It is made by soaking cotton threads in a starch and paraffin wax solution. Phase change materials such as paraffin wax and Glauber's salt are another thermal storage medium. They wax and wane, but are deadly when waxing. This liquid can be refined into spermaceti wax and sperm oil. Insects also produce useful substances such as honey, wax, lacquer and silk. This could be prevented by sealing the chambers with cotton, wax, or grease. The skin is covered in a protective layer of epicuticular wax. This oil is a mixture of triglycerides and wax esters. Bees store honey in wax structures called honeycombs. The latter is particularly important in candles made of harder wax. The techniques used were encaustic (wax) painting and tempera. An unusually dark colored ear wax (cerumen) may also be produced. It is made of cotton with feathers, wax and gold thread. Cobblers wore black to protect garments from the black wax used on shoes. Considerable trade in pitch, honey, wax, and cheese occurred.