The power of destroying the susceptibility of a photographic agent to the farther action of light, when the picture is completed by its influence, is absolutely necessary for the perfection of the art. Various plans have been suggested for accomplishing this, which have been attended with very different results; few, if any, of the materials used producing the required effect, and, at the same time, leaving the picture unimpaired. The hyposulphite of soda is decidedly superior to every other fixing material; but it will be interesting to name a few other preparations, which may be used with advantage in some instances.
The pictures formed on papers prepared with the nitrate of silver only, may be rendered permanent by washing them in very pure water. The water must be quite free from any chlorides, as these salts convert the nitrate of silver into a chloride, and attack the picture with considerable energy, and soon destroy it, by converting the darkened silver itself slowly into a chloride. Herschel remarks—" If the paper be prepared with the simple nitrate, the water must be distilled, since the smallest quantity of any muriatic salt present attacks the picture impressed on such paper with singular energy, and speedily obliterates it, unless very dark. A solution containing only a thousandth part of its weight of common salt suffices to effect this in a few minutes in a picture of considerable strength".
The great point to be aimed at in fixing any of the sun-pictures is the removal of all that portion of the preparation, whatever it may be, which has not undergone change, without disturbing those parts which have been altered in the slightest degree by the chemical radiations. When a picture has been obtained upon paper prepared with the nitrate of silver, or the ammonio-nitrate of silver, the best mode of proceeding is to wash it first with warm rain water, and then with a diluted solution of ammonia : if the ammonia is too strong it dissolves the oxide of silver, which in these processes is formed in the fainter parts of the picture, and thus obliterates the more delicate portions.
Photographs on the muriated papers are not, however, so easily fixed. Well soaking these in water dissolves out the excess of nitrate of silver, and thus the sensibility is somewhat diminished; indeed, they may be considered as half fixed, and may in this state be kept for any convenient opportunity of completing the operation.
Chloride of sodium (common salt) was recommended by Mr. Talbot as a fixing material, but it seldom is perfectly successful : as a cheap and easy method, it may be occasionally adopted, when the picture to be preserved is not of any particular consequence.
It may appear strange to many that the same material which is used to give sensitiveness to the paper should be applied to destroy it. This is easily explained : In the first instance, it assists in the formation of the chloride of silver; in the other, it dissolves out a large portion of that salt from the paper, the chloride of silver being soluble in a strong solution of chloride of sodium. The picture being first washed in water, is to be placed in the brine, and allowed to remain in it for some little time ; then, being taken out, is to be well washed in water, and slowly dried. After fixing by this process, the white parts of the photograph are often changed to a pale blue—a tint which is not, in some cases, at all unpleasant.
I have in my possession some pictures which have been prepared more than eight years, which were then fixed with a strong solution of salt, and subsequently washed with warm water. They have become slightly blue in the white portions, but otherwise they are very permanent; and they have lost but little of their original character.
The chloride of silver being soluble in a solution of ammonia, it has been recommended for fixing photographs. The ammonia, however, attacks the oxide, which forms the darkened parts in some preparations, so rapidly that there is great risk of its destroying the picture, or, at least, of impairing it considerably. The only photographs on which I have used ammonia with complete success are those prepared with the phosphate of silver; to many varieties it imparts a red tinge, which is fatal to their use for transfers. Still ammonia affords a ready means of partially fixing a photograph, and thus preserving it until a more convenient period for giving it permanence.
The ferrocyanide of potassium, or, as it is more commonly called, the prussiate of potash, converts the chloride into a cyanide of silver, which is not susceptible of change by light; consequently this cheap salt has been employed as a fixing agent, but, most unfortunately, photographs which have been subjected to this preparation are slowly, but surely, obliterated in the dark.
The iodide of silver, which is readily formed by washing the photograph with a solution of the iodide of potassium, is scarcely sensitive to light; and this salt, used in the proportions of five or six grains to four or five ounces of water, answers tolerably well where transfers are not required. It tinges the white lights of the picture of a pale yellow,—a colour which is extremely active in absorbing the chemical rays, and is therefore quite inapplicable where any copies of the original photograph are required. Bromide of potassium may be employed as a temporary fixing agent, and for this purpose is strongly recommended by Mr. Talbot, and constantly employed by many of the continental travelling photographers; since it will insure the permanence of an impression until an opportunity presents itself for giving the final permanence to the picture.
Of all the fixing agents, the hyposulphite of soda is decidedly the best. This was first pointed out by Sir John Herschel, who also recommended that it should be used warm in some cases, which was the plan adopted by Mr. Fox Talbot in the improvements of his calotype process.