Lesley Alexander on fri 10 mar 00
Can Ron Roy help with this? I use 'Insight,' but don't have an expansion
number for the clay bodies I've been using, Laguna's Los Altos and B-Mix.
The shrinkage given is 12.5 and 13.0, respectively. The slip I'm trying is
Johnson's from Clayart, with an expansion number of 4.83. (Grolleg 52.38,
epk 23.81, Silica 23.81). Results? As of the greenware stage, I've been
able to cox the slip to stay on B-mix even pretty thick, by spraying as it
dries with cmc gum solution and sticking it down. With Los Altos, only a
very thin layer wants to stay on. I have no results with bisque or glazing
yet and am not betting the house on it.
How does one compare shrinkage and expansion numbers? Thanks, I can't
believe how helpful Clayart has been! Lesley in So. Calif.
Ron Roy on sat 11 mar 00
Clay body shrinkage - in the area of 12% for a throwing clay - say 6% from
wet to dry and the rest as the body melts has nothing to do with
expansion/contraction of body and glaze. Shrinkage is not reversible after
firing - expansion/contraction is infinitely reversible depending on
temperature.
The problem here is keeping raw slip on the pot if I understand it. This is
probably because the slip is shrinking too much as it dries.
There are two approaches to this - calcine part of the slip so it will not
shrink so much or deflocculate the slip so it needs less water to flow -
and therefore shrinks less or both.
Calculated expansion numbers do not work with clays because they are
crystalline and crystalline silica inversions cannot be calculated - you
don't know how much crystalline silica is in the fired clay.
There are only two ways to solve the problem and that is measure the
expansion of the fired body, clay and slip and get an actual comparison -
probably too expensive for most potters - or experiment till you get good
results. If you can get the slip and body working together then your glazes
should follow along.
The slip you are using is 75% clay and no flux - I suggest defloccking it
to get it to stick on the raw pot. To get the fit right I suggest adding
Custer or G200 at 15, 20 and 25% for a cone 10 firing - for cone 6 I would
add the same amount of Neph Sy - which - because it will provide some
soluble sodium - just might deflocc the slip enough to do the trick.
I'm just guessing at this - make sure you test properly and lots. The slip
I use is the same body I throw with but deflocced. Hard to go wrong with
that approach unless you really work at it.
RR
>----------------------------Original message----------------------------
> Can Ron Roy help with this? I use 'Insight,' but don't have an
>expansion
>number for the clay bodies I've been using, Laguna's Los Altos and B-Mix.
>The shrinkage given is 12.5 and 13.0, respectively. The slip I'm trying is
>Johnson's from Clayart, with an expansion number of 4.83. (Grolleg 52.38,
>epk 23.81, Silica 23.81). Results? As of the greenware stage, I've been
>able to cox the slip to stay on B-mix even pretty thick, by spraying as it
>dries with cmc gum solution and sticking it down. With Los Altos, only a
>very thin layer wants to stay on. I have no results with bisque or glazing
>yet and am not betting the house on it.
> How does one compare shrinkage and expansion numbers? Thanks, I can't
>believe how helpful Clayart has been! Lesley in So. Calif.
Ron Roy
93 Pegasus Trail
Scarborough
Ontario, Canada
M1G 3N8
Evenings 416-439-2621
Fax 416-438-7849
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