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Properties of glass


#1

I recently attempted to enlarge the holes in a pair of glass beads
by drilling them out with diamond drill bits of progressively larger
sizes, while submerged under water for cooling. Both of the beads
split in half under moderate pressure. Fortunately, they were not
expensive beads. Since I’ve successfully drilled out glass beads
numerous times in the past, I was quite surprised by this result.

Are there any glass artists out there who might be able to shed some
light on why this might have happened to these beads, but not to the
majority of others I’ve handled similarly? Are there different glass
"recipes" or production methods that make some beads more fragile
than others?

Thanks in advance for any thoughts on this.
Linda in central FL


#2

Simple the beds were not annealed long enough to allow the extra
stress of drilling…


#3

Hi Linda,

I used to design glass for Steuben Glass and while I didn’t actually
make the glass objects myself I worked with the glass blowers to
realize my designs. There are many glass formulas, some stronger
than others. It’s hard to know what your glass was made of but if you
want some general help you might try calling Bullseye Glass in
Portland, Oregon - 503.232.8887.

Good luck, Taf


#4

I don’t know much about glass bead construction, but I have been
working with a local glassmaker modifying her beds to set into
jewelry as you would a stone. This has required that I cut beads in
half and use basic lapidary techniques to shape and polish the glass.
It works well, but I have noticed that, upon occasion, the bead will
come apart where the colored glass is added to the base glass. I have
no idea what these layers are really called. You may be drilling thru
this juncture and the stress causes the bead to break. Just a
thought, hope it helps. Rob

Rob Meixner


#5

Absolutely there are different glass recipes. Every mfg has their
own particular recipe. Then you have the Coefficients of Expansion,
not to mention how the beads were made. Were they in layers.
Generally speaking, the higherthe COE, the softer the glass. So a
COE of 104 is going to be much softer than a COE of 33, which is
typically borosilicate glass, such as Pyrex. Because I can’t see the
bead and how it was made, it’s difficult to conjecturewhy it broke
at this point. There is also annealing the bead to consider. It very
well might have been that the bead, when it was made, was not
annealed properly and still retained stress. This is certainly
possible, as beads and paperweights have been known to break or even
explode months after making, because they were not annealed properly
and retained stress within the glass.

I’m not sure how this info will help you in the future, but just
know that it probably wasn’t anything you did improperly, and least
it doesn’t sound like it. One thing you might want to remember is
that you should grind in short bursts, even under water, rather than
persistent drilling. That not only gives the water time to cool the
glass, but gives the burr a chance to get rid of sludge that might
cause breakage. Hope the info helps.


#6

Thank you to everyone who responded to my inquiry. Jackie, in
particular, was kind enough to offer an in depth understanding of
how various glass beads might differ.

While most of the beads I drill out are gemstone, occasionally I’ve
had clients provide glass beads, or request a particular style or
color of glass bead, that might need the hole enlarged. Knowing the
possibilitythat this endeavor might fail will allow me to advise the
client of this possibility, and reduce my own financial risk.
Fortunately, the pair that cracked were offered to me as free
tokens, and the attempt to drill them out was purely for spec
pieces.

So again, thanks to all who took the time to help me better
understand these attributes of glass. And Taf, your reference to
your time designing for Stueben Glass brought back memories of trips
to Corning long agowith my mother. Mom still loves glass, and still
has her very large collection of fine paperweights.

Linda in central FL


#7

The commonest cause of them breaking will be insufficient annealing
as already said. Another problem may be that the glasses are of
similar enough coefficient of expansion to stay in one piece as made
but being round have internal stresses that will cause it to break
once you start to do something else to them like drill them out or
grind flats on them.

Glass is what is called a supercooled liquid so doesnt have a
melting point but it does have a point where the viscosity is reduced
to be its liquidus temperature and it is the variations in this
liquidus point that gives you the COE number normally quoted by art
glass suppliers. The higher the number, the lower the temp at which
you can work it. Bullseye glass has a COE of the high 90’s, effetre
glass similar but different enough to make mixing the 2 together a
risky proposition in larger pieces. As they are cooled the 2 glasses
will shrink at different rates making the internal stresses that
annealing wont properly dissipate. So, you drill out the hole and it
give a new focal poiny for theses stresses and the item breaks.

You will need to have a chat with the beadmaker to find out what
glass was used and what annealing cycle was used. generally bead
makers use a short annealing as being small not much really builds up
in the way of stresses but that is often because the shape helps make
them hold the bead together rather than break it. If you have access
to a kiln you need to heat them up to 500 deg C and leave them for a
while and then cool slowly. If starting with hot glass anneal at 590
for a short while then cool down to 500 for 1/2 hour.

Bigger objects have to be heated or cooled in much longer
multi-stage cycles.

Nick Royall


#8

if you have access to polarising film, (e. g. glasses) set up a pair
of cross-polarisers. witht he polarisation at right angles so it lets
no light through

place the glass between them with a strong light source passing
through.

Stress in the glass polarises some light allowing it to be visible -
and allowing you to see how “stressed” the glass is. stressed glass
has lots of pretty. annealed glass should be much less so - almost
grey.


#9

the other thing is that coloured glass is often soda glass, which
needs annealing a lot more than pyrex (borate) glass.


#10

Have you heard of Prince Rupert’s Beads? They look a little bit like
glass tadpoles with a long wispy tail, and have a most unusual
property. You can hit them quite hard with a hammer, and nothing
happens, but if you break the tail, the whole thing shatters. It’s
due to the way they’re made, which is to drop blobs of molten glass
into cold water. The skin shrinks around the still hot interior and
makes a very tough shell which is under considerable tension. If the
tail is broken, the tension in the shell is released and the whole
thing shatters.

Could something like this be the cause of what is happening with your
beads?


#11

Had not previously heard of Rupert’s tears: thank you for the very
interesting education!

Linda in central FL