(1) A solution of shellac methylated spirit forms the basis of varnish, and a simple varnish so made will answer for all rough work; but where delicate results are wanted, it must be paler in colour, and for this purpose use "bleached shellac." Bleached shellac dissolved in spirit also makes an excellent varnish; but it is not nearly so hard and tenacious as that from the orange shellac. A good strong coating of it is readily scratched by the fingernail - a contretemps so likely to occur in printing that such a varnish cannot be recommended. White shellac is made by dissolving ordinary shellac in caustic alkali, and then treating the solution with chlorine, which at one and the same time decolorises and precipitates it. This process, though it produces a pale resin of great value for many economical purposes, causes the resin to lose many of those properties that specially fit orange lac for use in photographic varnish. One of the peculiarities of white lac varnish is the frequency with which it dries into a multitude of fine ridges, which no rocking of the plate to and fro during draining and drying will prevent.
But for paleness of colour in the coating obtained from it nothing can be better; and in a mixture of the two resins - that is, the bleached and the unbleached - the objectionable qualities of either seem either covered or greatly minimised This mixture in suitable proportions constitutes the chief part of the varnish recommended.
Experimenters with "bleached," or, as it is often called, "white lac," must know that unless it be properly stored it practically loses its solubility in spirit of wine; and many cases of failure in varnish-making are caused through the purchaser being supplied with a sample that had become insoluble. Of course this would not be likely to occur in a place where the lac was in great demand; but many of our readers live in places where photographic - indeed, any rare - chemicals are most difficult to get, and when obtainable are not always in good condition. However, in the case of white lac, where the experimenter is ignorant of the appearance it should present, he can easily test a small quantity if he have any doubt in the matter. It should be crushed or pounded into small pieces before adding to the spirit, as even in the best samples a large proportion entirely insoluble always exists, and a clear solution must not be expected. Its solubility or the reverse is soon discovered by noticing whether the small particles begin to disintegrate, as it were, or retain their sharp outlines.
A good indication of insolubility is the outer layer of the round pieces or sticks turning semi-transparent. The plan usually adopted to prevent this change taking place is to keep the bleached lac in the dark and covered with water, when, if it remain so covered, it will retain its solubility in spirit for a lengthened period.
The third and last ingredient in this varnish is sandarac. It is well known by varnish-makers that, when resins are mixed and "blended," the character of the solution or varnish is not by any means of necessity an average of the characters of the resins taken separately, and such is the case with sandarac. This resin taken by itself gives a varnish that is quite useless from its brittle-ness, but when added to a shellac varnish it confers a portion of its own quality of brightness of surface, which it possesses in a high degree, but does not, in moderate quantity, tend to make it "rotten."
The formula for a varnish devised on the principles above enunciated is as follows: -
Palest orange shellac .. 23/4 oz.
Bleached lac ...... 5 1/2 oz.
Sandarac........ 1/2 oz.
Methylated spirit .. .. 1 qt.
Bruise the bleached lac till reduced to small pieces. Powder the sandarac, and then add the whole to the spirit, putting in a few small pieces of glass to prevent the shellac caking at the bottom of the jar; stir or well shake the whole from time to time, till it is evident that solution is complete. All that is then necessary is to set aside to clear, pour off the clear, supernatant fluid, and filter the rest. It is best to allow a month or two for subsidence, for the insoluble part occupies so large a space that much waste through evaporation, Ac., is caused if an unnecessarily large quantity be passed through the filter.
(2) Quick-Drying Varnish For Ferrotypes
A very good and hard varnish used for negatives which have to stand far more handling than a ferrotype is composed of equal parts of white hard spirit varnish and alcohol. Warm the plate, and apply as collodion, pouring off the superfluous quantity; slightly warm again, and on cooling, which takes place in a minute or two, a fine hard coat of varnish will be found - so hard that it can scarcely be scratched with the finger-nail. The process used for ferrotypes is very similar to that for glass positives, with the exception that a special kind of collodion should be used so as to produce a thin deposit with considerable detail.
An elastic flexible varnish for paper, which may be applied without previously sizing the article, is prepared as follows: Crush transparent and clear pieces of dammar into small grains; introduce a convenient quantity - say 40 gr. - into a flask, pour on it about 6 oz. acetone, and expose the whole to a moderate temperature for about 2 weeks, frequently shaking. At the end of this time, pour off the clear saturated solution of dammar in acetone, and add, to every 4 parts varnish, 3 of rather dense collodion; the 2 solutions are mixed by agitation, the resulting liquid is allowed to settle, and preserved in well-closed phials. This varnish is applied by means of a soft beaver-hair pencil, in vertical lines. At the first application, it will appear as if the surface of the paper were covered with a thin white skin. As soon, however, as the varnish has become dry, it presents a clear shining surface. It should be applied in 2 or 3 layers. This varnish retains its gloat under all conditions of weather, and remains elastic; the latter quality adapts it especially to topographical crayon drawings and maps, as well as to photographs. (Pharm. Centralhatte.)