If a washer or other piece of wrought-iron is a trifle too large, fill the hole and part of the outside faces with fire-clay; heat the iron to a bright red, and plunge it in cold water. The contraction of the circumferential iron will squeeze the metal round the hole (which has been prevented from cooling so rapidly by the clay) inward, diminishing the size of the hole. To refit a bolt to a hole in which it has worn a trifle loose, case-harden it by the prussiate of potash process, which will increase the diameter of the bolt. If it fits into a hole of wrought-iron or steel, that too may be case-hardened, which will diminish its size, and thus refit it to the bolt.