Blistering on slipware?

Can u help?

I work with stoneware and have just been experimenting with slipware and
sgraffitto designs. At the moment I have three slips made up - white,
cobalt blue, and copper green. I decorated the ware when still 'green' with
the blue and green slip, bisque fired, then covered with a transparent glaze
to 'fix'. This was then returned to the kiln for a glaze firing.

When I emptied the kiln, the designs were very blistered. This has only
happened where the green slip has been put on - the blue is fine!

I generally bisque fire my ware to 900c...I know this seems low but the
products are easier to 'smooth' with glasspaper if there are any rough
edges. I have looked at my reference books, and one cause of blistering is
by the gases not being able to leave the clay before the glaze sets. Bisque
firing should remove most of these before any glazing is done. I am
wondering if biscuit firing to 1100c will help...but why did it only happen
on the green slip and not the blue as well?

Any suggestions? My brain cells aren't working too well at the moment - due
to head cold!

Many thanks
Jo

I work with stoneware and have just been experimenting with slipware and
sgraffitto designs. At the moment I have three slips made up - white,
cobalt blue, and copper green. I decorated the ware when still 'green'
with
the blue and green slip, bisque fired, then covered with a transparent
glaze
to 'fix'. This was then returned to the kiln for a glaze firing.

When I emptied the kiln, the designs were very blistered. This has only
happened where the green slip has been put on - the blue is fine!

Jo:

Check your recipe for the blue slip and compare it to the green slip.
There is something in the green slip that is reacting with your clay body
that
you do not use in the blue slip.
This assumes of course that the clay body you use for both is the same.

Hope that helps,
Wayne Seidl

Jo:

Check your recipe for the blue slip and compare it to the green slip.
There is something in the green slip that is reacting with your clay body
that
you do not use in the blue slip.
This assumes of course that the clay body you use for both is the same.

Hope that helps,
Wayne Seidl

Reply:-

Yes, both recipes were exactly the same; using the same clay that the wares
were constructed from. From my research it appears that my bisque firing
temp is too low for slipware - still strange that it only happened on the
green (it may be due to chemical properties of copper oxide that carbon
oxide doesn't have?). I will try increasing the bisque temp for my next one
and post my findings. Another suggestion was to add china clay to the
recipe. If the former doesn't work, I'll try the latter!
Thanks for the response.
Jo

Do you mean cobalt oxide (not carbon oxide - which would be either carbon
dioxide or carbon monoxide - both gases and ... yes, you must mean
obalt... )... the cobalt would give blue coloring and copper would give
green in oxidation... (and I just had to correct my having spelled carbon
when I meant copper). If you are using copper carbonate then the carbon
needs to burn off CuCO3 which would agree with your needing to fire
higher...anyway -

They are different chemicals so they are going to give different reactions
in the firing - therefore your slips are not identical.

I love digitalfire....

http://www.digitalfire.ab.ca/cermat/education/97.html
http://www.digitalfire.ab.ca/cermat/oxide/cuo.html
http://www.digitalfire.ab.ca/cermat/oxide/coo.html


"J M" wrote in message
news:K8b8c.25$%Z.16@newsfe1-win...
Jo:

Check your recipe for the blue slip and compare it to the green slip.
There is something in the green slip that is reacting with your clay
body
that
you do not use in the blue slip.
This assumes of course that the clay body you use for both is the same.

Hope that helps,
Wayne Seidl

Reply:-

Yes, both recipes were exactly the same; using the same clay that the
wares
were constructed from. From my research it appears that my bisque firing
temp is too low for slipware - still strange that it only happened on the
green (it may be due to chemical properties of copper oxide that carbon
oxide doesn't have?). I will try increasing the bisque temp for my next
one
and post my findings. Another suggestion was to add china clay to the
recipe. If the former doesn't work, I'll try the latter!
Thanks for the response.
Jo

Yes sorry...I meant cobalt.... fingers working faster than the brain...
JO

"dkat" wrote in message
. net...
Do you mean cobalt oxide (not carbon oxide - which would be either carbon
dioxide or carbon monoxide - both gases and ... yes, you must mean
obalt... )... the cobalt would give blue coloring and copper would give
green in oxidation... (and I just had to correct my having spelled carbon
when I meant copper). If you are using copper carbonate then the carbon
needs to burn off CuCO3 which would agree with your needing to fire
higher...anyway -

They are different chemicals so they are going to give different reactions
in the firing - therefore your slips are not identical.