The American Carbon Manual/Failures, Faults, and Remedies
FAILURES, FAULTS, AND REMEDIES.
Before proceeding further, a brief recapitulation of the causes of failure in Mr. Swan's process, and the remedies, may not be out of place.
Spontaneous Insolubility of the Tissue.—This, as has been said, arises chiefly from slow drying, or, long keeping in a damp place. The addition of substances to give elasticity, such as glycerine, which retard the drying of the excited gelatine film, also tend to produce spontaneous insolubility. Heat, in conjunction with the moisture, increases the tendency. The use of too much bichromate of potash, or too prolonged immersion in the solution of bichromate, will produce spontaneous insolubility. Immersion in very hot water, prior to development, is at times conducive to insolubility, also drying the tissue in an impure atmosphere, and especially one vitiated by the burning of gas. Some samples of gelatine are said to become readily insoluble; but this requires confirmation.
Tardy Solution of the Superfluous Gelatine in Development.—The same causes which will produce spontaneous insolubility, when present in less degree, cause tardy solution of the unaltered gelatine, and slow development. The more rapidly the tissue has dried, and the more horny and perfectly desiccated it appears, the more readily, as a general rule, the superfluous gelatine and pigment are removed by warm water, and complete development is effected. When the development is slow, hotter water may be employed; but care should be taken that the free soluble bichromate has first been removed by tepid water.
Bichromate of Potash Crystallizing on the Tissue in Drying.—If the tissue be suffered to remain too long in a saturated solution of bichromate of potash, the salt will crystallize on the surface during drying, and the tissue will be useless. The remedy, of course, is the employment of a weaker solution, or a shorter immersion in the full-strength solution.
Uneven Development.—If the print be suffered to float to the surface of the warm water, allowing portions to become dry; or if some portions of the paper forming the original basis of the gelatine, be suffered to become detached long in advance of the remainder, so that the warm water acts directly on the soluble matter in patches, the result will be uneven development, the portions last uncovered remaining darker than the rest of the print; and it will be difficult to equalize the tint, even by long-continued development.
Blisters During Development.—If, in mounting the tissue with the India-rubber solution, perfect contact in all parts be not secured, blisters will arise in the course of development, which will show as marks or defects in the finished print. They are also caused by small holes in the paper, or air which remained between the two varnished surfaces, or by want of contact in the rolling. If from the first cause, they will dry down, disappear, and do no harm. If from the second or third causes they may be pricked with a fine, sharp needle, from the back of the paper, and so rendered harmless. There is, however, once in a great while, a very refractory blister which will spoil a print, or which can only be removed by scraping, and retouching with Indian-ink, after the print is mounted. With great care in placing the two varnished surfaces together, before rolling, and care in rolling with a pretty heavy, steady pressure, the blisters may be entirely avoided.
Over-Exposure.—An over-exposed print will develop tardily, and continue, under ordinary treatment, too dark. After all the soluble chromic salts are removed, the temperature of the water may be raised, and by long soaking in hot water the depth may be reduced considerably. Mr. Swan has found that immersion for a short time in a very weak solution of chloride of lime, or of hypochlorite of soda, or in chlorine water, or peroxide of hydrogen, rapidly reduces a print, by decomposing a portion of the insoluble chromo-gelatine compound, and restoring it to its original condition of solubility. The action is, however, too energetic to be of much practical use in the reduction of over-printed pictures. Protracted immersion in hot water is the best remedy.
Under-Exposure.—An under-exposed print develops rapidly, the lighter half-tones rapidly disappearing. When this tendency is seen, quickly removing the print to cold water will arrest the progress of development, and by skilful manipulation and attention, and the after use of almost cold water (say under 80°), a brilliant print may be secured.
Weak and Flat Prints.—When a feeble print is obtained from a good negative, it may arise either from the use of a tissue containing too small a proportion of color, or from the tissue being old and partially decomposed by slow drying. If the negative be weak, the use of a tissue containing a large proportion of color will yield a vigorous image. Increased vigor may be obtained from an ordinary sample of tissue, by sensitizing it on the paper side of the tissue only, instead of immersing the whole. Printing in direct sunshine aids in obtaining a vigorous print.
Hardness and Excessive Contrast.—This may arise from the use of an unsuitable negative, or from the injudicious use of too hot water on a lightly exposed print, or from the use of tissue containing an excessive proportion of color, especially in conjunction with under-exposure. Sensitizing the tissue on the prepared side will tend to produce softness, even with a dense negative.
An Uneven Texture in the Finished Print, some portions looking more glazed than the rest.—This defect arises from unequal and insufficient pressure in transferring. This unequal pressure may arise from the coating of India-rubber being uneven, or, more probably, from the coating of clear gelatine being applied in uneven streaks, or from uneven texture of blanket, or uneven pressure.
Portions or the Image tearing off in Transferring.—This will arise from the face of the print being imperfectly coated with gelatine, or from the paper or board to which the print is transferred having an imperfectly moistened surface, or from not being dry when the paper is removed, or soiled by fingering or dust.
A Green Tint Pervading the Blacks is caused by imperfect washing of the print, by which traces of soluble chromic salt are left in the image.
Unequal Sensitiveness.—This arises from the tissue having imbibed the bichromate solution unequally. If, in immersing the tissue, one portion remains dry while the rest is wet, that portion will be least sensitive, and will form a light patch in the picture. If the tissue is raised out of the bichromate in such a manner that streams of the solution run down the sheet, there will be in the print patches or streaks of a darker color, corresponding to the streams of the solution. The attachment of a strip of paper along the lower edge of the tissue, immediately after it has been hung up to dry, helps to drain off the excess of solution, and tends to equalize the sensitiveness.
The Gelatinous Coating Runs in Sensitizing.—This will happen if the bichromate solution is too warm, and the tissue kept too long immersed. During summer it is necessary to keep the bichromate solution as cool as possible, and to sensitize in the coolest place that can be procured.
Dark Spots.—If a piece of tissue is printed under too heavy a pressure, dark spots or patches appear in the half-tones. This is most apt to occur if the tissue is limp, and the pressure of the back of the printing-frame not only strong, but uneven from coarse padding.
A Sparkling Appearance in the Print after final Transfer.—This arises from the transfer process being imperfectly performed, the paper being either too wet, or too slight pressure used, or the blanket not sufficiently yielding to diffuse the pressure equally over all the surface of the print.
Cracking of the Film.—This will occur sometimes after the print is entirely finished, generally only during cold weather and is caused by sudden changes of temperature from hot to cold, and vice versa, while the developed print is drying. Avoid this.
- Mr. Swan observes that in printing from negatives somewhat deficient in softness of gradation—in which case there is a tendency to abruptness in the transition from white to the lightest tint,—it is advantageous to expose the sheet for a moment to diffuse light, so as to produce a uniform tinting of the slightest degree possible. In vignetting, Mr. Swan regards this as almost indispensable. It may also be well to remark here, that the weakest tissue (No. 1) should be employed in vignetting, and that the vignetting screen should be very softly graduated in tint.