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.
Fixing. After the image is fixed it must be carefully washed. While this can be done in a very few minutes you must be careful that it is done thoroughly. If the cyanide is not washed out of the film the finished picture will in a very short time turn yellow and entirely disappear. Therefore, be careful that the plate receives a thorough washing. See formula for fixing bath, Par. 80S.
Drying. When the plate is washed it may be placed in a negative rack and allowed to dry. As a general rule, however, the picture is to be finished as quickly as possible; therefore, the plate should be dried over a gas or oil stove, or a lamp. You may heat this plate as hot as your hands will bear. While drying, continually move the plate over the heat so that the plate will be heated and dried evenly. When the plate is thoroughly dry it is ready for varnishing.
Varnishing. When dry, the image on the ferrotype plate is extremely soft and tender, and if your fingers or anything else were to come in contact with it, it would scratch or mar it. It is necessary, therefore, that it receive a hard coating, which will protect it, and for this purpose a transparent varnish is generally employed. You can purchase this already made, or, if you desire, you can prepare it yourself according to the following formula:
Gasoline Varnish For Tintypes. Place one gallon of gasoline in an ordinary oil can, and add one pound of powdered resin. Then place the can containing the gasoline and resin into a kettle of boiling water. To create a vent, loosen or remove the top of the can. Unless this is done there is danger of explosion. When the water becomes cool discard it and add more boiling water. Continue this until the resin is entirely dissolved.
Caution. Under no condition prepare this varnish near a stove or where there is fire, gas or lamp light, as the fumes of gasoline are extremely explosive. If the varnish is not heavy enough, or does not supply enough gloss, add more resin.
Drying - After Varnishing. Flow this varnish over your tin plate in the same manner that you would any other varnish. Then hold over a flame, allowing the varnish on the plate to catch fire. After it has burned for a second, blow it out and then dry over a gentle heat. The varnish, when dry, supplies an extremely hard surface with an exceptionally brilliant gloss.
1030. When other than the gasoline varnish is used, the varnish should be flowed over the plate in exactly the same manner as you did the collodion; and, then held over the heat to dry until the varnish is hard and perfectly set. If you have used four lens tubes and, therefore, made four exposures on the plate, they must then be cut apart to fit your ferrotype or tintype holder. Cut the proper size and trim off the corners. By trimming off the corners you can slide them into the folders more readily. They are then ready for delivery.