Powderless Deep Etching of Copper for Photoengraving and Chemical Milling.

Formamidine disulfide affords insufficient sidewall protection to permit control of sidewall etch for depth greater than .008 - .010. Ethylene thiourea provides protection to significantly greater depths, but sidewall structure is not suitable for letterpress printing. Mixtures of the two compounds provide protective effects and sidewall slopes intermediate between those of the individual constituents. By proper selection of additive composition and machine settings, a wide range of sidewall slopes may be reproducibly obtained, permitting application of this chemistry to letterpress engraving and chemical milling.