There is a formal test to measure wringability. First, the block is prepared for wringing using the standard process. The block is then slid across a reference grade ( flatness) quartz optical flat while applying moderate pressure. Then, the bottom of the gauge block is observed (through the optical flat) for oil or color. For Federal Grades 0.5, 1, and 2 and ISO grades K, 00, and 0 no oil or color should be visible under the gauge block. For Federal Grade 3 and ISO grades 1 and 2, no more than 20% of the surface area should show oil or color. Note that this test is hard to perform on gauge blocks thinner than because they tend not to be flat in the relaxed state.