what do they spray on the back of glass to make mirrors?

Answer 1

In the 'good old days' silver was the common backing for mirrors. Then they saw how the silver tarnished after years and changed to mercury. This was a toxic material and became a problem making sure it was sealed and not a health problem. So now the most common backing for a mirror glass is aluminum.. sometimes things like silicon oxide or silicon nitride.. Yea progress has taken the romance of the silver mirror... but they do look nice and give a more accurate reflection.. sigh..

Answer 2

silver