In the manufacture of enameled or glazed clay material everything depends upon the correct selection of the clay or clays that will be suitable to the process or method employed. The principal methods of manufacture at our disposal are two in number, single fire and double fire, but these are capable of subdivision, the single fire process into high heat and low heat, also into open burning, or burning where the material must bold its own weight and the weight above it, and burning in saggers, or where the material is supported by other and special material designed for that purpose. The double fire process can also be subdivided in the same way. This makes, in all, eight subdivisions - single fire high heat, open; single fire high heat, in saggers; single fire low heat, open; single fire low heat, in saggers, making four, and the same subdivisions of the double fire process making the other four. The selection of clay will be in some respects different for each of these subdivisions, or it probably would be more correct to say the use of one or the other of these eight plans would be indicated by the nature of the clay that is to be used. All English writers say that only refractory or high heat clays must be used for glazed work. With this opinion the writer decidedly disagrees. So long as the heat is high enough to enable one to make an acid-proof and weather-proof glaze, and the body under the glaze is burned to the proper condition, just as good material can be made as at a higher heat This can be accomplished considerably below the English high heat standard. The high heat used in England is, as nearly as can be ascertained, about the heat at which Albany slip clay begins to flow, or run thin on the sharp edges of trial pieces, and to lose its beautiful brownish black color and take on a partially transparent appearance and grayish hue. Good glazed material can be made from this heat down to the point where Albany slip clay begins to fuse and becomes a brown semi-glaze. If the clay is suited to it, the single fire, high heat, open burning method is undoubtedly the most satisfactory, but such clays seem to be difficult to obtain in the United States. The values of the other methods will probably appear as we advance with the subject. The question in this country is not so much What clay can I obtain to make glazed goods? as Can I glaze the clay I have? The design in this chapter shall be to treat the subject in such a way as to enable the reader to determine the proper answer to either of these questions.
- High Heat, Single Fire, Open Burning Method for Brick
Where the Material Carries Its Own Weight and the Weight of That Above It This method of manufacture requires for its perfect success a clay or mixture of clays, or clay and grit, that shall possess certain qualities. The clay must not only stand the high heat to which it is exposed, but it must stand this heat and, at the same time, stand a weight of about fifteen to twenty pounds to the square inch. This weight to be carried by the lower bricks would not be so great could they be set directly on each other, but as they have to be separated by rolls of clay, the weight acts upon a small surface. The clay should not only possess this quality, but should have a fairly large margin of safety. In order that crazing or shivering should not result, the proportion of silica to alumina should be about three and one-half to one. In order that the clay should stand the weight, it should not approach closely to the point of vitrifaction, but should derive its bond largely from the pure clay or silicate of alumina it contains; it should, therefore, contain little iron and fluxes. A fairly representative analysis of a clay suitable for this method of manufacture would be about as follows, and the more nearly the clay, or a mixture of clays, can be made to conform thereto, the more certain is success, other qualities being right: Alumina, 20 per cent; silica, 70 per cent; water, 6 to 7 per cent; fluxes and iron, 3 to 4 per cent
Such a clay will, as said before, derive its strength of bond from the silicate of alumina largely, the fluxes not being sufficient to take up anything like all of the free silica, unless the heat be beyond anything obtained in kilns. The pure clay (silicate of alumina) will begin to harden directly the combined water is driven off, and will harden slowly and evenly up to well beyond the heat contemplated, consequently such a clay is slightly affected by slight differences in heat and the bricks are likely to come from kiln nearly uniform in hardness and size under the variations that may occur in the fire. A statement of facts may strengthen this position. Analyses of five different makes of English brick showed that the original clay could not have varied 2 per cent from the above figures in the silica present in the most extreme case, and not more than 1 per cent in the alumina present, and the average was almost identical with the figures above. A mixture the writer was using was analyzed at the same time, and agreed almost exactly with the figures above. This mixture had been made without any previous knowledge of the chemical constituents of the English bricks, but had simply been the natural result of continued effort to secure a perfect enameled brick.
The physical characteristics of the proper clay are more difficult to describe and still more difficult to obtain in a single clay.
The clay should possess plasticity or toughness enough to enable the bricks to stand the large amount of handling they receive without showing it to too great an extent It should be as little subject to laminating in pug mill or brick machine as possible. It is better that it be a strong, hard drying clay that does not abrade or chip easily. A shrinkage of not over one inch to the foot is better than a greater shrinkage, as it is easier to adapt a good white coating to a light shrinking brick. If a high shrinking clay is brought down to proper limits by the addition of grit or grog it is apt to lead to a wavy-faced brick, unless the grit is ground extremely fine. Very fine grinding is expensive. Clay must be homogeneous throughout, or capable of being made so by thorough pugging, else unequal shrinkage will produce wavy face.
It is of the utmost importance that the clay, or a mixture of clay and grit, should possess at some point of stiffness, dryness, temper, temperature, or whatever it may be called, three combined qualities: It must not drag in pressing, so as to come from the repress with top corners depressed, making a crooked, untrue brick; it must not crack in pressing, and it must dry perfectly straight from that point until it is absolutely dry, without checking and without too much precaution being necessary. If the bricks are to be dipped while soft, this point must be while blanks are soft, otherwise it may be when blanks are stiff. Many clays possess one of these qualities, or, by selecting the proper point of stiffness, will possess two of them, but the three combined at any one point are difficult to obtain. The softer the blank the less apt to crack in pressing, but the more apt to come out of press with down corners, and the less apt to dry straight.This point will be fully dwelt on under repressing.