PRINTING PROCESS. By " printing " is meant the reproducing a positive, in which the lights and shades are true to nature, from a negative in which they are reversed. The operation not being at tended with the destruction of or injury to the negative may be repeated indefinitely, and therefore any number of prints may be taken from the same negative.
There are two methods of printing ; one consists in copying the ne,,mative by means of a lens, the other by pressing it upon a sen sitive tablet in a pressure frame, and exposing it to direct light. In both cases the light which produces the print is transmitted through the transparent parts of the nemative, and stopped by its opaque parts. The particulars of the fbormer method are described in the article on " Copying," (q.v.) It only remains therefore to describe the latter mode of proceeding, viz., printing by superposition.
There are two methods of printing by superposition of the negative upon a sheet of sensitive paper. One is called San-printing, the other Development-printing.
In sun-printing the paper is said to be either " plain" or " albu menized.' The latter process is minutely described at page 13.
The process of Sun-printing upon Plain paper is as follows :— Use the best Papier Saxe, or the papers of Marion, or Canson ; all of which are manufactured on the continent, and have a finer surface than English papers.
Float the face of the paper for a minute upon a bath composed of— Filtered rain water 1 oz Gelatine 3 grains Chloride of sodium . . . 6 grains.
The ingredients are to be boiled together, strained, and used when cold. Hang the papers up to dry by a pin at one corner.
Excite the paper by brushing over it, with a Buckle's brush, the following solution of ammonio-nitrate of silver Distilled water 1 oz Nitrate of silver . . . 50 grains When dissolved add ammonia, a drop or two at a time, until the brown turbidity at first formed is exactly redissolved, and the solution becomes again dear.
Brush the papers over twice with the above, and hang up to drain.
When as many have been excited as are required for immediate use, dry them before the fire at a moderate distance, and use them at once, for they turn brown by keeping.
Expose in the pressure frame until the picture is somewhat over printed.
On removing it from the pressure frame, wash it (in the dark room) in several changes of rain water, then with water to which a few drops of ammonia have been added, lastly with water again. It is now of a reddish purple tint.
Tone it by immersion in a very small quantity of a bath contain ing Distilled water . . 6 ounces Sel d'or 1 grain Hydrochloric acid . a few drops.
• From one to five minutes immersion in this bath is sufficient. The tint changes from a reddish purple to an indigo colour, while the whites are but slightly changed. The bath must not be used again.
Wash the print thoroughly in water changed a dozen times at least, then fix it in a bath containing 1 part of hyposulphite of soda to 20 parts of water. Let it remain 20 minutes in this bath, then throw the hypo away, and wash the print in water, and let it soak in water frequently changed and agitated, for 24 hours.
Press between cloths, hang up to dry, and the print is finished.
The simplest Development-printing process is as follows :— Use Hollingworth's thin photographic paper ; (the thick sort is useless.) Immerse it in the following bath : Filtered rain water . . 1 ounce Salt 6 grains Lemon juice 1 drop.
The time of immersion may lie between one minute and 24 hours without producing any marked difference in the result.
Excite the paper by floating it upon a nitrate bath made thus : Distilled water . . . . 1 ounce Nitrate of silver . . . 30 grains Lemon juice 6 or 8 drops.
Hang it up to dry, and use it as soon as possible.
Expose it in the pressure frame until a faint trace of the picture is visible.
Develop it thus : Turn up the edges of the paper all round so as to make it into a tray. Lay it, with a sheet of blotting paper underneath, upon a hori zontal sheet of glass, and pour upon the darkest part of the picture a little saturated solution of gallic acid, which spread with a bent glass rod. The development immediately commences and is com pleted in a few minutes. Do not stop it at too early a stage, before the blacks have acquired the proper intensity.
Wash the picture once or twice in rain water, and fix it exactly in the manner described in the preceding process.
When finished it should precisely resemble an engraving in ap pearance.
By adding a little iodide of potassium to the salt, the paper is rendered much more sensitive and the print more permanent, its colour is also improved by an admixture of grey or blue ; but the process is less manageable and certain.
These processes have been somewhat briefly described because the Author has learnt, within a few days, the particulars of a method of printing in carbon, which he feels sure must speedily supersede all the processes with the silver salts, from its economy and certainty, as well the undoubted permanence of the prints. This process, however, (the discovery of Mr. John Pouncy, of Dorchester,) he is not at liberty at present to make public.