Coldworking small castings, Part I
February 22, 2012 by Cynthia
Q: Is there a better (faster, cheaper) way to coldwork small glass sculptures?
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.
Making the wax, building the mold and setting it up with glass in the kiln for firing, steaming out and demolding the finished casting–averages out to maybe 2-3 hours per successful casting.* Even at $10/hour–pretty cheap, considering the skills involved–my cost is maybe $45 per pendant before coldwork.
Each one requires about 6-8 hours of coldwork so they sure as heck don’t MAKE money–at $10/hour, anywhere from 5 to 50 bucks is walking out the door with each casting, and that’s the kind of thing you don’t exactly make up on volume. So, I examined my options:
- Give them away as presents (my current solution, DO NOT ASK me to explain the logic)
- Sell them somewhere else for more money
- Find ways to streamline production and reduce my costs
- Make something else and forget about the stupid pendants
I’m working on #2. In the meantime, I’d rather not spend hours in cutting, polishing and grinding these things only to lose money on them, so I’m focusing on #3: Find a way to streamline production and reduce my costs.
One way to do this is to re-engineer the mastermold to reduce the level of coldworking required. For example, molds for the pendant at left feed glass from the back, so that I cut the back quarter-moon curve by hand, which takes time and wastes glass.
I’ve come up with a better way to do this, but optimizing mold design for glass casting is a massive, multi-post topic so I’ll save that for another time.
My now-cleaned pendants still need basic coldworking.. Excess glass must be removed (with a saw or cutoff wheel on my Foredom). Cut or chiseled areas must be roughly smoothed into a sensual, continuous shape. That’s handwork, it’s what makes each pendant hand-crafted and unique, and I’m not going to escape it.
Nor do I want to–I’m carving glass, and it’s as much a part of the sculpting process as making the original model. But once the pendants are shaped, the rest of the grinding/polishing cycle tedious, boring, stuff. How do I eliminate it?
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.
As 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, and I can’t afford it. So 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 the thing’s perforated basket, add water and cleaning solution, close the lid and turn it on. An hour or two later, your jewelry is sparkling clean.
Mine is bigger, holds about 2.5 litres 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 number of pendants.
It’s completely hands-off and except for the annoying whine it makes while working (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 simply make the devit very, very clean. If there’s any devit, it will need to be ground or blasted off.
I rinse off the casting as it pops out of the mold, rough out its final shape with the saw and a 60-grit diamond belt, then dump everything into the ultrasonic cleaner.* Four hours later, I’ve got sparkling castings, no mold crud anywhere.
If the glass comes out of the mold clean, with no flashing or sprues to remove, an ultrasonic cleaner is all you need. Straight-from-the-mold glass has a soft, semi-gloss surface that I love. It looks natural and is good to the hand.
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. If I’m making 24 cast pendants, I can pretty much count on losing one or two along the way.
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