A frisket is sheet of paper or metal used to mask of non-printing areas of the sheet. In this illustration from Wikipedia the pressman on the left is inking the type. His partner is changing paper with the frisket folded back. When the new sheet is in position the frisket will be flipped back over the blank sheet where it will prevent any stray ink from between the type pages from being transferred to the paper.


This diagram from Paisley Dog Press, Reading, PA, gives a clearer idea.

A frisket in operation may be seen on the video “The print workshop in the fifteenth century” from Cambridge’s University Library printing department at the bottom of my recent post on incunabula.