Don Goodrich on thu 8 feb 07
Ivy, and anyone else who's been bitten by the glass bug:
I don't pretend to be a glaze guru, but I've put glass on many
of my pots, normally only on surfaces that can be fired horizontally,
because it tends to run right off.
The colored frits that I use melt at around 1600=B0F, but will tolerate ^6.
On the other hand, some blue bottle glass bubbles at the higher
temperatures, leaving nasty sharp edges if it cools too fast for
them to heal over.
When it's only fired to lower temperatures, it tends to bead up, like this:
http://www.dongoodrichpottery.com/glasslotemp.jpg
Yes, the high coefficient of expansion means the glass will craze a lot.
This can be used to advantage if you want a deep sparkling effect.
I use this effect on the backgounds of candle sconces. See:
http://www.dongoodrichpottery.com/candle.htm
A closeup of the crazed surface can be seen here:
http://www.dongoodrichpottery.com/images/sc7close.jpg
When glass is stuck to clay, they're both subjected to a lot of strain.
As they're settling their differences when they cool, the glass solves
the problem by crazing, but the clay can also break if it surrounds a
large area of shrinking glass. I've found that stoneware can take this
treatment better than porcelain.
Using found glass is satisfying, but if there's a color you crave and just
can't find, check the Bullseye or Uroboros Glass catalogs. The advantage
to using colored frits all from one source is that the expansions are
matched so glass artists don't have to worry about the compatibility of
their colors.
Stephen Branfman has made raku pots with bits of glass embedded in
the clay. I haven't seen any close up, but the pictures look beautiful.
John and Ron have suggested in MC6G that glazes have a better chance
of being stable if the silica:alumina ratio is better than 10:1.
Since commercial glassware is mostly flint, cullet, and some flux (soda
ash), with negligible alumina, it fulfills that criterion. I don't think
stability in this case is a significant problem compared to crazing,
but I could be wrong.
Playing with melted glass can bring pretty surprises. Protect your kiln
shelves.
Have fun.
Cheers,
Don Goodrich
iglasgo on thu 8 feb 07
Here's a question for the glaze gurus. This may be a dumb one, but it
keeps popping into my head and it's really bugging me. Kind of an OCD-
thing- If there's something I don't understand I just can't let go of
it till I get it... Anyways...
Does glass make a stable glaze? Simplistically speaking, it seems to
me that glass would make a really good glaze. After all, glass
containers go through all kinds of abuse, and stand up to it, year
after year. I have a favorite drinking glass that I use every day, and
put in the dishwasher every night. Five years later, its surface isn't
even hazy, let alone pitted or degraded.
So out of curiosity, I looked at the analysis for glass in GlazeChem.
It doesn't *look* like a stable glaze as defined in MC6G or others...
no alumina to speak of, high in sodium, kind of low in silica. But,
can millions of drinking glasses be wrong?
Na2O 0.46
K2O 0.00
MgO 0.17
CaO 0.37
Al2O3 0.02
SiO2 2.46
Fe2O3 0.00
Expansion: 95.5 x 10e-7 per degree C
What happens if this glass is ground to 325 mesh and applied to clay
as a glaze? Or applied as glass bits? The COE shows me that this would
craze on my clay body. I'm not educated enough to know what cone this
formula would melt at. Would the low alumina make this a really runny
glaze?
After making a short story long, what I'm getting at is possibly
incorporating teeny little chips of glass into a Cone 6 glaze, to
create an effect of clear, crazed spots in a darker field of glaze. I
already have a glaze that does this, but it uses granulated borax. In
a quick vinegar soak, the borax melted right off of my test tile. Not
good! But it's a very pretty effect, so I'd like to find another way
to accomplish it.
Sigh
Go ahead, tell me I'm off the deep end already... lol
-Ivy G.
The Goodsons on thu 8 feb 07
Dear Ivy,
If your off the deep end, then I want to go too. I love the look
of colorful glazes- and glass really fascinates me. You wrote:
"After making a short story long, what I'm getting at is possibly
incorporating teeny little chips of glass into a Cone 6 glaze, to
create an effect of clear, crazed spots in a darker field of glaze."
I really hope this is possible too. I will be reading along with your responses, and certainly giving that theory a test myself.
I have melted smashed glass chips in the bottom of small ornaments - and yes it is completely crazed, but I like the way it looks.
Have a good evening,
Linda Goodson
208 East Rhodes Street
Lincolnton, NC 28092
goodfun@charter.net
Taylor Hendrix on thu 8 feb 07
Ivy,
When you say "stable" are you using John and Ron's definition? If so,
remember that an important part of a stable glaze is ability of the
glaze to keep its colorants from leaching. This is stated explicitly
in their book. What might not be as explicit but just as important to
us potters is the phrase "while staying on the pot." BTW, when you're
visualizing those millions of drinking glasses, are they clear glass,
no colorants?
As far as glass cullet goes, it is a great high alkaline frit, yeah?
I'm saving up glass myself to supply some raku glazes in the future.
Yes, it will flow off your pot if you're not careful, yes it will
probably craze, and yes others have certainly added bits of clay to
their pots for drippy, crazed spots. Hewitt does it at cone 10+ in his
salt kiln way up yonder in North Carolina and Charlie Pritchard does
it in his electric at cone 5 just up the road a piece (go Luling
Icehouse Pottery!). They both stick glass pieces into their soft clay.
I have a small Pritchard jug with a lone glass tear, yummy.
--
Taylor, in Rockport TX
William & Susan Schran User on thu 8 feb 07
On 2/8/07 2:54 PM, "iglasgo" wrote:
> Here's a question for the glaze gurus.
> Does glass make a stable glaze?
Probably not, as it would be hard to fit the glass to the clay.
You would probably gets lots of running on a vertical surface.
>
> So out of curiosity, I looked at the analysis for glass in GlazeChem.
> It doesn't *look* like a stable glaze as defined in MC6G or others...
> no alumina to speak of, high in sodium, kind of low in silica.
Glass is very high in silica, in fact glass is a silicate.
>
> What happens if this glass is ground to 325 mesh and applied to clay
> as a glaze?
This is called cullet and was often used in the past as a glaze constituent.
> After making a short story long, what I'm getting at is possibly
> incorporating teeny little chips of glass into a Cone 6 glaze, to
> create an effect of clear, crazed spots in a darker field of glaze.
Yes, some folks do this by sticking little bits of glass in the clay, in
little holes made in the clay or simply laying on top of handle connections.
But again, this will run quite a bit, so take care to protect the kiln
shelf.
--
William "Bill" Schran
wschran@cox.net
wschran@nvcc.edu
http://www.creativecreekartisans.com
Lee Love on sat 10 feb 07
On 2/9/07, iglasgo wrote:
> Does glass make a stable glaze? Simplistically speaking, it seems to
> me that glass would make a really good glaze.
Sometimes. Same as glaze. Depends on its makeup. Don't put
orange juice in your lead crystal glass.
--
Lee in Mashiko, Japan
Minneapolis, Minnesota USA
http://potters.blogspot.com/
"To affect the quality of the day, that is the highest of arts." -
Henry David Thoreau
"Let the beauty we love be what we do." - Rumi
Ivor and Olive Lewis on sat 10 feb 07
Dear Ivy:=20
You may get some of your answers from a research paper by Dr E. R. =
Segnit, "The Mineralogy of some Soda-Lime Silica Glazes" which is based =
on a study of three glazes made using Bottle Glass.
The reference is Australian Ceramic Society, Vol 1, No 1, August 1965. =
(Pages 9-13).
I found this very interesting because it speaks of using Chromium oxide =
to make a green glaze. The entities responsible for this are created in =
the glaze and identified as Uvarovite. This mineral is a very expensive =
gemstone which rivals emeralds in colour, intensity and rarity.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
=20
sacredclay on sat 10 feb 07
One of my popular projects to do with kids and some adults is placing
colored marbles (the ones we use for those flowers glass vase) on top
of challow bowls that has been already glazed with white glaze. That
color tends to emphasize the clarity of the marbles colors. I do warn
my student that some will break, because glass cools faster than clay
and glazees. I also tell them not to use them on containers that will
come in contact with food. namely because I don't know whether any of
these marbles contain lead or not. One small dish I made ended up
looking like the wooly caterpillar. God, I miss those creatures.
Can't remember if the wider the width of their brown band meant a
long winter or not. Kathryn with a vanilla almond tea.
.dongoodrichpottery.com/images/sc7close.jpg
>
> When glass is stuck to clay, they're both subjected to a lot of
strain.
> As they're settling their differences when they cool, the glass
solves
> the problem by crazing, but the clay can also break if it surrounds
a
> large area of shrinking glass. I've found that stoneware can take
this
> treatment better than porcelain.
>
> matched so glass artists don't have to worry about the
compatibility of
> their colors.
>
>negligible alumina, it fulfills that criterion. I don't think
> stability in this case is a significant problem compared to crazing,
> but I could be wrong.
>
> Playing with melted glass can bring pretty surprises. Protect your
kiln
> shelves.
> Have fun.
>
> Cheers,
> Don Goodrich
>
>
______________________________________________________________________
________
timothy knick on sat 10 feb 07
I fired a green beer bottle to cone 9, 15 minute hold, in a plate. The color stayed the same nice green, but it was extremely crackled. I suppose it could be ground up and you could add something to it.
Tim
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Ron Roy on sun 11 feb 07
Hi Don,
We do not use ratio as a criteria for durability - enough silica and
alumina, well melted and not overloaded with certain colouring oxides are
what to take into account.
It is true that some of the glazes in our book have a higher ratio than 10
to 1 but many are below that - and all are durable glazes.
As for glass - their are many varieties - just look at your drinking
glasses - notice how some of the older ones are starting to be not so shiny
and clear. Especially if you use a dish washer. Because glazes is always
short of alumina it is susceptible to attack by alkalies - like dish
detergent.
RR
>John and Ron have suggested in MC6G that glazes have a better chance
>of being stable if the silica:alumina ratio is better than 10:1.
>Since commercial glassware is mostly flint, cullet, and some flux (soda
>ash), with negligible alumina, it fulfills that criterion. I don't think
>stability in this case is a significant problem compared to crazing,
>but I could be wrong.
>
>Playing with melted glass can bring pretty surprises. Protect your kiln
>shelves.
>Have fun.
>
>Cheers,
> Don Goodrich
>
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Ron Roy
RR#4
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Canada
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