Mirrors are manufactured by applying a reflective coating to a suitable substrate.The most common substrate is glass, due to its transparency, ease of fabrication, rigidity, hardness, and ability to take a smooth finish . The tin(II) chloride is applied because silver will not bond with the glass. The activator causes the tin/silver to harden. Copper is added for long-term durability. The paint protects the coating on the back of the mirror from scratches and other accidental damage.