Dave Finkelnburg on sun 27 feb 05
Daniel,
It is my uneducated opinion that "aging" of clay is the process of dissolution of cations from the clay body constituents. This process changes the concentration of these cations around the individual clay particles. The dissolution process occurs faster as pH goes lower. Thus, adding acid formers (bacteria, nutrients that feed bacteria) tends to lower pH and promote "aging." Clays with high-sodium raw materials, like nepheline syenite, tend to age more because the sodium is somewhat soluble.
Adding epsom salts gives you "instant aging" of the body by supplying ions. The magnesium sulfate, by the way, has the benefit of providing both divalent cations and anions to the liquid phase of the raw clay body. The cations work on the alumina-like surface of the kaolinite in the clay, while the anions work on the silica-like surface. Research has also shown the divalent ion (magnesium is 2+ as opposed to the monovalent Na+) is several times more effective at dispersing the clay particles than a monvalent cation.
While this is beneficial for plasticity, you have soluble ions in the clay body that can conceivably move with the water as it evaporates, and accumulate at the drying surface, thus selectively fluxing that location. That's hardly ever noticeable, unless you have a lot of soluble ions present. The point is, a little Epsom salts can be good, but avoid too much of a good thing!
Good potting!
Dave Finkelnburg
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Daniel Semler on wed 2 mar 05
Hi Dave,
Thanx for this.
I looked up aging, plasticity, flocculation, workability, thixotropy and
souring last night in Hamer and Hamer. You can spend ages hopping from
reference to refence :)
They define aging in terms of wetting the particles, and define souring as an
acid based (hey I like that term - oooops) reaction. What they describe sounds
like what you have described here - basically, flocculation. I suspect like
many things in clay, they do not occur completely in isolation and thus
separating them and their individual effects is somewhat difficult. I expect
that aging a clay body that contains bacteria and so on results in what H & H
call souring. So in their terms both aging (wetting) and souring (flocculation)
occur together.
It seems that obtaining a good plastic mix is possible by a number of methods,
though some cannot be eliminated altogether - eg. one cannot eliminate water but
one does not have to deliberately flocculate. That said, to achieve the same
level of plasticity by one method over another has consequences on other
properties of the mix.
Thanx
D
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David Beumee on wed 2 mar 05
Aging clay: Not You!? HA! More mud facials Dave! See how many advantages there are to being a potter?
Dave wrote,
"While this is beneficial for plasticity, you have soluble ions in the clay
> body that can conceivably move with the water as it evaporates, and accumulate
> at the drying surface, thus selectively fluxing that location. That's hardly
> ever noticeable, unless you have a lot of soluble ions present. The point is, a
> little Epsom salts can be good, but avoid too much of a good thing!"
If Epsom Salts were added in solution to a clay body mixed as a slip, (I use either 0.3% or 0.4%) and dried out to moist consistency, grant it that some Epsom Salts are leaving with the extra water, but from experience watching the effect of Epsom Salts (in solution) flocculating a clay body slurry mixture, the effect is immediate as the mixture thickens noticably. Can it possibly be true that such flocculation would be lost or even lessened because some soluble magnesium sulphate is leaving with the extra water? Even if the flocculating effect is somewhat lessened, surely enough remains to "age" a porcelain body mixture?
All this wouldn't be so important if we were talking about a stoneware mixture where far more plastic clays are naturally included, but the flocculation/aging is very important to those of us trying to prepare an all kaolin porcelain clay body mixture that is workable enough for use on the wheel while at the same time trying to keep the fired result as white as possible so our glazes are able to reflect the greatest amount of clarity.
David Beumee
davidbeumee.com
Lafayette, CO
-------------- Original message ----------------------
From: Dave Finkelnburg
> Daniel,
> It is my uneducated opinion that "aging" of clay is the process of
> dissolution of cations from the clay body constituents. This process changes
> the concentration of these cations around the individual clay particles. The
> dissolution process occurs faster as pH goes lower. Thus, adding acid formers
> (bacteria, nutrients that feed bacteria) tends to lower pH and promote "aging."
> Clays with high-sodium raw materials, like nepheline syenite, tend to age more
> because the sodium is somewhat soluble.
> Adding epsom salts gives you "instant aging" of the body by supplying ions.
> The magnesium sulfate, by the way, has the benefit of providing both divalent
> cations and anions to the liquid phase of the raw clay body. The cations work
> on the alumina-like surface of the kaolinite in the clay, while the anions work
> on the silica-like surface. Research has also shown the divalent ion (magnesium
> is 2+ as opposed to the monovalent Na+) is several times more effective at
> dispersing the clay particles than a monvalent cation.
> While this is beneficial for plasticity, you have soluble ions in the clay
> body that can conceivably move with the water as it evaporates, and accumulate
> at the drying surface, thus selectively fluxing that location. That's hardly
> ever noticeable, unless you have a lot of soluble ions present. The point is, a
> little Epsom salts can be good, but avoid too much of a good thing!
> Good potting!
> Dave Finkelnburg
>
>
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Ron Roy on wed 2 mar 05
Hi David,
Yes - I recommend .2% so it would be wise to compensate and add more to
make up for that lost when dewatered.
Still I wonder - what is really lost and does having too much at one point
present any special problems?
Have you ever mixed porcelain with just the right amount of water to compare?
RR
> If Epsom Salts were added in solution to a clay body mixed as a slip, (I
>use either 0.3% or 0.4%) and dried out to moist consistency, grant it that
>some Epsom Salts are leaving with the extra water, but from experience
>watching the effect of Epsom Salts (in solution) flocculating a clay body
>slurry mixture, the effect is immediate as the mixture thickens noticably.
>Can it possibly be true that such flocculation would be lost or even
>lessened because some soluble magnesium sulphate is leaving with the extra
>water? Even if the flocculating effect is somewhat lessened, surely enough
>remains to "age" a porcelain body mixture?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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