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Re: Glass Teeth

Posted By: gavin
Date: Saturday, 24 June 2006, at 12:01 a.m.

In Response To: Re: Glass Teeth (SVP Neon Equipment)


> Dana, I know that for the most part you are
> teaching yourself, so you do not have the
> benefit of a seasoned bender looking over
> your shoulder giving you pointers. Even if
> they were, it has been my observation and
> experience that in situations such as this,
> most would not give the right advice anyway.
> (The advice to stretch the glass where the
> cut will be made is just one small example).

> It has always dumbfounded me that most
> veteran benders have never heard of a
> polariscope (much less used one) and why
> "neon schools" do not teach their
> students how to use one. It can be a very
> valuable educational tool for instructing
> someone. It will show you where stress
> occurs and the severity of it. The astute
> instructor can show what can be done to
> reduce or eliminate stress and why, and the
> results can be visually seen.

> If a polariscope were an expensive piece of
> equipment I could understand why some shops
> do not have one, but not schools.
> Admittedly, some commercial grade 'scopes'
> are rather expensive, but there are ones
> under $200.00 as well. For our purposes,
> viewing clear glass tubing, a home-made one
> is more than adequate and inexpensive to
> make if you don't want to spend a couple
> hundred.

> With that said, if you looked at your work
> with a polariscope you would notice that for
> simple bends, such as a 90* bend, double
> back, etc., the majority of the stress is
> adjacent to the bend - next to where the
> glass has been heated to the softening
> point, not in the actual bend itself. This
> is assuming you have not laid the bend on a
> cold table surface before the glass
> temperature was below the strain point.

> Example, 15mm: You may have noticed that
> when attempting to cut the glass 3/8"
> to 1/2" away from the bend the cut is
> usually not too good. (Beyond that point is
> obviously not a problem). Whereas if you cut
> it as close to the bend as possible it is
> usually more successful. This is because the
> majority of the stress is adjacent to the
> bend. In this case 3/8" to 1/2"
> away from it. The stress closer to the bend
> is more mild - sometimes almost
> non-existent. Again, this is assuming the
> bend was done in the air, not laid on a cold
> table and put in a rack to slowly cool.

> If a cut HAS to be made where you know the
> stress is going to be, there are a couple of
> "quick 'n dirty" things you can
> do:

> 1) After the bend is made and it solidifies,
> heat the glass in the area where the cut
> will be made, barely to the softening point.
> Do not stretch the glass as this will make
> for a thin seal. Let the piece slowly cool.
> Remember, the stress will be most severe
> adjacent to where the glass has been heated.
> What you are doing is "moving" the
> stress away from the heated area.

> 2) Immediately after the bend is made and it
> solidifies, thoroughly flame anneal the
> area. This takes more time and some practice
> to get it right, and you really don't know
> what you are accomplishing unless you look
> at the results with a polariscope. But if
> done correctly it is the better of the two
> methods because it reduces the stress on
> both sides of the heated area, thereby
> making the bend stronger.

> Heating the glass to a higher temperature to
> make a bend will actually increase the
> amount of stress adjacent to the heated area
> because it creates a greater temperature
> differential between the heated and unheated
> areas. This is assuming a decent bend was
> made to begin with. You obviously have to
> have the glass hot enough to *easily* make
> the bend and not force it.

> Mark

A polaroscope is a great tool to use, i made mine using some old binocular lenses, some polaroid plastic backlit and a thin sheet of mica to show stress in glorious colour. See the book - "Creative Glassblowing" by James E Hammesfahr and Clair L Strong, it has some good tips and techniques in there which you can attribute to neon glass bending.


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