A clay of a sticky nature - that is, one that does not easily polish - will be easier to make a slip or enamel adhere to than one of a fatty or easily polished nature. Such a clay is apt to contain or possess more of the requisite physical characteristics. It will possess plasticity, even when much grit is added; it will usually be a hard dryer and less subject to lamination; it may be a bad warper and checker in drying, may drag more in press, but these difficulties are all overcome by the addition of grit Therefore, as a rule, a light shrinking, sticky clay, of the proper chemical composition, mixed with grit will be found to more nearly comply with all the requirements than a fatty or greasy polishing clay.
All the physical characteristics must be the same, but the chemical composition may or must be somewhat different Slightly more alumina and fluxes are allowable, or more silica, with much more flux. There is a wider range in chemical composition allowable at a low than at a high heat At the extreme of the low heat range it is dangerous to place the bricks without extraneous support. The low heat clays are not apt to stand any excess of fire and support much weight at the same time.
Here we must have all the qualities of the open fire process up to the time that bricks are ready for the kiln. The burning qualities may be the same or may be very different A much wider range of clays may be used when bricks are supported than when they are not. The clay may even be a vitrifying clay and yet burn safely. The great point to watch under this process is that the composition of the clay shall be such that the finished bricks will neither craze nor shiver. This requires a clay whose alumina-silica ratio shall be about 1 to 3 1/2, a little more silica being allowable with much flux than with little flux. The amount of lime may vary much, depending upon whether a low or high heat is used, and whether the finished brick is to be porous or vitrified. This is also true of the alkaline fluxes.
Any clay that makes a smooth, straight, true brick; that will not craze or shiver its glaze, and that does not possess any of the few bad qualities that will be mentioned later, may be used for these processes. If the business is strictly a glazed business, the clay must possess such physical characteristics as will produce a large percentage of perfect bricks under the conditions existent If the business is glazed brick and common building brick, so that perfect bricks can be selected for glazing, this does not matter so much. The matter of lamination enters very slightly, if at all, into these processes.
Iron spots developing at heat used. Lime in lumps will produce flaws and breaks in face. Volatile matter in clay that stains enamel is fatal to the single fire process. Coal in clay is harmful, but not fatal.
In goods that are made by hand and not repressed many of the physical qualities are of less importance, and some are of greater importance. The tendency to laminate is usually ob-viated by pugging the clay in a very soft condition, and the handworking further eliminates it The properties that cause cracking and down corners in repressing have not to be considered, but the straight drying, and burning, and checking qualities are of greater importance in large, heavy pieces and in thin pieces. The selection, or rather picking and assorting, of clay for large pieces should be done with great care, as a little bad material will spoil a large piece of great value as easily as a brick of small value.
For sinks, baths and goods that undergo close examination, and that must, therefore, be perfect, more careful selection is necessary than for goods that are not submitted to close inspection.
So far as is known, this process has never been successful except with two fires. It requires a property in clay that it does not seem to possess, or, if it does, it is a very rare quality - that is, the capability of being wet after having been strongly pressed in the semi-dry condition, without cracking in drying. If machinery can be gotten up that will handle the clay in a quite wet condition, what might be called semi-plastic, this process might be made the most successful one. This process eliminates all waviness of face, and leads to the production of bricks with a beautiful surface. The experience with this process is so slight that it is impossible to say much about clay selection, except that it may contain more alumina without crazing, and must contain less silica to avoid shivering, than for the plastic processes.