Castor-Oil In Negative Varnish. Varnish made after the following formula will never check nor split on the negative.
1 1/2 "
Castor-Oil, about one drop to each ounce of varnish.
148. All the labor and all the art thus far expended upon the negative may be sacrificed by heedlessness in varnishing;. The varnishing - rooms should be absolutely free from dust. It is good plan to gently sprinkle the floor before beginning. Some varnishes require heal both for their application and for their drying. After the plates are varnished they should be placed in a rack to thoroughly harden, end the printing paper should never be allowed to touch them until the varnish is free from all tackiness. Above all, let no dust settle on the varnished surface before it is hardened. Beware also of flies and insects.
148. The method I have adopted fur some time past is simple, efficient, and reliable; and if not novel, is still, I believe, nut very well known. It is to flood the negative, in the first place, with dilute amber varnish, and when dry, to coat it with ordinary hard spirit varnish; by this means the latter does not come into hard contact with the collodion film, and the alcoholic solvent is therefore without any action upon the same. Moreover, double protection is thus afforded to the negative, and from the fact that the intermediate varnish film is of a pliant and elastic nature, there appears to me less chance of any injury resulting to the negative from change of temperature, splitting of the film, etc. The amber varnish does not attack the negative film, by reason of the solvent employed (chloroform) being incapable of acting upon collodion.
I may also remark, that where it may be desirable to add to the durability of a negative - such, for instance, as when a large number of copies are wanted - it will be found an advantage to dissolve in the ordinary spirit varnish a small quantity of shellac, which will tend to give a harder and more durable coating to the negative. The brown or orange shellac, not the blenched quality, should be used; the former makes the varnish somewhat darker in color, but this is of no consequence. - Wm. England.
The property which wire-gauze possesses of intercepting the passage of flame, as exemplified in the safety-lamp, may be turned to account in the varnishing of plates.
A clear fire, every one will agree, is the best source of heat in the operation of varnishing, but there are times when it is not possible nor convenient to have fire - heat, and, in that case, the spirit - lamp or Bunsen burner is usually resorted to, and with the not infrequent result of the vapor of the varnish catching fire, to the detriment of the coating of varnish.
Now, by forming a strong wire frame (somewhat larger than the plates to be varnished), bent on one of its sides so as to form a handle, and covering this with tine wire-gauze.(of either cupper or iron), a plate - holder is made, upon which a plate coated with varnish may be warmed without the slightest danger of catching fire.The plate heated on this "quid " will also be more equally heated than when held naked over the flame.
If the wire - gauze is simply stretched over the frame, and stretched to it by fine wire, so as to form a flat bed for the plate, then it is evident that there must be something prevent the plate from falling off when held in the nearly vertical position in which it is desirable the coating of varnish should "set." The iron frame, if formed of stot wire, will give a sufficient rim to keep the plate from falling off; or the same end may be gained by slightly dishing the wire - gauze, so as to make it like a flat photographic tray, or by even making it of the form of the dipping-bath, with, of course, in either case, a wire handle to hold it by. - J. W. Swan.