DAMP, PRECAUTIONS AGAINST Damp plays havoc with apparatus and sen sitive material, which should always be stored in a dry, airy place. Camera bellows become mouldy in a damp place, lenses spotty, cameras may come to pieces, and dark-slides refuse to fit or draw out, and shutters to work. Preventives are obvious. Where possible, it is advisable to use, for home-made articles, a glue that has been damp-proofed by mixing with it while hot one-quarter its bulk of linseed oil, stirring rapidly during the addition. The addition of part of potassium bichromate dissolved in the least quantity of water to 6 parts of melted glue, made with as little water as possible, makes a waterproof cement, which must be stored in the dark until required for use. A precaution against damp is to have cameras and dark-slides brass-bound. Dampness in a dark room may cause the detachment of labels from the bottles of chemicals, unless these labels are protected by a coat of waterproof varnish extending over their edges upon the glass.
Plates and papers deteriorate quickly if not kept dry ; and if damp is suspected they should be kept in a cupboard or in drawers, with some calcium chloride in a tin, either without a lid or with a perforated one ; the chemical absorbs moisture, and when it becomes wet it may be dried on a hot shovel or in an oven and used over and over again.
Damp must also be guarded against while printing, as damp paper will probably spoil any negative with which it is placed in contact (see " Silver Stains "). When printing is carried out in wet weather, and there is a likelihood of the paper absorbing moisture, it is advisable to use a pad of waterproof sheeting between the back of the paper and the frame back, or pads of blotting-paper may be used if dried before a fire after taking each print. Such a precaution
is particularly necessary when P.O.P or platinum paper is used ; and in very wet weather such papers may with advantage be dried before a fire previous to printing.
Carbon paper (tissue) is insensitive when wet and sensitive when dry ; but it is not generally known that the film is insensitive when in a state of absolute dryness amounting to complete desiccation.
The walls—plain or papered—of dark-rooms or work-rooms where photographic material is stored may with advantage be covered with a waterproof varnish formed of naphtha and shellac, in the proportion of 1 lb. of the former to r quart of the latter. The smell of the mixture is unpleasant, but soon wears off, and the wall is covered with a hard coating utterly impervious to damp, and to which wall-paper may, if desired, be attached.
A wooden erection used as a studio, dark room, or store-room, can be given a waterproof coating with tar, which is as effective as, and cheaper than, anything else. The following is also suitable, and can be applied with ts brush : equal parts of pitch and resin melted together in a bucket over a stove, and then, after removal, thinned with petroleum ether or paraffin oil. The fire risk in preparing this stuff is considerable, and the job should be done in the open air.