The following article shows iron gall ink as nearly transparent in 900-1500nm: You are not allowed to view links. Register or Login to view. (needs an account to access)
There are photos of test panels showing how 8 different writing materials are visible by themselves and when covered with 8 different paint types, in all possible combinations in visible light, 900-1500nm and 1500-2000nm. Iron gall ink looks nearly transparent in either IR band both when covered and by itself. Quoting from the article:
There are photos of test panels showing how 8 different writing materials are visible by themselves and when covered with 8 different paint types, in all possible combinations in visible light, 900-1500nm and 1500-2000nm. Iron gall ink looks nearly transparent in either IR band both when covered and by itself. Quoting from the article:
Quote:In summary, all the pigments studied are penetrated in the infrared, with the exception of lapis lazuli and chrome oxide green. The underdrawing materials that are rendered visible include mined black chalk, leadpoint, silverpoint and carbon black ink. The underdrawing materials that become transparent, and are not therefore visible, include red Conte crayon, charcoal and iron gall ink.
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