soda–lime glass is relatively inexpensive, chemically stable, reasonably hard, and extremely workable. because it can be resoftened and remelted numerous times, it is ideal for glass recycling. it is used in preference to chemically-pure silica, which is silicon dioxide (sio), otherwise known as fused quartz. whereas pure silica has excellent resistance to thermal shock, being able to survive immersion in water while red hot, its high melting temperature (1723 °c) and viscosity make it difficult to work with. other substances are therefore added to simplify processing. one is the "soda", or sodium carbonate (naco), which lowers the glass-transition temperature. however, the soda makes the glass water-soluble, which is usually undesirable. to provide for better chemical durability, the "lime" is also added. this is calcium oxide (cao), generally obtained from limestone. in addition, magnesium oxide (mgo) and alumina, which is aluminium oxide (alo), contribute to the durability. the resulting glass contains about 70 to 74% silica by weight.