To use the hyposulphite of soda with effect, there are several precautions necessary. In the first place, all the free nitrate of silver must be dissolved out of the paper by well washing. The photograph being next spread on a plane surface, is to be washed over on both sides with a saturated solution of the hyposulphite of soda. The picture must then be washed, by allowing a small stream of water to flow over it, at the same time dabbing it with a piece of soft sponge, until the water passes off perfectly tasteless. This operation should be repeated twice, or, in particular cases, even three times. The hyposulphite of soda has the property of dissolving a large quantity of several of the salts of silver, but particularly of the chloride, with which it combines, forming a triple salt of an exceedingly sweet taste. This salt is liable to spontaneous decomposition, accompanied with separation of silver in the state of sulphuret: hence the necessity of freeing the paper, by washing, of every trace of it, the sulphuret of silver being of a dirty brown. It might appear that the use of warm water would more effectually cleanse the paper; but it often occasions the immediate formation of the sulphuret of silver.
Some operators prefer leaving the picture in a bath of the hyposulphite of soda for some time, and then removing the salt by simple immersion in water, frequently changing it. The advantages of this appear to be, that the surface of the paper is not disturbed by any rubbing action, or by the mechanical action of water flowing over the surface. For fixing the calotype pictures, Mr. Cundell, to whom we are much indebted for improvements in this particular process, recommends the following mode of manipulation :—
The picture, or as many of them as there may be, is to be soaked in warm water, but not warmer than may be borne by the finger; this water is to be changed once or twice, and the pictures are then to be well drained, and either dried altogether, or pressed in clean and dry blotting-paper, to prepare them to imbibe a solution of the hyposulphite of soda, which may be made by dissolving an ounce of that salt in a quart of water. Having poured a little of the solution into a flat dish, the pictures are to be introduced one by one; daylight will not now injure them: let them soak for two or three minutes, or even longer, if strongly printed, turning and moving them occasionally. The remaining unreduced salts of silver are thus thoroughly removed by soaking in water and pressing in clean blotting-paper alternately; but if time can be allowed, soaking in water alone will have the effect in twelve or twenty-four hours, according to the thickness of the paper. It is essential to the success of the fixing process, that the paper be in the first place thoroughly penetrated by the hyposulphite, and the sensitive matter dissolved; and next, that the hyposulphite compounds be effectually removed. Unless these salts are completely washed out, they induce a destructive change upon the picture; they become opaque in the tissue of the paper, and unfit it for the operation of being copied.
Much has been said and written about improving the tone of the picture by the use of old hyposulphite of soda, and of the hyposulphite in which chloride of silver has been dissolved. My own experience, which is corroborated by that of many of the most successful photographic artists in England and on the continent, convinces me, that in aiming at peculiarity of tint by these methods, the permanency of the photograph is injured. I have pictures, produced by different artists, fixed after this method, and scarcely one of them remains free from change.
The object is to remove all the chloride or iodide of silver; and to secure this, as much hyposulphite of soda should be un-combined, with chloride or iodide of silver, in the solution as possible.
The hyposulphite of silver being formed, it has to be dissolved out of the paper, the fibres of which hold it with a strong capillary force; and it is only by very long continued soaking that all can be removed. The slight mechanical aid afforded by dapping the surface of the paper with a soft sponge well filled with water, greatly accelerates the removal of the salt; and when the paper ceases to taste sweet, we may depend upon the permanence of the photograph.
The hyposulphite of soda has been used for almost every photographic process, from the facility it affords for removing the silver salts. The following is the process of Gustave le Gray, of Paris, chiefly applicable to the calotype process on waxed paper, which is valuable as being the directions of one who has produced most beautiful pictures: but it does not differ, in any-important particulars, from the process already given :—
" Make in a bottle the following solution :—Filtered water, about a pint and a half; hyposulphite of soda, about three ounces; cover the bottom of a dish with this, and plunge in your negative proof, taking care to avoid air-bubbles : this dissolves the bromo-chloro-iodide of silver, but does not attack the gallo-nitrate of silver, which forms the blacks.—[Le Gray is in error here, and in a succeeding paragraph—the darkened portions of these photographs being metallic, silver in a state of extreme division.]
" Never put more than one proof at a time in the bath ; but you may use it for several proofs one after the other.
" If you examine the proof as a transparency after it has remained some time in the bath, you may be tempted to think it is lost, as in some places spots will appear from the iodide of silver not being completely taken away; but if you wait until it is removed, which you will know by the disappearance of the yellow tint, you will be astonished at the whiteness and transparency of the paper, as well as at the beauty of the blacks in the image.
" It will require for this, to remain in the bath from half an hour to three quarters; you will then wash it in several waters, and leave it in a basin of clear water for three quarters of an hour; then let it dry spontaneously by hanging it up; the proof is then quite unalterable by light, as there remains nothing more in the paper than the gallo-nitrate of silver, which is black. (See above).