This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Washing. After intensifying and fixing, the plate should be thoroughly washed in the usual manner. It may then be dried by holding over a gas or oil stove, or may be placed in a negative rack to dry. If drying is hastened by artificial means, the density of the negative will be slightly increased.
Varnishing. The surface of the negative even when dry, is very easily marred or scratched, as the collodion film is extremely delicate. It is advisable, therefore, to coat it with some hardening substance in the form of a varnish. Regular negative varnish may be purchased ready for use from any photographic supply house, or you may prepare it yourself by dissolving one pound of seed-lac in a gallon of pure alcohol. This should be kept in a warm place, and it may require a number of days for it to dissolve. The bottle should be shaken occasionally. When the ingredient is dissolved, decant and filter.
Varnishing The Plate. To varnish the negative grasp it firmly between the thumb and first and second fingers, in a similar manner to that described for flowing the plate with collodion. Hold the plate over a gas or oil stove until it becomes of a uniform blood-heat throughout; then apply the varnish in exactly the same manner as you did when coating the olate with collodion. Drain the su-perfluous varnish into another bottle. The object of draining into a separate bottle is. that particles of dust may collect while varnishing. If this dust were poured back into the stock bottle of varnish it would soon become charged with dirt; while if drained back into a separate bottle, after a sufficient amount of this varnish has accumulated it can be filtered and then added to the fresh stock, when it will be free from dust or dirt.
929. After the plate has been flowed with the varnish hold it over a gentle heat (not too close to it) until the back becomes uncomfortably hot. Keep the plate rocking so as to distribute the heat evenly throughout. To hold the plate too near the heat or flame might result in the varnish catching fire. Should it by accident catch fire, blowing over the plate will extinguish the blaze. As before stated, there are many good varnishes on the market, some of which may be applied to the plate when cold. Others require the heating of the plate. Usually a method for applying the varnish accompanies each bottle. Should the varnish become thick from age, such varnish applied to the plate may cause the loss of definition and should not be used. Always dust the plate before varnishing it.
930. When using varnish which requires the negative to be warm when applied, should the varnish have an all-over frosted appearance, you will know that the plate has not been sufficiently heated. Should the varnish form ridges over the plate, you have not taken care to tip the plate in one direction only when varnishing. Never allow the varnish to run back over the surface which has previously been covered, but have the flow all in one direction and keep the varnish constantly moving. Never allow the body of the varnish to remain on one spot while flowing the plate. When the varnish is too thin and the plate too cold, you will produce a frosted surface. When it is too thick, it is liable to cause ridges.