The object in this chapter will be to teach the reader how to slip and glaze or to enamel clay. This cannot be done by giving a lot of receipts and stating they are the best or worst in the country. The only course open to him would be to search the country for a clay that would fit the receipts. Were he lucky he might find it the first time, and were he unlucky he might not run across it at the one-thousandth experiment What must be done is to take the clay as we have it or as we can easily get it, and so mix it as to make it fulfill the conditions described in the selection of clay, as nearly as possible, and then adapt a slip and glaze, or a slip and enamel, or an enamel to it In the manufacture or mixing of slips, glazes and enamels we are not limited by distance or the amount of transportation, as they form such a small proportion of the total weight of the output so we can take our supply of materials from the whole country, or even import them, without running up expense materially.
A description of the different materials and their functions will probably be first in order.
This is usually a very tough, plastic clay, and a heavy shrinker in drying and burning. There are many different qualities, and they all vary as to the amount of shrinkage, color and hardness to which they burn, but, as a rule, they are cream-colored or slightly grayish when burned, burn hard and close, and shrink about one and one-half to two inches to the foot If the body complies with the condition that it should shrink one inch to the foot, it is easily seen that a pure ball day slip would be apt to crack on it The color of such a slip would be poor, but it should stick like wax to the body. There are some ball clays that do not stick well. The use of such a clay should be avoided. No. 4 English ball, from George Knowles & Son, Trenton, N. J., is a good ball clay.
This, as found in the trade, is a washed clay, usually a so-called koalin that has been mixed into thin slip and floated in such a way as to get rid of all sand, mica and coarse impurities. As only fine qualities of clay are worthy of the expense of washing, china clays are usually of good white color, but vary very much as to plasticity, some of them being very plastic and some of them being extremely short or lean. They burn to all degrees of hardness, from so soft as to easily scratch with a knife to vitreous. The shorter or leaner the china clay, usually the whiter it will burn. These clays are prepared principally for potters' use, and the plastic qualities are esteemed highly by the potters, so as the clay decreases in plasticity it must make up for it by increased whiteness in order to retain its value in their eyes.
Nothing very definite can be said about china clays on account of this wide variation in them. What would be true of one would be decidedly untrue of another. The china clays most used by the writer have been the National China Clay, obtained from the National Kaolin Company, Brandywine Summit, Pa., and English China Clay, No. 7, obtained from George Knowles & Son, Trenton, N. J. These two clays are opposites, the first being a plastic, high shrinking, vitrifying, cream-colored or cream-white clay, the second being a short, medium shrinking, soft burning, but extremely brilliant white clay. Were the typical brick body dipped in a slip of National Clay the slip would crack, probably, but would stick well and would glaze a creamy white. Were it dipped Into a slip of the No. 7 China, the slip would shell off in drying, or, even did it stick, it would be too soft to be of any use.
In this country ground quartz or quartz sand; in England usually the so-called flints, obtained in the chalk cliffs. Flint, when pure, should burn to a brilliant white color. It expands in burning, and can only be melted or vitrified by the heat of the oxy-hydrogen blow pipe. Its use in slips is for its whitening properties and to decrease shrinkage. In glazes this material furnishes the silica to form the compound silicates forming the glaze. Flint should be finely ground so as to pass through a No. 150 brass wire sieve.
Cornish stone and Cornwall stone are two English materials never used by the writer, and the properties of which are unknown to him. Whenever a receipt containing Cornish stone is to be tried he substitutes four parts felspar, one part clay, one part flint
This is orthoclase or potash spar, a double silicate of alumina and potash. Albite or soda spar is little met with. The feldspar of commerce varies somewhat in chemical composition, consequently in fluxing properties, but as a rule it melts at a bright yellow heat, sometimes into an almost transparent glass, at others into a white semi-enamel. It is esteemed most the more fusible it is, and the better the color of the fused samples. It should be perfectly free from dark specks. An analysis of the Brandywine Summit felspar shows: Silica, 65.21; alumina, 18.13; potash, 16.66.
This is principally carbonate of lime, with some little magnesia and other impurities. It should burn white. At high heats it combines with silica or with silica and alumina, and acts as a flux. At very low heats this combination does not take place unless aided by some low heat flux, as potash, soda, lead, etc., and even then, if much lime is present, the combination of all the lime is difficult to effect. The province of whiting, as well as other forms of lime, in a glaze is to toughen the glaze. It is an additional base, and makes a more complex silicate. The complex silicates are usually more durable than the simple silicates. Lime also prevents the coloring effect of iron to some extent
Anhydrous sulphate of lime, becoming a hydrated sulphate when mixed with water. Supposed to prevent cracking of slips when put on to dry clays, and is a whitening material when pure, probably through the lime action upon the iron of the clay.