3 i 4 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 immer- 

 sion in water holding in suspension a feldspathic rock, which has pre- 

 viously 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 compre- 

 hend 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 sur- 

 plus 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 dif- 

 ferent 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 suc- 

 cession from a dark red to orange, bright orange, and white. At the 

 white heat, which is reached in from twenty-four to sixty hours, ac- 

 cording 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 



