CHAPTER XVIII: HIGH-TEMPERATURE WARES

The subject of "Grand Feu Ceramics" has been so ably developed by M. Taxile Doat in his admirable treatise[P] that it will be unnecessary to go deeply into the matter, but in order that the reader may be aware of what is involved, some description of the technique will be given.

[P] Keramic Studio Publishing Co., Syracuse, N. Y.

Hard-fired wares are divided into two classes, porcelain and stoneware. The latter is called by the French, "Grès," an abbreviation of the name "Grès de Flandres," the stoneware made in the low countries in the sixteenth century. Both these wares are, technically, once fired, that is, the body and glaze come to maturity at one and the same burning. The biscuit ware is often given a low burn at first in order to facilitate handling, but this leaves the body very porous and is in no sense a maturing fire. The glaze is laid upon this porous ware, or upon the unburned clay if preferred, and then comes the high fire or "Grand Feu" of the French.

A mix for a porcelain body has already been given but if the ceramist means seriously to attack the porcelain problem he will have to do some experimental work for himself. The Georgia kaolin mentioned in the recipe on page 40 is a good, plastic clay but it is slightly off color. It may be necessary to improve the color by the use, in part, of another kaolin such as the Harris clay from North Carolina.[Q]

[Q] The Harris Kaolin Company, Dillsboro, N. C.

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.

[R] Stoneware clays may be procured from The Western Stoneware Company, Monmouth, Ill.; H. C. Perrine and Sons, South Amboy, N. J.

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--

K_{2}O .3 } CaO .7 } Al_{2}O_{3} .5 SiO_{2} 4.0

Which is carried out in the following mixture:

Feldspar 167 Whiting 70 Kaolin 52 Flint 108 ----

The glaze is ground for use.

The same glaze will also serve for stoneware but it will burn to a brilliant surface whereas stoneware is better when finished with a matt texture.

The following is a stoneware matt glaze:

K_{2}O .3 } CaO .7 } Al_{2}O_{3} .7 SiO_{2} 2.6

Of which formula the mixture is--

Feldspar 167 Whiting 70 Calcined Kaolin 66 Raw Kaolin 26 ----

The porcelain glaze is at its best when uncolored. The matt glaze will be more interesting when used as a colored coating.

The following are a few suggestions for colored matt stoneware glazes. To the glaze batch, 329 parts, add:

For blue: Cobalt Oxide 2 parts Nickel Oxide 1 part Ground Rutile 10 parts

For brown: Iron Oxide 6 parts Nickel Oxide 3 parts Ground Rutile 10 parts

For green: Chrome Oxide 2 parts Cobalt Oxide 1 part Iron Oxide 4 parts

For dark red: Iron Oxide 10 parts Chrome Oxide 2 parts Zinc Oxide 6 parts

Rutile has not before been mentioned. It is a crude oxide of titanium and is exceedingly useful in high temperature work for producing odd, mossy and crystalline effects.

These mixtures make no pretense to be complete, they are given as suggestions only because if the artist-potter is to be successful he must be prepared to compound glazes which are the expression of his own individuality.

For burning high-temperature wares the kilns already described may be used but upon purchasing it should be stipulated that the kiln is to stand burning up to cone 11 or 12. Successful porcelain can be made at cone 10 but better results are secured at cone 12, though, of course, the wear upon the kiln is proportionately greater. Stoneware requires a burn of about cone 9, higher or lower according to the clay used but fine results must not be expected below cone 7 nor is it necessary to go higher than cone 10.