In our studio we have obtained excellent results with two kinds of colour sensitive collodion. These are the isochromatic collodion emulsion prepared by Dr. E. Albert and that suggested by Major von Hübl. In using the former, the glass plates as soon as they are taken out of the acid bath and well washed must be given a substratum of the following solution:—

to which is added—

The solution should be filtered and coated whilst warm, and the plate dried in a room free from dust at least 15° R.

For the production of the sensitiveness, or for the production of the correct reproduction of the colours, two eoside of silver dyes, R. and P., are used. The P. dye gives great sensitiveness, and is especially suitable for portraits, landscapes, etc. In using this a {42} correct reproduction of the red must be abandoned, whilst the R. dye gives an absolutely correct colour rendering but requires about double the exposure. Ten ccm. of the eoside of silver solution are added to 100 ccm. of the emulsion. When coloured the emulsion will only keep for one or two days.

The time of exposure with the R. dye is normally from one-and-a-half to three minutes.

After exposure the plate must be well washed under a tap and then allowed to thoroughly drain, or otherwise developing streaks will show.

Hydrochinone developer is used for developing, consisting of three solutions:—

This developing liquid is concentrated, and is used as follows:—

To obtain hard negatives solution B is increased from 1 to 5 times. A mixture of 150 parts of the above solution with 1000 parts of distilled water forms the actual developer. By the addition of more concentrated developer or increasing the quantity of water the developer may be suited to the character of the subject. The developer must be freely flowed over the plate, and then the plate fixed with sodium hyposulphite. Intensification is effected with—

and when dissolved

are added.

Immediately before use 100 parts of solution A are mixed with 5 parts of solution B, and the intensification continued till the desired density is obtained. Negatives can be reduced by placing them whilst damp in a solution of—

When this has acted sufficiently, the plate is well washed, {43} flowed over with a five per cent. solution of potassium cyanide, and then well washed. When the dyed emulsion is used this must be done, after developing, in red light.

A very excellent method of preparing an orthochromatic collodion emulsion has been described by Hübl,2 which can be used either wet or dry. Repeated trials made in the Imperial Military Geographical Institute, in our establishment, and in the Photographic Institute in Vienna have proved the advantageous use of this method for orthochromatic work. The raw emulsion, according to Hübl, is prepared as follows:—40 g. of silver nitrate are dissolved in 50 ccm. of warm distilled water, and as much ammonia added as will give a clear solution; then 100 ccm. of alcohol are added and the whole allowed to cool. 30 g. of ammonium bromide are dissolved in 35 ccm. of water and 70 ccm. of absolute alcohol with gentle heat.

To make the emulsion 450 ccm. of four per cent. raw collodion are placed in a strong glass flask which will hold a litre, and to this in the dark room the silver solution added. Some of the wool separates out, but by vigorous shaking this can again be dissolved. A part of the silver salt also remains suspended in the liquid in the form of fine crystals. The ammonium bromide is added in three or four parts whilst warm. After shaking thoroughly for several minutes the emulsion is precipitated by distilled water, which is repeatedly added till the separation is complete. The emulsion, which is now powdery, is placed on a linen filter, washed several times with distilled water, squeezed to press out the last of the water, damped two or three times with alcohol, and then pressed. Whilst still damp with alcohol the emulsion is dissolved in from 800 to 1,000 ccm. of alcohol and ether, mixed with 0·5 grs. of codeine, and allowed to rest for some days, when it will be ready for use.

The preparation of the eoside of silver solution is effected as follows:—10 g. of yellow-shade eosine are dissolved in 250 ccm. of boiling water and 5 grs. of silver nitrate in 50 ccm. of water added hot. The solution is allowed to settle, then filtered, and then washed first with boiling water and then with alcohol on the filter, and then dried in a dimly-lighted room.

To prepare the dye solution 0·5 g. of the dry eoside of silver and 1 g. of ammonium acetate are gently heated together in 30 ccm. of alcohol till dissolved, then 120 ccm. of alcohol and 10 ccm. of glacial acetic acid are added, and the whole filtered. For plates to be exposed wet, 5–10 ccm. of glycerine are added. The emulsion is sensitized by adding one-tenth of its volume of dye, and should be well shaken before coating the plates. The glass plates should receive a preliminary substratum of gelatine or india-rubber, so that the frilling of the film and impurities of the glass plates may be totally avoided. {44}

For small sized plates the emulsion is used somewhat thicker than for larger, and for the latter it should be diluted with alcohol and ether. After coating they should be dried at about 15° C., and then for some minutes exposed to a temperature of 30° C.

The plates should be developed with the following mixture:—25 g. of sodium sulphite should be dissolved in 40 ccm. of warm water, then 10 g. of glycine added and 50 g. of potassium carbonate, at first in small quantities in consequence of the evolution of carbonic acid. When cool about 75 ccm. of a thin pasty liquid will be obtained, which may be kept as a stock solution, and before use must be well shaken and diluted with from 12–15 times the quantity of water. Wet plates should be washed before development, and the developer flowed on; dry plates should not be washed, and should be developed in a dish.

They should be fixed in sodium hyposulphite. The intensification may be effected with metol (see p. 39), and the blackening as described on p. 38.

Obviously the collodion emulsion may be used for ordinary work undyed.

The qualities of the cross-lined screen and the distance of the same from the sensitive plate will be described in Chapter V.