MODERN CERAMIC TECHNIQUES By ceramics is meant all production of which the final result is baked clay in different grades of hardness and purity.
composition of European hard porcelain has remained about the same since the 28th century. The ingredients consist of the plastic material, kaolin, and the non-plastic quartz and felspar, which, in the process of working, is used as a diluting element, and in firing, as a cement. The kaolin is principally found in Germany, but always mixed with another soil. It has to be purified by washing. The quartz and felspar must be cleaned. The fragments of iron, in particular, have to be removed, as they are dangerous to the product. By mixing these ingredients the plastic material is obtained. This is made homogeneous by a mechanical method, and it frequently lies unmoved for some months in a special cellar in order to be moulded once again. Then the shap ing can begin. It is not usual to shape this fine material by hand, but moulds are used which have been made of plaster after the models fashioned out by artists. The plastic material is pressed into these moulds or poured out in them as a pulp. In the latter case the plaster absorbs so much liquid that a sheath is formed alongside the inside of the mould, out of which the superfluous pulp can be poured. Shaping is followed by drying. After that there is the controlling and the removing of the casting seam and other unevenness. If the object consists of more than one part these are joined together. They are then put in the kiln in sagars or clay boxes so that the heat is more evenly distributed over the piece, and heated to about 95o degrees. The porcelain has then become hard and water-tight but dull. This is therefore followed by glazing. The objects are dipped into a liquid con sisting of the same ingredients as the material, but in which felspar and quartz predominate. The objects which have been glazed in this way must then be fired in a heat of again in sagars. This heating lasts from 20 to 30 hours, and is followed by a gradual cooling which takes three days. A full kiln never produces everything perfect, the many dangers in the firing— f or example all objects shrink about one-sixth of their volume— result in articles being spoiled by exploding, warping, etc.
The porcelain is then ready to be painted with dyes, with which a very lightly fusible glass-powder has been mixed. In order to make these melt together with the glazing and become durable the painted objects are heated in an enamelling-furnace to a heat of This heat must be reached quickly and the cooling takes place equally rapidly in order to keep the colours bright. The old porcelain bakers painted on the so-called biscuit-ware, i.e., after the first baking and before the glazing, but they could only make use of very few colours at this stage—really only cobalt blue, as most other colours could not endure the heat of the second firing. Modern ceramic technique, however, has consider ably extended this process of painting before the glazing. Since Copenhagen's success with it, it has been more generally used and imitated. In these cases the paint is put on the material in a dif ferent way, viz., as a liquid with a brush or squirt and then en graved in order to increase the plastic impression of depth. After that the objects are glazed. In a high temperature the colours un der the glazing mix with the material, except in the case of cobalt blue, which mixes with the glaze. This process has the technical advantage of making the colours durable, but on the other hand it has a disadvantage in the artistic sense, in that the covering colour reduces the lustre of the porcelain itself by breaking the rays of light which cause the minute crystals in the porcelain to glitter. This is not the case with painting after glazing. Another way of ornamenting, which, however, has the same artistic dis advantage, is to coat with coloured glaze, which is applied in one colour. By adding metals the glazing can be made what is known as "streaming"; or, by choosing a glazing compound, the coefficient of expansion of which is smaller than that of the material, irregu lar fissures or artificial crackles can be obtained with the cooling.