180. In the preceding- chapter the process was covered in a general way. We did not explain the whys and wherefores of any particular part of the process, believing that the worker, after having had some experience with the various manipulations, will more quickly comprehend the terms which we will use in explanation of the different phases of the work.

181. In future instruction each department will be taken up separately, and instruction given on how to produce equally good results by different methods. We will supply different formulae, so that should you experience difficulty with any one method the other can be tried.

182. In this instruction we will treat on the sensitizing of the tissue. We wish to dwell principally upon cleanliness and the care of the materials with which you are working. This being the foundation of all successful operations, we consider it advisable to guard you against expensive failures by cautioning you at this point.

183. It is very important that all chemicals and materials used in the carbon process should be kept in a cool, perfectly dry place. A stuffy room will not do. A room with good air, perfectly dry and free from gases or odors of any kind, is preferable.

184. All chemicals should be properly labeled. Your paper should be placed in boxes, and held flat by the weight of a piece of glass, or anything flat. Carbon tissue is apt to curl with age, thereby making it quite difficult to sensitize, and in trying to straighten it out you are apt to crack the film. To avoid this, keep the paper in its original wrapper. The best method of keeping carbon tissue in a pliable condition and ready for use is to store it in a tin receptacle which can be kept well closed.

185. The Work-Room

The Work-Room. The real causes of many failures in carbon printing are found to come from poor workrooms-rooms containing foul air. dampness, mustiness, odors from gas pipes, oil stoves, or the like. Such rooms are not fit for sensitizing the carbon. Steam heat is the best. Gas or coal stoves can be used, but see that no gas escapes, and allow no odor in the room from these gases. Never use kerosene. Few workers seem to understand the importance of a good, clean work-room. A properly arranged room, with the necessary apparatus, should be the first consideration. You require no elaborate, expensive apparatus. All that is necessary is cleanliness and plenty of fresh air, and a careful worker should have everything in its place. With every article in its place, properly labeled, the worker cannot help being successful.

186. The proper light for a work-room is similar to that of the ordinary toning-room. The only important difference is, that the light has no particularly bad effect on the unsensitized tissue, yet it is best to keep it in subdued light. The tissue in its wet state can be handled in good light, yet a subdued light is better. The paper grows sensitive only as it dries. When dry, it is two to three times as sensitive as printing-out paper; therefore, the drying-room must be absolutely light-tight.

187. The room used for sensitizing the tissue should not be used for the drying-room, as you are apt to spill some of the solution and cause dampness in the room, which will delay the drying of the carbon tissue. Moisture in the room is apt to cause mustiness and mold in the tissue, and long, slow drying will cause tear drops and streaks; so do not attempt to dry your paper in your sensitizing room.

188. The sensitizing bath given in Paragraph 130 will generally work under all conditions of weather. The bichromate salt is the actual sensitizing agent, but various other chemicals are frequently added to offset certain conditions of weather, or to improve the quality of the tissue. In cold weather the strength of the sensitizing bath must be increased, as the action of the cold decreases the sensitiveness of the tissue. Very frequently bichromate of ammonia is used in place of the potash, as the ammonia salt has greater sensitizing power for thin or weak negatives, so when troubled with weak prints we advise the use of the ammonia salt.

189. In damp weather, or in climates where there is considerable moisture, the following formula will work well:

190. Formula No. 3. -

Water.......................................

98

ozs.

Salicylic Acid............................

100

grs.

Bichromate of Potassium....................

2

ozs.

Plain Glycerin .......................................

1

oz.

Dissolve the acid in a little of the water and the potassium in the rest of the 98 ounces. Then add the glycerin, which acts as a restrainer, and filter through absorbent cotton.

191. With very strong negatives the bath should be increased to 5% strength of bichromate. On the other hand, if the negatives are weak and thin, the strength of the bath should be reduced to as low as 1%, if necessary. In this way considerable control is had over results from negatives of different densities.

192. In hot weather the tissue is liable to become insoluble before it is dry if a strong bath is used; therefore, a weak bath should be employed, averaging about 2% in strength.

193. A very good summer bath, one which has proven very satisfactory in the hands of competent workers, is as follows:

194. Formula No. 4.-

Bichromate of Potassium........................

2,

ozs.

Carbonate of Ammonium...........................

1

dr.

Pure Alcohol........................

2

ozs.

Water.........................

100

ozs.

105. If the negatives arc thin and flat, add 1 ounce of glycerin. For winter use this same bath can be increased to 3 ounces of bichromate. In summer the sensitizing bath should have a temperature of 50° Fahr., and certainly not over 60°.

196. Carbon tissue is much more soluble in a bichromate solution than in water, at the same temperature. If the sensitizing room is warm, the sensitizing bath should be set in a water bath, to which ice has been added. In winter the bath may register from 60° to 70° Fahr.

197. Use hot water for dissolving the bichromate salts, as they do not dissolve readily in cold. It is well to make up the different baths and label each bottle or jar with the formula printed thereon. The sensitizing baths will keep well if placed in dark colored bottles and protected from the light.