Carbutt's New Acid Fixing and Clearing Bath.

4 c.c.mSulphuric Acid1 drachm
480 grammesHyposulphite of Soda16 ounces
60 grammesSulphite of Soda2 ounces
30 grammes[D]Chrome Alum1 ounce
1920 c.c.mWarm Water64 ounces

Dissolve the hyposulphite of soda in 48 ounces (1440 c.c.m.) of water, the sulphite of soda in 6 ounces (180 c.c.m.) of water; mix the sulphuric acid with two ounces (60 c.c.m.) of water, and pour slowly into the sulphite soda solution, and add to the hyposulphite; then dissolve the chrome alum in 8 ounces (240 c.c.m.) of water and add to the bulk of solution, and the bath is ready. This fixing bath will not discolor until after long usage, and both clears up the shadows of the negative and hardens the film at the same time.

Let remain two or three minutes after transparency is cleared of all appearance of silver bromide. Then wash in running water for not less than half an hour to free from any trace of hypo solution. Swab the surface with wad of wet cotton, rinse, and place in rack to dry spontaneously. Then varnish with plain collodion.

CHAPTER III.

The Carbon Tissue—(Sensitizing and Exposure).

The carbon tissue used as a resist, which is mounted on the copper plate, is made by the Autotype Company, London, England. No. 100 Standard Brown is the right grade to use, though I have reached good results with No. 103. The No. 100 is a heavier grade than No. 103, and requires two or three minutes longer exposure than the latter. Use a deep printing frame with a screw pressure to secure absolute contact, which is known by iridescent markings appearing on the glass of the printing frame. A Johnson's actinometer is very useful to time the exposure. From 4 to 6 tints are necessary. Experience here is the only guide, as the light varies as well as the density of the negative and the sensitiveness of the tissue. If one does not have an actinometer, a slip of albumen paper may be used; as soon as the paper has reached the darkest point, which is then called one tint, extend it so that a fresh portion comes out to the light, and so on for the different tints. In September, for instance, the darkest tint is reached in about 3 to 4 minutes; two tints and a half or 8 minutes in the shade at midday on a clear day in September is about right,—this is understood to be with medium negatives and No. 103 tissue sensitized within three days.

You should over-expose rather than under-expose, allowance being made when the acid is used. Print deeply, so that, on development, the negative tissue on the copper plate shows all the detail clearly in the shadows. The tissue should not appear very dark on the plate. The copper should show up through the gelatine clearly and brightly. The thinner the negative tissue, the quicker the biting of the acid.