Introduction: Cast Pewter Coat Clasps

About: I run Neal's CNC in Hayward, CA, an expert CNC cutting and fabrication service. Check out what we do at I'm a founding member of Noisebridge, a hackerspace in San Francisco, and Ace M…

I promised my friend a big pimped-out fake fur coat, and he loves heavy metal hardware.  I had the idea to try making some gorgeous unique buckles for the coat.  After some testing I decided that a buckle, with its separate tongue piece, would be too complicated for my first pewter casting, and simplified it to a hook and eye clasp.  Took me a long time to complete, but I think they came out wonderfully.  I'll try a buckle ... later on.

Step 1: Pewter Experiments

First I had to learn how to cast pewter.  After researching on Instructables and other places, I decided to try a high temperature silicone mold.  Experimenting showed that a one-piece flat mold did not work well as the molten pewter has a very high surface tension and would not easily spread throughout the mold.  Pushing the molten metal further around in the mold with a spoon is a technique that initially showed promise but could not be done consistently.

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

I wanted a curvy, organic clasp design.  The coat I would put the clasps on is of black and white checked fake fur, with a purple lining, and I had made it without any overlap at the front.  I wanted clasps that would behave a bit like a frog closure, where the two pieces to be closed just butt up to each other, and the clasp does all the overlapping that's necessary.  The frog closure is of Chinese origin and I liked that feel, so I had that in mind while designing the clasps.  I decided they needed black leather circles to back them, to show off the clasps and prevent the pewter from getting lost in the fake fur.

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

Once I had a design laid out, I sculpted the two sides, hook side and loop side, in plasticine (oil-based clay) and stuck them onto a melamine board in preparation for the mold.  I built up a wall around each side to hold the liquid silicone while it cured.  I also added a "button" to the section of each clasp that I planned to be at the top during casting -- where I would pour the metal.  The button is basically just a built-in funnel so you don't have to pour into a tiny tiny hole, risking spills and failed casts.  One one of the pieces I also made a sprue between two sections of the mold as I had an upturned section and I was not sure the molten metal would push up into that area.  It is important to optimize the path of the casting material in any casting process.

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 keep my one-piece molds upright, I cut out two pieces of hardboard, one for each side.  I made a kind of sandwich of these, with the mold in the middle, and used a couple spring clamps to hold them together.  One piece actually touches the metal and acts as part of the mold; the other piece merely supports the clamp to keep it from squishing the silicone.  I was able to arrange the clamps to act as legs to keep the mold upright while I poured the pewter.

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
You will also need:
  • 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
Your first time, I recommend briefly walking through the gestures you'll make with the torch OFF, so you don't suddenly realize, say, that the bowl of water is on the wrong side and you have to move your arm through the flame to reach it.  It's also useful to have a friend around in case of emergency or just to say HERE HOLD THE TORCH while you arrange to not drop the mold.

Step 5: Casting With Pewter

I bought one pound of pewter from Rotometals, which cost about $20 plus another $15 shipping to my area.  From my experiments, I had plenty of pewter pieces from failed castings, and I was able to estimate the weight of the pewter needed for each piece, and gather scraps to that weight.  (Metal is a fantastic casting material as it is almost 100% reusable after a bad cast, just re-melt it.)  Each side of my clasp weighed about 50 grams, including the button, so I was able to melt and pour just the right amount.

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

I made three casts of each clasp side, for three closures on the coat.  The finish on each, due to the graphite powder, was far superior to the pitted surface I'd originally seen, but still not as pretty and shiny as I wanted.  I used a wire brush to remove any leftover graphite and slag on the surface, which helped a little, but not enough.  Also they were not totally identical; some had minor lumps or were slightly thicker at various points than others.  Again following gg1220's advice, I determined to sand and polish until I achieved a finish I found acceptable.

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 had not thought to make any small holes to sew through; I'm not sure, in any case, if the pewter would have been strong enough for a sufficiently small hole so that the sewing would not be noticeable.  I decided to make a virtue of necessity and sew the clasps on visibly, using heavy embroidery thread and large decorative stitches.  The other consideration was about the leather patches I planned to use as a base: to sew leather, you must make holes in it.  Holes weaken the leather so you can't do too many of them close together, or you end up with a kind of perforated effect that just begs to tear on the dotted line.  (Fabrics don't have this problem.  The needle, passing through the fibers of the weave, pushes them aside and slips past, rather than breaking the fibers.  Certain kinds of fabric seams are actually stronger than unseamed fabric, but this is never true of leather.)

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.

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