sighten means to add a fugitive colour to (a paste), to enable the printer to determine whether the figures are well-printed or not. Lexicurio rates it Rare gem — a strength score of 82 out of 100.
Etymology
From sight + -en.
verb
- To add a fugitive colour to (a paste), to enable the printer to determine whether the figures are well-printed or not.“The iron liquor should be sightened with logwood.”