Coldworking small castings

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


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 sell wholesale).

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

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: Reduce my costs by streamlining production, especially in the coldwork.

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 help 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.

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 the cutoff diamond wheel on my flex-shaft.

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. Compared to the rest of the coldworking, it’s relatively short, 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. 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.

Update: Ultrasonic cleaning works well for small pieces that fit in the basket, but aren’t much help for bigger sculptures. I purchased an inexpensive pressure washer, the kind homeowners employ to blast crud off driveways, and have been pleasantly surprised at how quickly–and well–it works to remove investment. If I could figure out how to secure small pieces so that the blast didn’t send them halfway down the block, I’d use it on jewelry, too.

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.

This involves lightly grinding the entire surface so that it’s even throughout the pendant, and then polishing WITHOUT losing any detail. Ugh. Ugh. ugh, but fortunately, THIS process is ripe for automation.

Automation doesn’t necessarily reduce the time it takes and may even lengthen it by days or even weeks.

What it DOES reduce (often considerably) is MY time, i.e., the hands-on time I must spend on per piece. I can dump multiple pieces into a hopper, add my grinding/polishing compounds, turn the thing on and forget it until it’s done. With luck, I’m hoping to reduce my actual time with each piece from 6-8 hours each to a more reasonable 30 minutes or less.

That would (barely) make these pendants sorta kinda profitable.


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 digging 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. You can pull them out of the mold in perfect condition…only to discover an uneven cloudiness on the surface that looks as though it needs to be cleaned.

Normally, I’d hand-sand off the scum with 200-grit wet-dry sandpaper backed by a sponge. Then I’d move through progressive sandpapers and polishes to take the piece to a final finish.

Instead, I can sandblast all the pendants at once, getting to the de-scummed 200-grit level in 5-10 minutes.¬†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.

Sandblasting is one of the few machines I investigated that shortens the process instead of simply making it hands-off. It’s not perfect; sandblasters don’t care if they’re blasting mold crud or glass. If you don’t pay attention¬† you can easily erase the detail in your work.

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.

Artist Peter Cummings has turned me onto soda blasting as a mold removal process. He says it cleans off the glass quickly and wonderfully well, removing crud but not 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 will 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 and I’m willing to accept that the back will pick up some kilnwash texturing. It’s not really an option for a piece that is shaped and textured on all surfaces–whatever is against the shelf will distort.

Many glass artists will add a “sealing” firing to sandblasted pieces, a very low firing in the kiln that stops just short of firepolish and is less likely to add shelf texture. It gives the piece a satiny finish without entirely losing the sandblasted quality, and makes it pendant less vulnerable to stains and fingerprints. It’s a great option, but it requires a full anneal cycle that ties up your kiln for a day or more (these pendants are nearly an inch thick).

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 can wind up looking plastic.

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.

Vibratory tumblers

At some point in my life I’ll finish my tests with vibratory tumblers (I hope), and have some definitive tumbler schedules for soda-lime glass. Until then…

You may have used rotary rock tumblers as a kid–you toss a handful of found pebbles into a plastic bin, along with some water and grit, set the bin on a rotating platform, and let it run for a few hours/days. The action of grit+other stones bashing into each other will knock off the sharp edges, round over the stones a bit, and, as you swap in successively finer grits and then polish, produce a beautifully shiny, rounded stone.

The problem with that, at least for my application, is the “knock off/round over” part: Rotary tumblers grind off the high points of your painstakingly detailed glass, trying to turn it into a smooth pebble.

Vibratory tumblers can do an amazingly good job getting an optical polish while preserving detail. Instead of bumping and grinding individual, grit-coated pieces against each other, a vibratory tumbler combines your material and grit-embedded pellets as a single mass. The action is much gentler on your pieces, tending to remove a thin layer of glass evenly all over the pendant.

They also work faster than a rotary tumbler (although I’m told a magnetic tumbler works even faster–haven’t tried that, so I wouldn’t know), although fast is a relative term: So far in my tests, going from smoothed sculpture to a beautiful, soft, shiny finish takes about a week.

My “sonic” vibratory tumbler (you can find these on eBay, and some lapidary supply places carry them, too) has a solid rubber hopper shaped a bit like an extruded, upside down light bulb.

You open the bin, pour in your pieces, pellets, 80-120 grit abrasive (I use silicon carbide), small polystyrene pellets, about a quarter-cup of water mixed with a tiny amount of dishwashing liquid and (not kidding) maybe a half-teaspoon of sugar. Then you pop the lid back on, turn on the machine, stick it in a box (to deaden the sound–these things make a huge racket), and walk away for 4 hours or more.

At 4 hours, you start checking by pulling pieces out of the pellets at random. They’re done when they look uniformly sandblasted. At that point, you dump the bin in a giant sieve, rinse material, pellets, and bin thoroughly, and move to the next grit. And the next, and the next, until you get to polish.

In my tests so far with soda lime** (

* I say “successful” because no matter how expert you become, casting can still go wrong, there are just too many variables. I’ve gotten pretty skilled at making these little sculptures, but I’m not surprised if I lose one every couple of dozen or so.

If you use lead crystal your glass processing time will shorter by 20-25% (or, if the glass is very high in lead content, even shorter). You’ll want to very carefully monitor it to avoid taking it too far, especially with the coarser grits.¬†




  1. spikeabell February 3, 2019 at 2:04 am - Reply

    I’ve not heard of soda blasting… but I snorted when you said ” but my partners in crime would probably kill me” as I’m fairly certain that would be my fate too if I tried that on at the Canberra Glass works sandblaster. Love your posts and sense of humour.

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

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

  3. 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.

  4. 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.

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

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

  6. 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!

  7. 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)

  8. 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.

  9. 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.

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

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

  11. 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.

  12. 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

  13. 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.

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

    what do you use in your sandblaster?

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