June Perry on sun 24 dec 00
I'm not sure they are the same thing. In my Insight glaze program, they are
listed separately. Maybe one of our experts like Tom Buck can answer that
questoin.
Isn't pumice ground tuff (a volcanic rock)?
Regards,
June
Gavin Stairs on sun 24 dec 00
At 12:47 PM 12/24/00, June Perry wrote:
>I'm not sure they are the same thing. In my Insight glaze program, they are
>listed separately. Maybe one of our experts like Tom Buck can answer that
>questoin.
>Isn't pumice ground tuff (a volcanic rock)?
Tuff is actually solidified ash, while pumice is froth. Both are
volcanic. Ash is what comes out of cinder cone volcanos as pyroclastic
fragments and smokes, which settle on the surrounding countryside, building
up the cinder cone. These volcanos are mostly found on the subductive rims
of the Pacific and elsewhere. Mt. St. Helen, Fujisan, Popacatapetl, etc.
Pumices also arise in such volcanos, as a result of gasses bubbling through
liquid magma, usually underground, I believe. When the mass cools before
all the gas is released, the result is a foam, which is called
pumice. Glassy pumices can form near the surface, or in parts of lava
flows. Natural glasses, called obsidian, and similar rocks, are also
formed near the surface, and may have vesicles or bubbles. These terms are
broad classifications covering much territory.
These rocks are formed in the subductive rims more often than in the
central ridge or rift volcanos because of the alumina-silica rations and
other features resulting from the origins of the rocks in the magma. The
central rift rocks have origins in the mantle interphase, and are pyroxine
rich. The subduction rocks are a mix of ocean floor and continental mass
rock, and have more like granite or andesite composition, richer in
fluxes. They are also rich in gas derived from water, carbonates and
suphides, etc. So the rift volcanos ooze lava, while the subduction
volcanos tend to blow their tops and vent stuff blown up with gas. The
Americas are full of old tuffs and occasional pumice and other vesicular rocks.
There was a recent discussion of the comparative chemistry of volcanic
rocks on this list. You might search under Michael Banks, Geology 101,
around September '99.
The analysis of ash and pumice will vary somewhat according to source, so
there's no trivial answer to the original question. They are both of
similar origin, and might well have similar composition. But they will
both vary quite a bit.
Joyous Holidays to all, Gavin
Gavin Stairs
Stairs Small Systems
921 College St., # 1-A
Toronto, Ontario, Canada M6H 1A1
phone: (416)530-0419 stairs@stairs.on.ca
Tom Buck on sun 24 dec 00
On Sun, 24 Dec 2000, June Perry wrote:
> I'm not sure they are the same thing. In my Insight glaze program, they are
> listed separately. Maybe one of our experts like Tom Buck can answer that
> question.
> Isn't pumice ground tuff (a volcanic rock)?
>
> Regards,
> June
>
Hi June, others,
Happy holidays. since you put me in the catbird seat I better
deliver.
the chem dictionary says: "pumice. a highly porous igneous rock, usually
containing 67-75% SiO2 [silica] and 10-20% Al2O3 [alumina]; glassy
texture. Potassium, sodium, and calcium are generally present. Insoluble
in water; not attacked by acids.
"Occurrence: Arizona, Oregon, California, Hawaii, New Mexico,
Italy, New Zealand, Greece." [and likely lots of other sites near old
volcanoes].
sounds like ancient volcanic ash to me. and the ancients did use
it as a household cleaning agent, among other uses.
Pumice is used in many industries, so it is "classified" by one
method or another, to be sold as lump, powdered (coarse, medium, fine),
N.F. (medicinal grade), technical [??] grade. it is used for its physical
properties mainly and it seldom is melted or otherwise chemically changed
in these uses.
The raw volcanic ash potters and others gather is essentially the
same stuff but potters would likely obtain fine powder because it is
easier for them to use it as such.
Now, volcanic ash will vary in composition somewhat, not all
magmas have the same makeup, but generally it will consist of silica,
alumina, and small amounts of metal oxides (K, Na, Ca, Mg, and ???).
Because pumice is used for its physical properities much of it is
sold unanalyzed chemically whereas ceramic industry users of volcanic ash
insist on having an analysis. So, volcanic ash tends to be a known
material with a specific chemical makeup whereas Pumice though likely
identical in most ways is a material of uncertain makeup.
But potters can safely use fine pumice or fine ash as equal.
bye for now. Peace. Tom B.
Tom Buck ) tel: 905-389-2339 (westend Lake Ontario,
province of Ontario, Canada). mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
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