This research focused on the identification of parasitic signal sources introduced by sintering under reductive atmospheres of oxide transparent ceramics. The examination encompassed a wide set of materials, like Mg-spinel, magnesia, alumina. YAG, yttria, zirconia and titania, so as to allow one to detect general rules if operating. The main finding is the dependence between the ability of such ceramics to generate, under reductive conditions, intrinsic parasitic light-absorbing centers and their composition and structure. Thus it was determined that oxides based on transition element cations have ,in this sense, abilities not present in the case of oxides of elements from the main periodic table groups. The reason is the possibility of non-absorbing transition cations to be reduced to oxidation states that absorb light. Drive toward reduced oxidation states is motivated by the stabilization energy associated with the ligand fields acting upon the such cations. It has been also found out that YAG lattice distortion, by dopants like tetravalent silicon or zirconium facilitates reduction of Y (3+) to Y 2 + (a light absorbing, d(1), cation). Oxygen vacancies do not significantly influence visible light absorption. Carbon atoms penetrate inside ceramics, even dense ones and cause achromatic tints formation.