Wednesday, May 16, 2018

Cast arm fibula

Three years ago, I fabricated an arm fibula out of brass rod.  I was never satisfied with the lack of depth afforded by filing the details into stock that was narrow to begin with, so I've always wanted to replace it with one that was properly cast.

Last semester I took foundry at BCCC again and took the opportunity to make a few more reenactment items.  This one, which I finished up just last week, is wax-cast (as the originals probably were) and based on an example found at the Harvard Art Museum.

Although this is a huge step up from the brass one, I still made a couple of mistakes and compromises.   First, I had been meaning to replicate one from Deve Hüyük, and ran a simple Google search for this purpose, but didn't read the article carefully enough.  This example is of unknown provenance, although the article does point out that it's similar to two Deve Hüyük examples.   All three are of unusually large size, each around 4 inches/10cm long, with "bead and reel" decoration, flared at the base, and having fingers filed in after forging.

I cast it as a blind vent on a larger assembly.  The hand was cast as a bulb.  Unfortunately, I didn't account for shrinkage, resulting in a mushy hole where the wrist should've been, and the bulb fell off during de-gating.  I forged the bulb out into a flared catchplate roughly 1mm thick (finding out, along the way, that the Everdur silicon bronze Bucks uses actually hot-forges nicely).  Luckily, crack lab technician Ray was able to weld the catchplate back on, but the result is a much thinner "wrist" than the original has (it's still thick enough, though).

You'll also note that the proportions are slightly off.  This probably had to do with my inability to sculpt the wax for the beads and reels thin enough, and of course I bent the elbow a little too sharply.  The rough patch on the elbow is a remnant of the chill ball, and I may grind it smoother later on.  The original might have been cast straight and then bent, but I don't think this is the case, since it's slightly thicker where it's bent than immediately above or below.

The other problem that occurred with casting was that the slurry, which is quite thick, apparently failed to flow between most of the very thin wax noodles which formed the reels on my wax positive.  The first pair of reels just below the elbow cast fine. The second cast part of the way.  The rest cast as single thick reels with only a short groove in one or two places around.  This was sufficient as a starting place for me to file the grooves in, but it means that the grooves are sharp and the sides a bit square, unlike the nicely rounded cross section on the original.

The fingers were filed before bending the catchplate over.  It proved too difficult to get the bend in the catchplate to align with the pin, but as you can see, this was a problem with the original as well, so it's perfectly accurate.

I made the hole for the pin with a drill press, unfortunately a bit off-center.  The pin itself is simply a piece of springy 3/32-inch steel welding rod hard-soldered into place.  The original used bronze, but the bronze welding rod at Bucks was too thin and I imagined that brass would contrast unpleasantly with the orangeish Everdur.  In any case iron was also commonly used for arm fibula pins.

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