To expand on this, the rainbow of colors which start at a straw then turn yellow, red, brown and then that vivid blue, are caused by refraction. The oxide layer on the surface is transparent or translucent, and the thickness of the layer determines what wavelength of light it scatters. The hotter it gets, the thicker the oxide layer forms, so you can fairly reliably tell the temperature the metal has been heated to by eye, and you might use different amounts of heating to achieve hard-but-brittle or soft-but-tough.

I've even seen it done by Chris of Youtube channel Clickspring for decorative purposes. It's how he made the steel hardware of his brass clock blue.

Exactly how you temper something the size of a sword using a forge is a bit outside my understanding; I've done it with relatively small bits of drill rod to make lathe tools with a gas torch, but that's about it.

[–] Rivalarrival@lemmy.today 1 points 4 months ago* (last edited 4 months ago) (1 children)

The hotter it gets, the thicker the oxide layer form

This is accurate enough for tempering of most cutting tools, but technically, the oxide layer will continue to grow if you hold a lower temperature for a longer than normal time, and might not fully develop if you reach a higher temperature for a shorter than normal period of time.

This property useful if you are trying to develop a specific color rather than achieve a specific metallurgy. You can heat to a lower temperature for a longer time to develop a deeper, more consistent color.

In my experience, it's easier to develop colors with an oven or propane torch rather than a forge or acetylene.

[–] captain_aggravated@sh.itjust.works 1 points 4 months ago

Yeah I referenced Clickspring, when oxidizing a part for decorative purposes he would put the part in a brass tray full of brass shavings apparently to function as a thermal mass so that the color comes out evenly.