These plastic jars and matching lids are both made of a highly durable polypropylene plastic. They can be steamed, pressure cooked or autoclaved repeatedly. The plastic jars themselves are straight walled and clarified for easy viewing. The lids are white and ribbed for tight gripping. Our plastic jars work great when growing cakes, sterilizing media or starting cultures. Compared to glass, they will not break or crack due to extreme changes in temperature. Also, unlike canning jars, these plastic jars come with plastic (no metal) lids. This means that you'll have no rusting or staining.