One may case harden (case carburize) steel or wrought iron by packing in wood charcoal and heating to some red color for a period of time, then dump the whole thing into water.
Whether or not the surface actually gets hard depends upon whether the part was in the pack long enough to absorb any amount of carbon, and how hot it was heated. Color case hardening as described by Oscar Gaddy is done at a rather low temperature, for a very shallow case.
Colors. As has been stated, pack hardening in charcoal often gives some amount of colors. Depends on more variables than I know, but one is whether or not air can get to the parts before they hit the water.
Guys who did this centuries ago had the same industrial outlook we do today. That is, I want to get this surface hardening done with less expenditure of time in the furnace, or maybe a deeper case. To that end people added a wide variety of Stuff to the charcoal in hopes of speeding things up. One, already mentioned, was leather charcoal. This adds nitrogen, making a case harder and more wear resistant than just plain charcoal.
Another is to add bone charcoal. In addition to, hopefully, speeding things up the calcium phosphate can give wonderful colors. These colors are not the thin, easily removed temper colors that come from hardening in wood charcoal. I might speculate that the colors were regarded as secondary to improving the process, before people decided they were pretty & went about developing the Art of getting good colors.
Back to speeding up the process. Rene Antoine Ferchault de Reaumur studied how to develop a reliable process for "cementing" (deep pack-hardening) iron. He found that sea salt and sal ammoniac both gave good results (I'll bet they ate the $#*! out of his pack container, whatever it may have been). While I cannot locate the reference this evening, I recall that pigeon dung and sea salt were a good combination, added to the charcoal. Salt was a popular additive to charcoal for case-hardening, it sure didn't give colors but it, well, cleaned up the iron surface so that it more readily absorbed carbon.
In the 20th century when pack hardening was still an industrial practice the chemical barium carbonate was added to charcoal to speed up the process. Not very poetic but it worked.
Here is a nice 1532 A.D. recipe for hardening files. Perhaps not cost-effective today but it would make a fine file:
The Temper of Iron for Files
It must be made of the best Steel, and excellently tempered, that it may polish, and fit other iron as it should be: Take Ox hoofs, and put them into an Oven to dry, that they may be powdered fine: mingle well one part of this with as much common Salt, beaten Glas, and Chimney-soot, and beat them together, and lay them up for your use in a wooden Vessel hanging in the Smoak; for the Salt will melt with any moisture of the place or Air. The powder being prepared, make your iron like to a file: then cut it checquerwise, and crossways, with a sharp edged tool: having made the Iron tender and soft, as I said, then make an Iron chest to lay up your files in, and put them into it, strewing on the powders by course, that they may be covered all over: then put on the cover, and lute well the chinks with clay and straw, that the smoak of the powder may not breath out; and then lay a heap of burning coals all over it, that it may be red-hot about an hour: when you think the powder to be burnt and consumed, take the chest out from the coals with Iron pinchers, and plunge the files into very cold water, and so they will become extream hard. This is the usual temper for files; for we fear not if the files should be wrested by cold waters. But I shall teach you to temper them excellently
G. B. Della Porta, 1589, Sources for the History of the Science of Steel 1532—1786, Ed. Cyril Stanley Smith
Soot added the carbon, Ox-hooves nitrogen as well as carbon. Melted glass + salt cleaned up, or fluxed, the iron surface so that it was quite receptive to reacting with carbon (really, with the carbon monoxide which broke down to carbon + carbon dioxide on the surface) Enjoy.