Coldworking small castings, Part I

>, coldworking, glass jewelry/tiny vessels>Coldworking small castings, Part I


Q: Is there a better (faster, cheaper) way to coldwork small glass sculptures?

A: Yep

A BEcon or two ago, Richard Whiteley, head of the Canberra glass school, said that glasswork fresh from the kiln was only half finished; coldwork was necessary to take it the rest of the way.


I happen to agree, but as much as I love HAVING coldworked, I hate DOING coldwork and seem to be on a neverending quest to avoid it. Right now I’m testing a bunch of machines to see if they can automate the finishing process for small cast glass sculptures, like pendants.

coveredmoonpendantReal-world costing

I cast sculpted pendants like this one. In the glassland market, you can sell them for right around $140.

Can I make a profit at that price? Let’s find out.

This pendant is about 3 inches long, with a sterling silver wire wrap. Not counting the chain, I invest about $20 per pendant’s worth of casting supplies, glass, and general overhead in one of these (it’s bigger than the picture).

So far, so good, right? You’re supposed to double your costs to get the wholesale price, double THAT cost to get the retail price. So…

$20 x 2 = $40
$40 x 2 = $80

Yay! I’m making $60 pure profit on each pendant, right? Uhm….no, because I haven’t added in my labor costs yet.

So let’s ignore the fact that I needed to create the original sculpture and silicone master mold, and just assume we’re talking about the costs of making the actual pendant. Here’s the labor involved:

  1. Cast the wax into the silicone pendant mold
  2. Demold and refine the wax pendant and prep it for refractory mold-building
  3. Build the refractory mold
  4. Steam out the wax, clean out the mold and give it time to cure
  5. Trim the mold, give it one last cleanout, inspection, and prep for filling
  6. Fill the mold, set in the kiln, and fire
  7. Demold the pendant from the plaster and clean away any stuck refractory from crevices
  8. Trim off excess glass to reveal the final pendant shape
  9. Coldwork the pendant to whatever level of polish I’m looking for
  10. Mount the pendant to be worn
  11. Package it in whatever gift box/pouch I’m going to use

Over the years I’ve gotten pretty fast at this; I gang up the moldmaking, jeweler-style, and make a couple dozen pendants at a time. That way I only take about an hour per pendant to complete steps 1-8, 10, and 11. Still, I’m starting to push the price envelope:

$30 x 2 = $60
$60 x 2 = $120

And here comes step 9: Coldwork. Done by hand, each of these pendants takes about 8 hours of coldworking to meet my standards. NOW let’s run that cost equation at $10 per hour:

$110 x 2 = $220
$220 x 2 = $440

Uhm…that means every time I sell a pendant for $140, $300 is walking out the door (well, I’m only losing $80 if I stick with wholesale).

Something tells me I’m not gonna make that up on volume. 😉

Obviously, some re-examination of options is in order. These are my choices:

  1. Give them away as presents (my current solution, DO NOT ASK me to explain this logic)
  2. Sell them somewhere else for more money
  3. Find ways to streamline production and reduce my costs
  4. Make something else and forget about the stupid pendants

Smaller cast pendants actually take more work (mostly because the back curve is carved out by hand from the casting, and the hole is very tricky to drill.

I supposed #2 is an option, and #4 is a real possibility. But the one that is most attractive, at least for now, is #3.

I don’t really enjoy coldworking my fingers to the bone anyway. If I can figure out some shortcuts that save time (or at least automate the process so I don’t have to do it all by hand), without compromising quality, that’ll translate into my more serious sculptural work, too.

Re-engineering to reduce coldwork

One way to do this is to re-engineer the mastermold to reduce the level of required coldworking. I originally designed the mold for the pendant at left to feed in glass from the back. That meant I had to cut out the back quarter-moon curve by hand, then carefully carve and polish it into shape.

So I redesigned the mastermold to feed glass from the top, which also allowed me to create a thicker drill pad for the findings hole. I can now drill the hole first, while there’s more glass to cushion any potential missteps, and then quickly smooth away the excess glass.

Even with a better infeed, these pendants won’t escape coldworking. The excess glass still needs removal, usually with a saw or cutoff wheel on my Foredom hand-shaft tool.

Then the cut or chiseled areas must be roughly smoothed into a sensual, continuous shape. That handwork is what makes each pendant unique, and I’m not going to escape it.

Nor do I want to. Carving the glass is as much a part of the sculpting process sculpting the clay and wax. Besides, it doesn’t take that long…30 minutes at most.

It’s the tedious other stuff–cleaning the casting, smoothing and polishing it–that takes all the time. THAT is where streamlining and automation will give me an advantage. So I’m trying several approaches, some involving machines you don’t usually see employed in glass artmaking.

Approach #1: Simplify cleaning

I’ve been digging spent material off the castings with brass brushes, tiny Foredom bits and a lot of elbow grease. Last summer I bought an ultrasonic cleaner, cheap, on eBay.

I already use an ultrasonic dental pick to remove mold crud from large castings. It’s sorta like a waterpik on steroids–it blasts a fine stream of water and pulverizing sound into crevices to clean out whatever’s there without harming the glass.

vibratory1-ultrasonicAs long as I can get the probe within a millimeter or two, glass crud and mold material will go sproooooinnngg and fly off the glass. It’s wonderful and tedious, since you must painstakingly touch the probe to EVERY crumb you want to eject. (BTW, it’s also very, very good at removing the worst stuck kilnwash)

It’d take hours to clean 10 pendants that way, so that’s no help. I invested in an ultrasonic bath.

An ultrasonic bath cleaner is probably better known as a jewelry cleaner, and you can buy a little one for $40-50 at any housewares store. You dump your jewelry into a perforated steel basket, add water and cleaning solution, close the lid and turn it on. An hour or two later, your jewelry is sparkling clean.

Mine holds about 2.5 liters with a basket of maybe 5x5x6 inches, and it heats the solution for faster action. Anything that fits in the basket can be cleaned, which means it holds a surprising amount.

It’s completely hands-off and–except for an annoying whine  (use it OUTside)–it’s a dream. (If I could afford one big enough to do my large castings, I’d buy it in a heartbeat.)

Ultrasonic cleaners do not remove devit, they only make it very, very clean. You’ll still need to grind off anything that’s actually embedded in the glass.

Now I simply rinse off the casting as it pops out of the mold, quickly shape it with the saw and grinder, then dump the lot into the ultrasonic cleaner* and do something else. Four hours later, I’ve got sparkling, crud-free castings.

Surface finishing

If the glass comes out of the mold clean, with no flashing or sprues to remove, no grinding is necessary, and it has a soft-natural surface I love.

Unfortunately, if you even touch that surface with any coldworking equipment, it’s done for. Coldworking dramatically changes the surface quality, and it’s tough to avoid with pendants–there are no hidden surfaces. Once you’ve started, you pretty much wind up doing the whole surface.

Ugh. This involves lightly grinding the entire surface so that it’s even throughout the pendant, and then polishing WITHOUT losing any detail. I hate this part.

Fortunately, this is the part that can be automated.

Automation doesn’t necessarily reduce the time it takes and may even lengthen it–some of the processes I’ll be discussing take days or even weeks.

What you actually reduce (considerably) is your own hands-on time with the glass, and you also enable processing in multiples. I’m hoping, by the time I’m done, to reduce total hands-on coldworking time from 6-8 hours to a more reasonable 30 minutes or less. That would (barely) make these pendants sorta kinda profitable.

Approach #2: Sandblasting

My friends Becky, Carol and I invested in a used sandblasting system which we keep at Carol’s house (thanks to my friend Bob, who sold his old one to us cheap, then not only set it up and gave us lessons, but donated an entire Saturday afternoon to dig fracas out of the frabistadjit when it stopped working).

Pate de verre pendants tend to scrub frit right up against the surface of the mold, so they’re particularly vulnerable to what I call “scumming out,” i.e., devit. Normally, I’d hand-sand the scum with 200-grit wet-dry sandpaper backed to a sponge. Once it was gone, I’d move through progressive sandpapers and polishes to take the piece to a final finish.

Sandblasting gets me to the 200-grit level very quickly, and it’s one of the few machines I’m discussing that shortens the process instead of simply making it hands-off.

Sandblasting removes the top layer of glass fairly evenly and leaves the surface at about a 200-grit level, ready for finer grits and polishes. It can also–if you don’t care about very fine detail (or the casting is pretty clean)–let you skip or reduce the ultrasonic cleaner step.

I can stop the coldwork at the sandblast stage, add a light coating of (thanks to Jeff Wright’s suggestion) shower sealer and wind up with a frosted, glowing pendant. The shower sealer brings out the color in the glass and also protects it from finger prints.

The sealer seems to set, or maybe cure, for a couple of weeks, and winds up at about half the sheen you start with, so it’s important to give glass treated this way enough time to settle before you give it to someone.

The sandblaster does have its cons: It’s really, really easy to sandblast away these small faces, especially if I’m trying to clean out a bubbleful of mold material. It doesn’t discriminate between glass and crud.

Peter Cummings has turned me onto soda blasting, is said to clean quickly and wonderfully without touching the glass. I’ll test that as soon as I have access to a soda blaster (I could put soda in our sandblaster, but my partners in crime would probably kill me). And I’m still building my blasting skills, so sandblasting’s ultimate usefulness remains to be seen.

But sandblasting has very definitely become part of my arsenal.

Approach #3: Firepolishing

Sandblasting gets the piece pretty far along, enough so that I can firepolish to a high gloss with only minimal loss of detail and very little drooping. It works well if the back of the piece is naturally flat, not so well if I’m looking for something that’s shaped on all sides.

It’s also very easy to overdo. Firepolish schedules for small cast pendants are tricky, especially if you multiple colors and types of glass in the same kiln firing. The harder glasses will keep their matte finish; the softer glasses will gloss up and start to lose detail. If you have multiple glasses in the same piece, the firepolish can be a bit uneven. Worse, if you overdo the schedule and firepolish too much, the piece will look as if it’s been dipped in varnish.

So I’ll firepolish some things but, frankly, the hand-polished look is far more attractive (to me).

So…next step is finding an automated polisher that (1) lets me control the level of shine in the glass and (2) is fast, cheap and effective and (3) doesn’t look like varnish.

Approach #4: Vibratory tumblers

I’m still running tests on vibratory tumblers, so stay tuned.


*I say successful because no matter how good you get at casting, stuff can still go wrong. I’m pretty good at this stuff, but if I’m making 24 cast pendants, I lose one or two along the way.




  1. Tosca June 8, 2013 at 8:54 am - Reply

    OMG ultrasonic cleaner!!
    I love, and hugely appreciate your tips and tricks, Cynthia!

  2. chaniarts November 19, 2012 at 11:27 am - Reply

    these work by causing cavitation bubbles in the liquid (the foreign debris are really nucleation sites), not by having the sound do the work directly. thus, anything submerged will be acted upon whether the lid is down or not.

  3. Peter Angel November 13, 2012 at 6:16 pm - Reply

    Cynthia, does the ultrasonic cleaner work with the lid removed?

    If so, that would allow you to clean a big piece by placing 1/2 of it into the cleaner, cleaning it, then flip and place the 2nd half into the cleaner and cleaning the second half.

    Big ultrasonic cleaners are very expensive.


    • Cynthia November 15, 2012 at 8:41 pm - Reply

      Peter, funny you should mention that. I’m working on a project right now where I’m doing exactly that, and it seems to work fine.

  4. Jen September 1, 2012 at 5:05 pm - Reply

    Thank you for sharing your knowledge, your discoveries and your findings!
    jenie yolland

  5. Kathleen Howard Piper August 1, 2012 at 12:45 pm - Reply

    Love this! Lots of great info. Now I’m looking for an ultrasound. Thanks!

  6. Cynthia February 27, 2012 at 10:36 am - Reply

    Thanks, Peter. I’m heads-down in the vibratory tumbler tests right now, but I’m hoping to pick up a soda blaster in the next couple of weeks for testing.

    And I’m definitely asking about the burrs. I’ve been told the grit size starts to limit out when you’re doing smaller and smaller tips, so that would be interesting to try. Thanks!

    (Thanks for the nice notes, Havi and Tami)

  7. Peter Cummings February 26, 2012 at 10:31 pm - Reply

    From my practice with glass beads they make the blast “whiter” . That is they make small fractures, you’d need a magnifier to see, and that reflects more light.Maybe useful for the picture type of blasting. Dangerous dust.

    For a practice run with soda, and I’d love a second opinion, you could use el cheapo from a cheap car place. A jar on top of a basic gun, as in the pic. I posted. Do it outdoors, perfectly safe, and easily wipes or hoses of things.

    I remember your frustration. My first cameos though small, were full layers and I carved the background away then handpolished it all back. Back about 2002 and I made my own burrs in the finer grits, from an old chinese jade carvers recipe. Ask if you’re interested.

  8. HAVI [HAVIVA ZEMACH] February 24, 2012 at 3:21 pm - Reply

    So I found again this page, Cynthia. I bookmarked it, and it will with me forever.
    Thanks for the interesting info.

  9. Tami February 23, 2012 at 5:32 pm - Reply

    Great post as always, Cynthia! Thanks for the refresher course on ultrasonic cleaners!

  10. Cynthia February 23, 2012 at 12:20 pm - Reply

    Ellen, we’re using SiC grit–would love to tell you the size but at the moment I don’t know it. I suspect it’s around 220, but…

    I’ve seen the glass beads and have been wondering if it makes a difference WHICH glass you use–bead comes in soda-lime and boro. Someone on warmglass said blasting with bead makes the piece “brighter,” which puzzles me, but I do want to try it.

    Jim, right now I’m testing the “medium” and “fine” grits, which I suspect are 400 and 600 but I’m trying to find confirmation. I’m using small plastic pellets as the filler medium, but I’m also looking at trying porcelain balls for burnishing, and (I’m also playing around with PMC) walnut shell for metals. Interestingly, several lapidary workers have suggested using the cross-shaped plastic spacers you buy for tilesetting, so I’m trying those, too.

    I’m really encouraged by what I’m seeing in tests so far, although it’s a long, messy process and there’s a definite knack to doing it correctly.

  11. jim simmons February 23, 2012 at 11:22 am - Reply

    The vibratory tumblers that I have SEEN WORK PRETTY GOOD.
    Use a variety of materials, everything from SiC to sawdust

  12. ellen abbott February 23, 2012 at 10:23 am - Reply

    You might try glass bead for the blaster. I’ve heard it works well without blasting away any detail.

  13. ellen abbott February 23, 2012 at 10:23 am - Reply

    what do you use in your sandblaster?

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