What is the process called that gives that blue color to steel. Looked at parkerizing but it does not seem to be the thing. I remember something about heating up a part and quenching in some sort of oil. Want to take old bolts etc, clean off the rust and then coat them with this black coating/covering/protection. Any help appreciated
Cold Bluing. CLEAN metal, apply. Generally has copper somewhere in the formula.
Hot Salts Bluing. Several formulas, ranging from simple nitrates to cyanide. CLEAN metal and cycle through several heated tanks.
Heat bluing. CLEAN metal. Heat to desired color. Quench. Works best on small parts---screws, pins etc.
All of the above require CLEAN metal. Really clean---see yourself in the shine. NO grease, fingerprints, etc.
For your rusty bolts the blacksmithing practice of swabbing on a coat of some mixture of beeswax/linseed oil/other oils/"secret ingredients" might do the trick. Done to hot metal, it leaves a smooth black surface.
The trouble with heating to achieve a certain color is that the surface is not protected. The part WILL rust as soon as the humidity goes up. Years ago, I took a basic blacksmithing course and made a half dozen hammers, fullers, etc.. The were hardened, polished, then tempered (the process you are talking about) from the faces inward toward the eye so they showed the full range of temper colors. They looked great for a year or two in my shop, but the first time I took them out for a weekend of demonstrating to the public, it rained and the humidity in the air rusted everything and my pretty colors were all gone.
What those colors are is extremely thin layers of oxide. Different compounds form at different temperatures and they create different colors. As you say, they don't really protect anything, but they look nice if they're kept protected.
Other methods of blueing, and some kinds of color case-hardening, create much deeper and more protective layers of color.
Commercial hardware uses a black oxide process, you can get kits to do that from various online suppliers. Not particularly rust resistant, more for appearance. Parkerizing isn't really that rust resistant, but the treated parts are usually dunked in various rust-preventative baths and the porous structure retains it. Or you can paint it with baking enamel like the Brits do with their small arms, very durable.
What you remember was an old blacksmithing process, heat your cleaned iron article up and wipe it down with linseed oil so that it smokes off, repeat until it's the color you want. Also not particularly rust resistant, but better than bare metal.
Cold blues are mostly for appearance, Oxpho-Blue from Brownell's works pretty well, but will still rust if allowed to.
If you've just GOT to have something that will prevent rust, look at LPS 3, this is like spray-on cosmoline and is good for quite a number of days under salt water. The surfaces you apply it to have to be rust-free to start with, though. Check out