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.

Last notes on this, I swear

The flower concho's petals and central circle in the OIP line art had a slightly rimmed appearance.  Assuming this to be accurate, I attempted to duplicate it by masking just the edges of these features on my newly-etched concho and giving it a short dunk of about 15 minutes in the chloride.

I'd hoped that this would make the features stand out more.  It didn't, but it does look a bit closer to the OIP drawing.  But the edges should be defined by gentle swellings, not sharp-edged borders.  I don't know how to correct this.

Interestingly, both the first and second etchings made a striated effect, always pointing toward the center, on all the exposed metal.  Internal crystalline structure after forging?  Something to do with the flow of etchant across the surface?

Lastly, I used some fine files to scallop the edges.  At this point, any further cleanup will be unnoticeable to anyone but myself, so the project is for all intents and purposes finished.

Tomorrow, final results on hide glue paint!

Improvements for the Northern-style concho

As I've discussed before, the early Achaemenid weapon belt included a shallow bronze concho which often had a flower design cast on.  In the absence of castings that could accurately replicate the originals, an acceptable substitute is a Northern-style concho.  However, it still has some differences, the most notable of which is that it has a small round loop on the back instead of a riveted bar.

 

Image from OIP 69, which you should go download right now.

You could, of course, leave your concho the way you receive it.  The alterations here are worthwhile if you happen to be set up for silver soldering (as of this writing, a low-end gas torch, tub of flux and coil of solder come to 20USD, give or take a few).

First the solder on the back is melted and the copper loop removed.  Unfortunately, it's too short to do anything with, so I tossed that one and used one of those huge copper-plated staples from a cardboard box, simply bent to shape with pliers and trimmed with wire cutters.

The advantages of this design, aside from a more accurate shape, are that it holds the concho closer to the belt, and that it can be used with either a thick, narrow leather lace or a flat, wide thong.

Before installing the back bar, I decided to beat the concho into a more conical shape with a small ball-peen hammer.  Unfortunately, my forging skills are mediocre and left it slightly lumpy, so I covered that up by going ahead and adding the classic flower design seen on some historical examples.

These designs were originally cast on, but a similar appearance can be affected by engraving or etching.  Since I don't know how to engrave, I etched it using some ferric chloride with permanent marker as a resist.  The chloride took about half an hour to make a good 3D design.

In order to let the flower stand out, I didn't burnish it as thoroughly as I could have.  Some of the permanent marker, dissolved with nail polish remover and scrubbed with a nylon sponge, tends to stick in the rough etched surface and provide a good contrast with the un-etched surface where the resist had been..