Why drawn iron wire curves and twists
A flat copper drawplate; wire curls because its concave side is shorter; twisting makes square rods concave
The page reasons about the shape metal takes as it passes through a wire-drawing die. Leonardo states that iron wire not drawn in a straight line becomes curved because its concave side is shorter than its convex side. Two drawplate (trafila) figures at upper right accompany a note on a flat die of copper clad in a non-tarnishable material, and a threaded rod is drawn beside them. He further explains that a rod square in section develops concave faces when twisted, because its corners crowd together as they lengthen in revolving around the body's central line.
On this page
Wire curls because its concave side is shorter
Iron wire that is not drawn through the die along a straight line becomes curved, because the concave side of the bend is shorter than the convex side.
A flat copper drawplate clad against tarnish
Two drawplate figures illustrate a flat die ('ignia piana') made of copper ('emar') clad in a non-tarnishable material.
Twisting turns a square rod's flat faces concave
The straight sides of a rod square in section become concave: as the corners revolve around the body's central line they gain length and crowd closer together, and in approaching one another they make concave the flat surface that lies between them.