Polarisation microscopy can detect stresses inside of transparent single-crystals, which is in particular important for monocrystalline diamond for jewelery [KLEB98, LENZ86]. Lattice defects in the crystals result in interference patterns and are recorded in double refraction pictures [MALZ00]. The results can be improved by embedding the grits into material with the same refractive index so that light refractions are avoided at the grit surface and only the internal refractions due to lattice defects appear.
2.9.2 Magnetic Susceptibility Selection and Analysis
ANSI B74.19 presents a test method to determine the magnetic content of conventional abrasive grits [MENA00, ANSI90]. A standardized magnetic analyzer is used on a sample size of 150 g. Its two coils, one as a reference, one for the sample, are excited with an odd frequency AC source (390 Hz) [ANSI90]. The unbalance is measured and translated to the relative magnetic content of the sample [ANSI90]. Other analyzers on the market work with 80 Hz excitation and are mobile [VOLL11a].
A sorting process for magnetic and non-magnetic grits can be based on lifting one sort of grits from a drum or a belt with a magnetic force higher than the grit gravity force [VOLL11b]. A related task is to recover abrasives from blasting material containing magnetic, metallic particles [DREN97].