Julius Schnauss

Negatives for Photo-lithography.

AN unconditionally good negative is required for photo-lithography, and those who have not seen one produced for the purpose by an expert should endeavour as a first step to do so, or they will scarcely be likely to devote the attention necessary for producing one of a thoroughly suitable character. Not only must the lines of the copy be represented in the negative by clear glass, and the white ground of the original be represented by opacity, but the utmost sharpness of definition of the finest dots and lines to the very corners of the plate is absolutely necessary to render the negative thoroughly serviceable. To produce such a negative a lens entirely free from distortion, and capable of giving microscopic definition, is, of course, a first necessity.[U]

The Wet Collodion Process is better adapted than any other for the production of suitable negatives, possessing the necessary density and freedom from fog or veil. The bath must work with the utmost cleanliness, and the collodion should not be used too soon after iodising. The developer works better when not quite fresh, and should contain a sufficiency of acid, and the negative, after fixing, should be subjected to a vigorous intensification.

Professor Husnik thus comments upon negatives for photo-lithography:—

“As in photo-lithography only subjects in black and white line or stipple can be reproduced, to the entire exclusion of half-tone, the negatives should represent the copy by clear lines and opaque spaces. This is seldom the case except in instances where the drawings are specially prepared, and the subjects somewhat bold and coarse. For example, in copying a proof from a fine steel engraving the graduated, grey, hair-lines, appear in the negative somewhat veiled, whereas the intermediate spaces between close shading, and which are somewhat toned in the print, are not developed actually opaque in the negative. On this account the fine hair-lines will take longer to print in the copying-frame than the bolder shadow lines. The latter may develop faultlessly, while the ink may leave the hair-lines. The intermediate spaces between close shading may not be sufficiently opaque in the negative, consequently the print is over-exposed in these portions, and during development remains covered with ink. Such faults must be overcome as far as possible by shading the negative during printing in the usual well-known manner.”

The Colour and condition of the original copy will greatly influence the class of negative it is possible to obtain, and if allowable it will be advisable

To Bleach old engravings or other copies, either yellow from age or stained, by immersing them in eau-de-javelle[V] 10 cc., water 100 cc., wash in water for two hours, and finally in a weak solution of hyposulphite of soda.

Intensifying.—Having developed a clear, sharp negative, wanting only in density, it must be intensified to the utmost limit—without fogging—first with citric acid, pyrogallic acid, and silver. This solution may be applied either before or after fixing, preferably the former; if this course is adopted, after washing and fixing, and again thoroughly washing, the final intensifying operations may be proceeded with. Opinions vary as to the actual best method to adopt, but preference is usually given to an intensification by a salt of mercury or lead; under any circumstances there must be a good deposit of silver to work upon, or the necessary density cannot be obtained. In fact, a negative too thin, submitted to mercurial combinations, particularly the bichloride, will become more bleached and spotty, and the after application of sulphide of potassium will not again restore it; the deposit will assume a dark brown appearance, but the spots arising from the mercurial salts will still be apparent. The two mercurial salts referred to are—(1) Bichloride of mercury, corrosive sublimate, a well-known and highly-poisonous substance used in solution in water, and (2) iodide of mercury, which, used in intensification, acts more evenly, though with somewhat less energy. This substance, which is of a beautiful red colour, is insoluble in water, but is soluble in sublimate, and also in a solution of iodide of potassium. It is recommended for intensification, and is easily obtained by dropping into a solution of bichloride of mercury—with agitation—a concentrated solution of iodide of potassium until the red precipitate as first formed is again dissolved. This yellow solution may be diluted as required by an addition of ten times its volume of water. If, instead of flowing the solution over the negative, a dipping bath be used, into which the negative is steadily lowered for a few seconds, a more uniform intensification is likely to ensue. Carefully watch the plate, and stop the action before it goes too far, or the silver deposit will lose strength in parts. An important gain in density may be obtained by exposing to sun or daylight a negative which has been developed and intensified with pyrogallic acid, fixation taking place after drying. The negative must be thoroughly washed before exposure to light, and with ordinary care there is little danger of fogging.

Illumination of the Copy should be perfectly even, a front light being made use of, and direct sunlight if possible. The latter is seldom admitted direct into the studio, and against outside work wind and rain are almost fatal objections. Where much copying has to be done it will be found an advantage, if at certain hours of the day the sun shines direct upon the studio, to remove all blinds, screens, or other obstructions, and so allow the rays to fall direct upon the print or drawing to be copied.

A Side Light, particularly if direct sunlight, must be avoided, as any unevenness or grain of the paper will be exaggerated and rendered more apparent if lighted in such a manner.

Under Exposure, if slight, is a fault much to be preferred to an error in the opposite direction.

New Transfer Paper for Photo-lithography.—In the photographic office, Calcutta, a new method of preparing the photo-lithographic transfer paper with arrowroot has quite recently been introduced, which, as is said, has the advantage of being much cheaper than the method of coating with gelatine usually adopted, and of giving results quite as good, if not even better, than by the old proofs. Bank post paper is coated in the usual way with two coats of the following mixture:—

After exposure to light, the prints are coated with transfer ink in the press as usual, the transfer ink used being composed of—

After inking in the prints are washed off with hot water, hotter than is required for gelatine transfers. The arrowroot transfer prints are found to go down well on the zinc without sticking, and give clear, sharp transfers.

Gelatine Plates have hitherto been considered almost useless for photo-lithography. Messrs. Mawson and Swan, however, have recently introduced a special make, known as the “Photo-mechanical” plate, which gives negatives of great density, and at the same time clearness in the shadows. The makers recommend the ammonia-pyro developer, with meta-bisulphite of potash. It will be found of considerable advantage if, before proceeding with the development, the plate be submitted for one minute to a bath of water 2 ounces, gallic acid 6 grains; this, without prolonging the development, will add considerably to the density of the resulting negative.