Cast iron is commonly adopted as a lap material because of its low cost and consistent accuracy on the lap surface. According to metallography, cast iron is a type of polycrystal metal, consisting of hard pearlite iron, soft ferrite iron, and free carbon. Such pearlite iron has a layered microstructure with the hard cementite and soft ferrite layers. It is said that fine irregularities are easily formed on a lap surface, which allows it to hold abrasives in the recessed regions and makes lapping advantageous.
On the other hand, in some cases tempered steel is used to meet the requirements for a hard lap. Glass or ceramics are also used to avoid the contamination of metal ions.