SPECIAL PRINTING PROCESSES 489 



It is advisable to store the gum solution in a wide-mouthed screw-cap bottle or, if 

 the bottle has been ground with reasonable accuracy, merely to lay a piece of glass over 

 the top, to prevent evaporation. The neck of the bottle should be wiped each time 

 that any of the solution is poured out, to avoid cementing the cap fast. 



The Sensitizer. — The sensitizer commonly recommended is potassium bichromate, 

 but the writer prefers sodium bichromate, as it is far more soluble than the potassium 

 salt, wherefore a given amount of a saturated solution contains much more of the 

 sensitizing agent and gives a much more rapidly printing film. The formula follows: 



Water, warm 500 . co. 16 oz. 



Sodium bichromate (technical) 500 . g. 1 lb. 



Filter, and store in a brown bottle. This will keep indefinitely if not exposed to 

 strong light. It is approximately a saturated solution. 



Sizing. — If it is found necessary to size the paper, this may be done by the 

 method which is used for making single transfer paper for carbon printing and is 

 described in the section on that subject. One, or at most two, applications will be 

 sufficient. 



Coating Mixture. — As the longest scale of gradation is secured when the coating 

 mixture contains the largest possible amount of pigment and as a long scale is usually 

 desired, it follows that the coating mixture should hold as much of the pigment as can 

 satisfactorily be used. But for every paper, every pigment, and every gum solution 

 there is a maximum relation of pigment to gum which can be used without staining the 

 paper — or rather, to be precise, there are two such maxima, one for automatic develop- 

 ment, the other for brush development. The method of determining these maxima 

 is as follows: 



Squeeze into a small mortar an inch length of the pigment, and rub this up with 

 K dram of the gum solution. With a fine brush dipped into the mixture, make a small 

 mark on the paper which is to be used, and opposite this mark, pencil "1 in. to 3^ 

 dram." Add }i dram of gum solution to the mixture, rub it up well, and make another 

 mark, labeling this "1 in. to 1 dram." Add another }4 dram of the gum solution, and 

 label the resulting mark "1 in. to IJ^ drams." Continue thus until a series of m.arks 

 extending to "1 in. to 10 drams" is reached. Then allow these gum-pigment marks 

 to dry thoroughly, and let the paper float face down in a tray of water at room tem- 

 perature for 3^2 hr. On inspection it will be found that some of the marks have 

 entirely disappeared, while others remain visible. Suppose, for example, that the last 

 visible mark is opposite the notation "1 in. to 4,^2 drams"; then it is known that, if 

 pure whites are to be secured with automatic development, the maximum proportion 

 of pigment to gum solution in the coating mixture must be 1 in. to 5 drams. Now with 

 a soft camel's-hair brush, brush over the remaining marks, when it will be found that 

 others will disappear. As an example, suppose that the last one visible after this 

 brushing is opposite the label "1 in. to 2 drams"; then it is known that, if brush 

 development is to be used and pure whites are to be obtained, the maximum allowable 

 proportion of pigment to gum is 1 in. to 2}^ drams. If a note is made of these propor- 

 tions, it will be possible at any future time to predict accuratelj^ the maximum gum- 

 pigment relationship for that pigment and that paper. This should be done for the 

 various pigments which are to be used and for the various papers. A table can then 

 be drawn up giving the sundry relationships at a glance, thus avoiding the "by-guess- 

 and-by-gosh" method so common in gum printing. 



This method serves also to indicate the possible maximum when two or more pig- 

 ments are mixed to secure variations in color. Thus, if it has been determined that a 

 certain black requires 5 drams of gum solution to 1 in. of pigment for automatic 

 development and burnt umber requires 4 drams to 1 in., then, if it is desired to mix 



