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Page:Popular Science Monthly Volume 21.djvu/326

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THE POPULAR SCIENCE MONTHLY.

flexible; in this condition it is submitted to the enameling process. That operation is of the simplest character, and consists in a quick immersion in water holding in suspension a feldspathic rock, which has previously been reduced to an impalpable powder. Yet it requires great care, for the thickness of the enamel must be adapted to the piece; it may be neither too great nor too little, under penalty of accidents; it must be as even as possible, with neither bulges nor thin places. These qualities can not be obtained by dipping alone, so the object has to be retouched with a brush. The next step is baking in the sharp-fire, where the temperature of from 2,880° to 3,240°, at which feldspar fuses, must be reached. A few facts will enable us to comprehend the steps and the difficulties of this operation. Porcelain-clay can not be baked in direct contact with the flames, ashes, and smoke, without being greatly altered. It must, then, for the biscuit-baking as well as for the sharp-fire, be inclosed in protecting envelopes called gazettes, or casettes—cases of as refractory clay as can be got, in which the pieces are adjusted with great care on suitably arranged supports. It must be remembered that porcelain is baked at the temperature at which it becomes soft; the softening must then be anticipated at the time of fitting the vessel in the casette, and all the parts of the object must be supported so as to prevent any possible deformation. At the same time the supports must be prevented from sticking to the piece; and it is only by the aid of many kinds of artifices that the object can be effected without the supports leaving visible marks of their having been applied.

The furnace is divided into two stories, the upper one of which is the dome, or biscuit-baking compartment, and is warmed by the surplus heat that escapes from the lower story and passes through the vent-holes in its roof. The lower story, where the baking with the sharp-fire is done, is called the laboratory, and is heated by a number of fires placed along the circumference of the furnace, called alandiers. The casettes filled with articles to be baked are arranged vertically and as symmetrically as possible, and properly supported in the interior of the laboratory; when the furnace is filled, the entrance is closed by a double door of refractory materials, and the fires are kindled at the different alandiers. The temperature should be raised very slowly and very regularly, in order to avoid unequal dilatations, which would develop breaks in the objects. The heating is watched through little openings left in the walls of the furnace for that purpose, through which the color of the fire is observed. It may be seen to pass in succession from a dark red to orange, bright orange, and white. At the white heat, which is reached in from twenty-four to sixty hours, according to the kind of furnace and fire, the porcelain is near its baking point. Since no apparatus has yet been invented for ascertaining the precise temperature within the furnace, the condition of affairs inside has to be determined experimentally by means of trial-pieces, which are