AMATEUR PHOTOGRAPHY. 



477 



AMATEUR PHOTOGRAPHY. 



GUM PRINTING. 



W. E. CARLIN. 



In October Recreation " C." speaks of 

 making gum prints by coating the paper first 

 and sensitizing afterward, exposing in the 

 sun and developing in hot water. I hope he 

 will pardon me for differing with him as to 

 the best method of working this process. 

 There is a well-known principle in photog- 

 raphy, that any colloid, such as starch, gela- 

 tine, gum, albumen, etc, when mixed with 

 the bichromate salts will become insoluble 

 in water, and rapidly so when exposed to 

 light or to undue heat. If a coloring matter 

 be mixed with the colloid and bichromate 

 and this mixture, coated on a sheet of paper, 

 exposed to light under a negative and then 

 washed in water, the parts which have not 

 been rendered insoluble by light will dis- 

 solve away, leaving a photographic positive 

 in the color supported by the colloid. The 

 best known form of this process is the stand- 

 ard carbon, in which gelatine is used. This 

 not being easily soluble in cold water, may 

 be sensitized after coating; but not so with 

 gum, which is easily soluble in cold water 

 and which will dissolve away, leaving little 

 strength of color on the paper. 



The gum process is an old one and was 

 formerly discarded for the very reason it is 

 now popular, i.e., a lack of sharp detail and a 

 softness not given by any other printing 

 medium. It also admits of the greatest con- 

 trol in selective development, thereby rid- 

 ding the photograph of its former more or 

 less mechanical reproduction and affording 

 the photographer a chance for individuality. 

 Unless one has the taste and artistic ability 

 to profit by this power of selective develop- 

 ment, he would better stick to the carbon 

 as being simpler and more satisfactory. 



An outline of the working methods are as 

 follows. 



Paper: Any well sized brand in which the 

 pigment will not sink into the actual fibres 

 of the paper. 



Glazed surface: Too difficult to coat and 

 a tendency to harsh contrasts. 



Rough surface: Almost too much loss of 

 detail and not easy to expose and develop 

 well. 



Smoothish and medium surfaces are the 

 most satisfactory. Use any ordinary writing 

 paper. 



The French charcoal paper — Allonge — is 

 excellent. For general work Michallet is 

 another good paper and easy to coat because 

 of its parallel lines or ribs. 



Sensitizing: The sensitizing solution is a 

 saturated or 10 per cent, solution of bichro- 

 mate of potassium. The paper may be sensi- 

 tized, first by immersing in this solution for 

 2 minutes, care being taken to remove all 

 air bubbles from the front and back of paper 

 as soon as possible, when it will be as sen- 

 sitive as carbon or nearly as sensitive as 



platinotype. Or, the bichromate, gum, and 

 color, may all be mixed and applied at once. 

 When the paper is very slow, about twice as 

 slow as P. O. P., strain bichromate before 

 use and see that the paper is bone dry before 

 sensitizing or coating. 



The pigment: This may be any that will 

 dissolve in water, pastels, chalks, charcoal, 

 etc. ; but that mostly used is pure water- 

 color paint, either in tubes, cakes, or pow- 

 der. The cakes and powder should be 

 ground into the gum solution thoroughly — 

 tube colors mixing more easily. If the 

 whole mixture is to be applied at once, the 

 bichromate is added, when the paint and 

 gum are thoroughly mixed, and the whole is 

 strained through medium fine muslin before 

 use. 



Some of the best and easiest colors to use 

 are Venetian red, light red, Indian red, lamp 

 black, burnt umber, ochre, and indigo, while 

 raw umber mixed with prussian blue gives 

 a useful range of tones. The dishes should 

 be porcelain or glass and perfectly clean. 



Gum solution: Dissolve 2 ounces pure 

 gum arabic in 5 ounces cold water. It may 

 be pulverized and dissolved in 20 minutes by 

 stirring, or may be put in in lumps and will 

 dissolve in about 24 hours. Gum in solution 

 does not keep long without becoming sour; 

 although a drop of ammonia or chloroform 

 will retard this. When sour it is apt to be- 

 come insoluble and produce stains. There- 

 fore it is best to use it fresh. 



Brushes: Two brushes are necessary. 

 The coating brush may be any soft brush 

 that does not hold too much color, such as a 

 thin pigs' hair, having thinly set and rather 

 rigid bristles, or a flat thin camels' hair will 

 do. The smoothing brush should be a 

 badger hair softener or blender, about 3 or 

 4 inches wide and having 3 or 4 rows of 

 hairs. 



To coat the paper: Having previously 

 sensitized the paper and having it bone dry 

 pin a sheet on a drawing board at the 2 up- 

 per corners. No exact formula can be given 

 to suit all purposes, Mr. Warren recom- 

 mends the following as an example: y 2 

 ounce of the 2 in 5 stock solution of gum 

 is mixed with y 2 ounce of water. This is 

 mixed with say 20 grains burnt umber, or 14 

 grains black dry paint, thoroughly ground 

 with a palette knife and strained. 



Moisten slightly the coating brush, squeez- 

 ing it to remove all superfluous moisture; 

 dip up a small quantity of solution and apply 

 rapidly, dipping up more if necessary until 

 paper is thinly covered. Do this as quickly ' 

 as possible. Then with softener held ver- 

 tically to paper make several firm strokes 

 downward, then several crosswise and in 

 every direction, the strokes getting softer 

 and softer. The sheet should be coated in 50 

 seconds or so, as the film sets in about that 

 time and thereafter should not be touched. 

 The result should be a level and very thin 

 coating. The texture of the paper should 



