The burning is the most delicate operation, and every care is necessary to secure the result at the very best point.
Take one of the tablets and place it upon a piece of fireclay in front of the muffle at a distance of about six or eight inches. The fireclay should be supported in such a way as to tip the enamel towards the heat, so that the rays may fall upon it as near as possible equally all over. When it is roasted in this manner it is moved a little nearer, and then a little nearer, examining it each time. As soon as the heat has turned the color of the film brown in the least degree it may be dealt with fearlessly. The •fireclay with the enamel on it may be placed level, just in the mouth of the muffle, where, in a few moments, the film will take all the shades of brown until it gets quite dark, when it is pushed into the heat. As soon as it is pushed in the heat it must be watched carefully. It will appear to get perfectly black all over, and then, all of a sudden, the whites of the picture will be seen coming out quite clear. Directly this takes place draw the tablet to the mouth of the muffle and remove it to the outside to cool a little gradually, and then take the tablet and place on a piece of wood to get cold. All the beauty of the enamel will by this time have disappeared, the whites will stand out, and the few tones near them will have some clearness, but all the other tones will be a dark and confused mass, hardly distinguishable one from the other. This is the true characteristic of a good enamel at this stage. It is now ready to glaze.
The enamel glaze is made in large quantities at Worcester, and can be bought in the form of a fine powder resembling flour. Place about a thimbleful of this in a bottle, and add an ounce of alcohol. To make the glazing solution, take a two-ounce bottle, and put into it half-an ounce of plain collodion, and add to it a quarter of an ounce of methylated ether and half an ounce of alcohol. Next add as much water as possible without precipitating the pyroxyline.
About six or eight drops will suffice. Shake up the bottle containing the glaze in alcohol, and let it remain for a few minutes, and then carefully add some of the upper part of the mixture to the diluted collodion—sufficient to make it slightly opaque.
When the enamel is quite cold some of this glaze mixture is poured over it, and it is im mediately tilted to allow the superfluous glaze to run off. When the collodion is set, place the tablet in a muffle on a piece of fireclay, and gradually introduce it to the full heat, keeping care ful watch that the burning does not proceed too far. The glaze should only just melt, when the enamel should be pulled out again, and placed on the wood block to get cold once more. The image is now fixed indelibly, but the picture is not, however, at its full beauty, as one glazing will not be sufficient ; thedark portions would still retain a matt appear ance. This is overcome by repeated glazing, no enamel being perfect until it has been glazed five or six times. A little trouble might be saved by lessening the number of bumings and making the glaze thicker, but the best results are not obtained in this manner.
By this process we obtained an image consisting of pure metallic silver, gold, and iridium in an extremely fine state of division fixed to the surface of an imperishable tablet, and protected by an equally hard, permanent and impermeable glaze.
Ceramic colors may be applied to enamels, and burnt in in the same manner.
Enamel may be retouched with a medium prepared by burning waste films and trimmings in the muffle, collecting the ash, and adding to it a small quantity of the glaze powder and a drop of an essential oil. The work being burnt into the enamel takes the same color and surface as the rest of the picture. With enamels that have been spoiled, the image can be removed with a little fluoric acid, and the tablet used again.