I.Lewis on mon 31 jan 00
------------------
Quartz and cristobalite problems.
Have others on the list noticed that problems associated with the changes =
which
occur to Silicon dioxide continue to cause concern because of their effects =
on
glaze/clay fit causing unwanted crazing and on the destruction of pottery
because of dunting. Given that inexperience rather than ignorance is one =
cause
of disaster and dispair, and I have had my fair share for pots going off
sounding like 3030 rifle fire, can someone explain why quartz or flint =
continue
to be a favourite additives to clay, especially the coarse varieties which =
give
feel and tooth when throwing. Is it because flint is cheaper than other more
suitable materials and ensures a better profit margin for manufacturers or =
are
alternatives unobtainable in sufficient quantities to make milling =
commercially
feasible. Could suggestions of suitable materials be made for people who are
prepared to hand crush and grind small quantities to make their own clay =
bodies.
Inquisitive as usual.
Ivor Lewis.
Ron Roy on wed 2 feb 00
Hi Ivor,
Good questions in my opinion. The role of quartz in bodies has been
generalized I think. There are a number of factors which we have not
taken into account.
You have heard me talk about Peter Songen's experiments and his
published results in the current Studio Potter magazine. I believe
his work on the subject will help us all in our attempts to make well
behaved clay bodies.
I don't want to take too long with this now but we are dealing with a
number of factors.
Micro fine silica - grinding machinery has become much more efficient
and ultra fine silica promotes cristobalite at our higher
temperatures in many stoneware bodies.
Large size (mesh) silica does not easily (at least 200 mesh down)
convert to cristobalite.
Many of our glazes have low expansions. This can be a problem even if
there is no quartz (I have never seen a body with none) or
cristobalite - and the problem becomes worse as the cristobalite and
free quartz increases.
Some authors recommend having some silica "to help fit" This is good
advice sometimes because the quarts and cristobalite inversions can
help to control crazing - but in other cases - causes the opposite
result - dunting.
By all means at least get a copy of this issue - and as Richard Aerni
has posted - a look at David Shaner - on of my favorite potters.
All this points to the wisdom of using glaze calculation software for
at least getting some useful expansion information about the glazes
we use and encouraging clay body manufactures to supply such
information about their products.
RR
>----------------------------Original message----------------------------
>------------------
>Quartz and cristobalite problems.
>
>Have others on the list noticed that problems associated with the
>changes which
>occur to Silicon dioxide continue to cause concern because of their effects on
>glaze/clay fit causing unwanted crazing and on the destruction of pottery
>because of dunting. Given that inexperience rather than ignorance is one cause
>of disaster and dispair, and I have had my fair share for pots going off
>sounding like 3030 rifle fire, can someone explain why quartz or
>flint continue
>to be a favourite additives to clay, especially the coarse varieties
>which give
>feel and tooth when throwing. Is it because flint is cheaper than other more
>suitable materials and ensures a better profit margin for manufacturers or are
>alternatives unobtainable in sufficient quantities to make milling
>commercially
>feasible. Could suggestions of suitable materials be made for people who are
>prepared to hand crush and grind small quantities to make their own
>clay bodies.
>
>Inquisitive as usual.
>
>Ivor Lewis.
Ron Roy
93 Pegasus Trail,
Scarborough, Ontario
Canada. M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Stephen Grimmer on wed 2 feb 00
Ivor,
You know the expression about too much of a good thing. Many bodies
benefit from the addition of fine flint (silica powder) to help alleviate
crazing of glazes. Most stoneware bodies need little, if any, as the clays
used in the body often add a fair quantity of free silica. Porcelains (of
the kaolin/ball clay/spar/flint variety), on the other hand, require
anywhere from 20-30%, depending on the ball clay and kaolin. Added flint in
this case also helps give the body 'bones,' both on the wheel and in the
fire. In addition, the flint acts as a filler, which helps the clay dry
evenly and with minimal warpage, and helps to whiten the fired clay.
Some alternatives to adding so much flint to a body for filler:
Pyrophyllite: Lowers the thermal expansion compared to flint, whitens the
clay, and makes for a "tough" clay, especially in the dry state. You could
substitute as much as 50% of the flint with Pyrophyllite.
Alumina: Whitens the clay, lowers thermal expansion, can cause some
deflocculation. Can use up to 5% (?? can anyone add to this?)
If your clay is dunting like a 30-06 rifle, then you probably need to add
feldspar and cut the flint some.
Here's a general method to arrive at a workable potting porcelain:
Start with equal parts kaolin, white ball clay, feldspar, and flint. A good
addition is 1.5% VeeGum T for plasticity.
Reduce ball clay and increase kaolin until lowered plasticity becomes
unacceptable. Ideally, you use no ball clay for a white, translucent clay.
Increase feldspar to get absorption below 1%.
Increase flint and fillers to eliminate crazing and slumping.
Try to diversify your kaolins to reduce the effects of material variation
and increase particle size distribution. Try to minimize use of kaolins that
are high in titania, which causes opaqueness and grayness. It's a worse
contaminant than iron!
There's some good information on Tony Hansen's Digital Fire sight
http://www.digitalfire.com/
steve
--
Steve Grimmer
Bowling Green State University
Bowling Green, OH
----------
>From: "I.Lewis"
>To: CLAYART@LSV.UKY.EDU
>Subject: Quartz and cristobalite problems
>Date: Mon, Jan 31, 2000, 1:28 PM
>
> ----------------------------Original message----------------------------
> ------------------
> Quartz and cristobalite problems.
>
> Have others on the list noticed that problems associated with the changes
which
> occur to Silicon dioxide continue to cause concern because of their effects on
> glaze/clay fit causing unwanted crazing and on the destruction of pottery
> because of dunting. Given that inexperience rather than ignorance is one cause
> of disaster and dispair, and I have had my fair share for pots going off
> sounding like 3030 rifle fire, can someone explain why quartz or flint
continue
> to be a favourite additives to clay, especially the coarse varieties which
give
> feel and tooth when throwing. Is it because flint is cheaper than other more
> suitable materials and ensures a better profit margin for manufacturers or are
> alternatives unobtainable in sufficient quantities to make milling
commercially
> feasible. Could suggestions of suitable materials be made for people who are
> prepared to hand crush and grind small quantities to make their own clay
bodies.
>
> Inquisitive as usual.
>
> Ivor Lewis.
Peter Jones on thu 3 feb 00
Is there a glaze/clay calculation software program out there that people use
which is freeware/shareware? I'd like to find one if there is. Any
suggestions would be appreciated.
Peter Jones
Stephen Grimmer on fri 4 feb 00
Peter
Insight is available for as a 30 day trial download from
http://www.digitalfire.com/ which expires in 30 days. It is, unfortunately,
no longer being developed for the Mac platform. If you run Windoze, you're
all set. Purchase price is reasonable, and it's pretty fast.
steve
--
Steve Grimmer
Bowling Green State University
Bowling Green, OH
----------
>From: Peter Jones
>To: CLAYART@LSV.UKY.EDU
>Subject: Re: Quartz and cristobalite problems
>Date: Thu, Feb 3, 2000, 12:19 PM
>
> ----------------------------Original message----------------------------
> Is there a glaze/clay calculation software program out there that people use
> which is freeware/shareware? I'd like to find one if there is. Any
> suggestions would be appreciated.
>
> Peter Jones
Mary Ella Yamashita on fri 4 feb 00
Peter-- For a comparison of glaze calc programs, including info about some
shareware, see Rick Malgrem's article in Ceramics Monthly, June/July/Aug
1998, p. 38. You can also do glaze calc for free on the net with
Interglaze at http://www.sonic.net/~wang/interglaze.html (this program has
also been available at the sdsu site, but I think it is temporarily down).
Hope this helps--Mary Ella
>Date: Thu, 3 Feb 2000 13:19:35 EST
>From: Peter Jones
>Subject: Re: Quartz and cristobalite problems
>MIME-Version: 1.0
>Content-Type: TEXT/PLAIN; charset=US-ASCII
>Content-Transfer-Encoding: 7BIT
>
>----------------------------Original message----------------------------
>Is there a glaze/clay calculation software program out there that people use
>which is freeware/shareware? I'd like to find one if there is. Any
>suggestions would be appreciated.
>
>Peter Jones
>
Michael Banks on sat 5 feb 00
Quartz and flint are actually of great utility in formulating ceramic bodies
from purified clays and fluxes etc. It would be quite unneccessary to wipe
this useful mineral from use, as well as making it almost impossible to
formulate a clay body that had a high enough expansion to fit most glazes.
The problems you speak of, generally arise in naturally dug clays that have
unusually large crystalline quartz contents or a bad balance of particle
sizes which promote dunting.
It is the responsibility of clay companies who dig and market the natural
stonewares and earthenware clays (in contrast to the "manufactured" white
blends), to not sell clays having troublesome quartz contents.
Quartz is one of the most common minerals on the face of the earth and it
would be uneconomic, as well as detrimental to screen it out of clay. The
coarse mesh sizes of quartz grains, say 30-80 mesh (180-500 microns) are
rarely a cause of dunting (cracking), unless the rest of the body is
woefully short of sintering strength. Such coarse quartz usually inhibits
cracking because it interferes with crack propagation (grog effect). Very
fine quartz, say 200-400 meh and beyond (75-38 micron and smaller) will be
consumed by fluxes during firing and removed from contention.
A high proportion of quartz in the intermediate range between say 100 to 200
mesh (75-150 microns) can cause body problems. This size fraction has little
grog effect because cracks can get around the grains and is incompletely
taken into fusion by fluxes because it is too coarse. Over-abundance of
this fraction can be big trouble when:
1. There is insufficient coarser particles present to inhibit crack
propagation
2. There are insufficient fluxes present
3. Sintering/fluxing is weak for any other reason
4. Firing is too short to build body strength
5. There is insufficient fine quartz (sub 75 micron) present to assist
eutectic fusion
It pays potters to be selective in the clays they buy, or dig and not
persist with rubbish that result in cracked pots and wasted effort.
Michael Banks,
Nelson,
New Zealand
----------------------------Original message----------------------------
------------------
>Quartz and cristobalite problems.
>Have others on the list noticed that problems associated with the changes
which
>occur to Silicon dioxide continue to cause concern because of their effects
on
>glaze/clay fit causing unwanted crazing and on the destruction of pottery
>because of dunting. Given that inexperience rather than ignorance is one
cause
>of disaster and dispair, and I have had my fair share for pots going off
>sounding like 3030 rifle fire, can someone explain why quartz or flint
continue
>to be a favourite additives to clay, especially the coarse varieties which
give
>feel and tooth when throwing. Is it because flint is cheaper than other
more
>suitable materials and ensures a better profit margin for manufacturers or
are
>alternatives unobtainable in sufficient quantities to make milling
commercially
>feasible. Could suggestions of suitable materials be made for people who
are
>prepared to hand crush and grind small quantities to make their own clay
bodies.
>Inquisitive as usual.
>Ivor Lewis.
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