| No. 1, 42 | deg. | Beaumé; | spec. grav. | 1.420 |
| No. 2, 38 | " | " | " | 1.375 |
| No. 3, 35 | " | " | " | 1.330 |
| No. 4, 31 | " | " | " | 1.285 |
The plate is now ready for biting. Keep a record of the bitings, and length of time for each one, for after-study; also note the time of exposure of the tissue, age of same, etc., etc.
Pour the acid from a glass graduate with one sweep over the plate, removing all bubbles with a feather, noting the time of immersion so as to guide you. Start with 42 deg., having ready the 37 deg. in another graduate, watch carefully the action of the acid, and if the resist has been properly printed, the action of the acid will show after a minute; if longer it means a generally longer biting for each bath.
Average Bitings.
| 42 deg., No. 1 | 5 minutes |
| 37 deg., No. 2 | 5 minutes |
| 33 deg., No. 3 | 2 minutes |
| 30 deg., No. 4 | 2 minutes |
| Temperature of bath at 70 deg. Fahr., with No. 103 tissue. | |
Total of different bitings, from 10 to 25 minutes, according to depth of printing. It always varies. There is no hard and fast rule; you must in time learn to judge by your eye alone. The acid will first attack the thinnest part of the film, wherever that may be, and when the darkening of the copper ceases to spread to the next thickest parts, instantly pour off the acid, and pour on the 37 deg. Do not allow the atmosphere to act on the gelatine while biting any longer than is necessary to pour off one bath and quickly pour on a new one. The 37 and 33 deg. baths are for the middle tones, the 30 deg. for the most delicate ones. The action of each bath is cumulative, the 37 deg. biting a little where the 42 deg. had bitten, the 33 deg. doing the same for those before it, besides taking care of itself, and the 30 deg. attacking all more or less. During the biting with the 30 deg. solution, it should be continued until the whites just turn color, and a minute beyond; that is, the copper should begin to show a very little under the thickest and darkest film.
(Note that in the carbon resist the shadows are transparent and the high lights are opaque.)
The length of the last biting very seldom is over two minutes. It is better to overbite your darks, and underbite your lights, if you vary any.
The amount of moisture in the air and the heat of the day influences the length of biting. In hot weather in summer it is very difficult to work the process, the walling wax being discarded and the copper (back and edges protected by varnish) placed in a porcelain tray, surrounded by ice-water and kept at 65 to 70 deg., and the acid pured over the plate to the depth of one inch.
