Mayssan Shora Farra on fri 31 oct 03
Hello All
I agree with all that is said about s2666 here, the good , the bad ....
But I wonder at what happened to one of my big totally flat trays (like a
big oval tile) with some vines around it.
I make those and have no problems usually ( once I figured how to dry them
but that is another story).
Now this tray I fired as usual to ^5 and all was well but I didn't like
the glaze too much and as is my habit I fired it back to ^04 with the
bisque.
Well the glaze came out perfect but the plate was split in two. Straight
line with a gap between the parts of at least 1".
Does anyone know what happened there? was it an explosion that pushed the
parts apart?
Any way I don't use this clay for this application anymore but still keep
wondering.
Thanks for anythought
Mayssan,
In beautiful West Virginia, Indian summer is here with great fall colors
but a hint of smoke in the air that brings feelings of sadness and worries
about California.
http://www.clayvillepottery.com
Ron Roy on sun 2 nov 03
Hi Mayssan,
Usually this happens because the ecpansion of the inside glaze is too low.
If you want me to confirm that I may be able to but l need the recipe of
the glaze.
RR
>I agree with all that is said about s2666 here, the good , the bad ....
>But I wonder at what happened to one of my big totally flat trays (like a
>big oval tile) with some vines around it.
>I make those and have no problems usually ( once I figured how to dry them
>but that is another story).
>Now this tray I fired as usual to ^5 and all was well but I didn't like
>the glaze too much and as is my habit I fired it back to ^04 with the
>bisque.
>Well the glaze came out perfect but the plate was split in two. Straight
>line with a gap between the parts of at least 1".
>Does anyone know what happened there? was it an explosion that pushed the
>parts apart?
>Any way I don't use this clay for this application anymore but still keep
>wondering.
>Thanks for anythought
>
>Mayssan,
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
Mayssan Shora Farra on mon 3 nov 03
Hello Ron;
The recipe for the glaze is:( It was called rutile at the school where I
got it. I don't know the origin).
15 whiting, 20 NC4 feldspar, 5 Gillespie Borate, 5 magnesium, 20 G200
feldspar, 20 spodumene, 15 KY ball clay. + 6 rutile + 6 zinc.
After I dip or brush this on I paint some on it using stains and some
glazes( I don't know how that affects the expansion. )
what puzzles me is that if it survived the ^5 firing why did it break in
the second firing to ^04. I didn't add any glaze after the first firng and
I fired it right away the second day so it had o chance to absorb any
moisture if that could be a contributing factor.
and why was it broke in 2 clean sections but sitting 1" apart on the kiln
shelf?? still scraching my head:).
Thank you very much Ron for taking the time to help me with this.
Best regard,
Mayssan
http://clayvillepottery.com
Ron Roy on wed 5 nov 03
Hi Mayssan,
Well I have to start guessing because I can't blame it on the glaze - the
calculated expansion is not low. It is very low in SiO2 however and will be
affected by acidic food - try the lemon test on it for starters.
Perhaps - as it was being heated - the outside of the plate got heated fast
enough - before the inside could catch up - and that caused the break. I am
assuming the glaze at the break is a little rounded (like melted) rather
than sharp. That would mean there was some melting of the glaze at even 04.
If the crack is very sharp then it would mean the crack happened on the way
down after the glaze had solidified - probably at the quartz inversion at
573C.
Plates - because the middles are in contact with a kiln shelf - can crack
because the rims heat (or cool) faster than the inside. If the plate was
heated too fast the temperature difference between the outside and inside
could result in that sort of crack. All I can think of,
RR
>Hello Ron;
>The recipe for the glaze is:( It was called rutile at the school where I
>got it. I don't know the origin).
>
>15 whiting, 20 NC4 feldspar, 5 Gillespie Borate, 5 magnesium, 20 G200
>feldspar, 20 spodumene, 15 KY ball clay. + 6 rutile + 6 zinc.
>
>After I dip or brush this on I paint some on it using stains and some
>glazes( I don't know how that affects the expansion. )
>
>what puzzles me is that if it survived the ^5 firing why did it break in
>the second firing to ^04. I didn't add any glaze after the first firng and
>I fired it right away the second day so it had o chance to absorb any
>moisture if that could be a contributing factor.
>
>and why was it broke in 2 clean sections but sitting 1" apart on the kiln
>shelf?? still scraching my head:).
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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