In the case of negatives which have only pass ing interest (§ 402) and of which prints are urgently required, the gelatine may be rendered proof against melting, even at the temperature of boiling water, by bathing the negative for about io minutes in a solution of formaline or formaldehyde.' It is often recommended to use for this purpose a mixture containing 10 parts of this solution diluted to roo parts with water. This is an excessive amount, for 5 parts in is ample to give the desired hardening (j. McIntosh, 1900), whilst it has the advantage that less formaldehyde is liberated from the negatives during drying. When frequent re course to this method of drying is necessary, it is best to obtain a sheet-iron drying box, through which air can be circulated and led to the outside, and in which the temperature can be raised to about F. 2 428. Rapid Drying with a Volatile Liquid. Many liquids evaporate much more rapidly than water, owing to their vapours not being com monly present in the air, to the fact that their boiling points are lower than that of water, and also because their heat of vaporization is much lower than that of water (half for methyl and ethyl alcohols, quarter for acetone). Such of these liquids as are miscible in all propor tions with water may be employed to hasten the drying of the gelatine of photographic negatives and prints ; in the case of films there is, of course, the additional requirement that they must not dissolve the film base. After some minutes' soaking in the selected liquid the latter will have almost completely displaced the water and drying will then be very rapid.
Considerations of cost only permit of the use of denatured alcohol (methylated spirit), which has the added advantage that it causes the gelatine to The alcohol, by gradually taking up water, becomes useless for this purpose after it has dealt with a number of negatives, and requires regeneration. 4 Too rapid dilution of the alcohol should be prevented by draining or wiping the negatives before placing them in it, and after the alcohol treatment is finished, the alcohol clinging to the negatives is collected. These precautions almost double the amount of material which can be dealt with by a given amount of alcohol (L. P. Clerc, 1917). Negatives of which some parts have already dried spon taneously should not be treated by this method on account of the risk of marks.
The extra speed of drying is obviously greater when concentrated alcohol is used, but the use of methylated spirit at its maximum concentra tion presents various difficulties. In addition to the precipitation of lime salts from hard water in the form of a white opaque fog (§ 420), it sometimes produces a dulling effect on the surface of the gelatine, which has been attributed to the dehydration of the gelatine cells (Liippo Cramer, Also, the use of concentrated alcohol, by softening the base of film negatives and dissolving an appreciable proportion of the plasticizers, may cause frilling of the gelatine or permanent deformation of the film.
The following table shows the time required for drying negatives (7 X 5 in.) after To minutes' treatment in denatured alcohol to which water has been added in different proportions. These times are of course only relative, for they must depend also on the thickness of the gelatine and on the atmospheric conditions— Pure denatured alcohol Pure water Water (added) % o 20 40 to 8o zoo Time of drying. 40 min. 8o mill. .5 rain. 175 min. 210 rain. 27o min.
It will be seen that whilst the effectiveness of alcohol decreases very quickly with dilution, alcohol diluted with water to a moderate extent (up to 20 per cent) is still considerably more rapid than spontaneous drying, and yet avoids the various troubles which may result from the use of too highly concentrated alcohol. 2 Negatives which have been passed through alcohol may, during drying, be subjected without harm, to a higher temperature than would be safe for negatives heavily charged with water.
After treatment by the alcohol method of drying, there is a temptation to consider as dry negatives which are really only dry on the sur face and in which the deeper parts of the gelatine are still wet. If such negatives are piled together in this condition it may be impossible to separate them afterwards.
429. Instantaneous Drying by Dehydration of the Gelatine. Certain salts which are very soluble in water may be employed in very concentrated aqueous solutions in order to bring about the rapid dehydration of gelatine im pregnated with water without causing any ill effects on the gelatine itself (Lumiere and Seyewetz, 1912). Among the salts which may be used (aluminium sulphate, sulphate of soda, hyposulphite of soda, etc.), carbonate of potash in saturated solution gives the best results without harming the negatives, even in case of prolonged contact.
This method of drying should be considered as suitable for the temporary drying of negatives, permitting of their immediate use for periods up to several weeks. They must be given a further washing in order to get rid of traces of carbonate of potash from the This substance, after a time, may give rise to stains, or, in certain cases, to the separation of the film from its support. Since the washing of the negatives must be repeated at leisure, the first washing and even fixing may be shortened, these processes being completed later.
The negative containing water is immersed for 4 or 5 minutes in a saturated solution of carbonate of potash (about 53 per cent at 60° F.) or 110 parts of the salt to Zoo parts of water ; (this solution registers 52-5' on the Ramie" hydrometer). The negative is rapidly wiped between blotting paper to remove the bulk of the adhering solution, and drying is completed by wiping the film with a soft cloth. The surface, which is very hard, is of glossy appearance. The negative may be used at once for printing.
The solution of carbonate of potash may be regenerated from time to time by dissolving more of the salt in it.