At one a watchmaker is blueing screws. First he mirror-polishes the tiny steel screw by hand rubbing it on a polishing plate until it is glossy. This process protects it from corrosion. Then he places the screw which is gray on a small heating device
link on the workbench. The screw soon changes color
link turning yellow followed quickly by brown red violet and then at a temperature of 290° C
link the color he desires a deep blue at which time he lifts it off the grill.