# Manufacturing coloured crystal glass using gold nanoparticles

I came across this video which shows how a crystal chandelier is made. I recommend that anyone reading this post should watch it; it is quite fascinating.

Anyway, at about the 2:15 mark, the video talks about how ruby red crystal is produced by adding 24-karat gold powder to the glass. However, they say the colour emerges after reheating in an oven at $1000\ ^\circ C$.

My question is, why is the reheating step necessary ?

• To nucleate and grow the appropriate sized gold nanoparticles. Both gold and silver were used in medieval cathedral stained glass windows. – Jon Custer Nov 1 '16 at 13:32