Paul Lewing on sun 25 jul 99
Alison Hamilton wrote:
>
> ----------------------------Original message----------------------------
> Emily,
>
> Bell Dark is a type of ball clay - I like to describe ball clays as poor
> cousins of kaolins, since not only does ball clay have kaolinite (pure clay
> mineral), it also has free silica and some other oxides in it (which act as
> fluxes). Okay, I admit it - I am now showing off to an audience of
> thousands, that I know how to plagiarize the Hamers'!
Allison,
The Hamers may be right about this kaolinite in ball clay business, but
my understanding, from taking clay mineralogy in grad school, was that
it's not quite like that. Admittedly, this one graduate school seminar
was my only geology course ever, and it was in 1971, so I could be way
off. But as I recall, we did X-ray diffraction and atomic absorption
tests on all of the clays that we could find that were used in ceramics
in any way- for throwing, for glaze ingredients, high-fire, low-fire,
commercial and locally dug. This class was taught by a brilliant clay
mineralogist, Gray Thompson, at the U of Montana. He was the best
teacher I've ever had for anything, and it was the first time he'd ever
taught this class for potters, so we got to watch him try to figure out
how his tests related to the ways we knew clay acted.
Anyway, we found out that, to a mineralogist, all the kaolins, all the
ball clays, and even all the fire clays were the same thing! They were
all kaolinite, specifically, B-axis disordered kaolinite, which means
that the layers do not lay perfectly aligned in the same direction with
each other. We were trying to find out the mineralogical basis for
plasticity, maturing temperature, and fired color. Turned out that what
made them act different was the impurities and the particle size (to
greatly simplify the matter). Which was fascinating to the teacher,
because after years of teaching this class to geology grad students and
soils grad students from the Forestry Dept., here was a group of people
who had a huge body of very detailed knowledge about clays that had
NOTHING to do with what the other two groups knew about. But he was
able to lecture to all three groups at the same time, eithout losing
anyone, and giving all of us as detailed stuff as we needed. Then he
designed serperate labs for each group to find out what each wanted to
know. It was a brilliant tour de force.
So... I know there are some geologists out there. Am I remembering this
right- that all the high-fire throwing clays, kaolin, ball clay and fire
clay, are all B-axis disordered kaolinite?
Paul Lewing, Seattle
Alison Hamilton on tue 27 jul 99
Paul,
uh oh, I knew I'd get into trouble if I posted another glaze recipe! :)
Lemme see, an analysis of ball clay as poor cousins of kaolins or as B-axis
disordered kaolinite - hmm, which sounds more impressive? The latter sounds
like a psychiatric disorder .... (and would probably sound impressive tripping
off my tongue so long as no one asked me to answer any questions about it!)
I'd like to understand this. Are you proposing that there are no differences
between the different kinds of ball clays? So then, what do you mean when you
say that the only differences were the impurities and the particle size? What i
defined as an impurity then? And did you really find no oxides or free silica i
your studies?
This is quite interesting; I'd like to hear more.
Alison
Trout Lake
Dorset, ON
Michael Banks on wed 28 jul 99
Yes, pretty much Paul. Kaolinite seems to be the king of pottery minerals,
but generalisations are dangerous. In New Zealand, we have a most unusual
white clay mined in Northland District which has only 0.28% Fe2O3 and 0.08
TiO2 which makes it the whitest-firing clay that I know of in the world.
And it's not a kaolinite (b-axis disordered or not). Mineralogically it's a
halloysite. It's funny stuff, not very plastic, not throwable on its own,
but it makes the most translucent porcelain imaginable. Here's the relevant
web site about this clay: http://www.nzcc.co.nz/
Generally though I would say that kaolinite is the most abundant clay
mineral in most ceramic bodies and by far the best behaved. As for b-axis
disordering, I have noticed that all the kaolinite grains I've ever seen
microscopically formed vermiform crystal "books", meaning the stacked plates
did not have perfectly parallel faces, top and bottom and tend to curl all
over the place. I guess that is what is meant by the phrase. But I'm also
aware that some kaolinites are almost totally disordered, a, b and c axes.
Of the very white clays, there is a virtual continuum from non-plastic china
clays (which are usually coarsely crystalline kaolinite) to very plastic
aluminous ball clays (which often have a high proportion of sub-micron
sized, poorly crystalline particles). These latter particles produce
subdued, to almost non-existent peaks on the graphical trace produced by XRD
(x-ray diffraction analysis).
The textbooks usually say that coarse crystalline kaolins are primary in
origin (in-situ replacement of feldspar in bed rock, usually granite) and
the fine-grained, disordered stuff; -secondary (from re-weathering and
milling of the primary kaolins by surface transport and sedimentary
processes). The coarse kaolinites are produced from in-situ conversion of
feldspar grains to kaolinite by acidic water. The water can be either hot
(from hydrothermal activity) or comparatively cool (in the case of
groundwaters). The water must be acidic to produce kaolins, as all the
alkali ions must be thoroughly stripped and taken into solution to be
removed from the products. If the waters are alkaline, the alkalis and
calcium hang around and produce smectite clays.
This picture is largely true for the true, blindingly white china clays
beloved by paper mills and paint manufacturers. But when it comes to the
plastic kaolins and ball clays, there are anomalies. Most ball clays are
secondary and occur in water laid sediments. But, some of these appear to
be primary clays as well, but instead of being flour-like, platy stuff good
for the pages of National Geographic, they are the sticky, mucky stuff
beloved by potters.
Why this is so is very complex and may be due to idiosyncrasies in the
original rock type, e.g. whether it was a granite, syenite, pumice,
rhyolite, rhyolitic ash, arkose, gneiss, or almost any feldspathic material
at all. Then there's the nature of the acidic water, what acid is present,
its temperature, gases available etc. Reactive gases such as fluorine are
known to have been present on the geothermal fluid which produced the well
known Cornish kaolins in south west England. Plastic kaolins can be
extracted from these otherwise low-plasticity clays by a variety of
wet-classification methods, which preferentially concentrate the finest
particles.
Whatever the reason, the right combination of natures foibles can produce
plastic kaolins with are characterised by very fine grainsize in some
districts. The fortuitous discovery of one such deposit led to the
invention of porcelain in eastern China about a millenium ago. Some
disordered poorly crystalline material is usually present in these clays,
but I don't think this is neccessarily a good thing. I've experimented with
disordered kaolin extracted from china clay slip by decanting small
fractions off the top of a large volume. And by and large it had very poor
charteristics as a clay, with massive dry shrinkage, a lot of drying cracks
and fired to a grey colour which I attribute to a high degree of titanium
substitution.
Michael Banks,
Nelson,
New Zealand
> ----------------------------Original message----------------------------
> Alison Hamilton wrote:
> >
> > ----------------------------Original message----------------------------
> > Emily,
> >
> > Bell Dark is a type of ball clay - I like to describe ball clays as poor
> > cousins of kaolins, since not only does ball clay have kaolinite (pure
clay
> > mineral), it also has free silica and some other oxides in it (which act
as
> > fluxes). Okay, I admit it - I am now showing off to an audience of
> > thousands, that I know how to plagiarize the Hamers'!
>
> Allison,
> The Hamers may be right about this kaolinite in ball clay business, but
> my understanding, from taking clay mineralogy in grad school, was that
> it's not quite like that. Admittedly, this one graduate school seminar
> was my only geology course ever, and it was in 1971, so I could be way
> off. But as I recall, we did X-ray diffraction and atomic absorption
> tests on all of the clays that we could find that were used in ceramics
> in any way- for throwing, for glaze ingredients, high-fire, low-fire,
> commercial and locally dug. This class was taught by a brilliant clay
> mineralogist, Gray Thompson, at the U of Montana. He was the best
> teacher I've ever had for anything, and it was the first time he'd ever
> taught this class for potters, so we got to watch him try to figure out
> how his tests related to the ways we knew clay acted.
> Anyway, we found out that, to a mineralogist, all the kaolins, all the
> ball clays, and even all the fire clays were the same thing! They were
> all kaolinite, specifically, B-axis disordered kaolinite, which means
> that the layers do not lay perfectly aligned in the same direction with
> each other. We were trying to find out the mineralogical basis for
> plasticity, maturing temperature, and fired color. Turned out that what
> made them act different was the impurities and the particle size (to
> greatly simplify the matter). Which was fascinating to the teacher,
> because after years of teaching this class to geology grad students and
> soils grad students from the Forestry Dept., here was a group of people
> who had a huge body of very detailed knowledge about clays that had
> NOTHING to do with what the other two groups knew about. But he was
> able to lecture to all three groups at the same time, eithout losing
> anyone, and giving all of us as detailed stuff as we needed. Then he
> designed serperate labs for each group to find out what each wanted to
> know. It was a brilliant tour de force.
> So... I know there are some geologists out there. Am I remembering this
> right- that all the high-fire throwing clays, kaolin, ball clay and fire
> clay, are all B-axis disordered kaolinite?
> Paul Lewing, Seattle
>
Paul Lewing on fri 30 jul 99
Alison Hamilton wrote:
Are you proposing that there are no differences
> between the different kinds of ball clays? So then, what do you mean when you
> say that the only differences were the impurities and the particle size? What
> defined as an impurity then? And did you really find no oxides or free silica
> your studies?
>
> This is quite interesting; I'd like to hear more.
Hi, Alison,
Actually, see the post from Michael Banks today about kaolinites for the
real lowdown- more than I ever knew about the subject. As I said, this
was in a class almost thirty years ago. But what I was really saying
was that the test we used, X-ray difractometry, could see no differences
in the mineralogy of kaolins, ball clays or fire clays. So to the clay
mineralogists in the group, there was no difference. We concluded that
there were impurities from the atomic absorption tests, but not how much
of anything there was. The conclusions about what the impurities were
and that particle size was making the difference in plasticity, we
inferred from our knowledge of ceramic behavior. Does that make sense?
Seriously, though, Michael's post was exactly what I was hoping someone
would post. Thanks, Michael.
Paul Lewing, Seattle.
Hoping the snow will melt out of the mountains sometime before October,
so I can go backpacking.
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