A gas muffle furnace is preferable for burning the enamels in, to one heated by coke; whichever is used, 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 unmanageable, and produces no good result to make up for the extra difficulty of work.

The burning is a most delicate operation, and all the care and attention of the artist are required to secure the result of its very best point; still, with care, 9 out of 10 enamels can be burnt to a successful issue.

Take up one of the tablets and place it on a piece of fire - clay in front of the muffle, but not too near, say at a distance of about 6 or 8 in. The fire - clay should be supported in such a manner as to tip the enamel towards the heat, so that the rays may fall upon it, as near as may be, equally all over it. When it has been roasted in this manner a little while, move it a little nearer, and then a little nearer, examining it each time. As soon as the action of the heat has turned the colour of the film brown in the least degree, it may be dealt with fearlessly; the fire - clay, with the enamel on it, may then be placed level, just in the mouth of the muffle, where in a few moments the film will take all the shades of brown till it gets quite dark all over; now push it into the heat. (A wire set in a wooden handle, and with about 1/2 in. at the other end bent to a right angle, is a most useful tool in manipulating the fire - clay plates when in the muffle.)

As soon as the plate is in the heat, watch it with great care; it will seem to get perfectly black all over, and then, almost on a sudden, the whites of the picture will be seen coming out quite clear. The moment this takes place, draw the tablet towards the mouth of the muffle, remove it to the outside to cool a little gradually, take the tablet then right away, and place it on wood to get cold. All beauty will by this time have disappeared from the enamel, the whites will stand out, and the few tones next to them will have some clearness; but all the other tones will be a dark and confused mass, hardly distinguishable one from another. This is the true characteristic of a good glaze at this stage. It is now ready to glaze.

The enamel glaze is brought, as prepared for photographic work, from Worcester, where it is made in large quantities. About a thimbleful of the glaze (which is a fine powder like flour) is placed in a small narrow bottle - say a 2 - oz. medicine bottle-and the bottle is filled - up about f with alcohol. This is marked "Glaze in alcohol." To make up the glazing mixture take a 2 - oz. medicine bottle, and put in it 1/2 oz. un - iodized collodion, such as would be used for negatives; add to this 1/4 oz. methylated ether and 1/2 oz. alcohol; now add as much water as it will take without throwing the gun - cotton down; to do this set the tap to drip very slowly; get one drop into the bottle, shake violently, and then get another drop in, and repeat the shaking; so go on till 6 or 8 drops are added, which is about enough. Shake up the bottle of "glaze in alcohol," and let it rest about 2 minutes for the coarser particles to subside, then carefully add some of the upper part of the mixture to the diluted collodion - enough to make it rather opaque and milky - looking will do.

This is the glaze ready for use; it must be well shaken up each time it is used.

When the enamel is quite cold, balance it on the top of one finger if small, or near the edge of a piece of fiat wood if large, and pour the glaze mixture over it; then immediately tilt the enamel up to the vertical position, letting the glaze run off on to soft blotting - paper, rocking the tablet in the meantime to prevent the formation of lines. When the collodion is set, place the tablet in a muffle on a piece of fire - clay, and gradually introduce it to the full heat; keep a careful watch now to see that the burning does not proceed too far. The glaze should only just melt; as soon as this is the case, which will be seen by. looking at the reflection of the bent wire held just above the tablet,'pull the enamel out, and when a little cool, remove to a block of wood to get cold again.

The image is now indelibly fixed, and it may be treated roughly with impunity. The picture is not, however, at its full beauty as yet, as, if all the baths, etc, have been in good order, one glazing will not be sufficient. The whites will be glazed, or have a polished appearance, but the darks will be still of a matt surface, and not transparent in effect, as they should be.

This is overcome by repeated glazings. No enamel is perfect that has not been glazed at least 5 times. The number of separate burnings (say 5 or 6), as here recommended, give a totally different effect to what would be obtained by one great burn with the glaze applied thicker. Those who wish to save themselves trouble will work in this way, but any one who wants to get the best results will not mind the trouble of 5 or 6 or even a dozen glazes.

When the glazing comes nearly to an end, there will be found some little points where improvement is needed in the way of retouching. This point is very easily gained. Collect all the trimmings of films after they have been through the toning and ammonia baths, and all waste or torn films as well; place them a few moments in the muffle on a piece of fire-clay - they will instantly burn, and the ash is to be carefully collected, and kept in a small bottle. A little of this may be placed out on a palette, with a minute atom of the glaze powder, and one drop of some essential oil, and then well rubbed down with a muller. The paint so obtained may be used with fine brushes dipped in turpentine, and the work being burnt into the enamel, will take the same colour and surface as the rest of the picture.

Ceramic colours may be applied to enamels, and burnt in with considerable success, but there is much difficulty in getting the red shades wanted about the lips and cheeks right. Use the colours made by Lacroix, of Paris.

When an enamel has failed, it may be put on one side; and when there is a sufficiency of them, the images may be dissolved off with fluoric acid, applied with a rag at the end of a stick, and then, after washing, the tablet may be fired in the muffle till it melts to a good bright surface. If this be carefully done, the tablet so renewed will be as good as new. In this firing, after cleaning, the image will often appear again when in the heat. If this be the case, the heat should be continued till a full glaze has been obtained, when the tablet, after cooling, may be again treated with the acid, and again fired.

When the successful enamel is finished up to the last heating, the name and address of the artist and other matter may be written on the back with a fine brush dipped in the same paint as used for retouching, but a little more thinned with turpentine.

The following are the main causes of failure, which should, however, never be met with by any one who follows the instructions accurately. Failures are of 4 distinct classes, which may be thus enumerated:- I. In development. II. In the direction of getting poor slaty - bluish colours, which glaze all at once when put in the muffle. III. In the direction of excessive blackness, just the opposite to the last. IV. In the glazing operation itself.

With regard to the first, it is imperative that the development proceed slowly - this seems to be the only condition of success. The photographer's knowledge of his business will enable him so to manage the light, lens, exposure, etc, of the film as to secure this necessary condition. The developer given is by no means the only one that will do, though in some hands it succeeds best.

The second class of failures arises from there being too much gold in the toning bath, or rather, perhaps, too much in proportion.

The third class arises from there being too much iridium, or too much in proportion. Both these may be avoided by a strict adherence to the formula given.

The fourth class of failure, the only one to be really feared, is the most difficult to deal with. It is much more difficult to describe than to show. The chief thing to avoid in glazing is the getting an unequal layer of glaze on the tablet the first time. Until the first glaze is burnt in, the picture will rub very easily; therefore a badly - laid glaze will be its ruin, as it cannot be removed. After the first glaze is burnt, the enamel is safe, and any further error in the matter of pouring on the glaze, etc, can be recti fied by simply washing it off again under the tap. Then, again, there is a possibility that when too much glaze is used the enamel will spoil by a sinking in of the image. The best remedies for all errors in glazing are to use plenty of alcohol in the collodion and plenty of water, and at the same time the smallest workable quantity of glaze, making more burns of it, but doing less work at each burn.