Stop Your Glass Jewelry From Cracking: Dealing with Cold Spots
If you’ve ever spent hours on a complex piece of glass jewelry only to have it spontaneously shatter later, you know the frustration. It’s heartbreaking. Usually, the culprit is a “dead zone”—those annoying little pockets in your kiln where the heat just doesn’t reach. Standard convection kilns are great, but they rely on air. Air is lazy. It struggles to get into tight curves or deep recesses. When one part of your piece stays cooler than the rest, you get internal stress. And stress leads to cracks. To fix this, we started bringing short-wave infrared (IR) lamps into the mix. Instead of waiting for hot air to drift into the right spot, IR lamps beam radiant energy directly into the glass. It doesn’t matter how weird the shape is; the heat just penetrates.
How it actually works
Think of it like the sun. You don’t feel the air getting warm before you feel the sun on your skin; you feel the heat instantly. By setting up these lamps in a multi-angle array, you’re basically wrapping your jewelry in a blanket of heat. No more guessing if the narrow neck of a pendant got warm enough. You’re hitting it from every side, which kills those cold spots that usually ruin a high-precision piece.
Picking your gear
You’ll probably want to go with quartz halogen tubes. They’re the go-to because they pack a serious punch of heat density. I usually suggest looking for R7s or SK15 connectors. Why? Because tubes eventually burn out, and you want something you can swap in seconds without a headache. One thing to watch out for: your power grid. High-wattage tubes get you up to temperature faster, but they pull a lot of juice. If you’re running a dense array of lamps, make sure your control cabinet has plenty of breathing room. If your electronics overheat, you’re going to trip breakers or fry your SSRs, and that’s a mess nobody wants to deal with on a Tuesday.
The catch (and how to beat it)
Here’s the thing: IR lamps are “line-of-sight.” If your piece has a wide flange or a bulky section, it’ll cast a shadow. And in that shadow, the glass stays cold. The trick is to wire your lamps in a “cross-fire” configuration. It means more wiring and a bit more space in the shop, but it ensures every single surface is getting hit. It’s a bit more work upfront, but it’s the only way to sleep soundly knowing your work isn’t going to crack the moment it hits the bench.