The Collaborative International Dictionary of English v.0.48:

Vachette clasp \Va`chette" clasp\ [Cf. F. vachette cowhide
   leather used for ligatures.] (Veter.)
   A piece of strong steel wire with the ends curved and
   pointed, used on toe or quarter cracks to bind the edges
   together and prevent motion. It is clasped into two notches,
   one on each side of the crack, burned into the wall with a
   cautery iron.
   [Webster 1913 Suppl.]