Step 1: Pewter Experiments
Video of melting pewter using a steel ladle as a crucible (it worked well for me due to the long handle):
Video of scraping slag off the top of the molten pewter:
Video of pouring pewter into a flat mold:
An additional problem was the pitted, moon-like surface result I got using untreated silicone. I had theorized that the mold was too cool and tried boiling it prior to pouring, but that did not help.
Taking advice from windswept and interesting's Simple Pewter Casting instructable, I tried using my 1-piece mold propped upright and backed with hardboard, to allow gravity to push the molten pewter fully into the mold. From gg1220's excellent signet ring instructable, I learned that a smooth pewter surface was possible by using a graphite powder mold release. I don't know why this works, nor why the surface is so pitted and lumpy without it; if you know more about this, please comment! Graphite powder can be purchased, but I just ran a pencil over some sandpaper for a couple of minutes, catching the result on a small plate. Prior to each pour, I brushed some graphite powder on the inside surface of the mold using a small paintbrush.
These changes allowed me to make some molds and castings I was satisfied with. The next steps will show the details of the successful process I worked out.
Step 2: Design
As I did the actual work to make these, I simplified this drawing a little. I made only one loop for the hook to go through, and removed the hook piece crossbar.
Step 3: Making the Molds
To make the molds, I used a 2 part silicone called Dragon Skin (10 Medium), and while this worked fine, a firmer silicone would have been a bit better. Also safer would have been a tin-cure silicone as they are less susceptible to sulfur, an ingredient in some plasticines. Basically, the cure of some silicones is "inhibited" in the presence of sulfur -- they don't harden up and just stay goopy. Not useful. I sprayed my originals with two coats of an acrylic sealant to avoid this issue and the Dragon Skin cured fine. (Always read the instructions for the materials you use in case there are important tips like this!)
I mixed the silicone and poured the molds according to the Dragon Skin instructions. It's a simple 1-1 ratio by volume. I measured the volume the first time by running water into the structure and pouring it out again into a measuring cup. It took a couple tries before I was able to do this without spilling enough of it to make the measurement clearly inaccurate!
I did not worry about degassing the mold to remove bubbles, as I didn't care if there were bubbles in the mold as long as they did not touch the casting surface. The best way to avoid this is to pour very slowly, starting from the bottom, and allow the silicone to find its own level. If you see any bubbles form against the original object, poke them with a stick until they pop. At a guess, it took me about ten minutes to pour the mixed silicone into each mold, and I mixed a different batch for each because the pot life was only about 20 minutes. ("Pot life" is the amount of time you have to work with any two-part curing chemical before it starts to firm up.)
Here is a video of the silicone pour of the test I did from back in Step 2. It's almost ten minutes long; skip through it. The first half has some useful discussion of technique but the second half is pretty boring. Shows a good pour speed: SLOW!
The 10 Medium Dragon Skin cures in about 5 hours so I just left these overnight. In the morning I pulled the plasticine out and cleaned the molds with a dental pick to remove as much plasticine as I could. If the original sticks at all, use care when peeling it away as you don't want to risk tearing the silicone. Plasticine doesn't stick much directly (to tin-cure silicone at least!), but the acrylic sealant I used did stick a little. I washed them with hot soapy water as well and got nearly all the plasticine out. With a mold release I probably wouldn't have had any visible plasticine leftovers, but I'd still have washed them to remove the leftover release. Here is a video of removing the plasticine from the same mold as in the previous video:
Readers with casting experience and/or topological knowledge may be wondering about the loop piece. In general, cast objects with genus greater than 0 must use a two-part (or more) mold, as the hole can't be removed from the mold without breaking either the casting or the mold. From my previous experiments with pewter, though, I knew that the molten metal would not enter a clean, closed slice in the silicone, as the surface tension is too high. Also, the silicone is flexible so even fairly severely undercut castings can be removed, and good at sticking to itself so a clean cut would "heal" very slightly, keeping the edges in place during the pour. My plan was to make the mold, remove the plasticine in parts, and slice the loop mold with a sharp knife to allow the two sides to bend back and release the pewter loop. See the last few pictures for more on this.
Step 4: Preparing to Cast
To obtain a smooth finish on the cast pewter, I used graphite powder as a mold release (see step 2).
Prior to casting, make sure you have the following supplies laid out and ready:
- leather gloves (DO NOT use synthetic gloves that might melt; go barehanded if you must. You don't want molten plastic on your hands, a clean burn is better!)
- tongs or large tweezers, depending on the size of your castings
- bowl of water for quenching castings, or dousing flames
- Torch or other heat source, and a lighter or matches if it's not self-igniting
- A crucible, which is just anything you can melt your pewter in that won't melt or burn itself. I used a steel ladle which was nice because it had a long handle and I didn't have to try and hold the cup part. But even a clean, dry tunafish can will do in a pinch (beware sharp edges of course, and remember it will get HOT)
- Clamps to hold the mold
- Small brush for the graphite powder
- Flat, non-flammable work surface
- Tinsnips are useful although trimming the casts can wait
Step 5: Casting With Pewter
I used my kitchen stove as a work surface; I have one of those glass-topped stoves where the heating element is set underneath the glass, and I do more projects on it than I do cooking!
To melt the pewter I used a small propane torch, and an old steel ladle as the crucible. I originally tried heating from the bottom, like a stove, but later realized there's no reason not to heat the pewter directly, holding the torch above it. This is much faster and you can direct the heat more precisely. Once the pewter was melted, I scraped any slag off (see video on step 2), and then poured into the top of the mold fairly fast. If your mold is large, keep the torch on the metal while pouring. A helper is nice for this but not essential. Be aware of where the flame is, as always; the mold will burn if you flame it too much. Silicone doesn't catch fire easily, though. I tried burning an earlier mold that I didn't need any more, and while I ruined the mold, it didn't cause a dangerous conflagration.
After pouring, the pewter was solid within a minute, and the mold cool enough to handle shortly thereafter. I did one of each clasp part at a time, letting the first one cool while I poured the second, and cooling the second while demolding the first. Using tongs, I quenched each piece in a bowl of water as it came out, primarily to speed up the cooling process so I could handle and set aside the finished pieces right away. As soon as each was cool, I cut off its button and sprue with tinsnips (and put the scrap pewter into the next cast!). The three clasps I wanted took about 20 minutes to cast.
Step 6: Finishing the Clasps
And wow did it take time! I hadn't originally thought about doing any finish work so I hadn't considered it when designing the pieces; as a result it there are inside corners that were impossible to reach into with the tools I had. Luckily the smallness also meant it was hard to see in, so I was able to get a finish on most of the visible surfaces that I was satisfied with. I used a Dremel sanding drum for the first two coarseness levels, but Dremel doesn't sell a sanding drum finer than 120 grit (nor do any of their competitors that I could find). After many hours of sanding with the two drum sizes, and various experiments with the blowtorch again (doesn't make a nice surface and melts too suddenly) and some of Dremel's sanding discs, which get finer, I ended up deciding that a brushed metal finish was very pretty. And the wire brush attachments could get pretty far into the small corners.
Step 7: Attaching to the Coat
I cut six black leather circles, of a size that would provide a nice visual frame for the pewter clasps, by first tracing a clasp set onto paper and then finding a bowl that fit around it well. I folded each circle in half around the side of the coat's front opening as tightly as I could, and clamped it in place so I could sew around the edge. This is an unusual technique; I wanted to support the heavy pewter with as much leather as I could, and also wanted the inside to look almost as good as the outside. I used a clamp support here too; some half circles I cut out of 1/4" plywood about an inch smaller in radius than the leather circles. I wasn't able to get a good machine seam going so I sewed the circles on by hand using a blanket stitch.
Once the circles were in place, I stitched the clasps down. I clasped two together, placed them on the coat centered on their circle, and clamped one side while I stitched. I made three large stitches, not too close, over several of the branches of the clasps, and tucked the ends underneath on the inside of the coat, then knotted them. To hold the knot (cotton sometimes works loose), I put a drop of Fray-check on each knot, and cut the ends to about an inch.