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Pure potassium sulfate single crystals are transparent in a wide spectral range up to 155 nm and do not show fluorescence and absorption over a wide spectral range of 200-800 nm before and after X-ray irradiation of the samples. The centers of luminescence in thallium-doped potassium sulfate crystals showed a maximum absorption 216 nm and emission 285 nm at room temperature. These centers are singly charged thallium ions. It is interesting to research the optical properties with decreasing size of crystals (size effects). For this, we used a supersaturated aqueous solution of potassium sulfate, in which the crystallization begins. The results of computer simulations using Scigress quantum chemistry package show that the occurrence of macromolecules in a cooled supersaturated solution should result in an increased absorption. In supercooled saturated aqueous solutions of ionic crystals nucleation occurs. A decrease of transparency of saturated solutions with decreasing temperature is clearly seen. This is due to absorption and scattering in the medium with the growing small crystals in the solution.