We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
June, 1930] STOP BATHS AND FlXING BATHS 697
the gelatin film has a definite basicity or alkalinity resulting after the acid in the fixing bath has neutralized the alkali in the developer retained by the film.
(b) Since all developers contain sodium sulfite and an alkali, the acidity and the hardening properties decrease with the addition of developer. The color of a plain chrome alum solution also changes from violet to green on the addition of developer or on boiling, but this has little effect on the hardening properties if the acidity of the solution is maintained between certain limits.
(c) Since most fresh fixing baths contain sodium sulfite their hardening properties fall off even without use at a rate depending on the particular formula used. Usually a high concentration of chrome alum insures a longer life and although the presence of an excess of acid or the addition of sodium nitrite tends to maintain the hardening properties, blisters are apt to form as a result of the excessive acidity or the liberation of nitric oxide from the nitrite.
(d) More uniform hardening properties can be maintained by processing at 85°F. than at 70°F. but this advantage is offset by the fact that the gelatin film tends to blister and to swell excessively at this temperature.
When developer is added to a chrome alum stop bath or fixing bath, the immediate cause for a drop in the hardening is a change in the alkalinity of the bath but the hardening properties can usually be restored immediately by reacidifying. With chrome alum stop baths containing developer a maximum degree of hardening is obtained with alkaline film (as defined in Part I) when the acidity of the bath is maintained between a pH of 3.0 and 3.8 and with neutral film between a pH of 3.8 and 4.0. With chrome alum fixing baths the alkalinity of the film has very little effect on the hardening action but the acidity of the bath must be maintained between a pH of 3.0 and 3.8 in order to obtain a maximum degree of hardening. The effect of sulfite in destroying the hardening properties is not thoroughly understood but this is probably due partly to the formation of a chromium complex which has no hardening effect on gelatin, and partly to the buffering action of sulfurous acid on the acidity of the baths.
Compared with potassium alum fixing baths, chrome alum baths do not sludge as readily but for normal temperature work potassium alum baths are equally satisfactory for use with motion picture film because they do not harden excessively and if revived at intervals with acid no sludging occurs.