THE means to be employed to clear off the layer of undecomposed iodide of silver from the collodion film, will now be considered. To effect this object, we make use of certain compounds having the property of dissolving the insoluble salts of silver, and which will do this effectually, without, at the same time, injuring the deposited image; which being formed of metallic silver in a state of minute division would be easily dissolved by any solvent having any great power over it.
The compound most commonly employed is the hyposulphite of soda. Iodide of potassium has the same power of dissolving iodide of silver, but it is inconvenient to use, as when water is employed to wash the plate after fixing, a portion of the iodide would be thrown down in the film, and cause thereby a disagreeable haze throughout the picture.
Hyposulphite of soda, on the other hand, forms, with the iodide of silver it takes up, a soluble compound consequently the whole is washed away on the application of water, leaving the plate clear and bright.
Cyanide of potassium also possesses great power over the iodide of silver, but it has one great defect that, unless it is used with great caution, it would dissolve the deposited image as well; to guard against this evil, it should be used very weak, and not left on the plate one moment longer than is necessary to clear off the iodide of silver.
I cannot recommend the use of a bath of hyposulphite of soda to fix the collodion picture; it would be liable to get under the film, and remain there after washing the plate, causing a partial obliteration of the picture; for even the hyposulphite of soda solution, when very concentrated, will slowly dissolve the image; the difference in this property between the hyposulphite of soda and cyanide of potassium solutions being only one of degree.
It may be remarked that pictures fixed with cyanide of potassium have a slightly whiter tone.
A very strong and even a saturated solution of hyposulphite soda can be employed to fix the collodion picture. On the other hand, a cyanide of potassium fixing solution should not be of greater strength than the following:
Cyanide of potassium
4 grs.
Water
1 oz.
After the application of the fixing liquid the plate cannot be washed too much.
It is better to be quite sure that no hyposulphite is left underneath, or on the film, than to be sparing of water in accomplishing its removal. The plate should be washed from the centre towards the edges.
Many an excellent picture has been destroyed by want of caution in this respect.
If sufficient water is not within reach when the picture is developed, it would be better to defer the removal of the iodide of silver, and only fix with salt and water a weak solution. Which latter method is altogether, sufficient in as regards the action of light, although it will not remove the iodide of silver.
If after fixing in a salt water bath the plate is washed, dried, and varnished, the picture will be thoroughly protected from any further influence of light.
Very often the fixing with salt and water will be all that is necessary, if the iodide in the film is not very dense, and not likely, by its yellow colour, to interfere with the printing of positive pictures.
The fixing with salt and water has another advantage. When the developed picture is found not to be black enough to print from, and requires a still further density of deposit, the application of a very weak solution of bi-chloride of mercury (three drops to one ounce of water), immediately after immersion in the salt bath, will have an excellent effect; the picture acquires considerable blackness.
This weak solution of bi-chloride will, however, give a yellow colour to the iodide in the film; but this can be got rid of by pouring over the plate a stronger solution of bi-chloride, and washing off immediately with water. After this, the plate can be dried and varnished, without having recourse to hypo-sulphite of soda at all.
If very large surfaces are to be thus treated, a bath of this bi-chloride of mercury can be employed with good effect, and perhaps in all cases it will be found the best mode of proceeding.
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