Originally Posted by
Neil Miller
Definitely not stamped, but acid etched.
I have seen the rubber dies used to coat the blade with the resist in some book or other - they look like a regular old-school printing stamp and were dipped in liquid resist (either wax or a gum - usually 'gum arabic' - to which a light-sensitive hardener like potassium dichromate is added) then stamped on the scrupulously clean blade.
Exposure to a strong UV light source - or even the sun - hardened the gum resist. Unfortunately the pot. di. also gave off carcinogenic vapours, despite being in use regularly when I was at college in the late 1970s and used for silk-screen printing and hardening felt in hat making. Seems like the poor old hat-makers either got it from mercury or pot di inhalation!
Photographic processes were also used, having the benefit of the whole blade being dipped in the resist, a negative applied, exposure to strong UV light which hardened the resist not covered by the dark areas of the negative, then washing off the soft areas and dipping in acid.
In the other process, once the rubber stamping had dried a resist was applied to the rest of the blade by hand - usually a bitumen-based printers resist.
Regards,
Neil