To use the bath, pour it into a dish to the depth of about half-an-inch, and have ready at hand another dish containing clean water, and a small dish containing some pieces of glass some what smaller than the films to be toned. Now take one of these pieces of glass, and slip it under the film containing a transparency to be toned, and gently raise the glass to the surface (at the same time manipulating the film with a camel-hair brush, held in the right hand) in such a manner that when the glass and film on it are lifted out of the water there will be an edge of film about a quarter of an inch wide lapping over one edge of the glass. The action of the water as the plate is taken out will wash this piece or edge of film round to the back of the plate, and by so doing will fix the transparency on the glass in a very satisfactory manner. If care be taken that the edge where the film laps over is kept uppermost or highest, a very considerable stream of water may be poured on the film without any danger of it slipping. Having got the film on the glass it should be rinsed under the tap, and can then be transferred to the toning bath. To do this, turn the glass over so that the body of the film is underneath, lower it gently - - under the surface of the solution, and with a brush disengage the lap of the • film where it had turned over the edge of the plate, now, of course, uppermost. As soon as this is done the film will move off into the solution free of the glass, which can then be removed. When the film has floated free for about a minute turn it over with a brush and note carefully if the deeper shadows are toned through, so as to give one uniform tint to the whole film. When this is effected it is removed with the piece of glass used in the same manner as before described and transferred to a dish of cold, clean water. Free it from the glass and gently agitate the water to remove the toning solution adhering to it As each film is toned it is placed in this dish of water, and when all are finished they are changed to another dish of cold water, taking each film up with the glass as before described.
The films are then ready for mounting on to the tablets. Have two dishes, one containing cold water, and another filled about half an inch deep with ammonia solution.
Ammonia .13130 6 drachms Water 12 ounces (Keep well corked.) Half an ounce of this mixture diluted with one pint of water makes the bath into which the films are to be plunged.
Place in the dish of clean water a clean glass, and on that an enamel tablet carefully washed. Take another glass, and with it remove one of the toned films from the dish in which it was washed, and plunge the same into the ammonia bath. As the film enters the solution the time is noted by the second hand of a watch, and after it has been in twenty seconds it is removed and plunged quickly into the water where the tablet is. Disengage the glass and slightly agitate the water in the dish to give the film a wash. Then take up with the left hand the piece of glass on which the tablet rests, and raise it about half way to the surface, then, manipulating with the brush, bring it into its proper position on the tablet. It is then removed
and placed slightly on edge to drain.
The tablets being curved, it is necessary to remark that a little care must be taken in the manipulation in order to make the films lie flat.
When almost dry the tablets are to be removed from the glass plates upon which they were lifted from the water. To do this, place the plate in a level position, and with a sharp pointed stick tear away the useless film around the edges of the tablet, slip a thin knife under the latter, and lift it off the glass on to a sheet of blotting paper, and cover over with a bell jar or similar contrivance to protect from dust.
The next process is the burning-in ; this may be done at once, or it may be left for weeks without any change.
A gas muffle furnace should be used in preference to one heated by coke. Whichever is used, however, it should be ready and at the full heat, a clear cherry red inclining to white, but by no means a full white heat. Too much heat is a mistake, as it renders the process quite unmanageable, and produces no better results for the extra difficulty in working.
Figures 329-330 illustrate the furnace used.* When enamels of small size only are attempted, the form here illustrated and described will do good work. Its construction is so simple that any good metal worker can construct it.
Fig. 529 is the front elevation, and Fig. 33o the side elevation. The muffle, M, is made of moderately thick sheet brass, permanently closed at one end, with a hinged or sliding door, provided with a peep-hole in front. The muffle slides into the furnace on two iron bars let into the walls. One of these bars is seen in Fig. 33o.
The furnace walls, A. B, C, D, Fig.
329, are of thick iron plate and doub led, the space between being filled with ashes or some other non-conductor of heat. The furnace is supported by four iron legs attached to the outer wall by screws, in order to be easily removed for convenience of transportation. A chimney, E, alsc detachable, fits into a hole in the middle of the furnace top. Heat is generated by a Berzehus or Liebig spirit lamp, having two or more wicks, which stands on an iron tripod.
The opening in the walls by which the muffle is introduced into the furnace must be of such a size and shape as to be completely closed when the muffle is in place. A shelf, N, of thick iron plate, is fastened by brackets just underneath this opening.
It is impossible to give any specific dimensions, as they are determined by the size of the muffle, which must be large enough to hold the largest piece of enamel likely to be fired.
The height of the muffle need not be more than three inches, nor its other dimensions more than seven by nine inches. A muffle four by five, by two-and-one-half inches, is, perhaps, as large as can be advantageously used in this simple form of furnace. The furnace proper should be about seven inches high, and the legs about the same. The chimney is a piece of iron pipe two inches in diameter, and about fifteen inches in height. The hot-air space surrounding the muffle may be one or two inches larger each way. The lamp must not be brought too near the bottom of the muffle, or it will not burn well.