Lately I have been seeing quite a few videos on YouTube about molten salt, specifically table salt (NaCl). One of the things that interests me about it is that regardless of what melting technique or type of crucible, the salt always ends up black afterwards.
Is the issue that the salt is highly reactive and is pulling chemicals out of the crucible regardless of what the crucible is made of? Wouldn't that mean that different crucibles would impart different colorations on the end result? That is the speculation I've seen on the comments of many videos (for example), but nothing definitive.
Or is it perhaps an optical change due to the change of the crystal structure? Or maybe something in the salt itself oxidizing or reacting with something in the air?