Glass is produced by combining sand, sodium carbonate, and lime (from limestone), which are fused together in a kiln or furnace. The molten glass may then be shaped by introducing air through a blow tube. The resulting product is then cooled. This process is done by a glassmith (one type of kilner). There are no records of when glass was first made, but it is believed to have been late in the first century. Sometime early in the second century, glassmiths began backing flat sheets of class with metals such as silver or aluminum, to produce mirrors. Over the centuries, the craft of glassblowing has become more refined, and more intricate glass products have been made, for various purposes.