(57) Acetylenic compound having at least two conjugated triple bonds in the molecule and polymerizable in the cyrstalline solid state to produce coloration, but relatively stable in liquid form, and having substituents of the group consisting of urethane, hydroxy, carboxy, carboxylic acid, carboxylic ester, carbazolyl and sulfonate, are used as printing ink. Preferred compounds are diynes, especially urethanes, diols and arylsulfonates melting in the range 40-150°C. They are employed as a melt at 40°-200°C adhering to the printing surface, whence they are transferred by contact to a substrate; and developed by cooling and exposure to a source of thermal or radiation energy.