480 HANDBOOK OF PHOTOGRAPHY 



sweeter print. This is by no means true of platinum alone but holds good with most 

 printing mediums. 



The Paper. — Practically any paper which is neither too absorbent nor too highly 

 sized can be used. Satisfactory results have been obtained on Whatman Cold Pressed 

 drawing papers, on Strathmore, Michallet, and Lalanne charcoal papers, on Strath- 

 more Alexandra Japan Vellum, on the Shidzuoka Vellum of the Japan Paper Com- 

 pany, and on various linen letter and typewriter papers. 



To determine whether or not any given paper will work well, pin a piece of it 

 about 8 by 10 in. on a table, pour on it % dram of water, and brush this water back 

 and forth with the sensitizing brush. If the paper is surface dry, or nearly so, in less 

 than 30 sec, it is too soft and should be sized (see page 483, under Multiple Printing) 

 before use; if it is still wet after 2 or 3 min. of brushing, it is too hard, and should be 

 rejected. 



It should be noted that different qualities of paper give very different contrasts. 

 This will be discussed later, under Increased Contrast, page 482. 



Sensitizing Brush. — This should preferably be a flat Japanese paintbrush, as sold 

 in various Japanese art stores. The advantages of this type are that the bristles are 

 short and are set in a thin row, causing little waste of sensitizer, and that no metal is 

 used in its construction. Three inches is a convenient width for prints up to 16 by 

 20 in. 



If such a brush is not readily obtainable, a painter's graining brush may be used, 

 though if this is bound with metal, some steps should be taken to protect the sensitizer 

 from contact with the metal ferrule. This may be done by diluting Duco cement with 

 a mixture of acetone and alcohol and introducing a small quantity of the dilute 

 cement among the hairs, at their base. Capillary attraction will spread it, forming a 

 base past which the sensitizing solution will not go. It will also shorten the effective 

 flexible length of the hairs, but this does no harm, since the hairs in such a brush are 

 longer than necessary. 



The Solutions. — Make up three solutions, as follows: 



Solution I 



Water (distilled), warm 55.0 cc. 2 oz. 



Ferric oxalate 15.0 g. 240 gr. 



Oxalic acid 1 . g. 16 gr. 



Solution II 



Water (distilled), warm 55.0 cc. 2 oz. 



Ferric oxalate 15 . g. 240 gr. 



Oxalic acid 1 . g. 16 gr. 



Potassium chlorate . 3 g. 4 gr. 



Solution III 



Water (distilled), warm 65 cc. 2% oz. 



Potassium chloroplatinite 13.0 g. 219 gr. (equals J.^ oz. avoir.) 



Note that ferric, not ferrous, oxalate is used. This should be in the form of dry 

 bright-green scales. If the scales have a brownish tinge or show any tendency to 

 stick together, the sample is stale and should be rejected; this chemical must be 

 perfectly fresh. Note also that potassium chloroplatinite, not chloroplatinate, must 

 be used. This has the form of bright ruby-red crystals, the chloroplatinate being 

 yellow. No trouble is likely to be caused by the other chemicals, provided reasonably 

 pure samples are obtained. 



These solutions are the basic ones from which the actual sensitizer is made, and 

 they should be protected from strong light. A convenient plan for storing and 

 handling them is to keep the solutions in three properly labeled brown-glass 2-oz. 

 dropping bottles, with a medicine dropper in each. These droppers should be of the 

 guaranteed type, giving drops of equal size. If a single dropper is used for all three 



