The influence on degradation and mechanical properties of selected core materials with different binder variations and different thermal and atmospheric conditions was investigated in this work. For various tests, standardized specimens were prepared from quartz sand, Kerphalit and Cerabeads with organic or inorganic binders. Thermal ageing at defined temperature conditions was used to assess the heating behaviour of the different core materials. In preliminary tests the ideal thermal ageing parameters were determined. For thermal ageing cylinder and bending samples were used in an oxidizing or reducing atmosphere. A separate test device was used for the reducing conditions, in which sand was used to achieve a reducing environment. Samples were examined after thermal ageing for bending, shear, compressive, splitting and tensile strength and compared with untreated reference samples. In addition, evaluations were made regarding the Weibull probability and the chronological strengthening progress of the samples. Therefore the optimum core handling time was determined. The different results of the mechanical analyses were summarized in the Mohr circle and described in detail. In this work, new insights were gained regarding the degradation of the core material, the atmospheric influence and the strength evaluation by Mohr's circle.