Michael Banks on thu 1 jul 99
Carolynn,
You've just rediscovered froth floatation! This is the process where
metallic ores are separated from gangue (waste rock) in separator plants at
mines. To beat the frothing problem (actually caused by the increased
suspending power of deflocced clay grains -which is what you want), add the
epsom salt (as a solution) after stirring.
Like Tom Buck, I prefer Epsom salts to acid for the reasons stated in my
post yesterday (hydrochloric [-muriatic] acid eats-up your ingredients!).
Muriatic acid is good for loosening concreted frit in long-settled glaze
though.
Here are some general thoughts on suspending frit-rich glazes:
Suspending these can be tricky as they tend to naturally
deflocculate due to the release of Na+ -due to slight solubility of the
frit. Intuitively, one would expect this self-deflocculating tendency to
result in improved dispersion as it does in clay slips. But in practice
frit-rich glazes have a well-known tendency to settle rock-hard. The
problem is the very low clay content, which when deflocculated has less
ability to enclose the larger frit particles and suspend them.
Flocculated frit-rich glazes have better suspension qualities because the
low proportion of clay particles present are in a strongly attracted
edge-to-face "boxwork" configuration which can effectively enclose and
entrap frit grains and dramatically increase their effective surface area.
This enhanced surface area allows Brownian motion in the fluid to slow the
settling process, meaning that the glaze ingredients stay dispersed much
longer.
I think that the best way to suspend a heavily-fritted glaze is to add
sufficient smectite-based clay, e.g: bentonite, hectorite (macaloid), V-gum
etc and flocculent, such as Epsom salts.
My personal favourite is hectorite (lithium-magnesium-montmorillonite),
because it is white (not disturbing glaze colours), it swells strongly
(-forming a dispersed gel) and it is the only clay mineral with no natural
alumina (i.e: does not disturb the glaze maturation of low-melting point
glazes). It is not cheap however. Otherwise bentonite works well.
Michael Banks,
Nelson,
New Zealand
> ----------------------------Original message----------------------------
>
> In a message dated 6/29/99 01:33:39 PM, you wrote:
>
> >Epsom Salts (MgSO4). If you have an option, choose MgSO4.
>
> My experience with Epsom Salts in my heavily fritted glaze was that it
caused
> it to froth unbelievably when mixed with my Jiffy Mixer - this bubbly
froth
> even stayed on the top of glaze bucket for days, and when a pot was dipped
> through it, it caused the glaze to have a very rough uneven surface.
>
> I haven't tried the muriatic acid yet, but I was hoping that it might be
the
> thickener (suspesion agent) that I have been searching for.
>
> Carolynn Palmer, Somerset Center, Michigan
>
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