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A bone folder is a piece of bone, used to apply pressure for creasing paper and pressing glued surfaces together. It is shaped vaguely like a knife, but with only smooth, dull surfaces.
The correct term for this instrument is a 'bone folder'. It is traditionally fashioned from dense wood or cow-bone. Commonly it is a double-sided dull knife with one dull pointed pyramid-apex at one end. The traditional name for a paper knife is a 'shoe knife', used to cut paper a bookbinder uses this in conjunction with the bone folder to fold and cut paperinto 'signitures' (grouped pieces of 2 or more sheets of paper folded against with the grain)
Rather than using a guillotine to cut a books pages all at once, a plane can be constructed to slice pages a few at a time, with many cuts all on the same plane. Douglas Jones described making such a plane from a wood chisel:
I attached the chisel to my plowing jig with a pair of countersunk wood screws that grasped the narrowest part of the shank of the chisel between the handle and flat part of the blade. Most of the flat part of the blade is countersunk into the body of the jig in a slot I cut with the same chisel. The final adjustment of the blade height is critical. I do this by trial and error, adding scraps of paper behind the chisel blade or shank, as needed, to bring the flat of the blade exactly into the plane of the bottom of the jig.
In retrospect, I should have made one jaw of the clamp about twice as thick as the other, because by the time I've plowed off half the pages, the far end of the jig holding the chisel falls off the jaw of the clamp and it's a bit harder to keep the cutting edge of the chisel exactly on the plane I'm trying to follow. The problem isn't serious enough to make me go back and make a new clamp, though.
It is crucial that your chisel be very sharp, and there must be no bevel at all on the flat side of the blade! Careless sharpening will frequently put a slight bevel on the flat side, and this will make your plowing ride up as you work across the book instead of allowing you to hold to the plane established by the jaws of your book clamp.
The plowing jig against the spine of the book.
- H: The chisel handle
- C: The two clamp jaws
- J: The hardwood jig holding the chisel.