Blue sheet metal.

[Michael S.]

I heated the iron on the drawbar of my steam engine with a gas flame, but didn't heat it to a glow, and then dipped it in old motor oil. This produces a nice dark color. But not blue.

Michael

[Jo]

New engine oil will blue the metal, old engine oil will burn whatever contaminates are in the oil onto the metal leaving the carbon deposits on the metal surface hence the black.

Jo