FADING. it is unfortunately by no means an uncommon thing for photographs to become altered by time, even when preserved with the utmost c,are. In the case of daguerreotypes, or collodion positives or negatives, or negatives upon paper, this misfortune is, however, comparatively rare ; but in the case of paper positives which have been printed by direct exposure to light, and fixed and toned with hypo-baths, fading may unfortunately be said to be the rule, and permanence the exception.
Experience has sufficiently proved that the black material of a developed argentine photograph on glass or paper is permanent when the fixing agent (hyposulphite of soda or cyanide of potassium) is removed by copious and thorough washing in water many tunes renewed. The same may be said of the thin white metallic precipi tate which forms the lights of a collodion positive. It is however necessary, in the case of a collodion positive or negative, to varnish the film, for if this be not done, the pyroxyline is liable to be decom posed, and give off an oxide of nitrogen which destroys the image.
The most serious case of fading is therefore that which occurs where the material of the image is a red compound of organic matter and a low oxide of silver, as in a sun-print, or a red developed print stopped in too early a stage of the development,— the picture being then fixed in a hypo-bath, containing sulphurating salts. Positives produced in this way may be considered certain to fade in from two or three months to as many years, no matter how carefinly they may be preserved, or how thoroughly washed after their fixation. The faded appearance is produced by the dark brown tints of the shadows turning yellow.
The chemistry of fading is at present very obscure, but the phenomena may be produced at pleasure by immersing a fixed red sun-print in a weak bath of hydrosulphate of ammonia. The red
tones first darken to a brown or purple brown, then to a purple black, and ultimately to a yellow, the entire series of changes only occupying a few minutes. If then the hypo-bath contains, as it always does, a sulphurating compound analogous to the sulphide of ammonium, and any of this should remain, as it probably must do, in the pores of the paper, it is easy to understand why the thin red material of the print becomes yellow by time.
As nothing certain has yet been proved with respect to the com position of this yellow substance, we will not offer any conjectures with respect to it.
When sel d'or is used to tone a print, it is more likely to be permanent, because the effect of this mode of toning is to substitute gold for silver in the material of the image. This is done most effectually by toning the print in a bath of sel d'or before fixing it in hypo. When chloride of gold is added to hyposulphite of soda in excess, and a toning and fixing bath made in this way to act together, the deposition of gold takes place simultaneously with the sulphuration of the silver, and the image is less permanent than in the former case.
But even metallic gold will not entirely resist the action of an alkaline sulphide, for this combines with it, and forms a double sul phide of gold and the alkali. This is probably why prints toned with gold have sometimes been known to change from a purple tint to reddish yellow, particularly in the finer details of the lights.
The liability of argentine photographs to fade when sulphur is present in the paper renders it therefore very desirable that a pro cess of printing in carbon, or some permanent pigment, or dye, should be brought to sufficient perfection to be substituted for the present methods.