Gretchen Woodman on tue 13 nov 01
First I will describe more fully what I am making:
My bowls are built up of thin slabs wrapped over a
form. The base of the bowl is all one piece that
curves up around the sides of the bowl about 1-2".
From there I add slabs around the bowl until the form
is covered. I dry the bottom slightly with a blow
dryer and then invert it to remove the form. I
bisque, rub iron oxide in H2O on the outside, wipe off
excess, then glaze the inside. I pour some glaze into
the bowl and rotate it around for coverage. It is
very possible that it gets too thick in the bottom,
even though I am trying to avoid this.
The cracks are only across the bottom of the bowl,
large enough to feel with your fingernail and see well
at the right angle. There are 4 to 5 on the bottom.
They don't seem to continue up the sides. The clay
structure of the bowl is fine, nothing there is
cracking or giving way.
The smaller bowls do not seem to have such a problem,
although a couple flasks I made did.
The glaze is a combination of two glazes. One
selectively applied over the other for color
variation. I only apply narrow rivulets of the second
glaze to get the pattern I want.
These glazes are from "The Ceramic Glaze Handbook" by
Mark Burleson
Ocean Green Glaze ^6 ox
Kaolin (EPK) 5.4
Feldspar (G-200)_ 25.3
Gerstley Borate Sub(I used Gerstley Borate) 20.0
Nepheline syenite 22.0
Silica 18.0
Red Iron Oxide 1.8
Light Rutile 2.8
Copper Carbonate 3.5
The above is the first glaze I coat the inside of the
bowl with.
Turquoise Glaze ^6 ox
Dolomite 12.0
Kaolin (EPK) 5.0
Gerstley Borate sub (Gerstley Borate) 12.0
Feldspar (G-200) 12.0
Nepheline syenite 23.0
Silica 5.5
Strontium carbonate 15.0
Zircopax 10.0
Copper carbonate 4.0
This is the glaze I apply in rivulets.
I am using Miller 45 clay.
Thank you all for responding to my question so
quickly.
Sincerely,
Gretchen Woodman
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Cindy Strnad on wed 14 nov 01
Dear Gretchen,
Do I remember your saying that your clay body was
porcelain? The expansion of your base glaze is
7.60 according to Insight. This would probably
work fine on my dark stoneware, but I can use
glazes with a fairly high expansion and get away
without crazing. You couldn't do that with
porcelain. Even though the smaller pieces don't
craze immediately, they would do so eventually.
Others more capable than I are sure to chime in
with suggestions for altering your glaze.
That said, if you don't want to alter your glaze,
keeping it thin is the key. You might consider
constructing a glaze fountain to do the insides of
your bowls. You can do this for around $60-70 US,
and in a few hours time. You'll find instructions
on Janet K's Chapel of Art site. Just follow her
links to find the Clayart stuff. I think my site
is under my name: Cindy Strnad, but it may be
under Earthen Vessels Pottery. If you don't want
to do that, thin your glaze out a bit, or sponge
the inside bottom of your bowls so the damp clay
won't take up so much glaze.
Good luck and best wishes,
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
Cindy Strnad on wed 14 nov 01
Oops!
I forgot to include Janet's URL:
http://www.the-coa.org.uk . My earlier message
referred to plans for a glaze fountain which Janet
has generously posted on her site for me. Just
follow the links to the clayart stuff and you'll
find it.
Cindy Strnad
Earthen Vessels Pottery
RR 1, Box 51
Custer, SD 57730
USA
cindy@earthen-vessels-pottery.com
http://www.earthen-vessels-pottery.com
Ron Roy on thu 15 nov 01
Hi Gretchen,
I just calculated the expansions of the two glazes - but just to make sure
- the first one totals 98.8 with the colourants included - right?
The second one - every thing included totals 98.5 right?
It's probably the second glaze that is causing the crazing - as with most
glazes that are short of silica - it's expansion is by far the highest.
Probably not a stable glaze by the way - too low silica and a lot of
copper. I suggest you do the vinegar test on it - or leave a slice of lemon
on it over night.
You can start adding silica to it in 5% increments at a time - when the
total amount of silica gets around 27 you are starting to get into the
stable range - and it will be much less likely to craze. Adding silica to
this glaze will also make it more shiny.
Remember - the thicker the glaze the more likely the chances of crazing -
if it's going to craze at all.
RR
> First I will describe more fully what I am making:
>My bowls are built up of thin slabs wrapped over a
>form. The base of the bowl is all one piece that
>curves up around the sides of the bowl about 1-2".
>From there I add slabs around the bowl until the form
>is covered. I dry the bottom slightly with a blow
>dryer and then invert it to remove the form. I
>bisque, rub iron oxide in H2O on the outside, wipe off
>excess, then glaze the inside. I pour some glaze into
>the bowl and rotate it around for coverage. It is
>very possible that it gets too thick in the bottom,
>even though I am trying to avoid this.
>
>The cracks are only across the bottom of the bowl,
>large enough to feel with your fingernail and see well
>at the right angle. There are 4 to 5 on the bottom.
>They don't seem to continue up the sides. The clay
>structure of the bowl is fine, nothing there is
>cracking or giving way.
>
>The smaller bowls do not seem to have such a problem,
>although a couple flasks I made did.
>
>The glaze is a combination of two glazes. One
>selectively applied over the other for color
>variation. I only apply narrow rivulets of the second
>glaze to get the pattern I want.
>
>These glazes are from "The Ceramic Glaze Handbook" by
>Mark Burleson
>
>Ocean Green Glaze ^6 ox
>
>Kaolin (EPK) 5.4
>Feldspar (G-200)_ 25.3
>Gerstley Borate Sub(I used Gerstley Borate) 20.0
>Nepheline syenite 22.0
>Silica 18.0
>
>Red Iron Oxide 1.8
>Light Rutile 2.8
>Copper Carbonate 3.5
>
>The above is the first glaze I coat the inside of the
>bowl with.
>Turquoise Glaze ^6 ox
>Dolomite 12.0
>Kaolin (EPK) 5.0
>Gerstley Borate sub (Gerstley Borate) 12.0
>Feldspar (G-200) 12.0
>Nepheline syenite 23.0
>Silica 5.5
>Strontium carbonate 15.0
>Zircopax 10.0
>
>Copper carbonate 4.0
>
>This is the glaze I apply in rivulets.
>I am using Miller 45 clay.
Ron Roy
RR# 4
15084 Little Lake Rd..
Brighton,
Ontario, Canada
KOK 1H0
Residence 613-475-9544
Studio 613-475-3715
Fax 613-475-3513
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