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nitrate which substance, though apparently fixed, undergoes a slow transformation, while the gelatine containing the emulsion is absolutely fixed, provided the impression, washing and drying are properly carried out.
Transformation or deterioration in the nitro-cellulose basis will be of no importance, if a constantly low temperature is maintained, but will become increasingly marked through the influence of high temperature as the latter favours the formation of peroxide of nitrogen which acting on the physical properties of the basic compound may penetrate the emulsion and gradually destroy the picture.
An impression insufficiently fixed, imperfect washing of the film allowing traces of hyposulphite to remain, greatly increase deterioration, producing a typical yellowish decoloration of the layer of gelatine. This is what makes the fixing of the impression and the washing so extremely important, and it is essential with a view to greater care that, after the ordinary washing after fixing the impression, the negative be once again fixed in new hyposulphite solution, then lavishly washed, — if the negatives are of great value, it is advisable that the water used be distilled water!
Again, when negatives are being prepared for a long period of preservation, it is advisable to reel them on cylinders made of wood, packing in chemically pure paper ; they should then be placed into corrugated boxes, preferably of fibre or of vulcanite, which in turn should be enclosed into metal boxes. Contrary to what is almost always done in actual practise, it is advisable that during its time of storage, a negative should not come into direct contact with metal.
As has already been said, owing to the influence of temperature on the nitro-cellulose basis, it is advisable that negative boxes should be stored in cool, dry and properly ventilated premises. The ideal conditions for the purpose can be imagined as those of a good house refrigerator Such precautions tend to effectively avoid physical alterations of the basic compound, of the layer of gelatine, as well as any shortening or crinkling of the film, so disastrous to any subsequent impression or projection, because of possible misplacement of perforations.
In conclusion we would say that care should be taken not to let films lie untouched for any considerable number of years. At least one impression should be made every year in order to allow of the dissolution of any gases that might form ; in case this is impossible, it is advisable to unreel the cylinders once a year, which will also enable all traces of deterioration to be controlled.
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