Ditmar/Gayle on sat 4 oct 97
Definitely put the glass in after the firing. There are lots of problems
in combining glass with clay. Different coefficients of expansion
guarantee cracking. Temperatures that will fuse the glass to the pot will
already have the glass flowing and sagging. Then there's the problem of
annealing it properly to keep it from cracking at a later date, even if
you've gotten that far.
Just think of glass as a glaze, after all they're related. Imagine how
difficult it would be to fire a clear glaze to cover an opening in a pot !
Temperature wise, glass is slumped and fused at 1100 - 1400 degrees F.(
more or less, depending on which glass and what you're doing ) which is
about cone 021 to cone 016.
The annealing temperatures are lower, so you don't even have cone
equivalents, which you would have to control on the way down.
From Alohaland, Ditmar.
VICTOR JOHNSTON on tue 7 oct 97
Hey, if glass fuses at a lower temp than you're firing glaze, why
not try a third fi;ring at a lower temp (say around 1400) with the
glass in place then?
Experament a little. I hear a lot of opinions, but no sure
witnesses. I've found I can do a lot of things that the "experts"
say can't be done. Throw something together that you aren't
going to sell and try your ideas. Use the information provided by
others (like the fusing temp of glass) and see what happens.
Different clasy exjpand at different rates. maybe you can find one
that matches the expansion of glass. Didn't we just read that
glass is like glaze?
There are different kinds of glass, too. Maybe you'll need to look
at them. When I was a teen (many years ago) I worked in a
technical glass shop. I made those cute little double stemed
vases from a single tube of leaded glass and cut other tubes to
sjpecific lengths and ground and polished them for flow meters
for dialisis machines. The owner made neon signs and built
low cost gas lasers up to 10 feet long. We used many deifferent
kinds of glass, silica, quartz, i forget the others (it's been too
long) with different lead and other element contents. we had a
wide variety of melting temps and expansion coefficients. Some
went to liquid almost instantly without having a plastic/tacky
stage, others would remain relatively in tact in shape and
become very tacky for quite a while before even starting to sag.
My boss had to fuse 4 foot lengths of glass tubing together and
not alow them to get even slightly out of round when building
lasers, for example. Incidentally, he was told by the then "laser
experts" that it couldn't be done.
Vic in ogden, where it's ok to throw caution to the wind once in a
while and try new things in spite of the expert opinions.
>>> Ditmar/Gayle 10/04/97 07:38am >>>
----------------------------Original message----------------------------
Definitely put the glass in after the firing. There are lots of
problems
in combining glass with clay. Different coefficients of expansion
guarantee cracking. Temperatures that will fuse the glass to the
pot will
already have the glass flowing and sagging. Then there's the
problem of
annealing it properly to keep it from cracking at a later date, even
if
you've gotten that far.
Just think of glass as a glaze, after all they're related. Imagine
how
difficult it would be to fire a clear glaze to cover an opening in a
pot !
Temperature wise, glass is slumped and fused at 1100 - 1400
degrees F.(
more or less, depending on which glass and what you're doing )
which is
about cone 021 to cone 016.
The annealing temperatures are lower, so you don't even have
cone
equivalents, which you would have to control on the way down.
From Alohaland, Ditmar.
Karl P. Platt on wed 8 oct 97
The glass/clay topic is back I see.........
Only highly unusual clay bodies will have a thermal expansion equal to
that of common colored glasses one might readily encounter, i.e. "stained
glass", colored wine bottles and the like.
As such, while it is entirely possible to fuse glass onto the surface of
clay it will in all liklihood craze real badly. Common (soda-lime) glasses
are quite fluid at temperatures in the area of 850 C, it would be best to
apply the glaze in a firing dedicated to that purpose. The alternative is
to have all of the glass flow to the lowest point on the pot.-- in bowls
it will accumulate on the bottom in a thickish layer (and craze).
The glasses most likely to survive whole on the surface of typical potters
ceramic bodies are "Pyrex" ovenware -- available in the lovliest of colors
-- none and Ti-Fe amber. This glass has an expansion much closer to that
of ceramic than the others. I have been seeing, however, a Ni rose here
and there in the stores.
Powdered glass can be used as a glaze frit, but one needs to be aware that
there will likely be a bit of soluble alkali which can meddle with the
fluid qualities of the slip.
Fay & Ralph Loewenthal on wed 8 oct 97
Dear Vic in ogden, where it's ok to throw caution to the wind once in a
while and try new things in spite of the expert opinions.
What you are suggesting will have glass all over the shelf
and inside of the pot and not in the windows. You should
know from your past experience that glass slumps long
before it fuses no matter what glass you use. The
simplest and safest way is to just cut glass into the size
required and glue into place. Ralph in PE SA
Many Mused on thu 9 oct 97
reading some of the earlier posts on this topic, I would think that the COE
33 (approx.) of borosilicate (pyrex) glass is farther from the COE of most
clays and glazes than the fusible compatible 90 COE glasses. There is an
Australian glass company making sheet glass to fit the COE of float (window)
glass~87 at http://www.wt.com.au/~rdeint/higlass.htm but this is still too
far from the COEs in the 70s that I recall from earlier posts (in fusing, the
COEs have to be within one or two points.)
Don't stop now!
This thread is quite intriguing.
Karl P. Platt on fri 10 oct 97
There are many types of glass sold under the brand name of "Pyrex". All of
these are high silica borosilicate glasses which have expansions ranging
from 32, as mentioned, up to about 50 or so. The "Pyrex" ovenware -- and
its kin sold under names like "Fireking" -- have expansions on the order
of about 50. This is relatively near the expansion of many clay bodies --
at least its a lot closer than Pyrex labware glass or float glass.
KPP
P.S. I looked at the page of the Austrailians making colored float glass
sheets and have to wonder when they'll figure out how to make a square
sheet
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