Moulding - Wood mouldings contain lignin, the same acidic material found in wood pulp matboard. Even if the frame is separated from the artwork by a mat, acid can still migrate over time, possibly reaching the art. To prevent this, the frame's rabbet can be lined with an acid-free barrier tape produced for just this purpose. The rabbet can also be shellacked before taping, providing extra protection.

Note: Artwork that has already been contaminated by acid can sometimes be de-acidified. This process is especially useful for older items such as engravings that may have been improperly framed many years ago. By neutralizing the acids in the artwork, further deterioration and discoloration can be prevented. Paper that has been badly discolored can often be bleached without harming the pigments.