The composition of a sample of historical plaster with polychromic residues was established on the basis of analysis of absorption spectra in infrared conditions. They found that the samples contained chemical compounds corresponding to the medieval technology of production of polychrome-based mortar – calcium carbonate (CaCO3) with little addition of calcium sulphate (CaSO4). Additionally, they found presence of pigments that were used at those time: visible residues of ochre (Ug), including silicates (Si-O) and aluminosilicates (Si-O-Al) and little addition of lead white (LW). One should pay attention to presence of calcium oxalate ((COO)2Ca), which indicates processes of plaster destruction as a result of presence of microorganisms. Microorganisms generate oxalic acid, which, reacting with calcium carbonate, which leads to precipitation of calcium oxalate. Conclusions from the tests made include recommended delivery of works aimed at removal of microorganisms that cause degradation of the historical substance.