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Re: bombarding darkening

Tom,

  I'm in the "soupy hot" category.  But with Voltarc I had to be careful because 
of the darkening phenomenon.  What I'm talking about here is just a "flick" back 
though the fire several times during the course of the bend, which keeps the temp 
up, as opposed to heating sufficiently before bending begins to keep it plastic 
long enough.  

  Just as a sidebar to this discussion, for a strong bend in standard glass, your 
glass should continue to move for quite a long time after the bend is done.  I 
can't describe how long, for while I've showed this to people countless times, 
I've never actually timed it.  If your glass seizes up during the bend or directly 
after, you are looking at problems down the line.  This is also a good arguement 
for several layers of non-bestos on the table for its refractory properties.  The 
wood of the table directly under the paper can chill the glass.  I believe that 
the thick asbestos paper we used to use helped prevent this chilling.  Although 
there is a screen between the glass and the table, this is little protection 
either way, especially if you only use one layer.  Just look at how burnt paper 
patterns get with only one layer of screen.  If you are transmitting that much 
heat downwards, there isn't much barrier there.  Double screening helps also.

  As to how charred to get the paper while bombarding, for most standard glasses, 
when the paper turns visibly brown you can then start the final bombardment of the 
electrodes.  You need to experiment with how hot you can get different glass, 
usually the hotter the better except for cases like this.  But if you've charred 
the paper at all with normal glasses you should be ok.  Try using EGL glass and 
see if you encounter the same problem.  Also, you remark that the powder only 
turns during bombardment...while that is the case it is very likely at least 
partially caused by overheating during bending.  

  Electrodes:  I had a little exhibit on the wall of my shop.  It had several 
different electrodes removed from the worst repairs to come into my shop.  The 
best ones were a set of electrodes from one unit that came in with horrible 
mercury stains, having been repaired several times, it was a real disaster.  
Before remaking it I removed the electrodes and started the exhibit...one 
electrode was a Voltarc double shell cone one that was barely there, and the other 
was an EGL double shell (little capsule shapes) one that was completely intact.  
That was a situation that I've seen time and time again, I still can't figure out 
why anyone uses those other ones.  Unless they like doing repairs.   It's been 
several years so I can't speak to the quality of something I haven't even seen in 
that length of time, but I hope they are still of good quality.  

Bettina

NeonThom@aol.com wrote:
> 
> Ted-
> 
> >Maybe also look at the way you're heating the glass in the torches, some
> phosphors react easily to overheating. I have a local competitor who loves to
> really get the glass soupy-hot, and his stuff ALL has "prestained" bends on
> his blue and white tubes.<
> 
> Could you tell me more about how to avoid overheating.  I seem to heat the
> glass the least amount I can to make a good bend...  I wouldn't say I get the
> glass soupy-hot.  My bends also don't appear "pre" stained.  It doesn't show
> until during the bombing...?  It seems to start showing up when glass temp
> hits 100C.
> 
> Bettina-
> 
> >With Voltarc, I had to institute a technique of heating several times during
> the bend to maintain the sort of plasticity I needed without overheating the
> powder. <
> 
> Could you also say more about this technique...?
> 
> >it's my opinion that gauges vary and can be misleading<
> 
> Yes, I almost always use paper and a temp gauge.  I, too, find inconsistency
> between the gauge and the paper.  Even after testing my gauge against other
> gauges.  Using both seems to be the best way to get a "feel" for the glass
> temp.  I do, though, have some confusion as to how "charred" the paper is
> supposed to get to be up to proper temp.
> 
> >Give EGL double shell electrodes a try also<
> 
> I'm not familiar w/ these.  I've seen many of the Voltarc double-shell
> (mostly when the shells have fallen apart and the unit comes in for repair -
> I tend to not use them), but not EGL.
> 
> Thanks much - all info is helpful!
> Tom
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