#sandcasting

"I will grant you my emblems of trust. Do not lose them, for I only grant them once." She puts her hand up, and one of the nearby assistants comes up to the side of the throne on the right hand side puts aside a curtain and pulls out a small box, opens it, rummages through, and you see putting a stack of these small, silver medallions.  Closes the box, shuts it, and comes forward and presents one to each of you.  You can see within the center of each of these emblems - they're about that big, they're not incredibly huge, a little bit larger than a gold piece - but in the center, you see the carved symbol of the dodecahedron with what looks to be a circle around it, but the circle is broken in places and twisting inward to form almost like a protracted sideways eight that merges with it and extends.

This project was an example of the workflow process I've been working towards for the better part of a year, starting with a 3D model and carrying it through to a finished metal casting.  I started with a little post-it note sketch and thought about what shapes would be easiest for me to get a good polish on - I had considered making the dodecahedron outline raised and the curved infinity lines embossed, but having a series of very thin raised areas would have been difficult to polish well without stripping off the raised areas. The 3D design was fairly straightforward in Inventor, just a couple of rotated shapes joined together to create the base of the medallion and the jump ring.  I decided not to model the dodecahedron pattern digitally because my 3D printer isn't capable of producing enough detail to make it worthwhile, and sanding the FDM layer lines out of the design seemed like it would be a huge headache.  If you would like to print a medallion blank of your own, the STL is available on Thingiverse here!

Once the medallion blank was printed out of PLA, I primed and sanded it to get it as smooth as I could, then made a mold out of MoldStar 20T from Smooth-On.  This didn't have to be too fancy so I made it an open-backed mold.  From there, I melted down some blue Maves Inlay Wax and spooned it into the mold to make a wax copy of the blank.  I'm not sure how much use this particular type of wax gets outside of dental circles, but I had a couple sticks left over from dental school, and I'm sure jewelers make use of wax with very similar properties for their work.  The Maves wax is a very hard, brittle wax at room temperature and can be scratched and carved, and is capable of taking on an extremely smooth finish if handled well.  We used this stuff to wax up crowns and onlays for our lost wax casting projects in school, and it was actually pretty cool to be able to work with it again. 

Making a wax blank allowed me to use one of my favorite techniques to get a smooth finish on a sculpt: heat polishing.  It works with any sculpting material that behaves like wax, including Monster Clay, that will turn into a liquid when a quick burst of heat is applied.  When you have your sculpt very, very close to your final desired shape but want to smooth out minor imperfections, this is a great trick to use.  The goal is to apply a quick, intense burst of heat to the surface of wax to melt a very thin layer - think 0.5mm - over the surface of the sculpt, which in its liquid state will smooth itself out and then solidify within a couple seconds.  For this project I used a Hanau torch (shown in the above left image), which is basically an alcohol lamp with a trigger-controlled air puffer right next to the flame, to melt and smooth the wax on the surface of the blank.  (When working with Monster Clay, I'll usually use a heat gun for this because the area of effect is a lot bigger.)  The trick is to melt just enough to let the sculpt smooth itself out - too much heat and the wax could bubble or drip, and too little heat and it will be ineffective.  If you've been doing a lot of heat polishing on a piece all at once, you'll also need to let it rest a bit if the core of the piece starts accumulating a lot of heat, which will make the surface wax stay liquid longer and increase the chances of unwanted dripping.  Check out how much smoother and shinier the wax blank got between when I pulled it from the mold vs. after I heat polished it: 

For the next step, I used a glue stick to paste a paper template of a dodecahedron over the surface of the wax medallion, then poked through the paper with a sharp instrument to create a series of dots I could connect to form the pattern.  I removed the template and carved some deep grooves for the dodecahedron, then went back and freehanded some lighter curly lines for the infinity symbol passing through the dodecahedron.  I alternated between carving and heat polishing until I got something that I was pretty happy with. 

Next, I made a two-part squish mold of the wax pattern so that I could make some resin duplicates of the master sculpt.  Sand casting requires you to have a durable positive to pack sand around, and since I wanted to do batches of multiple medallions at a time I needed several copies of the same thing.  I chose to go with a two-part mold here so that I could reliably reproduce the thickness of the medallion, since open-backed molds can produce a lot of variances in thickness depending on how much resin you put in the them. The day that I did this I was under a time crunch and wanted to get four or so resin duplicates ready within a couple hours, so I used some old bottles of Amazing Casting Resin, which has a very short cure time (10-20 minutes).  I'm not exactly sure what happened, but despite my efforts to have good mixing technique, three out of my five casting attempts had HUGE air bubbles which made the casts unusable, so I ended up with only two resin copies.  I've since made more duplicates with SmoothCast 326, but the cure time for that is an hour and I didn't have time to wait for those to be ready for use.  One side effect of the squish mold was that the jump ring sections had a small film over the hole where the bails would need to go.  Rather than cutting through the film at the stage, I left it in place so that the metal would have an easier time flowing into the area, and chose to reestablish the holes with a dremel tool later. 

Next was the fun part - packing the casting flask with sand!  This was my first time using a very small cast iron flask that I got for small projects, and it was the perfect size to fit two medallions in at once.  I use petrobond sand, which is oil based, for my sand casting projects because it's very shelf stable and easy to store for long periods of time without suffering from changes in the way the sand handles. (There are also water-based sand mixtures that are less costly than petrobond, but since water evaporates you have to fuss a lot more with making sure you sand has the proper consistency every time you use it.) The resin positives weren't completely undercut-free at the edges which meant that the sand crumbled around them a bit as they were removed from the flask, but it was a relatively minor issue that could be fixed with a grinder once the medallions were cast.  I was really pleased with how well the petrobond picked up the fine lines of the sculpt, since I wasn't sure how well it would capture small details.  I used a length of copper pipe to punch pouring holes in the back of the sand molds, and everything was ready to be cast! 

I used Zamak 3 alloy for this project, which is 96% zinc and 4% aluminum and melts around 725°F.  I'd love to work with more metals in the future, but for a beginning sand caster I found that it's great for two reasons: 1) it polishes VERY well and 2) it's pretty inexpensive.  (I paid about $18 for a 4 lb ingot, as opposed to some other alloys I was looking at that were $30-50 for a 1 lb ingot!  Makes you feel way better when you get to the grinding stage and you see all that unreclaimable metal dust you produce go in the trash...)  I'll talk a little bit more about my furnace and crucible setup in another post, but at this stage I managed to push through four successful rounds of casting before the afternoon rain caught up with me and I had to pack everything up and go inside.  There actually ended up being a tornado warning later in the day, so uh... just imagine how supremely Midwestern the Darwin award would have been if I had stayed outside and died of tornado/exploding molten metal-related reasons. 

Once the medallions were cast, I had to finish and polish them.  I have a dental lathe that I got off of ebay last year that makes this SUPER easy, but if you're looking to make your own setup then any rotary tool that is capable of holding grinding wheels and polishing pads will do.  I use a set of grinders and polishers with a 1/4" shaft that I got from my local hardware store, similar to this kit.  I took a time lapse video of me polishing some of the medallions, which you can view here!

I chose to leave the sprues on until the end of the polishing process, because they gave me a convenient handle to hold on to while using the polishing wheels.  I found that the best tool for cutting the sprues off was a regular old hand saw, rather than a dremel with a grinding wheel like I originally tried. 

After that, it was bail and chain time!  I used some standard silver-plated bails I got from Joann's, although I had to grind off part of the back in order for them to fit through the rings on the pendants.  I really loved how the chains looked at the store, but might have chosen something else if I had known how fussy and prone to breaking they would be if they got kinked in any way.