The Potter's Craft: A Practical Guide for the Studio and Workshop

by Charles Fergus Binns

Let us suppose that the work is intended to be carried out at a temperature of Cone No. 01. The numbers run both ways from this. The higher or less fusible cones are, 1, 2, 3, 4, etc., up to 36, and the more fusible numbers are 02, 03, etc., down to 022. If the firing is to be to Cone 01, numbers 02, 01 and 1 are selected and set upright in a small strip of soft clay. Eight or ten of these groups of three cones are to be prepared for the first firing, so as to test the kiln, one group is placed in each corner, at the bottom, and another in each corner on a shelf, which is arranged opposite the spy-hole in the door. In the middle of this, where it can be well seen through the hole, one of the groups of cones is placed. They must be set so that all three cones are visible as the kiln is being fired.

The kiln is now filled up on both levels with pieces of pottery. To burn an empty kiln is not a reliable test. On the first occasion the fire should be started in the morning because no one can tell just how long the burn will take. When this time is ascertained it is best to start the fire so that the kiln will be finished by early evening. The cooling then takes place at night and there is no temptation to open the door too soon.

The fire is started slowly and the flow of oil is gradually increased as the muffle begins to glow. The work here needs practice, nerve and judgment. A good deal of smoke will be seen at the chimney at first but this should disappear as the kiln grows red. If the fire be urged too strongly at the beginning fuel will be consumed to no purpose, the only result being the choking of the flues with carbon. As the red becomes visible through the spy-hole, more oil may be supplied, but notice must be taken that the smoke at the chimney does not increase. The ideal firing is where there is no smoke but this cannot be reached until the kiln is hot enough to cause the smoke to burn.

Persons who have burned kilns for overglaze work will find the method of burning pottery very different. Instead of a fire brought as rapidly as possible to the finishing point, there must now be a slow soaking burn in which the heat shall have time to saturate the ware.

The cones in front of the spy-hole must be observed from time to time and presently as the kiln reaches a bright cherry red, number 02 will begin to bend at the tip and will gradually arch over until the point touches the shelf upon which the cones stand. By this time number 01 will have begun to bend and when the point of this has touched the shelf, the firing is over and the oil is shut off.

It requires some resolution to leave a kiln until morning but it is conducive to early rising anyway. The kiln need not be quite cold but it will help the kiln itself to wear better and the pottery will be better if nothing is done until everything can be handled without gloves.

The cones are now taken out and a diagram is made of each level with the bend of each cone accurately drawn. This diagram should be mounted and hung on the wall for reference. It is not well to trust to memory. It will probably be found, in the type of kiln we are discussing, that the cones on the bottom have bent further than those on the shelf. That is, the bottom is somewhat the hotter.

The variation in the kiln is not necessarily a disadvantage. It may be utilized in burning wares of different kinds. For example, if the bottom prove much the hotter, the biscuit ware may be placed below and the glazed pieces on the shelf. In such case the shelf itself should be washed with a good coating of clay and flint in order to protect it from casual drops of glaze.

If a number of small pieces are being made, more than one shelf should be set up. The legs may be just a little taller than the tallest of the small pieces, but the art of placing or filling a kiln economically consists in making selection of pieces which fit well together both as regards height and shape. Thus, pieces which are large at the base may be dovetailed in with others of which the base is smaller than the upper part. In the case of clay ware the pieces may be set close together or even piled one upon another. There is no danger of sticking unless the ware is burned to complete vitrification. The glazed pieces must not, of course, touch each other.