oxidizing fire, which is the most economical way of firing, since by it all the gases are completely burnt inside the oven without any waste of fuel. By a better application of this principle, Messrs. Minton have introduced a new oven, in which the fuel is so completely utilized, that it requires only one half of the usual quantity of coals, besides doing away with the dense smoke, which is the annoyance of the district.
By the first fire to which it is exposed, the ware is converted into what is termed, from the French, biscuit– an incorrect name, as it seems to imply that it has already been fired twice, when, in fact, it has been only fired once. Some classes of pottery do not require more than a single firing, as, for instance, the common terra cotta and stoneware. However, for all our English ware it is necessary to have two fires, for the following reasons: First, the necessity for getting a denser texture of the ware by submitting it to a strong heat, lest the glazes which are to be melted on their surface, and which thereby become very dense and most contractible, should not agree with the more open texture of the body, and should crack or craze when exposed to changes of temperature. Secondly, that for coating the ware with the glaze, it is necessary to dip the article in the vitreous mixture finely ground, and kept in suspension in water; consequently, if it were in the raw state when this was done, the adhesion of the particles would be so small, that they would readily dissolve in the liquid. It is customary, therefore, to expose the goods first to a hard fire, which, according to the size of the ovens and the quality of the ware, may last from forty to fifty hours.
From the biscuit oven, the goods, if they are to be left white, may be sent to be glazed; but if they are to be decorated with a printed pattern, they must be forwarded to the printing department. Printing on pottery is comparatively a modern invention, its chief advantage being the cheap rate of production. Up to the last century, the goods were always painted by hand: a slow, but it must be confessed, a more artistic process, as the work executed in this way, even of an inferior kind, will exhibit a freedom of touch and facility of execution, which will make it attractive and preferable to the formality of a printed pattern, however rich or complicated it may be. This superiority is sufficiently illustrated by comparing monochrome patterns of Italian majolica, Delft, and Chinese, with the modern printed ware of the same colour.
Public taste has so wonderfully improved lately, that, for my part, I have no doubt that we shall soon have a special class of artists trained to execute, by hand, cheap and simple decorations for those purchasers who are not satisfied with printed decoration.
To what extent the introduction of printing on pottery has hindered the progress of art education in Staffordshire, is a question on which people may entertain different opinions; but we might ask, what amount of artistic work we might not do, if at the present time we had some hundreds of artisans trained from their early years to that style of painting? However that may be, the process of transferring printed patterns to biscuit ware was considered a great step, and one which contributed largely to the extension of the earthenware trade.
Liverpool and Worcester claim the priority for this invention, towards the year 1752. It is a fact that shortly after that date, Staffordshire potters used to send their wares to Messrs. Sadler and Guy-Green, of Liverpool, to be printed; and there is also every reason to believe that about the same time it was introduced at the Worcester works, then under the management of Dr. Wall, by an engraver named Hancock.
The process of printing on pottery does not differ very materially from that used for transferring to paper a design from an ordinary copper-plate. There are, however, these differences, that a metallic colour is used instead of lampblack, and that a fine tissue paper is specially made for that purpose. When that paper, with the pattern printed upon it, is laid on the ware, face downwards, the colours adhere strongly to the biscuit, which, being porous and aluminous, has a great affinity for the oil with which they have been mixed. After rubbing the back of the print with a roll of flannel, to secure the adhesion of every portion of the pattern, the biscuit piece is plunged in water, and the paper comes off quite freely, the whole of the colour sticking fast to the ware.
Previous to glazing, the printed ware must be brought to a red heat, for the sole object of burning the oil mixed with the colour. This is done in kilns, called hardening-on kilns.
The colours in use for printing under the glaze are not many; as few only of the preparations made with metallic oxides can, when brought to a red heat, stand the action of the glazes under which they are laid. Most of them in this case will be dissolved and considerably weakened, if they do not even completely disappear. Cobalt, and the preparations made from chromates, are the most resisting, and, when well prepared, the glaze in melting over them will bring out the colour with increased beauty.
The necessity for covering the biscuit with glaze to stop the absorption of liquids or greasy substances, which would find their way into its interior and would stain it, is so obvious, that I do not think it necessary to dwell on the importance of this operation. I have stated already that it was used by the Egyptians and Assyrians, who knew most of the saline mixtures by which white and coloured glazes could be obtained; but these, which for the greatest part were alkaline silicates, could not have resisted the action of time as they have done, if a certain amount of silicate of lead had not made them permanent. They found this material in the sulphide of lead, which by the silica it contains, or that which it meets on the body of the ware, gives a glaze, which stands exposure to damp better than any other. That this mineral was used in remote antiquity, proofs are numerous. I recollect, amongst others, some small shalti, or sepulchral figures, made in Egypt more than two thousand years ago, of which the red parts, such as the faces and hands, have been glazed in this way. My opinion is, that it was used by the Greeks, in connection with the black oxide of iron, to produce the black colour used in the decoration of their vases, and it might some day prove that it was an indispensable material in the preparation of the red smear, which is the characteristic feature of the Samian ware. At all events it is with this single material, stained with metallic oxides, that the Arabs glazed their rich-looking pottery, and the same was used afterwards for our encaustic tiles and our common pottery, from the time of Elizabeth down to the middle of the last century. Lately, however, the science of making glazes has considerably improved, and a variety of new substances have been introduced. To prepare a glaze is one of the most delicate operations possible, and failures are attended with most serious consequences. The conditions to be fulfilled are many. It must not be too fusible nor too hard, either of which conditions would make it dull or apt to craze; and it must be transparent, otherwise the colours underneath would not be clear. It may happen that a glaze which apparently seems good when it comes out from the oven, will craze when a few months, or perhaps years, have elapsed. Generally, the less alumina that there is in the biscuit, the easier is the adaptation of the glaze, and this accounts for the soft porcelains being easier to manage in this respect than ordinary earthenwares.
The materials used for the foundation of glazes are in principle the same as those for the body, viz. silica, in the form of flint, or sand and felspar, pure or mixed with other components in the granitic rocks, called Cornish stone. These are the hard materials to be vitrified by the fluxes, which are carbonate or oxide of lead, boracic acid or borax, potash or soda, carbonate of lime or barytes. There is no definite receipt for mixing, and they may be combined in a variety of ways. Every manufacturer has receipts of his own, and I must say that some make their glazes a great deal better than others. They are rather expensive, chiefly owing to the increased price of borax, a material of comparatively modern use, which, being apt to promote the brilliancy of the wares and the beauty of the various colours, is now extensively used. When the components of the glazes are not soluble in water, it may be sufficient to have them finely ground in water. But if any soluble salt, such as borax, nitre, or soda, is employed, it is necessary to render them insoluble, by vitrifying them together with other substances. This may be effected in crucibles, or, still better, in reverberatory furnaces, where a large quantity may be melted more conveniently. In this case, when the mass is well liquefied by the intensity of the heat, it is run into cold water, which, cooling it suddenly, causes it to break into small fragments. This is called a fritt; and when it is sent to the mill, any other insoluble material may be added to it
