Question about 'bluing' of a surgical instrument from the late 1800s/early 1900s

I recently acquired a Galt trephine produced by the C. Truax company. It looks very similar to this version, or this one, except that the brass of the shaft is nickel plated. It came from an estate sale at the home of a long-retired veterinarian. There’s no other provenance for the piece, but apparently the Truax company was in business from the 1880s through 1920.

On my piece the conical cutter and sliding plate along the shaft have traces of very vibrant blue coloring, almost like Dykem layout fluid blue. The linked images of Galt trephines shows this bluing as well, although those examples are darker than my version. The remaining blued portions are still nearly mirror-bright, while most of the cutter and sliding plate are pretty pitted with rust. It isn’t the blue of

Anyway, I’m curious about the bluing. Does anyone know what process might have been used to color these steel bits so brightly ~100 years ago? I know that there are now multiple ways to achieve the effect, but what would they have been using back then? The cutter has two set screws holding it to the brass shaft, but even with the screws removed the cutter shows no sign of moving - so it’s either press fit or brazed on somehow, in case that would suggest a different form of bluing. The cutting surfaces were blued as well, so the process must have been done to an already-sharpened part.

Could this have just been heat-treating the pieces to the appropriate temperature, as is described here, in reference to watch screws? Or would it have been a chemical process?

Yeah, it’s pretty certainly from the heat treat to harden it for being a cutting device. Bonus that that heat treating process is, pretty much, the same as blueing for rust resistance.

Bluing (or, more properly, ‘browning’) is a form of controlled rusting which protects the metal from corrosion; the browning solution is applied and the item left to rust overnight, sometimes in a ‘wet’ cabinet, and the rust carded off the next day. The process is a labour-intensive one which is repeated numerous times to get the degree of finish desired, and the protection obtained is somewhat limited and easily damaged by scratches. Nowadays it is limited to high-end firearms. Better/cheaper methods of anti-corrosion finish have mostly superseded it for mass-production firearms.

Clickspring YouTube channel has several videos of the process of heat-bluing parts, and yes, it does produce that vibrant peacock blue.

Well, heat bluing did the trick quite nicely. It isn’t perfect, but compared to the rusty mess is was before, I’ll declare this a victory. I filed all the pitting out and polished the steel bits to bright metal, cleaned them repeatedly with 91% isopropyl alcohol, then did the heat treating. Since the cutter wouldn’t come off the shaft, I chucked the whole thing into a drill and spun it over a propane torch with a low flame. A straw color appeared after about two minutes, and the blue appeared about 5-10 seconds later. I quenched it in DI water. The sliding plate and thumbscrew were held with some stainless steel wire and done a few inches over the burner on the kitchen stove; I did those a few times to try to get the best color match to the cutter. It’s hard to capture the jewel tone of the metal; you have to pick up reflective highlights because the color is so deep otherwise.

Doing it in a sand bath or brass chip bath like Clickspring did would probably have given me better control, but the torch works too. None of my ovens or hot plates goes above 500° F, or I might have tried that.

Victory indeed!

I have done this on a few different parts with varying success. You have achieved a beautiful shade of blue.

Nice! And fast work.

I got the sense from Clickspring that the mirror polish is the key to getting the best effect.

I would agree. Although the color is the same everywhere, the vanishingly thin oxide layer does nothing to conceal pits or scratches. In fact, I think it accentuates them compared to an uncolored surface. On the rougher areas it looks like a surface coating, while on the more polished surfaces it looks deeper, like a sapphire.

Now that I know how quick and easy the process can be, I’m tempted to try this on a bunch of metal scraps to see if I can get a reliable purple - it’s a very quick transition from brown-purple-dark blue, with a little more time between dark blue-light blue- gray

It’s not as easy as I would like to get a good blue. Too many factors: kind of steel, geometry of part, method of heating, time, etc.

Here’s a thumbrest I made for one of my bass guitars some years back.

I’m happy with the purple hue on that part.

In contrast, look at the remaining parts I made for the bass, including the knobs and bridge. The knobs turned out nice, and the little round things that support the strings blued to a perfect dark color, but the bridge itself took on a mottled look that appeared similar to a color case hardened part.

I did the thumbrest using a handheld torch.
For the other parts I used one of those little stainless steel tabletop furnaces.

(obligatory gallery of the bass project)

I suspect it works best on small parts that can heat evenly. In my limited experience, I was continually moving the parts into and out of the flame to try to let the heat equalize throughout. I was worried that the thin bits of the cutter would heat faster than the base, where the brass shaft was acting as a heat sink - so I only applied the torch at the base and let the heat flow toward the thin bits. Those clickspring videos of tiny screws and thin, flat hands are probably the best case scenario for getting uniform results.

I can see how a more massive part with a complex geometry put into a very hot oven (800°, if I’m reading it right) would quickly hit the critical temperatures at the corners and edges before the faces or core. I’d bet that for larger parts you’d have to be paying very close attention and actively moving a flame around, or conversely using an oven that crept up in temperature slowly.

Beautiful work on the bass!