A collodion film, when dry, is very delicate, and, except for line work in the hands of an expert, will require protecting. This is usually done by slightly warming it (when quite dry) in front of a fire, and then pouring over a sufficient quantity of thin spirit varnish. After the superfluous varnish is well drained off, dry by again holding it in front of the fire. The spirit {21} varnish answers perfectly for ordinary work, but for printing upon metal for relief blocks, or upon glass for collotypes, it will be best to use a benzole varnish, such as is sold by dealers for protecting the films of glass positives or ferrotype pictures. Anthony’s “diamond” varnish is good.
But the best plan of all is to use a water varnish, made by dissolving half an ounce of borax in a quart of boiling water, then adding a few pieces at a time (stirring meanwhile), one pound of ordinary shellac; if the quantity of borax named does not suffice to make all the shellac dissolve, add more, a grain at a time, until the shellac is dissolved; then add half an ounce of white sugar, cool and filter it.
When the negative is washed for the last time, allow it to drain, then flood with the above solution, and let it dry. This will be found a quicker, cleaner, and more satisfactory method of protecting the delicate film than with the spirit varnish.
THE GLASS PLATE.
The glass used as a support for the collodion film should be thin flatted crown of the best quality, for the negatives, as a rule, are not required again after once the zinc is etched, so that the film may be washed off, and the glass used again, which cannot be done so often if the glass be of bad quality; flatness is a sine qua non, as it has to be subjected to heavy pressure between a zinc plate and the front glass of a printing frame.
As a good and suitable negative is the foundation of success in the subsequent processes, so is a properly cleaned glass plate the foundation of a successful negative; unless the glass is chemically clean, the collodion film will either be stained or marked, or it will split off in drying, therefore care and attention are requisite; even in preparing a piece of glass for the reception of the film of collodion care is especially requisite, as the want of it cannot be seen until after all the operations, therefore the labor and time are not thrown away.
There are two methods of cleaning the glass plate: one quite mechanical; the other is a mixture of the mechanical and the chemical. Both are good. The second is perhaps the best, as it is simple and more certain.
In the first method the glass, when new, merely requires polishing with clean wash leather kept for this purpose.
The best way of doing this is on a flat board 24 by 18 inches, or thereabouts, {22} covered with good American oil-cloth, glazed side out. A plate laid upon this will not slip about when being polished.
Both sides and edges of the plate must be well cleaned, as one of the most prolific causes of derangement of the silver bath is from organic matter introduced by imperfectly cleaned glass plates.