If the plate has by accident been in contact with mercury, it must be heated over a spirit lamp until the whole of the mercury has been volatilized and expelled.
Then proceed to clean it in the following manner :— Cut the Canton flatmel into pieces about two inches square. Lay one of them on the plate, and pour on the back of it a few drops of dilute nitric acid ; rub the damped flannel all over the plate, going well up to the corners. Then powder a little rotten stone on the plate, and rub it all over with the bit of flannel, with a circular motion, until it is clean. Beware of scratching the plate. By scratches are not meant the fine circular marks left by the rotten stone, and which are removed on the buff, but the deep cuts made by a crystal of sand or dust on the flannel. When the plate is clean, it presents the grey appearance of unpolished silver, and is free from stains or marks. When you lave cleaned the face of the plate, clean the back with the same piece of flannel, and then with a fresh piece of flannel rub the face again until it is dry. The clean plate must not be touched by the fingers.
To polish the plate, procure some prepared charcoal and a couple of buffs. Tie up about an ounce of the charcoal in a piece of cambric, and keep it in a box with a lid, in order that no dust may stick to it. The buffs are made in the following manner. Procure two pieces of wood 16 inches long, 6 inches wide, and 1 inch thick. Screw a strong handle on the back, at the end of each. Cover one with fine velveteen, and the other with cotton velvet. The eord of the velveteen is to be put acrou the buff. When not in use keep them face to face.
Powder some charcoal all over each buff with the dabber, and rub them together. Then, taking the velveteen buff in the left hand, and resting the end of it on a table, lay the clean plate face down wards on it, and with the fingers of the right hand on the back of the plate, rub it up and down the buff vigorously, in every direction, sideways, longways, diagonally, until you have polished it, and re moved the circular marks of the rotten stone. Change continually the position of your fingers on the back of the plate, and in order to prevent them from slipping off on to the buff, breathe on them occasionally, or wear a leather glove. If they slip on to the buff,
immediately powder some charcoal upon the spot.
When the plate is thoroughly polished it looks black, and not grey. Polish well up to all the corners and edges, and then put it away into the plate box. Clean and polish enough plates for the day's use, before proceeding to manipulate. A plate may be cleaned and polished in two or three minutes in the manner described, but it is hard work, and may be handed over to an intelligent assis tant. The remaining processes are extremely neat and elegant ; much more so than the collodion or paper manipulation.
2nd Operation.—To render the Plate Beneitive.—Powder some charcoal upon the velvet buff, and rub the polished plate lightly up and down for half a minute. The fine lines left by the buff across the plate, must be horizontal, when you are looking at the picture ; remember this when giving the last rub to the plate.
The plate is now ready to receive the sensitive coating.
Procure a double coating box, containing two compartments, one for iodine, the other for bromide of lime, and furnished with re flectors. See that the sliders fit air-tight over the glass pans. Put an ounce of iodine into one pan, and four ounces of bromide of lime into the other. Spread them evenly over the bottom, and lay some cotton wool thinly divided over the iodine, in order that the fumes may be filtered, as it were, and 'rise more equally.
Place the coding box between yourself and a small window, shaded by a white blind, and open the back shutters of the coating box so that a feeble white light may enter, and be reflected from the mirrors.
Place the plate face downwards over the iodine, draw out the glass slider, and observe by means of the mirrors the _ changes of colour which the plate assumes. These occur in the following order, viz., pale yellow, deep yellow, rose colour, steel colour, the original colour of the plate; after which the colours recur in the same order as before.