| From the field of printmaking, ruby lith is thin, flexible, translucent, dark red material, used for masking areas of a light sensitive medium that shouldn't be exposed. This is useful for adding a pattern to a burn or dodge, or fixing mistakes that have already been committed to master. It is slightly tacky on one side, and mostly stays in place on acetate or film or whatever. It comes in individual sheets stuck to a piece of acetate, or on a roll like tape. To use it, you usually use an x-acto knife to trace out whatever mask shape you want, then peel off the shape and put it wherever it's needed.
There are machines that use lasers to cut pieces of ruby lith very finely and in intricate shapes, but shops that can afford them have mostly gone digital nowadays. In fact, the use of ruby lith by any print shop is pretty archaic, since digital processing has become so cheap and common. This is too bad, as cutting and aligning a mask by hand is something of an art, or so I've been told.
Ruby lith fun facts:
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