I've done that on many small parts, drop into cutting oil when the blue is right. I drop the parts into oil to stop the color change and expect the oil saturates pores to add corrosion protection. I've used old engine oil too. On many guns I've seen a brilliant light blue that must be furnace heated to get the color all over. But I'm not a gunsmith, so someone may offer more insight.
Search Luger forums for info on heat-bluing and 'strawing', which is just another heat color. You can see the heat colors easily on a chrome motorcycle exhaust up at the cylinder. This will show which color comes first (hottest) closest to the engine - I forget whether it's blue or straw. There are several other methods to get heat colors involving charcoal, bone etc. Definitely fun to play with on old parts. Like any finish, much of your success depends on surface prep - 'fire blue' really jumps out at you on a highly-polished surface. These colors are constantly appearing on many things, such as a lawnmower blade you have overheated on a grinder. Like color case-hardening, they are not very durable and benefit from clear (gloss or matte) baked-on coatings. I think you can do it with a propane torch, but an industrial electric fiurnace is better, as you control the temps and times so much easier. One of my 'dream' guns is a 1911 with Damascus slide, color-case frame and fire blue parts.
There's an old watchmaker and gunsmith method that works really well in a small shop.
There are two ways this method can be done: The sand bath, and the lead bath.
To do either, the part MUST be polished to a brilliant shine and totally degreased with a solvent that leaves NO residue.
The sand bath method:
Get a metal baking pan about 2 to 3 inches deep.
Get some ordinary sand and wash it thoroughly to remove all dust and dirt.
Dry the sand and mix it to a loose consistency.
Put the sand pan on a burner and heat the sand HOT.
When the sand is hot enough, bury the part to be colored in the sand, leaving a small area alike a corner just sticking up out of the sand.
Watch the part as it changes color from a light straw, to a gold color to purple, to a brilliant DEEP blue.
Pull the part out of the sand and quench in water (or oil).
The Lead method:
Heat up a pot of lead in a bullet casting furnace.
Drop the part in the molten lead and HOLD IT UNDER. (Steel is lighter than lead and will float).
Leave a corner up out of the lead and watch for the color you want, quench.
Using these methods, you can closely control the color.
For tiny parts, you can flame-color the part, but it's much harder to control.
Simply hold the part in a pair of large tweezers and pull it through a flame and out. Repeat until the part turns the color you want.
Pulling the part through the flame allows you to more closely control the color versus just sticking it in the flame and holding it there.
The USFA 1910 is a great example of nicely done fire bluing. Here's one on Guns America; https://www.gunsamerica.com/Classifieds/View/_976998925.aspx#
I've read a lot about the "depth" of different types of bluing but doubt it. Since it's an oxide it can't be more than a few molecules deep. As for durability I'd like to read more about it from a scientific standpoint. It would be pretty easy to test different finishes and see what is most durable. I love blued guns but stainless sure makes life easier.
Does anybody know the maximum temperature that I can heat parts without compromising the structure and hardness of the metal (I am thinking frame here). I have an oven that is capable of 750 degrees F. With this I have achieved some nice color on scrap steel of different grades. It doesn't matter how good it looks if I ruin it in the process though.
I'm not a metallurgist, but I know above 350 degrees subtle changes start to happen, above 400 they are more pronounced. Any good modern metals shop textbook should show a color chart with the temperatures at which both colors and temper changes occur. If I remember right it was like 450 degrees from just chilled glass hard to chisel hard, depending one how hard you want it to become. Many parts on a 1911 are harder than that, so I'd consult further if coloring hammers for instance.
A 1911 hammer hardness is specified at RC46-52, safety RC 44-50, slide stop RC 43.5-50 (43.5!! as a lower limit, but that's asking a lot of a tester, and current production slide stops are not that hard in my experience) so use your own judgment as to the effect of flame bluing. Heat treaters and metallurgist say you can't use color to temper, but as a guide to the effect you are having on parts already heat treated it should be useful. You can find a machine shop, ask them to test the hammer hardness in a non visible area, blue the part and test the opposite side (stone the first test dimple before bluing so it's burr won't effect the second test), the test dimple is so small it shouldn't effect function. Different materials temper at different heats, for instance, hammers are specified as 1060-1090 carbon steel but also 4140 and 4150 chromoly steel, so without knowing which it is the effect may vary, but also, there may be no effect on either at the color you heat it to. The frame is specified at RC 22-27 but may be as high as 27-30, at these hardnesses I would be amazed if any change occurred at the temperatures you'd use for blue, though I've never heard of flame bluing a frame. The materials specified were 1035, 1127, 4130 and 4140 for non stainless frames. I would talk to a heat treater before trying flame bluing a hammer. Please let us know what you decide as this is very interesting stuff to some of us.