[en] It is shown that oxide crystals contain cluster defects of variable composition, which cause absorption of light in the transparency region of crystals. The model based on the presence of cluster defects in oxide crystals explains well the experimental data on the thermal and radiative coloring of these crystals. It is noted that cluster defects accumulate oxygen in oxide crystals. These defects are responsible also for the photochromic effect in them. Application of the noted model made it possible to fabricate lead tungstate scintillators at North Crystals Company for the ALICE project (CERN) with almost 100% reproducibility of their operating characteristics