I want to turn some milk-bottles into drinking glasses. I need to cut off the neck of the bottles. Then I need to remove several glass blemishes. Can anyone give me a clue as to how to do this?
Ficer67
Some glass cutting methods. The cut edge can be fire-polished with a propane torch. depending on the nature of the blemishes, these can also be fire-polished out, or ground down with a small rotary grinder (I use my Dremel), then fire-polished.
I’ve tried a variation on the cutting methods in Q.E.D.'s cite. Basically, scratch the bottle with a glass cutter where you want it to break, heat it with a torch, and dip in water, letting the thermal differential crack the glass. It worked, but let’s just say that you should have a few extra milk bottles for practice. My cuts were pretty jagged.
If there’s a glass shop in town, they might have a diamond bandsaw. You’d need a jig to hold the bottle so it doesn’t rotate when cutting, but that would be the ideal way.
You can polish the glass with a belt sander and some wet/dry sandpaper, with all the caveats about using water near electrical appliances being in full force.
The fire polishing is probably a better approach but I wouldn’t be surprised if you have to anneal the glass afterwards to keep it from cracking.
IIRC, there used to be hobby kits that made it easy to do this. With a little googling you should be able to find them. Ebay, maybe?
But it shouldn’t be hard to do it yourself. All you need is a glass cutter and a way to roll the bottle while making a straight, even cut. I think you use a rod with a weight at the end to tap the cut from the inside of the bottle and break off the neck.
Then you use emery cloth or grinding polish to smooth the edge of the cut. Be very careful, obviously.
WEAR SAFETY GLASSES - WEAR LEATHER WORK GLOVES
I used to do this all the time.
First, get a decent carbide wheel glass cutter. A good glass cutter makes all the difference in the world. With the bottle clean and dry, wrap PVC electrician’s tape around the neck of the bottle with one edge of the tape at the place where you want to make the cut. The thicker the tape, the better. Using the edge of the tape as a guide, oil the cutter and then scribe a line around the bottle. If the ball on the end of the cutter will reach the line from inside the bottle, then use it to tap the inside of the bottle at the line you just scribed. Take your time and slowly increase the pressure of the tapping as you go around the line. You should see the crack starting to form as you work your way along. If the cutter ball won’t reach, you will need to fashion something that will.
The aluminum oxide emery cloths (cloth backed sandpaper) made for use with metals will work better for smoothing than the paper backed sandpapers for use with wood. It is best to set the emery cloth flat on the bench and rub the bottle against the cloth than the other way around. Polishing glass by hand takes a lot of time and patience, but is fairly easy to do if you are willing to take the time. Work your way down to 400 grit (crocus cloth) and by then you will have a very smooth surface that won’t shine but it will be smooth to the tongue and lips. You will need to move to a polishing wheel and rouge if you want to go further. Don’t forget to bevel the edges as you polish.
The glass used to make bottles usually has fewer fluxes in it than the soft glasses used, for example, in scientific glass that can be easily worked in a flame so flame polishing will be tricky. The entire bottle will probably get very hot by the time the edge gets hot enough to melt, so if you try flame polishing, be sure to work on a heatproof surface. Don’t set your workbench on fire! But the time required for hand polishing will pretty much guarantee that you will try flame polishing at some point so be very careful. Did I mention that you must wear safety glasses the entire time you are working with glass?
Good luck!
Two suggestions:
First, all of the previous posters recommend scribing a surface cut with some sort of tool and applying a mechanical force or shock to propogate the crack. To this, I would only add that after you make the initial scribe, wet it. For small diameter tubing, spit works good enough for me, although this might not be practical for something the size of a coke bottle. Personally, I’d say that a glass-cutting blade and polishing wheel is the way to go. If you don’t have these, well, practice, practice, practice.
Second, as regards QED suggestion to use a propane torch to flame polish the rough edges, be very careful. Simply lighting up and blasting the cut edge will most likely result in a hand- or faceful of shrapnel. You’ll want to heat the cut slowly and evenly to the softening temperature. Keep the flame moving at all times!! Similarly, after polishing, you should reduce the heat slowly so as not to produce internal stresses which will make it very prone to cracking at the slightest shock. A nifty way to do this is to have a small metal pail handy, half-filled with vermiculite. After polishing, drop your new coke-bottle mug into the pail and cover it with a generous amount of additional vermiculite. This stuff is safe, non-flamable and a great insulator, allowing your piece to anneal slowly.
Bizzwire (who just finished putting a flameworking bench in his basement)
DaffyDuck
one small, insignificant, teensy-weensy nitpick:
You have it backwards: Most scientific glassware is borosilicate or similar which has a much higher working temperature, requiring an oxygen/fuel flame and didymium glasses. Soda glass, with more flux, can be worked at lower temps.