To test the amount of silver nitrate in solution take, say, half an ounce, and precipitate by adding excess of hydrochloric acid. Filter off, dry the filter paper, and weigh the amount of chloride. Multiply this weight by 1.18 and it will give the amount of silver nitrate.

Exposure

Ordinary dark slides are not suitable for wet-plate work; the most useful pattern is a single slide opening at the back. After draining the plate a piece of blotting should be put in the lower edge of the dark slide to receive any further drippings; otherwise these drainings may in hot weather creep up the film by capillary attraction, and produce odd markings. With regard to exposure, it is difficult to lay down any precise rules; with experience, the operator will learn to guess the time required fairly accurately.

Development

The usual developer is the ordinary copperas or sulphate of iron.

Ferrous Sulphate (saturated solution) . . . 5 oz.

Acetic Acid (glacial)....... 2 ,,

Alcohol......... 1 1/2 ,,

Water......... 35 „

The exact amount of alcohol is not important, but more must be added if the developer does not flow evenly over the plate, which is not immersed as in dry-plate development, but held over a sink in a pneumatic holder, and a quantity of solution poured evenly over the surface. As little as possible of the liquid must be allowed to escape into the sink, as it will carry away much of the free silver, which should be redeposited on the image; and the developer should cover the whole film at once, or markings will result. Rock very gently until the fine detail is visible, when it is rinsed carefully and fixed, not in the usual hypo bath, but in a ten per cent. solution of potassium cyanide. Other developers are:

Ferrous Ammonio-sulphate . . . . . 150gr.

Acetic Acid Glacial....... 150 min.

Copper Sulphate....... 15gr.

Alcohol . . . . . . 1 oz.

Water......... 9 ,,

An acid pyro developer is sometimes employed, but requires longer exposure than ferrous sulphate, and bromides must be omitted from the iodiser. However, it gives a very dense black image which is most excellent for copying purposes.

Pyrogallic Acid.......10 gr.

Acetic Acid........150 min.

Alcohol.........1/2 oz.

Water.........10 „

Intensification

Before fixing, the plate may be examined and, if too thin, redeveloped with:

A. Ferrous Sulphate....... . 10 gr.

Acetic Acid........ 10 min.

Water......... 2 oz.

B. Silver Nitrate........ 15 oz.

Water........ 1 "

Mix A and B immediately before using. If the weakness of the negative is not discovered until after fixing, either of the mercury or copper intensifies used for dry plates may be applied.

Collodion Emulsion

This may be procured sensitised ready for use, and it is especially coming into favour for colour work, as the colour sensitisers can be added to the emulsion before coating. With the ordinary wet plate, collodion emulsion has little in common, except that it has to be coated on the glass plate before using. The chief difficulty in its manufacture, as compared with gelatine emulsion, is in the washing out of the soluble salts.

Care Of The Collodion

Always keep the collodion bottle in a cool place, and use ordinary care in decanting it, or when coating plates, with regard to any gas jets or artificial light near adjacent. It is inadvisable to perform these operations near the fire, in view of the fact that ether vapour is heavier than air. In the days of the wet plate disastrous explosions were not uncommon, owing to such accidents as a cat upsetting a bottle of collodion, the contents of which leaked into other apartments. For this reason we would suggest that it should be stood in some safe cupboard and not on an open shelf; above all, not in the dark room.

For fuller particulars regarding collodion the following works may be consulted : Wet Collodion Photography, C. W. Gamble; The Wet Collodion Process, Arthur Payne; Collodion Emulsion, H. O. Klein.