THE PRINCIPAL FAILURES IN MAKING POSITIVE PRINTS ON SILVER PAPERS 6i6. Preliminary Note. Before enumerating some of the failures which may occur in the various stages of the production of positive prints, the reader should realize that the great majority of failures are entirely due to mistakes made, often unconsciously, by the worker himself 430).
The failures already described in connection with the production of the negative need not be repeated here, but they apply as such when printing on development papers.
It should also be borne in mind that any defects in the negative are necessarily repro duced on the positives made from them, and are even occasionally exaggerated to a certain extent.
617. Failures Common to Various Printing Processes. The Outlines of the Image are Doubled. The printing paper has shifted in relation to the negative during printing. This occurs more frequently with print-out paper as a result of insufficient pressure during inspection of the image while printing.
Lack of Sharpness in Parts of the Image. Localized defective contact between the negative and the sensitive material, chiefly caused by warping of the back of the frame or printer, badly-fitted hinges, inequalities in the thickness of the cloth or felt with which the back is covered, or, in the case of transparency plates, by imperfect flatness of the surfaces in contact. The effect of defective contact is exaggerated by printing in diffused light and lessened if a point source of light is used.
General Lack of Sharpness. The negative has been placed in the frame or printer back to front, the positive material being in contact with the support. The picture is then reversed. The thicker the negative, the more marked is this defect ; it is lessened if the printing is carried out with a point source of light in a fixed position relative to the negative.
No Image Appears, or only Very Faint after a Long Time. The back of the sensitive paper has been placed in contact with the negative. Light Spots of Irregular Shape. Light or white marks may be caused by a shadow thrown on the frame during printing, by the presence of foreign bodies (pieces of paper, bits of the cloth or felt of the pressure pad, etc.) between
the glass support and the negative, or between the negative and the paper, or by opaque spots (fragments of gelatine, etc.) on the back of the negative.
Cracks in the Paper or Sensitive Coating. The prints have been rinsed or washed under a too violent jet of water, or the wet prints have been handled incorrectly. Large sheets of paper should be lifted by one corner, and the strain taken off by lifting also the opposite corner. A print should never be taken hold of by the middle of one of its sides.
White Spots Appearing During Fixing. Local solution of the silver image by particles of rust suspended in the fixing bath (especially in a used bath) due to iron or cast-iron pipes or to the vessels (iron casks, etc.) used for the storage and handling of hypo.
Blisters. Blisters which occur during the manipulation of paper prints consist of pockets of gas or liquid between the actual paper and the layer of emulsion. The first can be caused by transferring prints on development paper from a developer which is very rich in carbonate to a very acid stop-bath or fixing bath without intermediate rinsing. They may also be formed by using water supersaturated with air (very cold water under high pressure), which, on attaining the temperature of the atmosphere, liberates the excess of dissolved air in the form of bubbles in the substance of the paper. Blisters, which consist of pockets of liquid, are usually formed during the washing which immediately follows fixation in a very concen trated bath, especially on papers coated with insufficiently tanned gelatine, the water pene trating into the gelatine faster than the thio sulphate solution can get out Any factors which tend to weaken the gelatine locally, increase its permeability, or lessen its adherence to the support (contact with warm hands, folds, kinks, and creases in the print), favour the formation of these blisters. This tendency can be reduced by properly tanning the gelatine before or during fixation (the use of alum is effective even for collodion papers, as it hardens the gelatine of the baryta coating), and by avoiding very sudden changes of temperature and concentration in the handling of the prints.