THE WET-PLATE PROCESS The Glass first essential for the glass support is that it must be flat, or it will not sustain the heavy pressure of the printing frame, possibly in contact with a metal engraving plate. Sheet glass is often slightly curved, and therefore only suitable for very small plates ; plate glass has been ground and polished, generally to the injury of its outer skin. Flatted crown, although expensive, is the best and most economical in the long run, especially as the negatives for engraving purposes are not stored, and after once printing may be cleaned off, and the glass used again. The smooth side of the glass must be very carefully cleaned, by soaking in a weak solution of spirits of salt, say 5 per cent., and then washed and rubbed, after drying with a clean wash-leather. To prevent splitting of the collodion on the glass, the latter is sometimes flooded with the following solution, and then allowed to dry : Albumen . . . . . . . . . I oz.
Ammonia ('88o) to min.
Water . . . . . . . . . so oz.
or sometimes the margin is run round with rubber solution.
Collodion.—It is a question whether collodion is worth manufacturing at home, the time and skill involved being considerable. Pyroxyline or gun-cotton is made by dissolving cotton wool in a mixture of sulphuric and nitric acids, or, alternatively, in a mixture of sulphuric acid and pure potassium nitrate, which latter is rather the easier process. In either case the operation should not be undertaken by those who are not experienced in the handling of sulphuric acid.
_Hardwick Formula.
Sulphuric Acid (1'845 at 6o°) . . . . . 18 oz.
Nitric Acid (1'457) . . . . . . . 6 „ Water Cotton Wool . . . . . . soo gr.
Alternative Formula.
Sulphuric Acid . . . . . 18 oz. (fl) Potassium Nitrate (dried) . . . . . Lai „ Water . . . , . . .t „ Cotton Wool . . . . . . . . t8o gr.
The sulphuric acid must be mixed very cautiously with the other chemicals, when the temperature will rise to somewhere near boiling point. Stir the mixture with a glass rod, and commence to add the cotton wool in small quantities with the aid of the glass rod, when cooled to iso°, which tempera ture must be maintained to ensure success. The cotton wool must have been previously purified from resinous and greasy matter by boiling in an alkaline carbonate and then drying.
When the cotton wool has been immersed about ten minutes it may be removed with the glass rod, the acid drained away, and then washed for twenty-four hours in running water. For use as collodion it is dissolved as follows : Pyroxyline . . . . . . . . . to gr.
Alcohol ('82o) . . . . . . . t oz.
Ether (.72o) , . „ ,In winter rather less alcohol is used, and a proportionately greater quantity of ether, in order to promote more rapid setting.
In practice, the worker will either purchase celloidin, or some other form of collodion, ready-made, and will be able to proceed direct to the iodising.
Iodising.—Dissolve the celloidin in a solution which will differ slightly according to whether for use in summer or winter. For winter : Alcohol ('8o5) . . . . . 6 oz.
Ether (•2o) . . . . . 8 „ Celloidin . . . . . 13o gr.
In summer the amounts of ether and alcohol may be equalised. Then add the iodising mixture : Zinc Bromide . . . . . . 3o gr.
Zinc Iodide 70 „ Alcohol . . . 2 oz.
or : Cadmium Bromide . . . . io gr.
Cadmium Iodide . . . . . 3o „ Sodium Iodide . . to „ Ammonium Iodide . . . . . 15 „ Alcohol . . . . . . . 2 Oz.
The mixture must then be set aside to ripen for about a week, after which it may be bottled off for use. It will improve with age, and keep good for about six months.
Coating the Plate.—This must be done immediately before it is required for use, and is not a very easy task for the beginner, even with the proper collodion pourer. Dust the plate with a badger-hair brush, and then, holding it in a horizontal position, pour upon it on the right-hand side a good quantity of the iodised collodion ; incline the plate so that it runs to the right-hand further corner, then to the left corner, then towards the operator, and finally, via the last corner, back into an extra bottle provided with a funnel. Rock sideways so as to prevent the formation of stringy ridges while draining. Some considerable practice will be required with a steady hand before the operator can get a perfectly smooth homogeneous film, such as is necessary for fine half-tone work.