Although Dragon's Blood has several different sources and several different uses, this consignment was to be used in photoengraving. Photoengraving is the act of taking a flat copper, zinc or magnesium plate, burning an image on it, and then etching the plate with chemicals to eat away the unwanted portions of the plate material. This would leave a raised area on the plate that would be inked and printed from.
Dragon's Blood is acid resistant, and is used to reinforce the coating that the engraving plates have on them. This photo sensitive coating is exposed to a bright light, and any imperfections in the plate are touched up with Dragon's Blood before they are etched.
In the 1921 Ostrander Seymour Company catalogue, Dragon's Blood is sold with this description: The use of dragon's blood as an acid resist in the burning-in of plates is universal, and the brands we carry are in general use in the most progressive photo-engraving plants in the country.
No. A, dark red, 1 pound can, $2.00
No C. pink top powder, 1 pound can, $.75
Special red, our leader, 1 pound can, $1.50
(From the prices, I am assuming that the dark red was fairly pure, the special red a kind of house blend, and the pink top powder was the economy version.)
Brown Brothers & Company was the consignee for this shipment of Dragon's Blood. They were -- as far as I have been able to find out -- distributors of printing equipment and supplies on the east coast of the United States. (Not to be confused with the banking interest of the same name.) I have precious little on this firm, but am personally very active in the trade that once used Dragon's Blood quite extensively.