" Fixing by means of the bromide of potassium is not so durable, because it does not remove any of the materials used in preparing the paper. It may, nevertheless, be of great use in travelling, and when it is required to make several proofs one after the other; because then you avoid touching the hyposulphite in preparing the negative paper, which spots at the least contact with it.
"You may thus place the whole of your negative proofs together in this bath.
"Water, a pint and three quarters; bromide of potassium, 360 grains.
" In taking the proof out of the bath, you must wash it in several waters, and dry it; it should be kept in the bath at least three quarters of an hour, but if you leave it in two or three hours, it will not injure it".
Such is M. le Gray's statement, and so it is rendered by his English translator, Mr. Cousins; but I believe the quantity of the bromide of potassium to be by far too large, and that the pictures would sustain less injury by using a solution of one half the strength indicated. His process for fixing the positive pictures contains some important hints.
"Dissolve in a bottle hyposulphite of soda, 1500 grains ; " Filtered water, nearly a quart.
In another bottle dissolve 75 grains of nitrate of silver in a wine-glass or two of water; when well dissolved, you add to it a saturated solution of chloride of sodium, until the white precipitate ceases to fall; allow it to repose a short time, and then decant the clear liquor, and gather the precipitate of chloride of silver, which you dissolve in the other bottle of hyposulphite of soda; by means of this solution you obtain directly black tints upon the picture. The older the hyposulphite of soda is, the better; when it gets thick, you must add a fresh solution of hyposulphite alone, without the chloride of silver, the old containing an excess, which it has taken from the proofs already immersed in it. You must not filter it to take away the deposit, but only let it repose in a large bottle, and decant the clear liquid for use, leaving the sediment to be re-dissolved by fresh solution.
" By leaving the proofs a longer or shorter period in the bath, you can obtain all the tints from the red to the black, and clear yellow; with a little practice, you will be sure to get the tint you desire. You must not leave a proof less than an hour in the bath for it to be sufficiently fixed, and it can remain three or four days to obtain the sepia and yellow. By heating the hyposulphite of soda I accelerate the operation; but we must not then leave the proof for an instant to itself, as the rapidity of action is so great, that the picture might be completely effaced.
" By adding to the preceding solution about one fluid ounce of liquid ammonia, I obtain pretty bister tints, and very pure whites. The English paper is exceedingly good for these tints.
" I obtain also fine velvet-like tints by putting the photograph (when taken out of the hyposulphite of soda) upon a bath of a salt of gold, using 15 grains of the chloride of gold to one pint and a half of distilled water.
" Fine yellow tints are obtained by placing the proof (if too vigorous) first in a bath of hyposulphite, and then in a bath composed of one pint and a half of water, and one fluid ounce and a half of hydrochloric acid; washing it perfectly in water. Liquid ammonia, employed in the same quantity as last mentioned, gives remarkably fine tints.
"When the proof is the colour you desire, wash it in several waters, and leave it two or three hours in a basin of water, until, touching it With the tongue, you perceive no sweet taste, which indicates the presence of hyposulphite of silver; then dry it by hanging it up, and it is finished. The bath may contain as many proofs as can be conveniently placed in it".
Experience has shown, that however beautiful may be some of the tones given to a photograph by the methods recommended by M. Le Gray, these are obtained with some sacrifice of permanency. Many choice productions prepared by this photographer, which I have had for but a few months in my possession, are showing indications of decay : the change taking place first at the edges, and gradually creeping over the whole picture.
The following fixing processes are rather more curious than useful: they were first indicated by Sir John Herschel, from whose memoir on the Chemical Agency of the Rays of the Solar Spectrum, I quote:—
" By far the most remarkable fixing process with which I am acquainted, however, consists in washing over the picture with a weak solution of corrosive sublimate, and then laying it for a few moments in water. This at once and completely obliterates the picture, reducing it to the state of perfectly white paper, on which the nicest examination (if the process be perfectly executed) can detect no trace, and in which it may be used for any other purpose, as drawing, writing, etc., being completely insensible to light. Nevertheless, the picture, though invisible, is only dormant, and may be instantly revived in all its force by merely brushing it over with a solution of a neutral hyposulphite, after which, however, it remains as insensible as before to the action of light. And thus it may be successively obliterated and revived as often as we please. It hardly requires mention that the property in question furnishes a means of painting in mezzotinto (i.e., of commencing on black paper and working in the lights), as also a mode of secret writing, and a variety of similar applications".
There is a remark which ought not to be omitted in regard to this part of our subject, viz., that it makes a great difference, in respect of the injury done to a photographic picture by the fixing process, whether that picture has been impressed by the long-continued action of a feeble light, or by the quick and vivid one of a bright sun. Even supposing the pictures originally of equal intensity, the half-tints are much less powerfully corroded or washed out in fixing in the latter case than in the former.