VARNISHES. An excellent practical account of the manufacture of varnishes, by Mr. J. W. Neil, will be found in the 49th volume of the Transactions of the Society of Arts. But the photographer is not so much concerned with knowing the particulars of this manu facture in its various branches, as with knovving the best formuhe for making the two or three different kinds of varnish which are used in photography. These are as follow :— Benzole Varnish. This may be applied to glass plates without the application of heat, and it dries very quickly, leaving a tolerably hard film, which does not become sticky at ordinary temperatures. It is made by adding finely pulverized gum dammar to pure benzole. The dammar readily dissolves, and the varnish may then be filtered through cotton wool to separate any solid particles there may be in it. The proportions are about one ounce of dammar to a pint of benzole. The varnish is applied to the plate exactly in the same way as collodion.
Instead of dammar finely powdered amber may be employed, and this varnish will be found better than amber dissolved in chloroform, though probably not so good and more costly than that made with dammar.
Spirit Varnish. This is the best varnish for photographs upon glass, but there is some little trouble and risk in applying it. It is made thus :— Put into a glass flask Alcohol S. G. 825 • . 20 fluid ounces.
Pulverized white lac . . 1f ounce.
Sandarach . . . . 1-1 drachm.
Put the flask into hot water, having previously wrapped a piece of paper loosely over the mouth of it. - The ingredients are soon dissolved, and may then be filtered by passing the varnish thragh cotton wool.
Instead of white lac, seed-lac may be employed, but the varnish is then of a darker colour. Methylated spirits may be used as the solvent. French polish diluted with an equal part of alcohol makes a good spirit varnish for negatives.
Before applying the varnish the plate must be dried and heated before the fire to a temperature of about 100° ; not more, or there will be a risk of destroying the picture by causing it to run in smears down the glass when the superfluous varnish is poured off the plate into the bottle. On the other hand, if the plate be not
heated sufficiently, the varnish will be chilled, and produce the effect of ground glass. The plate must be warmed again while the varnish is drying. When spirit varnish is properly applied it forms an exceedingly hard and tough film, which it is difficult to scratch or injure during the process of printing, and which does not become sticky by heat.
Spirit varnish must never be applied a second time to a plate. If this be attempted the picture will most probably be spoiled.
Black Varnish. This is made by dissolving in one bottle powdered asphaltum in benzole, and in another, india-rubber in benzole ; then adding the latter to the former in such proportion as may seem best, the object of the india-rubber being to prevent the black varnish from cracking.
The india-rubber should be cut into small pieces, and left two or three days to dissolve in the benzole, which it does without heat.
Varnishes may be divided into two classes,—viz. fat varnishes, and spirit varnishes ; and the latter class may be subdivided into two, in one of which alcohol is the solvent, and in the other turpentine.
The principal fat varnish used by painters is copal, and the principal spirit varnish mastic, but neither of these is suitable for photographic purposes. Varnish for maps and drawings may be made by adding turpentine to canada balsam in about equal parts, and gently heating the mixture. The paper should be sized with gelatine before applying the varnish. Another paper varnish may be made by digesting together—amber 300 parts, camphor 1 part, alcohol 1500 parts. White lac dissolved in borax may also be used as a paper varnish. Another kind of pgper varnish may be made from Xyloidine ; (q. v.). Paper may be varnished by floating it upon albumen, drying it, and then coagulating the albumen by floating the back of the paper upon boiling water. See also " Leather Varnish."