Arnold Howard on sun 1 aug 99
It is a common belief, in hobby ceramics, that lead in glaze helps the
glaze to fit the clay body. Therefore, unleaded glazes craze more than
the older leaded glazes. I don't see how lead could help with
glaze/clay fit. What is your opinion?
Arnold Howard
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Tom Buck on tue 3 aug 99
I suspect Mr Howard is just waving the red flag at glaze calc programs,
and hoping someone will charge.
Being solid, fired clay will expand and contract with variations
in temperature. How much the body changes in size is a function of several
factors. But in vague general terms, a porcelain body will expand/contract
5x10-6 (5 one-millionths) in length units per unit of length per degree
Celsius; a stoneware body (nearly vitrified) will show perhaps 6x10-6; and
a terracotta (earthernware) body could be anywhere below or above these
two, depending on its firing history.
People who do science studies of ceramic materials/systems have
demonstrated that the common ingredients in the glass we call glaze also
have expansion/contraction properties, and the individual oxide in the
glass of a glaze will contribute a specific value of expansion or
contraction, that these individual values combine together (their sum) to
yield the "coefficient of thermal expansion, linear" (aka COE in North
America) for a specific fired glaze.
If on cooling, the glaze contracts more than the body, cracks will
likely appear should the difference between the body's contraction and the
glaze's contraction rise above a certain value (I usually set this at
one-millitonth of a length unit). Likewise, if the body shrinks much more
than the glaze, at some point the glaze will bunch up and "shiver", that
is, burst off the pot's surface.
So, what's so good about PbO (lead oxide) as a lowfire flux
oxide.
The literature usually cites Coefficients of Thermal Expansion for
the various oxides in these units: change in length (delta) L per Length
per degree Celsius (Centigrade), and the size is one-millionth of a length
unit (any), or 10 to the minus sixth power, 10-6, or sometimes 10E-6.
Others sometimes quote the values 10 to 100 times larger by changing 10-6
to 10-7 or 10-8. The actual expansion/contraction has not changed, just
the way we describe it. (NB: measuring COEs is prone to error, so values
vary in the literature).
Let's use 10-7 this time. One set of Values for the flux oxides
that I use is as follows:
CaO 1.48; MgO 0.62; K2O 3.31; Na2O 3.87; ZnO 0.94; Li2O 1.0;
PbO 0.83, B2O3 0.31; for comparison: Al2O3 0.63; SiO2 0.35.
Since it has the lowest COE value for what may be considered the
"standard" flux oxides, then PbO will help to keep a glaze
expansion/contraction low, a desirable property in lowfire pottery.
Boria, boric oxide, B2O3 is a special case. Up to a certain value
in a glaze mix (most think it is 10 mole per cent of a glaze's
composition), Boria behaves as a good low-cone flux, and thereby can
displace Lead Oxide as the low COE flux oxide. But above this 10 mol%,
B2O3 changes its "spots" (crystal pattern), and exhibits quite a high COE.
So one cannot say with assurance that B2O3 will always provide a good
low-cone glaze for earthenware.
Here endth the discourse today. Bye.
Tom Buck ) tel: 905-389-2339 (westend Lake Ontario,
province of Ontario, Canada). mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
Linda Arbuckle on wed 4 aug 99
> From: Arnold Howard
>
> It is a common belief, in hobby ceramics, that lead in glaze helps the
> glaze to fit the clay body. Therefore, unleaded glazes craze more than
> the older leaded glazes. I don't see how lead could help with
> glaze/clay fit. What is your opinion?
>
> Arnold Howard
>
Historically the choice was between lead glazes and alkaline glazes for
lowfire. Thermal expansion of lead is moderate, while thermal expansion
of sodium and potassium (alkaline fluxes that melt well at low-fire
temperatures) is high. This means sodium and potassium expand a
relatively large amount upon heating and contract a similar amount on
cooling. If the glaze shrinks more than the body, you get crazing.
Alkaline blue glazes are an example of a lowfire transparent glaze that
is unleaded (and it crazes from high sodium/potassium).
Fortunately, many glaze materials form eutectics, melting lower in
specific combinations, so that lowfire glazes are not limited to only
those materials that melt by themselves at lowfire temperatures. Boron
is another material that works as both a flux and viscosity agent at
lowfire temperatures.
Another big problem with crazing of lowfire glazes is delayed crazing.
Lowfire bodies don't usually vitrify (i.e. become fused enough to hold
water w/o a glaze as some stoneware and porcelains do), and the body can
absorb water thru the foot, swell slightly, and cause delayed crazing.
Using terra sigillata on the foot helps reduce porosity and counter this
problem.
Firing a bit higher, say to 03, also helps, as you just reach the
temperature where cristoballite is formed from free silica in materials.
Cristoballite inverts (becomes about 2% smaller) around the temperature
paper burns, 451 F. A bit of cristoballite is good for lowfire as that
inversion in the clay body puts a glaze under a bit of compression and
counters crazing. Too much cristoballite shrinkage of the body (more of
a problem for high-fire potters), and you get dunting, (the piece cracks
in cooling) and maybe shivering (the glaze pops off the pot in cooling).
It's difficult to make a lowfire glaze that will NEVER craze, but it's
possible to cut down on the problem quite a bit. Thermal shock
aggravates crazing (e.g. making iced coffee by pouring hot coffee over
ice cubes in the cup), as does dishwasher-ing and microwaving. I do
microwave tea in my terracotta cups. They don't seem to heat up any more
than some stoneware cups I have.
Back to the lead issue.... lead and the alkaline fluxes (potassium,
sodium, lithium) make a soft glaze that is not very durable. If you look
at old lead or alkaline glazes that have been exposed to weathering,
they begin to break down. Adding alkaline earth fluxes like calcium,
magnesium, or strontium (barium, too, but it's toxic) contribute to a
stronger glaze. Issue is that these would also stiffen up the glaze
some, and probably make is more toward frosty if thick than transparent
like lead or alkaline glazes. The majolica glaze I use is high in Ca for
a lowfire glaze, very durable, fired to 04-03, lead free.
--
Linda Arbuckle
Graduate Coordinator, Assoc. Prof.
Univ of FL
School of Art and Art History
P.O. Box 115801, Gainesville, FL 32611-5801
(352) 392-0201 x 219
e-mail: arbuck@ufl.edu
Stephen Grimmer on thu 5 aug 99
Tom,
Thanks for the good read. Unfortunately, I seem to have missed all the
lectures concerning mole percent. I'm guessing that its the oxide in
question's percent of the whole amount of molecular equivalents in a given
glaze? It's different than the percent by weight, right?
Oh boy, so much more to learn.
steve grimmer
marion, illinois
----------
>From: Tom Buck
>To: CLAYART@LSV.UKY.EDU
>Subject: Re: A question about unleaded glaze
>Date: Tue, Aug 3, 1999, 1:04 PM
>
>Up to a certain value
>in a glaze mix (most think it is 10 mole per cent of a glaze's
>composition), Boria behaves as a good low-cone flux,
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