Timothy Sullivan on tue 5 oct 04
I've had a terrible problem with cracking in a very peculiar situation. I
use Laguna B-Mix fired in reduction to cone 10, and I do a lot of glaze
trailing and spraying of multiple glazes. About 8 months ago I began to get
very fine cracking through the center of vertical pieces, all the way
through the pot, but only in an area of the pot glazed with a particular
saturated iron glaze. Thinking that it might be due to an inside glaze/
outside glaze expansion issue I tried several different liner glazes with
higher COEs, but still kept getting the cracking. I also tried
reformulating the glaze with different materials i.e. Custer for G-200 etc.,
changing iron from RIO to Spanish RIO, ball milling and a host of other
things. Ultimately I used some software to compare several different sat.
iron glazes and found most of them to be very low in silica. Thinking that
perhaps silica content may have been borderline all along, but perhaps
worsened with the inevitable changes in materials, I decided to increase the
silica content of the formula. The results were excellent. Often this
glaze will be the best orange color when vertical and would often turn an
ugly brown and develop a matt finish rather that the preferred glossy
surface. When adding the additional silica it stayed glossy, with improved
color and the cracking stopped.
These can be very difficult problems to solve, and when they involve your
favorite glazes it's hard to just walk away from the look you want.
Timothy Sullivan
Creekside Pottery
www.creeksidepottery.net
tim@creeksidepottery.net
Ron Roy on fri 8 oct 04
Hi Timothy,
Something strange here - and if you would be willing to share some of your
information I would like to try and figire this out.
Usually cracking is due to a glaze with a low expansion/contraction on the
inside of pot. If that were the cause of the cracking - then adding silica
to that glaze would have made it worse - by lowering the COE.
I am assuning the edges of the glazes are sharp indicating that the glaze
and clay cracked during cooling.
You can email any info you are willing to share directly to me - I don't
keep records of the glazes I work on and I will guarentee strict confidence
if you prefer that.
Just copy my address into your header when replying.
ronroy@ca.inter.net (Ron Roy)
RR
>I've had a terrible problem with cracking in a very peculiar situation. I
>use Laguna B-Mix fired in reduction to cone 10, and I do a lot of glaze
>trailing and spraying of multiple glazes. About 8 months ago I began to get
>very fine cracking through the center of vertical pieces, all the way
>through the pot, but only in an area of the pot glazed with a particular
>saturated iron glaze. Thinking that it might be due to an inside glaze/
>outside glaze expansion issue I tried several different liner glazes with
>higher COEs, but still kept getting the cracking. I also tried
>reformulating the glaze with different materials i.e. Custer for G-200 etc.,
>changing iron from RIO to Spanish RIO, ball milling and a host of other
>things. Ultimately I used some software to compare several different sat.
>iron glazes and found most of them to be very low in silica. Thinking that
>perhaps silica content may have been borderline all along, but perhaps
>worsened with the inevitable changes in materials, I decided to increase the
>silica content of the formula. The results were excellent. Often this
>glaze will be the best orange color when vertical and would often turn an
>ugly brown and develop a matt finish rather that the preferred glossy
>surface. When adding the additional silica it stayed glossy, with improved
>color and the cracking stopped.
>
>These can be very difficult problems to solve, and when they involve your
>favorite glazes it's hard to just walk away from the look you want.
>
>Timothy Sullivan
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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