Michael Banks on thu 18 nov 99
This is an interesting one Chris. When I addressed this as a deflocculation
problem rather than a flocculation problem, I focussed on two parameters:
1. The glaze symptoms (cracking/crawling) indicated to me weak binding
forces, high shrinkage and;
2. there appears to be few sources of soluble flocculents in your recipe,
but an abundance of potential deflocculant (soda from the Darvan, frit, CMC
and F4). In other words, four deflocculants, versus NO obvious flocculants!
I wilfully ignored your other observations and interpretation that the glaze
was over-flocculated (even though this is the superficial appearance)
because the normal remedies for this were patently not working. I would say
you were stymied and had a lot of complex factors operating here (reflecting
the material complexity). It is actually a nightmarish mixture of kaolins,
smectites, CMC and sodium ions you have here. Thickening of such a mix can
be due to deflocculation of the EPK (your main binder), as I will explain:
One percent of bentonite (and the other smectites) can increase plasticity
(binding force) more than 10% of ball clay. But under extreme swelling
conditions (where bentonite absorbs 5 times its volume of water) in
relatively dilute slip, e.g: glaze slops , the binding forces are so
attenuated by distance, that deposits of such slips are only weakly bound.
In my experience, bentonite cannot be deflocculated and exerts a powerful
thickening action in the presence of sodium ions. So in a mixture of
bentonite and kaolins (e.g: EPK), the kaolins can be in a low-plasticity
deflocculated condition, while the bentonite acts as a powerful thickener
(appears flocced but is not). The rules do not apply to bentonite.
Bentonite absorbs massive amounts of interlayer water and this "water of
plasticity" is at a maximum in the presence of sodium ions. The particle
structure of the thick thixotropic gels formed by bentonite in the presence
of sodium are in face to edge boxworks. I.E. Indistinguishable (to the
casual observer) from a flocculated slip, but having very different
characteristics. Those characteristics include VERY HIGH SHRINKAGE, leading
to inevitable cracking and spalling of a drying glaze layer.
To summarise: If you keep out the smectites, Darvan and CMC and keep the
glaze flocculated to counter the soluble sodium, you may have less trouble.
Smectites like bentonite and Veegum can over-expand in the presence of
soluble sodium leading to thick glaze, weak glaze deposits on the bisc with
high shrinkage and consequent cracking, spalling and crawling. BTW, CMC
makes all this worse! Phew... sounds gruesome.
So, I still blame soluble soda and deflocculation of the EPK content.
Cheers,
Michael
Chris Schafale wrote:
> ----------------------------Original message----------------------------
> Michael,
>
> Thanks for your thoughts on my glaze problem. Perhaps I need to
> clarify a couple of things and ask a question.
>
> First, I should make clear that the glaze appeared to have a
> flocculation problem before I added anything beyond the
> basic recipe. All the additions were attempts to address either the
> apparent flocculation (Darvan), or the shrinkage/cracking (bentonite,
> CMC, calcined kaolin, oh yes, and there is Veegum in there too).
> Second, the symptoms it exhibited seemed to be those of flocculation
> (glaze thickening despite high water content, glaze not settling)
> rather than deflocculation, unless I'm totally confused (not an
> impossibility) about what a deflocculated glaze would look like.
>
> What in my description made it sound like the glaze was
> deflocculated?
>
> Thanks for any clarification.
>
> Chris
>
> >
> > The EPK in your glaze is there (in part) Chris, to act as a binder. It
also
> > contributes silica and alumina, but these oxides can be got from other
> > ingredients, so it's main role is a binder.
> >
> > Now, I think you are DEFEATING THE BINDER by defloccing the glaze with
> > Darvan 7. Also the F4 feldspar (a soda-rich feldspar) is probably
> > contributing soluble Na ions which also deflocculate the EPK and also
the
> > soda in 3134 Frit may be exacerbating this. The strength of plasticity
of
> > EPK (the source of its binding power) is directly related to whether
> > deflocculant ions (Na, Li) dominate in the glaze water or flocculant
ions
> > (H+, Ca, Mg). Deflocculant ions cause the clay particles (EPK in this
case)
> > to repel each other, flocculants maximise the binding force. Soda
feldspars
> > are a common (and overlooked) source of unwanted deflocculation of glaze
> > suspensions and this effect increases with age.
> >
> > Suggested remedies to your problem are:
> > 1. Flocculate the glaze with Epsom salts (MgSO4), calcium chloride or
acid
> > (vinegar, HCl).
> > 2. If (1) doesn't work, add a more powerful smectite-based binder
(V-gum,
> > bentonite, hectorite).
> >
> > Michael Banks,
> > Nelson,
> > New Zealand
> >
> >
> >
> Light One Candle Pottery
> Fuquay-Varina, NC
> candle@intrex.net
>
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