Microtomy is a common method for preparing thin sections of the sample for TEM analysis. A diamond knife slices out a block of the material. This can be done under ambient conditions if the material is sufficiently hard, or under liquid nitrogen conditions if, for instance, a polymer needs to be frozen. Packaging materials, labels, multilayers and, in general, any plastics or rubber compounds can be cut and will often require staining with a heavy metal to provide contrast in the TEM. Some metals and oxides can also be microtomed, but most ceramics and minerals are too hard, have a tendency to chip the knife or are too brittle themselves and, therefore, break apart during the “controlled fracture” of the advancing knife. These materials require ion milling.