Lugz,
That definitely deserves a response.
SO NOW YOU’VE NOTICED HOW MUCH TIME MR. COBB SPENDS DOING THINGS HE SAYS NEVER TO DO.
I do not count Fe2O3 (rust/hematite) as “patina;” I believe your position is similar. Rust is bad.
“Patina” I understand to mean the accumulated grime of countless dirty hands, dried sweat, and in some cases toddler saliva found on old wooden objects. (Numismatics venerates the same film on coins as an indicator of authenticity.) I recognize there’s a romanticism for some in making physical contact with the dried body fluids of our predecessors, but personally, I’m not really a fan. By and large, a properly-applied coat of shellac (bug juice), boiled linseed oil (plant juice), or both can have the same visual and tactile effect (minus the stickiness), and DOESN’T come off with a damp cloth. I am willing to use the word in the context of a general discussion of surface condition on steel tools, but I here I think I should distinguish it from what I referred to earlier as “the work of decades.”
What I meant was the natural production of Fe3O4 (black oxide/magnetite) of iron exposed to oxygen. Unlike rust, it does not swell, is hard and slippery, so is a desireable surface for mating parts. If salts (from sweat) are removed, a light coat of oil will cling to Fe3O4 with adhesion superior to bright steel, and protect the underlying steel from further corrosion. That is the surface I desire for my tools.
The problem is, Fe3O4 and Fe2O3 commingle freely (often in unevenly-distributed layers and concretions) and react similarly to chemicals & electrolysis, making removal of only the Fe2O3 difficult.
In my experience the best result is achieved by hand-scraping the commingled layers, effectively crushing the Fe3O4 shell(s) that surround the pockets of Fe2O3, exposing the hidden rust for removal with a wire brush (carding). Wire brush alone leaves clusters of rust “geodes” - not desireable, particularly on a smooth or polished surface. These adherent geodes tend to resist the brush even after the rest of the Fe3O4 has been removed, exposing bright steel in uneven patches - ugly! That’s why we should scrape first - to crush these nodules.
The danger in using power tools is that they work faster than we observe and react. Injuries and damage to the workpiece can result. Or as one of my teachers said, “power tools make power mistakes.”
Despite what I may have implied, I resort to the wire wheel frequently, but I hope, judisciously. First, the wire wheel alone simply accellerates the work of the brush alone. It can ruin the surface in very little time. Use a coarse wheel as “most guys” do, and it’s no time at all. While I have both a coarse and a very fine wheel on a benchgrinder, I only use the coarse wheel on coarse, working edges that are likely to be sharpened or reshaped anyway (like a cold chisel).
When do I elect to “cheat” with the very fine wheel?
1. When the tool is chrome or satin nickel, or one of the alloys that just don’t darken.
2. When the tool has been dressed or requires it, resulting in blotches of bright spots.
3. When the surface is textured or contoured in a way that makes hand-scraping impossible (grainy castings and rough, unpolished forgings come to mind). A light touch, single passes and frequent visual inspection can help avoid breaking through the last layer of Fe3O4 on smoother surfaces.
4. When the tool was originally blued AND hardened. I am not sure of any science behind this, but it seems to me that the harder the steel, the more tenacious the Fe3O4. I had a saw blade I was working on, and couldn’t seem to crush up the rust conglomerates. I stepped up the hardness of my scrapers a few times and finally chucked a wire wheel into an electric drill (admittedly lower RPM than a bench grinder), and despite abandoning my original intent, COULD not buff off the Fe3O4! That saw remains the
Black Prince of my collection. Hex-shaft driver tips are another item I don’t hesitate to skip straight to the wire wheel.
5. When the underlying steel will darken again quickly anyway, like gray iron, semi-steel, carbon steel. Leaving the raw surface exposed to air overnight, rinsing with hot water, or a coat of BLO (since it contains oxidizers) can help.
6. When I am in a hurry, impatient, lazy, in a foul mood, or just don’t care enough about the object or final result to take the proper steps. I am only human, after all.