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such as to resist the entrance of water. Their glaze, the composition of which will be hereinafter described, is much superior to any used in European potteries, but requires the most intense degree of heat for its fusion, and considerable art is consequently required for the
management of the fire, as well as in the construction of their ovens. These are built in a most substantial manner, so that when the fire is at its greatest height, the hand may be applied to the outside without any fear of burning. The draft is promoted by placing the oven at the extreme end of a long narrow passage, which acts as a funnel in supplying air for supporting combustion, the intensity of which is regulated by four or more side apertures, or registers, which are opened or shut according as the heat is required to be augmented or moderated. The hearth is placed in front, and occupies the whole breadth of the oven. The pieces which are placed in seggars, are artfully disposed in the oven, in
the manner already described as followed in England. When the fire is lighted, the furnace door is walled up, leaving only an aperture large enough for the introduction of fuel. The heat is raised gradually during about thirty hours, after which time fuel is incessantly supplied by two men, who relieve each other at intervals. The wood used for this purpose is very carefully dried and cut into slender billets about one foot in length, that their combustion may be effected with greater rapidity.
Great attention is necessary for properly conducting the operation of baking. Vauquelin observes that the heat must be sufficient to expel all the moisture, and occasion the cohesion of the parts whereof the paste is composed ; but that, if carried too far, the texture of the ware becomes too homogeneal, and it is rendered brittle. It requires a degree of heat sufficient to melt silver (4717°Fahrenheit) in order to expel the last portion of water from clay: when this has been effected, it is found that the weight of alumina is diminished forty-six per cent. · The process of baking usually lasts from forty-eight to fifty hours, during which time the heat is gradually increased; as it would be injurious to apply a very high degree at first. In order to ascertain when the baking has been carried far enough, the oven-man places trial pieces in different parts of the oven, but so disposed that they can readily be taken out for examination. These pieces are rings made of common Staffordshire fire clay, which is found to have the property of changing its colour with each accession of temperature. By comparing these rings, therefore, with pieces of the same clay which have previously been sufficiently baked, and which serve as a standard, the actual progress of the wares in the oven may at any time be ascertained precisely, and with less trouble than attends the use of Wedgwood's pyrometer. When the appearance of these trial pieces is judged satisfactory, the firing is discontinued, the furnace and ash-pit doors are closed, and the oven, with its contents, left to cool gradually during
twenty-four or thirty hours. It is not necessary to delay the withdrawing of the pieces from the oven, until they have become quite cold; but the sudden alteration of temperature would occasion them to crack, if they were taken out while their heat was greatly above that of the atmosphere.
Some potters are occasionally tempted, when the fur. nace contains articles of small value, to risk the damage here mentioned, and to withdraw the seggars with their contents without delay, their object being to profit by the heat of the furnace either for introducing a new charge, or for drying a fresh set of seggars. No one, however, would be so improvident as to expose the finer descriptions of porcelain to this hazard, in order to gain any such immaterial advantage.
From the similarity of its appearance to well-baked ship bread, the ware is now called biscuit. Its permeability to water when in this state fits it for being employed in cooling liquids. If previously soaked in water, the gradual evaporation from its surface by means of the air, causes an absorption of heat from the surrounding atmosphere, which is again supplied by neighbouring objects, until an equilibrium of temperature is restored.
The proprietors of potteries are accustomed to furnish vases, urns, and other pieces of ornamental shapes, in the state of biscuit, to ladies who exercise their taste and ingenuity in embellishing them by painting and gilding. Being then returned to the manufacturer, the glaze is applied, the baking is finished in the gloss oven, and the gilding is burnished by means that will be described hereafter.
If it were attempted to apply the glaze to articles of porcelain and earthenware, without their previous conversion into biscuit, their texture and shape would be injured by the absorption of water from the glaze. Neither would it, for the same reason, be possible to ornament the ware by painting, or to transfer patterns to their surface by printing. There is another reason
given for the necessity of this previous baking, in the greater contractibility of the clay than of the glaze, which would crack or peel off if the limit of contraction had not been previously attained. It will be remembered that the shrinking of clay upon the application of heat is permanent, and that no alteration of its bulk will occur, unless it be subjected to a still higher degree of temperature. By limiting, therefore, the heat of the gloss oven in which the baking is finished, below that applied to the biscuit, the evil of cracking the glaze, through the contraction of the ware, is avoided.
The glaze usually employed for common kinds of earthenware is compounded of litharge of lead and ground flints, in the proportion of ten parts by weight of the former to four parts of the latter. Cornish granite is sometimes substituted for flint, and used in the proportion of eight parts to ten of litharge. This method of glazing is objectionable on account of the injury which, notwithstanding every precaution that can be taken, it occasions, in its application, to the health of the workmen employed, who frequently are seized with paralysis; and because the lead, which is soluble by means of acids, and highly poisonous, renders vessels thus glazed improper for preparing or containing many articles of human food.
The bad effect of raw glazes upon their health, is greatly lessened to the workmen when they can be brought to the frequent use of ablutions. In every pottery the men employed in glazing should be, and in most establishments they are, plentifully supplied with soap, which they are enjoined to use on every occasion of quitting their work. Unfortunately, however, the workmen themselves have become erroneously impressed with a belief in the superior efficacy of ardent spirits in warding off or counteracting the poisonous effects of lead, and fly to the use of this as a specific, to a degree which too often proves, both physically and morally, worse than the evil which it is intended to prevent.
The mixtures just mentioned are called raw glazes; their employment is convenient to the potter because of their cheapness and extreme fusibility. Flint, which remains unaffected in the focus of the most powerful lens, is, when combined with 'lead, melted and vitrified at a comparatively low heat. The method of using this glaze is to reduce the ingredients to the state of a fine powder, and throw them into as much water as will make them of the consistence of thin cream. The mixture must be well stirred, that the powders may he always kept uniformly blended throughout the fuid. l'he pieces are first brushed to free them from dust, and then merely dipped into the liquid and withdrawn, when they must be turned rapidly about in all directions, that the glaze may flow equally over the whole surface. The superfluous liquid having been allowed to drain off for a few seconds, and the pieces having been set on a board during a few minutes, they are ready for insertion in the seggars.
Chaptal, in his “ Chemistry applied to the Arts," has given a process for forming white enamel, which answers well for glazing the superior kinds of earthenware and tender porcelain Equal parts of lead and tin are kept in fusion until completely oxidated. The powder thus formed is ground with water, all impurities are removed by repeated washings, and being dried it is kept for use. The whitest flints are then chosen, and fused with carbonate of potash, the latter being in such proportion to the flint, that the mixture will be soluble in water. To the solution of flint thus made, muriatic acid must, from time to time, be added, until no further precipitation occurs. The precipitate thus obtained is pure silex, which, being washed and dried, is also fit for use. If then one part of this silex, and one part of the metallic oxide, be added to two parts of carbonate of potash, and the whole be fused in a crucible, the mass need only be reduced to a fine powder to prepare it for use in glazing.
Mr. John Rose, of the porcelain works at Coalport in Shropshire, speaks in commendation of a glaze for hard