The glass lid will not release chemicals when heated, which is quite safe. Moreover, the transparent appearance can directly observe the changes of internal ingredients without opening the lid, and easily grasp the cooking degree of the dishes.
Glass is a poor thermal conductor, that is to say, after local heating, heat can not spread quickly and evenly throughout the glass. Therefore, the temperature varies greatly from place to place. When glass is heated unevenly, it will expand and contract, resulting in internal tensile stress.
However, the ability of glass to withstand tensile stress is very poor. On the contrary, pure compressive stress, glass is still very durable. When the glass jar has defects such as scratches, cracks and deep explosion edges, it is easy to cause stress concentration or cracking of tempered glass.