what oil for blackening steel?

Linseed oil works. Dab it on with a small piece of cotton rag. The steel needs to be hot enough that the oil smokes and carbonizes while you're applying it. Wipe off the excess when the piece is cools. A coat of paste wax adds some protection and seems to make the finish darker.

Does it need to be hot? I have had good success with Birchwood Casey's Gun Bluing paste. It is an easy cold application and with the paste (rather than the liquid) the results are less patchy. Some sort of oil/wax finish over it is recommended.

Boiling the steel in sodium thiosulphate solution, photographers' "hypo", also works. I just saw a reference to "black oxide" that mentioned caustic soda at 140C.

And if you use Linseed oil on a rag do something like burn the rag after use. Linseed oil on rags is prone to spontaneous combustion an can light up later on if you just chuck it in the bin.

Used motor oil works too. I've quenched in this from red heat and you get a great black finish after buffing.

Karl

Old-timey blacksmiths used to "blacken" iron by heating it to about dull red and quenching it in HORSE MANURE. Then, wiping it with oil of some sort.

Yep, the softer it (the manure) was, the better. The thought then was that the carbon that was added to the surface made it black. Blacksmith's shops smelled that way for a reason !

The most modern method for it is used motor oil, natural, not synthetic. Back in the days before there were so many "non-petroleum additives" to the gasoline and the oil, it worked better.

But it still works. Hardens (depending somewhat upon the alloy) and blackens at the same time.

Flash

When I was in tech. school, we had a 45 Imperial Gallon drum of motor oil for this.

Steve R.