I have a framing job in the workshop. It’s a print from the 1960’s signed by the artist and is of some value. On taking the print out of the existing frame I found the barrier board seen in the photograph below.
The barrier board sits between the print and the backing board, isolating the print to avoid acid burn from the backing board.
10 smarty points to anyone who can explain the reason for the marks on the board (answers will be posted here in a few days).