Another kind of fog which may occasionally be met with in some makes of plates is c,alled green fog. When the negative so affected is examined by reflected light, it will appear quite green, while if looked at by transmitted light it will appear red. The defect is due to the preparation of the plates, and is more often noticed in the case of under exposed negatives which have been considerably forced with :Ammonia in development. Its presence, however, will not materially interfere with the printing qualities of the negative. It may be removed by rubbing the dried film with a pledget of cotton-wool moistened in alcohol.
Staining of the Film—This must not be confounded with any of the varieties of fog previously referred to. If the formuke for development which have been given are used, this defect will not be met with ; but when a simple solu tion of pyrogallic acid is employed, without sulphite of soda, or other preservative, a more or less deep staining of tho film results. Such negatives being non-actinic in colour take a long time to print; therefore it is well to remove the stain by placing them, after removal from the fixing bath and washing, in a solution composed cf alum, 1 oz; citric acid, I oz. ; sulphate of iron, 2 oz. ; water, 10 oz. After a few minutes' immersion the yellow stain will dis appear, and the colour of the negative will be changed to a bluish-black.
Frilling is a defect which has been referred to incidentally in the chapter upon " Development," and it consists in a puckering up of the film, generally near the edges of the plates, and a want of adhesion to the glass. It is rai ely met with now, except in very hot weather, when the temperature of the solutions and the washing water is high. At such times the plate should not be handled more than is absolutely necessary, and, whenever possible, the tempera ture of the solutions reduced by means of ice. The use of the alum bath directly after development will tend to pre vent frilling during fixing.
Blisters.—These are really a modified form of frilling, and are sometimes caused by allowing the negative to re main soaking too long in the washing water. Directly it occurs the plate should be removed, and placed in a tray containing methylated spirit ; after it has been allowed to soak for about ten minutes it may be removed, and, without further washing, placed in the rack to dry.
Spots.—The formation of these upon the negative are due to various causes, which, in the majority of cases, are preventable. Their appearance, although a minor defect, is one of a most annoying character, for they generally show themselves in a most conspicuous part of the negative ; for instance, in the case of a portrait, upon the nose of the sitter, or, in a landscape, in the middle of the sky. Spots caused by dust show as transparent markings ; but if the advice previously given be always followed, namely, to brush the surface of the plate before exposure and develop ment, spots due to this cause will never occur. Round, transparent spots, more regular in outline, and larger than the foregoing, are caused by air-bubbles forming on the plate during development. Some plates are more subject to this defect than others. If, during the progress of development, a camel-hair brush be lightly passed to and fro over the plate while it is in the developer, they will be removed before harm is done. It is unnecessary to add that the brush must be kept clean, and for this particular purpose.
Opaque Spots are sometimes met with, and are generally due to some defect in the manufacture of the plates. When they occur, the plates showing the defect should be submitted to the manufacturers for examination.
Developer Markings are often caused by applying the developer to the plate unskilfully, instead of allowing it to pass over the entire surface of the plate in an even wave. They may be also occasioned by not thoroughly mixing the different ingredients forming the developer. The solution should always be thoroughly stirred before applying it to the plate. A general mottling of the plate is also frequently caused by allowing the plate to lie motionless in the de veloper. When development is protracted the dish should be constantly rocked.
Insufficient Density may result from under-development, over-exposure, or the employment of too weak a developer. The first fault is fully dealt with in the chapters upon development, while the chapter upon intensification provides a remedy for the second, Excessive Density sometimes occurs, and is the converse of the defect just referred to. It is caused by continuing development too long, or b3‘, employing a developer con taining too great a proportion of pyro. Some subjects require much less pyro. than others. Assuming the ex posures in each case to be correct, a view taken in bright sunshine would require considerably less pyro. to develop it than the same view taken in a dull or diffused light. Subjects showing great contrasts, or a large proportion of white, whether they be whitewashed houses, a chalk road, or a lady's dress, all require a less proportion of pyro. than that given in the chapter upon development. Probably in such cases 3 gr. or 30 minims of the 10 per cent. solution of pyro. to the 3 oz. of developer will be found ample.
Halation is a defect which is very often met with in certain makes of plates which are but thinly coated with emulsion. Such plates the reader should avoid. It is usually only seen when strong contrasts of light and shade occur in the negative. It generally makes its appearance when such subjects as the interior of a church with light streaming through a window, or a landscape with heavy masses of foliage, through which bright patches of sky appear, are attempted. It is probably caused in part by reflected light from the back of the plate ; but it seems also to- be due to a lateral spreading of the light from a portion of the plate which is brilliantly lighted to an adjacent portion which is in shadow. The evil may be mitigated, if not prevented, by backing the plates before exposure. To do this, mix yellow ochre to a paste with weak gum-water, and apply to the back of the plate with a brush ; this, of course, naust be done in the dark-room. The plates may then be reared up on end, in a large box, to dry, which they will do in a short time.
Insensitive Markings of an irregular shape, sometimes of an iridescent character, round the margins of plates, are met with after development, in plates which have been kept too long, or which have been exposed to damp or to the funaes of gas. Plates should always be stored in a thoroughly dry place, not on the upper shelves of a cupboard, or room, in which gas is burnt. Exposed plates must never be packed in paper which contains printed matter of any kind, as contact with the printing ink will inevitably ruin the negative.