Bob Masta on tue 4 may 04
I have been reading about prehistoric Native American
pottery on the Web, and it seems that it was quite common
in places to use crushed shells as "temper", which I gather is
what potters would call grog. But none of the sites
discussed the issue of lime pops. This has me
completely mystified, since I'm sure these pots
weren't fired high enough to cause the lime to flux.
Anyone know how they pulled this off?
Thanks!
Bob Masta
potsATdaqartaDOTcom
Vince Pitelka on tue 4 may 04
> I have been reading about prehistoric Native American
> pottery on the Web, and it seems that it was quite common
> in places to use crushed shells as "temper", which I gather is
> what potters would call grog. But none of the sites
> discussed the issue of lime pops.
Bob -
I know that lime inclusions need to be a certain size before they cause lime
pops. I do not know the specific size, but when crushed seashells were used
by tribal potters as tempering material, the coarser particles were sifted
out, and only the finer material was mixed into the clay - thus no problem
with lime pops. If you think about the lime pops you have seen, you can
always spot the little inclusion of lime, and they are of considerable
size - a minimum of a sixteenth inch or so, and that would still be very
coarse for tempering material.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Ivor and Olive Lewis on wed 5 may 04
Dear Bob Masta,
If the shells were well crushed then the problem of lime pops is
reduced. The second point is that the temperature has to get up above
about 850=BA C to cause the chemistry that changes Calcium carbonate
into Calcium oxide which would later react with atmospheric moisture
to cause lime popping.
Might be another reason though.
Best regards,
Ivor Lewis. Redhill, South Australia
Ivor and Olive Lewis on thu 6 may 04
Dear Friends,
If we think carefully about the reactions that take place in a clay
body as it is heated then after the dissipation of chemically combined
water, the next reactions are those that cause the Group 2 Alkali
Earth element Carbonates to dissociate. This happens as the
temperature rises above 850=BA C (~1500=BA F). This is also about the
lowest temperature that Sintering will commence. The next step is the
decomposition of kaolin minerals which releases free silica.
Now free silica has an affinity for calcium oxide, or is the other way
round. They can interact in th solid state, without becoming liquids,
in a chemical way to produce a compound similar in chemical
composition to Wollastonite. This effectively locks the Calcium oxide
up and so there are no lime pops.
As Vince says, grading of Shell Temper would be fine to dust and
larger fractions would have been removed.
Best regards,
Ivor Lewis. Redhill, South Australia
----- Original Message -----
From: "Vince Pitelka"
To:
Sent: Wednesday, 5 May 2004 12:00
Subject: Re: Shell temper and lime pops
> > I have been reading about prehistoric Native American
> > pottery on the Web, and it seems that it was quite common
> > in places to use crushed shells as "temper", which I gather is
> > what potters would call grog. But none of the sites
> > discussed the issue of lime pops.
>
> Bob -
> I know that lime inclusions need to be a certain size before they
cause lime
> pops. I do not know the specific size, but when crushed seashells
were used
> by tribal potters as tempering material, the coarser particles were
sifted
> out, and only the finer material was mixed into the clay - thus no
problem
> with lime pops. If you think about the lime pops you have seen,
you can
> always spot the little inclusion of lime, and they are of
considerable
> size - a minimum of a sixteenth inch or so, and that would still be
very
> coarse for tempering material.
> Best wishes -
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft, Tennessee Technological University
> Smithville TN 37166, 615/597-6801 x111
> vpitelka@dtccom.net, wpitelka@tntech.edu
> http://iweb.tntech.edu/wpitelka/
> http://www.tntech.edu/craftcenter/
>
>
______________________________________________________________________
________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
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> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Bruce Girrell on thu 6 may 04
Ivor wrote:
> ...This effectively locks the Calcium oxide up and so there are no lime
pops.
OK, so you've explained why no lime pops occur for the shells, but now why
_do_ we (some people anyway) get lime pops at all? The same reaction should
take place whether the lime came from aragonite in shells or from any other
source, should it not?
Bruce "pop goes the weasel" Girrell
Dave Finkelnburg on thu 6 may 04
Bruce,
If you go back and read the posts from Vince and Ivor, Vince stated and
Ivor acknowledged the use of crushed shells for temper in a clay body
involves screening out the very coarse material. Lime pops come from coarse
bits of shell, limestone or gypsum. All of these calcine in the kiln to
calcium oxide, CaO. The critical size seems to be around 2-3-mm or 1/16th
to 1/8th-inch and larger. Fine particles of CaO go on to react with the
clay body to form some form of calcium silicate. The coarse CaO, though,
absorbs moisture over time and swells when it does so, causing the pop.
High firing temperatures allow more heat and time to cause the calcium
oxide to react. Lime pops are more of a problem at low fire and in bisque
ware.
Good potting!
Dave Finkelnburg on a beautiful day in Idaho, where yesterday it
"snowed" in the orchard every time the wind blew a cloud of petals off the
apple trees, and where the drive is still pink from the windrows of petals
from the crab apple trees, and where now the yard smells heavenly with the
scent of just-blooming lilacs...
----- Original Message -----
From: "Bruce Girrell"
Sent: Thursday, May 06, 2004 9:28 AM
> OK, so you've explained why no lime pops occur for the shells, but now why
> _do_ we (some people anyway) get lime pops at all?
Logan Oplinger on fri 7 may 04
On Tue, 4 May 2004 09:33:51 -0400, Bob Masta wrote:
>I have been reading about prehistoric Native American
>pottery on the Web, and it seems that it was quite common
>in places to use crushed shells as "temper", which I gather is
>what potters would call grog. But none of the sites
>discussed the issue of lime pops. This has me
>completely mystified, since I'm sure these pots
>weren't fired high enough to cause the lime to flux.
>Anyone know how they pulled this off?
>
>Thanks!
>
>Bob Masta
>
>potsATdaqartaDOTcom
In addition to what Ivor and Vince have already said here, I will add that
on the island of Guam where I live, the early Chamorros (Guamanians) made
pottery, a form of "low fire" earthenware, using different forms of sand
temper, everything from stream sediments, magnetite sands, and medium fine
calcareous beach sand. Pot shards can be found strewn about the ancient
village sites. More information is available in the archeological
literature about the research done in the use of sand tempers in pottery.
See literature references cited at:
http://home.earthlink.net/~gdescantes/jas_2001.pdf
and the publication:
Dickinson, William R. and Richard Shutler, Jr
1979 Petrography of sand tempers in Pacific Island potsherds. Geological
Society of America Bulletin 90(2):1644-1701.
Logan Oplinger
Another Tropical Island
Ivor and Olive Lewis on fri 7 may 04
Dear Bruce,
Why do larger fragments of originally insoluble Calcium Carbonate and
also Calcium Sulphate distributed in clay cause to much bother ?
(Aragonite is just Calcium Carbonate with a differing crystal
structure for those who have not met the word before. It makes very
durable sea shells and coverings for other molluscs).
I know about it. I have a terra cotta sculpture in the garden which is
so afflicted it looks like "White Pox". In this case the clay became
contaminated when fragments of Plaster from a sculpture class spread
to places where they should not have been.
Here goes :-
When both chemicals are fired there comes a point in time where acid
radicles of these compounds are discharges as "volatile Oxides"
leaving nodules of Calcium Oxide distributed throughout the clay.
After being fired above 825=BA C (1517=BA F) for Aragonite or 899=BA C
(1650=BA F) for Calcite, atmospheric moisture diffuses into the clay if
it is not fully vitrified, meaning it is still earthen ware. A new
chemical forms called Calcium Hydroxide ( CaO+H2O =3D Ca(OH)2. ) The
extra 2 H and 1 O means Calcium Hydroxide has a larger volume. So it
exerts a pressure on the continuing clay. Near the surface of the clay
there is not enough back force so the clay fractures and a little
piece of clay spalls from the surface.
So when ware which contains visible fragments of limestone, chalk or
sea shell is fired below let us say 800=BA C ( 1472=BA F) as might happen
in an open firing, there is no chemical change so no problem with
spalling and the dreaded "Lime Pox". Take the temperature up to cone
04 and expect problems. Achieve full vitrification and there is a
possibility the ceramic will survive.
An interesting historical aside :- In ancient times, before the
invention of gunpowder and similar compounds, this chemistry was used
to mine hard rock. Tamper Drills would be used to cut holes in a line
along the face which was being won. they were filled with"Quick Lime"
which was tamped into place. Water was poured onto the QL and the
miners waited for the inevitable fracture. More effective than wooden
wedges.
Hope that gives everyone a good enough picture.
Best regards,
Ivor Lewis. Redhill, South Australia
----- Original Message -----
From: "Bruce Girrell"
To:
Sent: Friday, 7 May 2004 12:58
Subject: Re: Shell temper and lime pops
> Ivor wrote:
> > ...This effectively locks the Calcium oxide up and so there are no
lime
> pops.
>
> OK, so you've explained why no lime pops occur for the shells, but
now why
> _do_ we (some people anyway) get lime pops at all? The same reaction
should
> take place whether the lime came from aragonite in shells or from
any other
> source, should it not?
>
> Bruce "pop goes the weasel" Girrell
>
>
______________________________________________________________________
________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your
subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
melpots@pclink.com.
Ivor and Olive Lewis on sun 9 may 04
Hello Logan,
Margaret Tuckson did a lot of work in this direction with pottery form
Papua New Guinea. Presented a paper on the topic at the 4th NCC in
Melbourne 1985.
Best regards,
Ivor
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