Determining the fibrillar orientation of bast fibres with polarized light microscopy: the modified Herzog test (red plate test) explained

polarized light microscopy, red plate test. Summary The identification of bast fibre samples, in particular, bast fi-bres used in textiles, is an important issue in archaeology, criminology and other scientific fields. One of the characteris-tic features of bast fibres is their fibrillar orientation, referred to as Z- or S twist (or alternatively right- and left-handed fibres). Anempirical test fordetermining the fibrillarorientationusing polarized lightmicroscopy has been known in the community for many years. It is referred to as the modified Herzog test or red plate test. The test has the reputation for never producing false results, but also for occasionally not working. However, so far, no proper justification has been provided in the litera-ture that the ‘no false results ’ assumption is really correct and it has also not been clear up till now, why the method some-times does not work. In this paper, we present an analytical model for the modified Herzog test, which explains why the test never gives a false result. We also provide an explanation for why the Herzog test sometimes does not work: According to ourmodel, the Herzog test will not work if none of the three distinct layers in the secondary cellwall is significantly thicker than the others