rounding over the sharp edge of a cut neon tube

hi, wondering if i could ask a question, worried it's been asked many times before, apologies if it has. i got a few 4 foot long broken lengths of neon tube. i want to cut them into approximately 16 inch long pieces and heat the cut ends to round over and smooth off the sharp broken edges. i was told i could use a ("bernzomatic") propane torch to heat the glass enough so it melts (and rounds over the sharp edge). i tried it and much to my surprise it worked. i tried it on a few more pieces and i got erratic results. on some it worked fine, on others it caused a crack to run down the length of the tube, about 3 or 4 inches. this is what i don't want to happen. can anyone give me tips or pointers on how to heat the end to round off the sharp edge without causing the tubes to crack? i am so much hoping i'm not going to need an annealing oven (cause i don't have one!).
Reply to
William Wixon
Welcome to the wonderful world of glass! The best you can hope for is to lightly play the flame over a good bit of the length (4 - 6"?) to prevent just the end from getting hot, causing the sharp transition from hot to cold, and the resulting thermal shock. If you have an OA rig (and since you post to rcm, there's a chance you do), try using it (careful - it's a lot hotter!) gently, and "insulate" the warmed areas by turning down the O2, to coat the glass with soot (after you warm it); this will help the glass to cool more slowly & evenly. Don't get soot on the end when it's molten, though, or it might get embedded into the lip.
Generally, the thicker the glass, the more problems you will see with trying to do this sort of operation. The soot coating is a great way to anneal thin pieces - regardless of their overall size. For somewhat more mass, dropping the finished piece into a container of vermiculite will do much the same. Above that, you need an annealing oven.
What are you gonna do with the tubing? Is it real neon tubing (clear glass) or does it have the phosphor coating? Just curious.
Reply to
Hi, there is another trick you can use - get one of those single electric burners - really cheap at walmarts - or try out the local thrift store - you can also use a cheap crock pot for this. Fill a can vermiculite (for a crock pot skip this step, and just fill the pot). Turn your heating on to warm and place the tip of the rod into the vermiculite, let this sit for about 30 minutes then turn up to hot, let sit for another 30 minutes - take the rod out of the warmer (use a heat glove if the other end is hot) and then slowly introduce the tip into the flame, this will greatly increase your odds on not cracking. Place the heated end back into the warm vermiculite and let the rod slowly cool (this really doesn't anneal as well as a kiln, but it is better then nothing). Your other option is to just score the glass and do a clean break, then take a dremel tool with a glass polishing disk (they have them at the hardware store) and polish your edge.
Reply to
One thing people overlook is the simplicity of just doing things by hand: a simple piece of fine sandpaper rubbed at ninety-degrees to the cut will quickly remove any hazard, and it can also be used inside the lip, should that be a concern. Any grinding routine will result in dust, so the inside of the tube should be blocked if dust is going to be a problem. Also, to avoid dust inside the tubes, store them horizontally or put plugs in each end.
If you use a torch, one problem which might occur is that the glass might become "reduced", causing it to turn black. This is caused by the lead in the glass (neon tubing is technically "crystal", as used for glassware, and crystal is more than a quarter lead). This can be avoided by using more oxygen (if using an oxy-acetylene or oxy-propane - the latter being used by lampworkers), but the only way to avoid this using a Bernzomatic torch is to avoid playing any part of the flame over the glass which contains unburned propane, as the propane will "steal" the oxygen it needs to burn from the glass, reducing the lead oxide in it down to plain lead, which is black in glass. Just use the edges of the flame, and keep the flame moving so that no one spot gets too hot - the hotter the glass, the easier it is for the chemistry of the flame to react with it - if you just try to warm the glass and don't rush it, you should get good results.
As for annealing: use very diffuse heat to warm the glass near the area to be worked before you start to apply melting heat to the lip. After the glass has rounded, apply more diffuse heat to the glass near that area and a little down the tube, causing the heat to form a gradient and avoiding having too great of a difference between any adjacent areas of the tube (keep rotating it - always!). Gradually withdraw the heat and you should reach a state of balanced stress which will not break the tubing. All of this problem is caused by the fact that most materials expand when heated: add that to the fact that glass is brittle, and you can see how expansion can caused problems Using a can of hot vermiculite for control of the cooling is nice, in theory, but it really doesn't help for something as thin as neon sign tubing: it is already too cold to be affected by the warm vermiculite - that cooling technique is often used by bead-makers just getting started and unaware of how to make their own annealing oven on the cheap, but this only helps if the piece is thick enough to hold enough heat between torch and bucket (or pan - an electric frying pan is sometimes used to keep the vermiculite hot) so that the piece can self-anneal.
Unless you have a lot of this tubing, or unless you have a specific project in mind, you might find it less costly to just buy the tubing from a neon supply house - they might charge you to cut the glass, and they might not be able to fire-polish the ends, so a trip to a glassblower (a neon shop) might be in order: sometimes, if the worker is interested in what you are doing, you might catch a break on the price (some glassblowers are willing to give a break to those doing odd non-profit projects while making their bread-and-butter off of those with commercial interests - but most are poorly-dressed tightwads uninterested in anything but the bottom line and who would take offense that you would even consider taking up a torch on your own, so it might be best not to mention your own trials unless you are copacetic or know his or her temperament: there are glassblowers who have a reputation for verbally bringing grown men to tears, just for the sport or because of a (possibly permanent) bad mood (one name stands above the rest: Gunther Weiss, founder of Weiss Scientific Glassblowing and once, arguably, the best glassblower in the US - probably dead by now, but he could make a complete stranger feel like trash). Hopefully, if you complete your project and don't do any more glassblowing, you will be safe from whatever causes glassblowers to be this way.
Reply to
Douglas Wiggins

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