Have you ever seen the trick of using a string soaked in fuel and lit on fire to cut a glass bottle?
The thing I immediately thought the first time I ever saw this trick was that it seemed like it’d be a lot more controlled, repeatable, and safe to do with a band of nichrome wire wrapped around the bottle instead of a flaming thread. Today I finally tried it out.
As a first test, I tried the neck of the bottle. I used my existing hot wire tool as a frame, tied nichrome off to one end, wrapped it around the bottle neck, and tied it off on the other end under tension so it’d be just the right length and tension in the as-applied state.
I attached the wire to a bench supply in constant current mode and cranked up the current until the wire just started to glow. I picked constant current mode so that if the wire shorted at the wrap point, the voltage would just immediately drop and the wire wouldn’t get any hotter elsewhere, so no risk of it breaking by surprise due to a short. I insulated that cross point with a touch of kapton tape though, and that did the trick so shorts were never a problem.
I heated the bottle this way for maybe 30 to 40s of glowing wire, then dunked the neck in a cup full of ice water.
The results were good but concerning. The bottle neck cracked cleanly and didn’t actually detach. When I went to the recycle bin and tapped it off with a hammer, I got the nice clean break you see above. The problem is that there were hairline fractures extending away from the break. I didn’t test this, but I’m worried that if I tried to grind the edge smooth with a diamond wheel, it might just crack down into the rest of the bottle from the stress.
With that initial success I re-tried it on the body of the bottle with a longer bit of wire and more or less the same strategy. I increased the current a bit to get a hotter wire.
This time, my cup of ice water wasn’t big enough, so I just used the garden hose and tap water temperature. This resulted in an almost-clean break, but with two flaws. For one, it was not perfectly under where the nichrome had been resting and it wasn’t perfectly planar around the bottle.
For two, the offshoot cracking problem is much worse on this cut, with many tendrils reaching down into the rest of the bottle.
Conclusions and Next Steps
Unfortunately I’m calling this one a failure, although I’m quite pleased to walk away from it neither slashed nor burned. I can’t say it’s any more effective than the also tedious and error-prone process of mechanically cutting a bottle. If I had to try to improve it, I’d next try a thicker bit of nichrome, heated much hotter and for less time, to try to drive a sharper thermal gradient into the cut zone. The tricky part is that the wire in contact with the glass gets cooled by it while the wire hanging in air doesn’t, so maximum wire power is limited by how hot the free-air wire gets before snapping, rather than the wire in contact with the glass.
Anyway, I WOULD try that, but I don’t cut bottles most days of the week, so I probably won’t bother. Hope you enjoyed the experiment!
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