Abstract—Annealing of thin silver films on oxidized silicon substrates in different furnaces is studied. It is shown that identical temperatures and durations of thermal treatment do not guarantee reproducibility, i.e., the annealing provides different results, e.g., shape and size of nanostructures in different furnaces. To clarify the source of the variation, morphology and optical properties of the samples are analyzed. Spectroscopic ellipsometry is used to measure thickness and composition of the oxide layer before and after annealing. Reflectance spectra, obtained for different angles of incidence and polarizations, demonstrate the dependence of sample plasmonic properties on the furnace design. Additionally, a numerical simulation of the heating process in a diffusion furnace has been performed. It is concluded that uncontrollable overheating of silver film with regards to the substrate produced by thermal radiation of the environment leads to variation in annealing results.