Visualize Invisible Overheating Risks in Gold Jewelry
A layer of oxidized copper that rises to the surface when heated. It creates reddish/black blotches that are extremely hard to polish off. 24K gold does not get fire scale because it has no copper.
Metals are made of interlocking crystals (grains). Overheating causes these grains to grow abnormally large. Large grains have weak boundaries, making the metal brittle and prone to cracking.