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Re: European-style sharp doublebacks
Posted By: SVP Neon Equipment In Response To: European-style sharp doublebacks (Romarrk)
Date: Wednesday, 30 January 2013, at 12:51 p.m.
> Hello!
> Can anyone give some practical
> ideas/recommendations on that technique,
> like shown in this french video:> http://www.youtube.com/watch?v=CvThdxtwRgI
> or this> http://www.youtube.com/watch?v=H1zcrQ2Llng
> As I understand this is usually associated
> with boro glass and oxy torches. I tried to
> imitate that technique on regular 10mm
> lead-free glass with air-propane torch,
> surprisingly got a bend that at least did
> not crack. Still, many things are not clear,
> like when to gather the glass? Looks like
> they gather when doing the first 90 degree
> pre-bend?> Roman.
The first video has to be borosilicate. You could not lay any type of soft glass on a cold hard table like that and leave it there like he did without it cracking. Even with boro, there HAS to be a ton of stress in it by doing that. Beyond that, if you look closely, the "double back" looks very thin. I didn't see any type of gathering happening. The finished product really didn't look all that good compared to some "square" bends I've seen.
In the second video, he's just doing simple bends with a one sided gas/oxy hand torch. The clip states it's Pyrex.
You can do the same things with soft glass and a good hot double head gas/air splicing hand torch. Try 15mm instead of 10mm. I think you will find that it is more controllable. Remember to keep some heat on the opposite side of the glass from where you are working. It will help keep it together. When you are finished with the bend, evenly apply heat to the entire area (around the circumference of both tubes) and then use a good, well established flame annealing technique. When done right, you should not have ANY problems with cracking. The only way you will know the effectiveness of your flame annealing is to first practice with clear glass and look at your results with a Polariscope. Otherwise you are just guessing at what you are doing. Glass with significant stress may not crack immediately. It may happen after months or longer while it is in service.
Btw, there are other ways to make a "square" double back, other than what is shown in the videos you posted.
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