Paper Adhering to the Gelatine—Those layers which yield the best and most brilliant impressions, showing plenty of contrast, which have been fully printed under a vigorous negative, most often exhibit a disagreeable tendency to adhere to the paper in the high lights. The greater the proportion of isinglass present in the film the more will this fault manifest itself, particularly with an enamel paper. An application of a diluted solution of ox-gall to the film, the use of a thick well-sized or glazed paper, lighter pressure while printing, and less frequent damping will all tend to diminish this evil. Layers of this nature are extremely liable to damage, and the adhesive portions will soon suffer unless the greatest care is exercised.
Ink Remains upon the Plate.—Through too light or uneven pressure it sometimes happens, especially when using a stiff ink, that the latter will, after the impression has been pulled, partially adhere to the darkest portions of the printing surface, particularly on the edges of plates deeply copied. If this fault is not corrected at once, and the printing allowed to proceed, these portions giving a light, faulty impression will gradually increase in size, and the corresponding portions of the Collotype plate gradually darken. When first observed the whole plate must at once be cleaned with turpentine and again rolled up with thinner ink, a greater and perfectly even pressure being applied. Small faults of this nature occurring on the edges of the plate can be corrected after each faulty impression by means of a small piece of sponge moistened with gum water or with a turpentine rag.
Displacement of the Paper under the scraper will create a double impression, and sometimes wrinkles, the latter invariably leading to the destruction of the plate. By adopting the use of the bridge now used in conjunction with most Collotype hand presses, this fault is avoided, as the paper is only in contact with the printing surface immediately under the scraper.
CHAPTER XII.
Investigations on Collotype.
AUGUST ALBERT[L] is of opinion that it is by no means a matter of indifference whether bichromate of potassium or bichromate of ammonia is used in the preparation of the Collotype plates; on the contrary, he believes that the latter salt materially conduces to interruptions in printing, especially in unsettled weather. The results of his experiments with the two salts in the Collotype process, and of their behaviour in combination with different sorts of gelatine, are as follows:—
To 1000 cc. of water are added ten drops of a saturated solution of chrome alum; this mixture, after having stood for a quarter of an hour, must, if shaken up, show an opalescent appearance, and a slight milky turbidity; if too much chrome alum has been added, the water acquires a greenish colour. The gelatine is now added, and, for about fifteen minutes, kept at a temperature of 134°, when the chrome salt is added, stirring all the while, and the temperature of the water bath raised to 150° to 160°. The temperature and time must strictly be adhered to. The jelly thus prepared should be kept at least ten hours in a solidified state previous to using.
[L] This experimentalist has evidently been confused with the late Joseph Albert, of Munich, by some writers. It may be mentioned that Joseph Albert, whose business is still carried on by his widow, was father of Dr. Eugene Albert, known in the photographic world for his orthochromatic collodion emulsion. August Albert, of Vienna, is of another family.