The strength of Polish plaster varies within: for bending 50-70 kg / cm2, compressive strength 100-150 kg / cm2. Some small Polish factories produce lower quality gypsum, however, it constitutes a relatively small percentage of the total domestic production. According to the scale. Mohs, the hardness of plaster casts is 1-2. The admixture of borax or potassium alum has a positive effect on increasing the hardness. C. obj. the casting is approx 1200 kg / m3 at the water-gypsum ratio 0,6. Gypsum is not very resistant to moisture; plaster cast loses 70% strength under the influence of 24-hour water exposure. However, it is not very sensitive to the action of acids and bases. If they can be harmful, it is only by the property of blurring the plaster mass. The highest gypsum solubility is recorded at 40 ° C.