Cast belt concho

Another item I cast this spring was a concho for a weapon belt.  I've done a number of posts on this subject and I won't go into the background yet again, so here goes.

As with most of my bronzes, I modeled it in Victory Brown.  I formed the cone by rolling out a sheet of wax to roughly 1/10 to 1/12 inch/2-2.5mm thick, cutting a circle, removing a small pie wedge from it, and bringing the cut edges together.  I cut the scalloped edge with an X-Acto and formed the rims around the petals by rolling ultra-thin wax "noodles" and pressing them into place, separating the petals by dragging the tip of a hobby awl between them.

On the back, I attached six wax gates of around 1/8 inch/3mm near the edge and two of perhaps 1/16 inch/1.6mm a bit in from the edge to serve as the rivets (as no rivets are visible on the original's face, I assume they were cast in place on the back, since adding them and then grinding them smooth on the front might undo the peening).  These all branched out from a central main gate of about 1/3 inch/8-9mm.  There were no gates or vents on the model's face.

The back bar was cast straight as a section of one of the thicker gates, with room on either end so angle grinder could cut through without touching either the bar or the concho itself.  Since I had doubts about whether it would cast, I didn't poke holes all the way through for the rivets before casting, but instead just made small pits to serve as guides for the drill press (which has an annoying tendency to slip a bit to one side when first entering).  I then drilled fully through the pits after de-gating, and formed the kink through repeated bending with needlenose pliers and annealing.

BCCC lab tech Nick investment-cast the model using a centrifugal casting machine.  This technique, also called spincasting, allows for very fine and detailed models which are almost guaranteed to cast whole, unlike gravity casting, as the centrifugal force throws the molten metal into every nook and cranny.  It also obviates the need for venting, since the weight of the molten metal being sent into the mold forces the air out the same way, through the pouring gates.

One drawback is that the investment, which as I understand it is basically plaster of paris with some sort of strengthening agent, traps a lot more small bubbles than ceramic slurry does.  Where these bubbles are in contact with the wax, they form little warts all over the model, making finishing more tedious.  You can see one large bubble on the bottom center there, and close examination will reveal the remnants of a number of smaller ones.

On the plus side, the investment is much easier to remove than ceramic shell.  Most of it crumbles away with a dunk in water, that which is left can be softened if necessary with a baking soda solution, mostly scrubbed away, and any remaining traces scraped off with metal tools or buffed off with a wire Dremel brush.

The rims around the petals are almost certainly too prominent.  Going by the OIP drawing, I interpreted them as sort of a gently rounded lip, but because I didn't spend enough time smoothing the noodles down, they instead look high and sharply differentiated from the petals' faces, with crevices that I couldn't successfully polish.  As a result, there's still a significant amount of brown as-cast finish there, giving the face a slightly antiqued appearance.  The dip around the middle cone is probably too deep as well.

Dry-fitting the back bar was the most tedious part of the process, since the rivets were crooked where the met the concho's back, and I still failed to get it perfect.  Once the fit was sufficient, peening the bar into place only took a couple minutes.  I did so with the face resting on a thick carpet on top of concrete, so as not to damage it.

As you can see, after being bent, the back bar sticks out considerably more than the original.  I should have made it only a little bit longer than its desired final length.

As I mentioned before, I think this is a low-strain application, and the thin bar and small rivets will be good enough.  Should they break, a replacement can be silver-soldered on.

If possible, I intend to make a whole new belt with a strap of buff cowhide in place of my old belt's veg-tan.  I will probably still use latigo lace for the time being, however, since it doesn't stretch like fat-cured leather, which is likely to become an issue when supporting the fully-loaded gorytos.

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.