[LMB] OT: glass shattering: was dick francis, was trashy books for guys

sylvus tarn sylvus at rejiquar.com
Fri Apr 4 03:38:27 BST 2008

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Marilyn Traber wrote:

I have some 4500 unread messages in this list so it's pretty amazing I
found this one, but...

> Mark Allums wrote:
>> sylvus tarn wrote:
>>> And I just had a really hard time believing a hot glass piece would
>>> explode, and the artist would know exactly when.  Break, yes.  But
>>> shatter?  
> It  may not be predictable, but I made beads a few times at SCA events 
> and believe me, when you are making a compound bead of 2 or more 
> different glass rods, they can explode if not tempered and cooled down 
> properly. 

In over 10 years of making beads I've never, ever had this happen.  No
matter how many colors I've used.  Granted, I make mostly hollow beads;
 but the closest I had to this experience was when I accidentally mixed
boro and effetre, which as you probably know are (nominally) coe 32 and
104 (coe standing for linear coefficient of expansion, as measured in
10-7 m units from 0 -- 300dC...I think)  Nor have I ever heard of this
happening to any of the other beadmakers I know and I know some who make
pretty honking beads.  Large or complicated beads will certainly *crack*
if they're not put right into a kiln;  but they don't shatter, sending
glass popping everywhere.  (Now, putting an unannealed, or even a
perfectly well annealed glob of mixed glass or even just a plain fat
rod directly into the flame without preheating it is entirely another
matter, but that's not what happens in the story.)

>The additives that color the rods [and even different
> manufacturers of rods have different formulary for what look to be
> identical colors] change the working temperatures of the rods, and  when

Which can be rather a pest, sometimes, putting transparents, on say,

> tou get enough degrees of difference in the rods ccooling, that compound
> bead will shatter as the different rods shrink at different rates. That

Er, no.  If the reason your bead is breaking is because the colors are
incompatible, no amount of annealing is going to help.

> is why with glass you put it into a tempering oven to cool down at a
> specific rate. Depending on how complex the piece is it may be a gentle
> pflump to a very explosive experience.

There are a number of heat related reasons can break.  One is
incompatibility:  if the glasses shrink at different rates, and there
are no compensating factors (e.g. high lead content, balancing
viscosities) and the shrinkage is enough different, then yes, the piece
will break, and quite possibly shatter, depending upon the design.
However, hot-glass artists learn to deal with compatibility issues early
on, particularly if they batch their own base glass (don't recall
whether the protagonist did this, but he was cast as a highly
technically competent character, so I presume he knew what he was doing.)

Pieces are *not* tempered by artisans, since the tempering process, as I
understand it, deliberately induces stress in the glass, by creating a
compression layer on the outside.  Glass artists do, however, *anneal*
their work, and this is to *remove* out of the piece the working stress,
which comes out of uneven cooling.  Glass generally cools down faster on
the outside than the inside---it's an insulator, after all.  And, as it
cools, it shrinks.  This uneven shrinkage introduces stress, which can
be prevented by soaking the piece until it's a uniform temperature; then
the kiln ramps down slowly until after the piece is below its strain
point (which is typically between 700 and 1000 deg F, depending on type)
at which point it's fine, unless heat shocked. Heat shock is reason you
don't want to put glass objects on hot electric burners.

> Someone with a lot of experience may be able to predict when something 
> will shatter from many years of experience exploding stuff made out of 
> specific companies glass, and different colors, but other than asking 
> the people in the glass museum in Corning I have no idea where to 
> research it...but the people down in Corning are really fun to get to know.

I'm not saying categorically that a large piece of furnace glass
couldn't shatter;  I haven't the experience.  But while hot glass folks
don't like to waste time before getting their pieces into the kiln,
neither have I ever seen, nor been warned about glass shattering in the
way that was described in the novel.  It struck me as implausible, just
as ordering unpopular stone chips from a big supplier in Straight struck
me as implausible---not impossible, but not the way things really are.

I remember thinking at the time, why didn't he just grab a blowpipe, or
better yet, a punty (preferably with molten glass on the end, which
comes out of the furnace at 2500 d F) and bash his enemies with that?
Those things are 5' long and solid steel, plenty heavy enough to crack
someone over the head even without glass on the end---and by the time
the glob is down to 1000 d F or so, it *looks* cool to the touch, but
trust me, will still give a very nasty burn.   Or stab them with the
jacks?  (Pity the poor furnace worker who tries to take his, or her,
fave tools on a plane...) But, see, those didn't have the visual appeal
or exotic-ness of the shattering glass.  It just seemed awfully contrived.

Mark Allums:

>Tempered glass can shatter.

Tempered glass is a whole 'nuther category, and not a process easily
achieved by the artisan.

> However, I doubt highly that ordinary flint glass used by a glass
artist  would behave like that,

I think tempered glass actually does start out as ordinary flint glass
(aka soda-lime, aka most soft glass.)

and I seriously doubt the artist would be able  to predict when, either.

I think lampworkers learn when glass is going to heat shock, because an
awful lot of us get impatient and stick stuff in the flame too quickly.
 But, of course, heat shocking cold glass doesn't tend to be so much of
an issue for furnace workers.  And that's not what happened:  the item
was cooling down not being heated up.  Usually, if something is worth
saving, it goes into the kiln;  and if it isn't, it goes into the bucket
of water.  You don't leave hot glass lying around just to watch it
crack, burn something or someone...it's not good studio hygiene.

> --Mark Allums

sylvus tarn

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