They have been hardened which makes them stronger which in turn makes the Duralex glass resistant, this is the tempering process. It consists of sudden cooling by the ventilation of cold air on the whole of the glass product which was previously brought to a temperature of approximately 700°C. This treatment creates controlled stresses in the glass that improve most of its physical qualities: Resistance to mechanical stresses (bending, shock) and thermal shock. 2.5 times stronger than an annealed glass product and able to withstand 130°C heat shock, our products are easily transferred from the freezer to a microwave. Directions for use: – Wash before use – Do not use in the oven. – Do not expose to open flame. -Do not subject the products to a sudden temperature change of more than 130°C. -Do not use a damaged or chipped product (risk for the user). – Avoid scratching the surface of products.