The beta source calibration is shown to be directly dependent on the transparency of the particular quartz grains under study. As the transparency of the quartz grains varies from sample to sample, especially after etching in HF, so the apparent beta source dose-rate varies. The influence of different backscattering materials is described, and it is shown that the firing temperature of the quartz grains before etching with HF also plays an important role with respect to grain transparency. The consequences for TL dating are discussed.