Furthermore, in the preparation of a fine porcelain it is necessary to grind the whole mix upon a mill. The mill used for glaze grinding will answer every purpose and care must be taken that the grinding, while carried far enough, be not too long continued. A certain amount of fine grit in the body mass is necessary but only by constant practice can the right point be reached. In making these experiments each step should be faithfully noted in a handy book. The amount of water to a given weight of clay and the duration of the grinding should be accurately observed and written down. It is most unwise to trust to memory.
The process of casting may be used for porcelain as already described, but the very best of workmanship is necessary. The hard fire to which the porcelain is subjected reveals every error which has occurred in the making. The same thing applies to wheel work. Not only is great skill required in order to shape the tender porcelain clay on the wheel but the very essence of the porcelain is its lightness, to produce which by craftsmanship a long and arduous course of training must be endured.
Stoneware is free from many of these difficulties and, consequently one who attempts the conquest of high-temperature wares is advised to begin with this. Stoneware clay need not be a mixture. There are many clays which can be used for the manufacture of grès with no more preparation than that laid down for common clays.[R] It sometimes happens that a clay will need the addition of a small quantity of flint or spar but this does not amount to a difficulty.
Stoneware does not present the same manufacturing difficulties as are found in porcelain. The clay is quite plastic and can be easily shaped on the wheel; casting is scarcely a suitable process for this ware. The essence of stoneware is strength and virility, just as that of porcelain is lightness and grace. Each ware has forms suited to itself and it is a mistake to depart from these essential characteristics.
After shaping and drying the technical manipulation of both wares proceeds along the usual lines. The first fire is at a very low temperature. The melting point of silver (cone 010) is enough in nearly every case. This leaves the ware in a soft and porous condition but hard enough to resist the action of water. The process of glazing has already been described but the composition of the proper glazes differs from that of low temperature glazes.
Porcelain is always burned in a reducing fire; stoneware may be burned either reducing or oxidizing. The temperature at which the glaze is burned is very high, it must be, in fact, the maturing point of the body itself.
The simplest form of porcelain glaze is that represented by the formula—
| K2O | .3 | } | ||
| CaO | .7 | } | Al2O3 .5 | SiO2 4.0 |
Which is carried out in the following mixture:
| Feldspar | 167 |
| Whiting | 70 |
| Kaolin | 52 |
| Flint | 108 |
| —— |