lili krakowski on sun 17 jan 10
Anne Elisabeth Maurland writes:
"....I'm not sure what COE means, or the numbers you mention. I can't =3D
tell=3D20
any difference between my glaze and Ron's on the surface, and since I=3D20
can't (yet) find any real flaws in my own, I'm not sure I need to do=3D20
much more glaze searching. Perhaps you (or Ron) can convince me to do=3D20
his version. Actually, I probably will, to be sure."
To get that out of way first. COE =3D3D "coefficiant of expansion" There =
=3D
are several published "scales". RonJohn's discussion of COE, glaze =3D
expansion and fit is tops..As is Michael Bailey's in "Glazes c.6".
In terms I can understand: Most of us find a February shoe size too =3D
small/narrow in August when our feet are very hot, and Summer shoes are =
=3D
loose when our feet are cold. So we try for shoes that fit perfectly at =3D
both seasons. Same idea with glaze. A glaze expands when heated and =3D
shrinks to final size when cooled. It should be added that because some =3D
materials LOSE ingredients in the fire, the shrinkage can be =3D
considerable. However. We try for glazes that fit the pot perfectly =3D
after firing.
My theory about what happened to your pots:
When you pour something through a sieve or filter, the little bitsies of =
=3D
stuff stay in the sieve, and the liquid goes through. When we put glaze =3D
onto a pot (and spraying, about which I know naught, may be a partial =3D
exception) the glaze solids remain on the surface, but the liquid seeps =3D
into the clay body. Soluble materials in the glaze--materials that =3D
dissolve in the water-- are carried by the water into the pot.
As the glaze dries on the pot the evaporating water may carry some of =3D
these materials to the surface. Just as the water carried them INTO the =3D
pot at application.
When the soluble materials are carried INTO the clay, they change its =3D
composition. We talk a lot about "interface" (well, some of us do!) =3D
which is the layer of the clay body where the materials from the glaze =3D
have melted into the body. (In some cases the materials in the bodywill =3D
modify the glaze--or why a glaze may look different over and iron rich =3D
body, and a white one.)=3D20
The interface determines the final result of a glaze. Remember: this =3D
interface is created by glaze melting into the pot, and some of the clay =
=3D
/pot melting and merging with the glaze.
Soluble materials penetrate into the clay body on their own, BEFORE =3D
anything has melted. So, AS AN EXAMPLE AND EXAMPLE ONLY, if your clay =3D
body has kaolin and ball clay and feldspar--and you pour borax, or =3D
washing soda, or unwashed wood ash on the pot you are changing the =3D
composition of the clay: it now is kaolin, ball clay, feldspar AND boron =
=3D
or soda. This before firing, before an interface is formed.
My idea is that when you dipped very thin pieces of clay (i.e. very thin =
=3D
pots) into a glaze containing water soluble material the liquid from =3D
both sides carried soluble stuff into the clay through and through. In =3D
my opinion you turned the "middle" of the clay into a vitreous =3D
engobe--so no surprise it is fragile. In the thicker pieces--a.k.a. =3D
plates--there was "pure"/original clay in between the two glazed =3D
areas...some part unaffected/untouched by transported fluxes.
In town we are in a house built 1904 or so, on a paved street. When--as =
=3D
now--a really heavy truck or snow removal machine comes down the street, =
=3D
the house trembles. When a big load of snow crashes down from the roof, =
=3D
all the pictures on the walls go askew. If, in a solid house, =3D
vibrations can cause such "rearrangement" it seems to me fragile, =3D
internally stressed pots, could, after years of vibration, crack.
As to Your Glaze, Ron's Glaze, MY glaze...to spare Mel's nerves I will =3D
make that into a separate post.
=3DE3=3D80=3D80
Lili Krakowski
Be of good courage
Anne Elisabeth Maurland on sun 17 jan 10
Lili,
Thank you for the explanation. When I first asked about this last fall,
I did mention that the customer lives in San Francisco. I don't know
how common tremors are there, and she's only had the pots for four
years, but maybe that had something to do with it. I've only done one
art fair in California and live so far away that I don't have many
other customers there either.
I put an old, thin plate (has been used for ten years, and not been
handled particularly carefully) through the freezing/cooling test (a
few days in the freezer, then immediately doused with boiling water.
Still no cracking.
I don't think the California customer had plates, at least not all of
them were.
Do you think it makes a difference, according to your theory, that I
glaze the insides first (of the pots that are not plates), wait a few
hours, and then glaze the outsides? That's how I do it.
Elisabeth
On Jan 17, 2010, at 3:46 PM, lili krakowski wrote:
> Anne Elisabeth Maurland writes:
>
> "....I'm not sure what COE means, or the numbers you mention. I can't
> tell
> any difference between my glaze and Ron's on the surface, and since I
> can't (yet) find any real flaws in my own, I'm not sure I need to do
> much more glaze searching. Perhaps you (or Ron) can convince me to do
> his version. Actually, I probably will, to be sure."
>
> To get that out of way first. COE =3D "coefficiant of expansion" There
> are several published "scales". RonJohn's discussion of COE, glaze
> expansion and fit is tops..As is Michael Bailey's in "Glazes c.6".
>
> In terms I can understand: Most of us find a February shoe size too
> small/narrow in August when our feet are very hot, and Summer shoes
> are loose when our feet are cold. So we try for shoes that fit
> perfectly at both seasons. Same idea with glaze. A glaze expands when
> heated and shrinks to final size when cooled. It should be added that
> because some materials LOSE ingredients in the fire, the shrinkage can
> be considerable. However. We try for glazes that fit the pot perfectly
> after firing.
>
> My theory about what happened to your pots:
>
> When you pour something through a sieve or filter, the little bitsies
> of stuff stay in the sieve, and the liquid goes through. When we put
> glaze onto a pot (and spraying, about which I know naught, may be a
> partial exception) the glaze solids remain on the surface, but the
> liquid seeps into the clay body. Soluble materials in the
> glaze--materials that dissolve in the water-- are carried by the water
> into the pot.
>
> As the glaze dries on the pot the evaporating water may carry some of
> these materials to the surface. Just as the water carried them INTO
> the pot at application.
>
> When the soluble materials are carried INTO the clay, they change its
> composition. We talk a lot about "interface" (well, some of us do!)
> which is the layer of the clay body where the materials from the glaze
> have melted into the body. (In some cases the materials in the
> bodywill modify the glaze--or why a glaze may look different over and
> iron rich body, and a white one.)
>
> The interface determines the final result of a glaze. Remember: this
> interface is created by glaze melting into the pot, and some of the
> clay /pot melting and merging with the glaze.
>
> Soluble materials penetrate into the clay body on their own, BEFORE
> anything has melted. So, AS AN EXAMPLE AND EXAMPLE ONLY, if your clay
> body has kaolin and ball clay and feldspar--and you pour borax, or
> washing soda, or unwashed wood ash on the pot you are changing the
> composition of the clay: it now is kaolin, ball clay, feldspar AND
> boron or soda. This before firing, before an interface is formed.
>
> My idea is that when you dipped very thin pieces of clay (i.e. very
> thin pots) into a glaze containing water soluble material the liquid
> from both sides carried soluble stuff into the clay through and
> through. In my opinion you turned the "middle" of the clay into a
> vitreous engobe--so no surprise it is fragile. In the thicker
> pieces--a.k.a. plates--there was "pure"/original clay in between the
> two glazed areas...some part unaffected/untouched by transported
> fluxes.
>
>
>
> In town we are in a house built 1904 or so, on a paved street.
> When--as now--a really heavy truck or snow removal machine comes down
> the street, the house trembles. When a big load of snow crashes down
> from the roof, all the pictures on the walls go askew. If, in a solid
> house, vibrations can cause such "rearrangement" it seems to me
> fragile, internally stressed pots, could, after years of vibration,
> crack.
>
> As to Your Glaze, Ron's Glaze, MY glaze...to spare Mel's nerves I will
> make that into a separate post.
>
> $B!! (B
>
>
>
>
>
> Lili Krakowski
> Be of good courage
>
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