. Dirty ware, chipped ware, kiln dust, rubbed ware (i.e. colours rubbed off by careless handling), are all kiln faults that are due to carelessness, and should be closely checked by the manager of the department. Polishing. If a kiln happens to be at all overfired, it often results in stilts leaving marks on the surface of the ware which are very objectionable. These marks have to be removed by experienced workmen called polishers.'' The polishing is done on a lathe driven by machinery. The polisher is supplied with a series of grinding and polishing wheels, some of them made of carborundum having various grades of fineness, and also wood and cork wheels. It will thus be seen that an expert workman, with the aid of these tools, can remove all slight faults, and bring back the surface of the ware to its former condition.