" When speaking of the reduction of silver, it is associated in most minds with the idea of great heat, when the reduction is said to be by heat ; this is liable to mislead : a great heat is not only unneces sary but objectionable ; a dull red heat is the most required, for a low heat and a longer time does the work more effectually. (See Notes.) " That the silver does become so reduced is very curious ; it shows the metallic deposit to be in a peculiar state, and I imagine that the contraction of the ffim must take a meehanical part in the opera tion.
" If the operator allows the burning process to go on slowly, (say half-an-hour or more, as it varies with different impressions,) he can not fail in this part of the process.
" When sufficiently burned, push the box containing the picture to a cool part of the stove, and, when cold enough to be taken hold of by the hand, the plate may be removed, and placed in a plate-box.
" It is better to engrave the picture as quickly as possible, or a film of oxide will soon damage it if not used.
" Materials and Apparatus for th,e application of Hydroilluork dad Gas:— " Fluor spar, " Common sulphuric acid, " A leaden pan and cover.
" Grind the fluor spar into fine powder, and put ounce into the leaden pan, upon which pour 2 ounces of common sulphuric acid, and stir with a leaden rod. The size of a pan for a picture 31x 24 inches, should be 4 x 3 inches in width and breadth, and 4 inches in depth.
" The frames holding the plates over the vapour should be waxed or varnished, otherwise the wood absorbs the vapour ; and it acts upon the edges of the plate before it receives the vapour from the pan, which should be placed upon a stand high enough to admit a spirit-lamp underneath. The stand holding the pan of hydro-fluorie acid should be placed upon the hob of a kitchen range, when there is a fire and the draught generally good, as this will carry away the fumes, which are suffocating- unless the operator is fortunate enoug,h to have a laboratory with all the requisites.
" If an ounce or so of water be used with the sulphuric acid, in the pan with the fluor spar, the spirit-lamp will not be much required ; but the action is so rapid, that I prefer adding water some time pre viously, and letting the mixture become cool before putting in the fluor spar. Using the spirit-lamp, and not adding water gives the operator most control, and is the method I have adopted ; but, in one or, two cases, when water was added to raise the temperature, there was so much energy as to eat half through the glass. But this plan might really prove the best, provided it could be so modi fied as to be a little more manageable.
" When the hydro-fluoric acid is made to act upon the plate, it permeates the transparent coat first, except in the false action (mentioned in the notes) ; and in proportion as the silver, forming the opaque parts of the picture, increases in density from the half tone to the high light, so is the time the acid takes to act ; the high lights being the most opaque, resist the attack longest. When this part yields to the action of the acid, the whole of the picture is loosened from the surface of the glass, and any further action of the hydro-fluoric gas tends only to destroy the engraving which has been made, so that a few seconds will form an engraving, and a few seconds more, if the action be continued, will entirely destroy it, by eating too far into the body of the glass. It will thus be seen how great a nicety is the timing of the plate over the hydro-fluoric gas ; it varies according to the nature of the glass, and the strength of the impression and of the gas, the latter being regulated by the size of the flame of the spirit-lamp, so that, like the timing of a picture in the camera, it would be difficult to state any time nearer than from five seconds to five minutes.
" The best plan is to take the plate up after about ten seconds, and if the picture has a uniform grey appearance by transmitted light, and a prismatic appearance by reflected light, there is every reason to suppose it will bear the action no longer. It is now to be washed off with clean cold water; but should the plate not be sufficiently cool for this, use warm water, and wipe dry. The engraving will be distinctly seen, and if strong will be easily felt ; but if only a surface impression, it will be necessary to hold the glass in the same position as you would a daguerreotype, that the picture may be seen.
" It is evident that this paper, so far, applies to direct camera pictures : the lights of the picture being formed by the metallic de posit, the process of engraving being founded on the well-known resistance of silver, as well as other metals, to the action of hydro fluoric acid on the one part, and the affinity which that acid has for glass on the other. If, therefore, the law holds good, the trans parent parts of the proof will be bitten out, and the lights of the picture will be untouched, or nearly so. This is the kind of proof required to produce a plastic cast in relief, from which a plate for the press may be obtained by the electrotype process.