WASHING (Fr., Lavage ; Ger., Waschen, Reinigen) Negatives and prints that have been fixed in " hypo " must be freed of this substance by wash ing with water before they can be safely dried ; otherwise the permanency of the result is im paired. " Hypo " being of greater specific gravity than water, it follows that as part of the washing water removes " hypo " from the gela tine and paper it becomes a solution which is denser than plain water, and therefore tends to sink to the bottom of the washing vessel. Agita tion is necessary to cause the plain water and the " hypo " solution to mix thoroughly, although slight admixture must, of course, occur. It follows, then, that the worst method of wash ing is to place a negative or print film side up wards in still water, and that the quickest and best way is to support the film in such a way that the " hypo " may, as it were, fall out of the film to the bottom of the vessel. For washing a single negative quickly, it may be held in the hand, face downwards in a pail of water, where it will be freed from " hypo " much more quickly than if laid in a dish and water allowed to run upon it. A convenient accessory for holding a negative film side downwards is shown. To a piece of wood the same size as the negative are nailed strips of tin-plate, as shown, these being bent under so as to hold the negative. The last named is slid into place, film side outwards, and the whole floated in a vessel of water ; in this way the suspending of a negative film side down wards in water becomes a very simple matter. Another method is to use a dish with sloping sides, which support the negative when this is placed horizontally. Most of the commercial washers hold the negatives in a vertical position, in which position they wash quickly and well if space is left below the negatives. When a negative is held vertically, the " hypo " solution flows to the lower edge and congregates in a dense mass at the bottom of the vessel ; this demon strates the necessity of having a space below the negative if still water is used. The prop ping up of a negative on a brick placed in a pail of still water is a simple method of washing when running water is not available, as by frequent changes the " hypo " may be quickly got rid of.
The time taken to wash a negative or print depends upon the method employed and upon other considerations. Several experimenters, after exhaustive trials, have stated that a washing of twenty minutes is enough for negatives under favourable conditions.
Complete removal of " hypo " from negatives occupies about twice the time the plates take to fix, in favourable circumstances. Negatives do not hold " hypo " as prints do, as in the latter there is the porous paper to consider. Lumiere and Seyewetz concluded that the use of running water for washing was both wasteful and in efficient, and that the best way to wash negatives is to immerse each negative, vertically or upside down, in five successive baths for five minutes each (twenty-five minutes in all), allowing 17 oz. of water to each half-plate. Gaedicke discovered a number of useful facts, the most important of which is that when the water is changed every five minutes, three times as much " hypo " is extracted as when the water is changed every half-hour.
As regards the washing of prints, practically all the above remarks apply. Prints and celluloid films should be kept on the move and the water frequently changed or carried away from the bottom of the tank. If they are soaked in still water, this must be frequently changed, or the prints should be transferred from one dish to another. From data published by Messrs. Lumiere, it seems that eight changes, using 31 oz. of water for each 7-in. by 5-in. print, are suffi cient; 90 per cent. of the " hypo " is soaked out by the first two baths. In the case of a print placed for twenty minutes under a tap passing about 14 pints per minute, and then allowing it to soak for five minutes in 3i oz. of water, the latter was found to contain about the same quantity of " hypo " as the water after one-fifth the washing in the manner above described. The experimenters state, however, that these methods do not remove the last minute traces of " hypo," and that even twenty-four hours in running water will not do it. Haddon and Grundy stated in x894 that they had proved that gelatino-chloride prints, washed for ten minutes in running water and under proper con ditions, had lost the whole of the soluble salts, and that there was no necessity whatever to wash longer ; further, they also found that six changes of water after five minutes' soaking were equally effective. Thus it is obvious that the amount of washing necessary for both negatives and prints depends entirely upon the conditions under which they are washed, no two experi menters agreeing.
For methods of testing washing water for " hypo," see " Sodium Hyposulphite, Testing for.