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help with slips again and glazes

updated wed 23 jun 04


tammy brown on wed 16 jun 04

I successfully bisque fired my first piece of
sculpture and I am so excited I could burst. It is a
large bust constructed on an armature. I applied white
slip to the piece while at the greenware stage. It
fired nicely but I am a bit unsure where to go from
here. The white silp flaked off a lot! I may have
applied it too thickly but I dont think so. I would
like to apply a wash of stains and borate (gillespie
maybe?) to the recesses and leave it white. I think I
need to apply a matte glaze over it but am not sure
what product to use. I am willing to mix from raw
materials...but frankly being new to this I could use
recipes or direction. I would definetely need a five
gallon bucket of glaze to cover the size of this bust.
I want a completely matte finish...something that will
not I know is a problem with clear
finishes.But I also have the issue of flaking slip. I
sanded the piece down so that there are no more flakes
apparent, but will I encounter problems with this if I
glaze over it ? Also, I fired this to a cone 04
bisque. The clay body I am using is cone 04-7. What
would anyone suggest that I fire it too on the second
firing considering all the factors I have added....the
wash with a matte glaze?
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Snail Scott on tue 22 jun 04

At 08:26 PM 6/16/2004 -0700, you wrote:
>...The white silp flaked off a lot! I may have
>applied it too thickly but I dont think so.

If it was applied when the piece was leather-hard or drier,
then thickness is a big issue. If it's applied to damper
clay, it's a lot more compatible. If a slip must be applied
to drier clay, using an engobe instead will give better

Any discrepancy between the dampness of the clay and the
dampness of the slip will be magnified with thicker slip
coatings, and minimized with thinner ones. Since ALL slips
will always be wetter than the underlying clay, thickness
can never be disregarded, and if a slip needs to be thick,
modifying it with non-plastic ingredients (i.e. making an
engobe) may be necessary to avoid flaking due to the
inevitable shrinkage differential of plain raw-clay slips.
Using deflocculants to reduce the amount of water needed to
produce a brushable liquid can also help, but I'd still
prefer to start with a low-shrinkage recipe first.

>...I think I
>need to apply a matte glaze over it but am not sure
>what product to use.

Do you 'need' to for artistic reasons, or is that an
assumption that everything 'needs' a glaze, or do you feel
that 'saving' the piece using a fired surface is more
legitimate than another finish?

You could try waxing it, which would give a low sheen and
help protect against grubby fingerprints - always an issue
with white slip finishes! If you don't like the look, just
refire to burn it off. Paint can also be a great finish for
sculpture. (You can apply wax over paint, too, or use other
finish coatings.) I don't suggest using it as a way to avoid
learning ceramic processes, but if it's what the piece ought
to have, there's no rule against it.

As for applying glaze over a peeling slip - if all the loose
slip is removed, it may work OK. The glaze will tend to make
any remaining 'slip flakes' harder and less likely to break
off, though it can also tend to cause previously 'borderline'
areas to pull loose more then before. You can certainly give
it a try, but if you didn't want to glaze it before, don't
do it now just as a salvage effort.

-Snail Scott
Edwardsville, Illinois, USA, Earth

(Mostly just skimming and lurking for now, on a borrowed
laptop, 'til I get unpacked from the move!)