Brian O'Neill on sun 28 dec 03
John, Ron, and all the merry cone sixers,
Before addressing my little problemo, I just have to say that after
reading many of the seasonal threads here on Clayart, it is so very
apparent that we grow through our intimate contact with those things
that we love. On the eve of this new year I wish you all close
connections with the people and pets, plants and clay you love.
E l p r o b l e m o :
I made a handful of large serving bowls for gifts this year and was
planning on glazing them with Ron and John's majolica as my tests of it
had convinced me that it was "bomb proof". I scratched it with nails and
silverware and they left no mark. Wonderful white that takes my oxides well.
Oh, oh! I looked at my tests again a few days out of the kiln with a
magnifying loop, and there is some very fine crazing going on. Rats!
Clay body is from Seattle Pottery Supply--CKK6. Described as a ^6 bright
white porcelain. I don't have the recipe.
Any suggestions to remedy this problem would be welcomed.
Cheers,
Brian
in the NW corner of the Northwest, where it is snowing buckets, but will
no doubt turn to rain in short order. Hope not!
Jan L. Peterson on sun 28 dec 03
Re-glaze? Maybe marbleize? Jan, the Alleycat
Dave Finkelnburg on sun 28 dec 03
Dear Brian,
Hope you also enjoy "close connections" with those you love as the New
Year approaches.
The problem of crazing you encountered is familiar to any of us who make
pottery. Unless you strive for crazing as decoration, it can be
frustrating. The glaze shrinks more than the clay until the glaze cracks.
This works much the same as how the seam splits out of the seat a pair of
too-tight trousers. :-( The solution, of course, is to make the glaze
shrink less as it cools, so it shrinks at a rate similar to the clay body
you use.
The easiest way to make the fired glaze shrink less is to add silica to
the glaze. You can use different fluxes, say magnesium or lithium instead
of calcium or sodium, but you are more likely to change the glaze color and
appearance and you need more experience and testing.
For simple silica addition, if you make a few test tiles, and glaze each
with your glaze mixed with a fixed amount more silica, say 2,4,6,8 and 10%
added silica, you can fire those tiles and test each. For the one that is
closest to working for you, you can get a more precise recipe by making
another set and test by increments of 1/2-percent silica.
In the archives you will find information about testing the
tiles--basically freezing them, then tossing them into boiling water,
cooling them, looking with your magnifying glass for crazing, and then
putting them back into the freezer for another round. If you find a tile
that takes this for a half-dozen cycles without crazing you are getting what
you want.
Do keep in mind that a craze-free glaze is not necessarily the Holy
Grail. There is the demon called "shivering." That's when the glaze pops
off the pot because it doesn't shrink enough! That is far more dangerous to
users of functional ware than crazing. If you ever make functional pots
with a glaze that shivers, destroy (hammer!) the pots immediately. You
don't want to ever worry about feeding someone broken glass!
So, being as close as possible to crazing without quite having crazing
is really your goal.
Regarding clay bodies, most porcelains don't shrink as much as stoneware
clay bodies. In fact you generally want different glaze recipes for
stoneware and porcelain. You can use the same glaze on both...but it
usually doesn't fit one or the other...or both...well.
Did I mention that you do need to test your clay body for vitrification?
Underfired clay can cause glaze crazing by absorbing water. This is usually
a low-fire issue, though, not a common mid-range problem. All the same, it
would be good if you glaze fire an unglazed test tile, weigh it very
accurately while still warm and dry, then soak the tile in water for at
least 24 hours or boil it for 2 hours or more, cool in cold water, pat dry
and weigh very accurately immediately. Then calculate the weight gain as a
% of the weight of the dry tile. In general the "absorption" of water
should be 1/2-percent or less for your porcelain, less than 3% for
stoneware.
I hope this helps. I know this does just scratch the surface of the
subject of "glaze fit."
Dave Finkelnburg, in high, dry southeast Idaho...still digging out
from Friday's snowfall...the heaviest ever recorded here in one
day...amounted to more than a third of our annual average....can now reach
the kiln but I hate to load as it continues to snow...so...what's a person
to do? I'm going skiing!!!!!!!!
----- Original Message -----
From: "Brian O'Neill"
Sent: Sunday, December 28, 2003 12:21 PM
> I made a handful of large serving bowls for gifts this year and was
> planning on glazing them with Ron and John's majolica....
> Oh, oh! I looked at my tests again a few days out of the kiln with a
> magnifying loop, and there is some very fine crazing going on. Rats!
> Clay body is from Seattle Pottery Supply--CKK6. Described as a ^6 bright
> white porcelain. I don't have the recipe.
> Any suggestions to remedy this problem would be welcomed.
Ron Roy on fri 2 jan 04
Hi Dave,
Just a note on your comments.
The standard way - when we talk about crazing is to say the glaze
contracts more than the clay .............
If we say it shrinks we risk confusing potters becuase they get the
shrinkage that happens with drying and firing confused with the
contraction that happens as clay and glazes cool.
I know they mean the same thing but - but it will help if we make a clear
distinction between the two processes. Every book I have read on the
subject uses those different terms - no doubt to simplify explanations -
and not have to qualify which they are talking about each time.
>Dear Brian,
> The problem of crazing you encountered is familiar to any of us who make
>pottery. Unless you strive for crazing as decoration, it can be
>frustrating. The glaze shrinks more than the clay until the glaze cracks.
>This works much the same as how the seam splits out of the seat a pair of
>too-tight trousers. :-( The solution, of course, is to make the glaze
>shrink less as it cools, so it shrinks at a rate similar to the clay body
>you use.
> The easiest way to make the fired glaze shrink less is to add silica to
>the glaze. You can use different fluxes, say magnesium or lithium instead
>of calcium or sodium, but you are more likely to change the glaze color and
>appearance and you need more experience and testing.
> For simple silica addition, if you make a few test tiles, and glaze each
>with your glaze mixed with a fixed amount more silica, say 2,4,6,8 and 10%
>added silica, you can fire those tiles and test each. For the one that is
>closest to working for you, you can get a more precise recipe by making
>another set and test by increments of 1/2-percent silica.
> In the archives you will find information about testing the
>tiles--basically freezing them, then tossing them into boiling water,
>cooling them, looking with your magnifying glass for crazing, and then
>putting them back into the freezer for another round. If you find a tile
>that takes this for a half-dozen cycles without crazing you are getting what
>you want.
> Do keep in mind that a craze-free glaze is not necessarily the Holy
>Grail. There is the demon called "shivering." That's when the glaze pops
>off the pot because it doesn't shrink enough! That is far more dangerous to
>users of functional ware than crazing. If you ever make functional pots
>with a glaze that shivers, destroy (hammer!) the pots immediately. You
>don't want to ever worry about feeding someone broken glass!
> So, being as close as possible to crazing without quite having crazing
>is really your goal.
> Regarding clay bodies, most porcelains don't shrink as much as stoneware
>clay bodies. In fact you generally want different glaze recipes for
>stoneware and porcelain. You can use the same glaze on both...but it
>usually doesn't fit one or the other...or both...well.
> Did I mention that you do need to test your clay body for vitrification?
>Underfired clay can cause glaze crazing by absorbing water. This is usually
>a low-fire issue, though, not a common mid-range problem. All the same, it
>would be good if you glaze fire an unglazed test tile, weigh it very
>accurately while still warm and dry, then soak the tile in water for at
>least 24 hours or boil it for 2 hours or more, cool in cold water, pat dry
>and weigh very accurately immediately. Then calculate the weight gain as a
>% of the weight of the dry tile. In general the "absorption" of water
>should be 1/2-percent or less for your porcelain, less than 3% for
>stoneware.
> I hope this helps. I know this does just scratch the surface of the
>subject of "glaze fit."
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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