Tom Buck on sat 14 oct 00
Chris:
I will offer my take on this, as I am sure John H will too.
The common glass we see around us is a melt of silica (silica sand
usually) fluxed by sodium oxide (from sodium carbonate, soda ash) and
calcium oxide / lime (from limestone). The process yields a quartz-like
solid (actually a super-cooled liquid ...but that's another story) with
14% sodium oxide and 5% CaO incorporated into the glass matrix. When
formed from the raw materials, the glass has a distinctive melt
temperature (Cone), and it is 1090 oC 2000 oF. Once formed and shaped it
likely will remelt even lower. As many potters have learned, container /
bottle glass is rather incompatible with pots formed from clay... the
glass, ground into "cullet", will most times melt and run off the pot
surface before the claybody reaches maturity. To offset this behaviour,
you add alumina to the cullet and get a "glaze".
So, to be happy with the claybody, the glaze mixture needs alumina
and silica in its molecular makeup. The question is: How much of each
along with the flux oxides to melt them?
Since the days of Hermann Seger in Germany in the late 1800s, many
ceramic scientists and engineers have used his formulas, and explored many
aspects of alumino-silicate melts. The technical literature (see ACers
archives) is full of reports on various types of glass/glaze. As a result
of this accumulation of hard data on test results, there has been
established a "standard" of sorts for glazes... this standard is called
"Limit Table(s)" for the range of firings used to bring claybodies to
maturity. Limits are not Ten Commandments, they are excellent guidelines
which a resourceful potter will follow to produce durable food-safe
pottery, the essential activity of industrial potteries around the world.
The usual Seger Formula is written thusly:
1 mole flux oxides to x moles alumina to y moles silica.
This flux unity approach allows direct comparison of recipes mixed from
differing raw materials.
Let us not go into the complex behaviour of a few special oxides
that will form glass under special conditions -- these oxides include
Boric Oxide, Phosphorus Oxide, Titanium Oxide. Instead, let us stay
strictly with the 1:x:y Seger type glaze. And look up in the Limit Table
what the variations can be:
Below 1000 oC 1800 oF, a successful glaze needs a lot of Boric
Oxide and 0.1-0.15 moles of alumina (Al2O3) and 1.25 - 2.0 moles of
silica (SiO2). If you fire this much above 1000 oC it will run like crazy.
At Cone 06, the glaze still needs lots of B2O3 and the alumina is
0.1 - 0.25 moles and the silica is 1.5 - 2.5 moles.
At Cone 6, the Al2O3 should be 0.3+ moles; SiO2 3.3-3.5 moles.
At Cone 10, the Al2O should be 0.5+ moles and SiO2 5+ moles.
Why these Cones? Because claybody designers like Ron Roy formulate
their clays to become mature (2% porosity or less) at these cones. So, the
glaze has to follow suit to be compatible.
Ok, now we have the background to say what happens when we use as
lot more or a lot less Al2O3.
Take C6 mixes. as the Al2O3 falls below 0.3 moles, the glaze
becomes more mobile (less viscous, lower viscosity), and it will run badly
as the Al2O3 mole content approaches 0.2, and below that level we start to
see behaviour much like container glass. Let's now increase the Al2O3
level above 0.3: at 0.35 the alumina content is enough to raise the
viscosity markedly and the glossy glaze edges into satin matt; if we go
higher again, heading for 0.5 moles, the glaze will not fully melt and we
will get an "immature" dry matt, a glaze that can be then be rendered
mature by adding significant amounts of Calcium Oxide (whiting)...it seems
two glasses then exist on the surface of the pot, the expected flux oxides
alumino-silicate glass and a calcium aluminate type glass. But the
CaO.Al2O3 mix has to be kept sufficiently low (as part of the overall
recipe) to prevent severe discontinuities from occurring.
So, in summary, if we take the alumina too low, we lose the glaze
off the pot. If we take the alumina too high, the glaze surface starts to
break up and will be unacceptable for foodware, the bread and butter
product for many studio potters.
Ok, Chris. Does this make sense to you?
til later. 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
John Rodgers on sat 14 oct 00
Tom,
In the darkness of my miniscule knowledge of the realm of clay, you have again,
through your message to Chris, provided me with illumination!
Thanks!!
John Rodgers
Birmingham, AL
Tom Buck wrote:
> Chris:
> I will offer my take on this, as I am sure John H will too.
>
> The common glass we see around us is a melt of silica (silica sand
> usually) fluxed by sodium oxide (from sodium carbonate, soda ash) and
> calcium oxide / lime (from limestone). The process yields a quartz-like
> solid (actually a super-cooled liquid ...but that's another story) with
> 14% sodium oxide and 5% CaO incorporated into the glass matrix. When
> formed from the raw materials, the glass has a distinctive melt
> temperature (Cone), and it is 1090 oC 2000 oF. Once formed and shaped it
> likely will remelt even lower. As many potters have learned, container /
> bottle glass is rather incompatible with pots formed from clay... the
> glass, ground into "cullet", will most times melt and run off the pot
> surface before the claybody reaches maturity. To offset this behaviour,
> you add alumina to the cullet and get a "glaze".
> So, to be happy with the claybody, the glaze mixture needs alumina
> and silica in its molecular makeup. The question is: How much of each
> along with the flux oxides to melt them?
> Since the days of Hermann Seger in Germany in the late 1800s, many
> ceramic scientists and engineers have used his formulas, and explored many
> aspects of alumino-silicate melts. The technical literature (see ACers
> archives) is full of reports on various types of glass/glaze. As a result
> of this accumulation of hard data on test results, there has been
> established a "standard" of sorts for glazes... this standard is called
> "Limit Table(s)" for the range of firings used to bring claybodies to
> maturity. Limits are not Ten Commandments, they are excellent guidelines
> which a resourceful potter will follow to produce durable food-safe
> pottery, the essential activity of industrial potteries around the world.
> The usual Seger Formula is written thusly:
> 1 mole flux oxides to x moles alumina to y moles silica.
> This flux unity approach allows direct comparison of recipes mixed from
> differing raw materials.
> Let us not go into the complex behaviour of a few special oxides
> that will form glass under special conditions -- these oxides include
> Boric Oxide, Phosphorus Oxide, Titanium Oxide. Instead, let us stay
> strictly with the 1:x:y Seger type glaze. And look up in the Limit Table
> what the variations can be:
> Below 1000 oC 1800 oF, a successful glaze needs a lot of Boric
> Oxide and 0.1-0.15 moles of alumina (Al2O3) and 1.25 - 2.0 moles of
> silica (SiO2). If you fire this much above 1000 oC it will run like crazy.
> At Cone 06, the glaze still needs lots of B2O3 and the alumina is
> 0.1 - 0.25 moles and the silica is 1.5 - 2.5 moles.
> At Cone 6, the Al2O3 should be 0.3+ moles; SiO2 3.3-3.5 moles.
> At Cone 10, the Al2O should be 0.5+ moles and SiO2 5+ moles.
> Why these Cones? Because claybody designers like Ron Roy formulate
> their clays to become mature (2% porosity or less) at these cones. So, the
> glaze has to follow suit to be compatible.
> Ok, now we have the background to say what happens when we use as
> lot more or a lot less Al2O3.
> Take C6 mixes. as the Al2O3 falls below 0.3 moles, the glaze
> becomes more mobile (less viscous, lower viscosity), and it will run badly
> as the Al2O3 mole content approaches 0.2, and below that level we start to
> see behaviour much like container glass. Let's now increase the Al2O3
> level above 0.3: at 0.35 the alumina content is enough to raise the
> viscosity markedly and the glossy glaze edges into satin matt; if we go
> higher again, heading for 0.5 moles, the glaze will not fully melt and we
> will get an "immature" dry matt, a glaze that can be then be rendered
> mature by adding significant amounts of Calcium Oxide (whiting)...it seems
> two glasses then exist on the surface of the pot, the expected flux oxides
> alumino-silicate glass and a calcium aluminate type glass. But the
> CaO.Al2O3 mix has to be kept sufficiently low (as part of the overall
> recipe) to prevent severe discontinuities from occurring.
> So, in summary, if we take the alumina too low, we lose the glaze
> off the pot. If we take the alumina too high, the glaze surface starts to
> break up and will be unacceptable for foodware, the bread and butter
> product for many studio potters.
> Ok, Chris. Does this make sense to you?
> til later. 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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