John Baymore on wed 29 oct 97
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Hi all.
This is a copy of a private e-mail I sent off list to a person with whom I
was conversing who had questions about soluble materials and integrating
them into predictions from glaze calc and Seger formulas and the like. It
is pertinent to the discussion of carbon trapping, so I thought I'd post it
up here. Edited a couple things out....like the name to protect the
innocent =3Cg=3E.
---------- Forwarded Message ----------
From: John Baymore, 76506,3102
TO:
DATE: 10/17/97 10:42 PM
The soluble materials issue is a very intertesting one, and one that you
can't take into account in general Seger format chemistry calculations
(except by guestimates). The only way to know is by serious testing like
chemical analysis of the glass areas, electron microscopy and the
like...... way out of the craft potters field. That's why God created
ceramic engineers and scientists =3Cg=3E.
There are a lot of variables that Seger calculation doesn't take into
account.
First, lets take an oxidation glaze fired in the =5E9 range with minimal
soluble materials and look at it. Picture the glaze and body sliced in
two, and you are looking side edge on in a magnified XC view. Glaze
surface is =22up=22 and clay body layer is =22down=22.
On the outer surface of the glaze (top), the composition is pretty much
what you would calculate out. As you move toward the body the composition
remains pretty constant for a while. Then you get near the body and the
composition changes as the effects of the materials in the body start to
appear. Crystals developed from the body grow into the glaze, the glaze
erodes into the body, bits of melting spar and silica from the body bleed
into the glaze, and so on. So the composition in this =22interface=22 or
=22boundary=22 layer is different from that which is predicted by calculated
formulas. The change is gradual.... not like a distinct line or plane. In
some area of the slice, it is hard to tell where the glaze stops and the
body begins. Then you truly reach the body (down).
As a broad generalization, this effect is less and less pronounced as the
firing temperature goes lower. It is also greatly dependant on the exact
composition of the body. This is why glazes can look SO different on
differing clay bodies, and why defects can occur on some bodies and not on
others. A body that has a lot of =22glassing materials=22 will have more of
these bleed into the glaze. A body that develops lots of mullite will tend
to grow more crystals into the galze. And so on.
If the body formula contains a chemical that has some unique properties,
those properties will tend to then affect the glaze too. For example......
if a body contains spodumene supplying lithium oxide as a flux, that
lithium will tend to cause some affects promoted by lithium into the glaze
layer, and can change things like CTE or color rendition with certain
colorants. Ditto for many others, including the common Fe2O3.
Now lets change the firing atmosphere to reduction.
On the outer surface the glaze =22skin=22 (to a depth that is determined by =
the
permeability of the glaze to oxygen during the reactive period during the
cooling cycle) now will have a composition like that calculated, except
that all or most =22reduceable=22 oxides (like Fe2O3) will have be in the =
most
stable oxidized state (iron will be red not grey or black). Note that
this is assuming you cool the kiln in an oxidising atmosphere. Below this
skin layer, the glaze materials will be in the reduced state, and will have
acted as they do in the melt when considered by their reduced formulas
(iron will be gery or black and will have acted like a flux).
The interface layer will still be there, but compounds containing
reduceable materials in the body will have affected the amount of and color
rendition of this layer. Compared to the oxidized sample previously
looked at, this interface layer will be different in composition and
extent.
Now let's add some soluble soda ash to the glaze batch to supply all of the
Na2O (and adjust the rest of the composition to remain constant
chemically).
The soda ash is completely soluble in water in the concentrations typically
used in glazes. (Try mixing soda ash into straight water by itself.) So
it will be in solution, not suspention, in the raw glaze batch. It is
evenly disbursed in the mixture. Now we dip in a bisque pot and get a
glaze layer. The suspended materials form a layer on the bisque, but the
water tends also to penetrate into the bisque....... taking some disolved
soda with it.
Now, the water has to evaporate out of the pot (either before being placed
into the kiln, or during the early stage =7Bup to 212F=7D of the firing). =
As
the water migrates toward the outer surface to evaporate, the soluble soda
goes with it. But the soda can't go with the water when it changes from
the liquid to the vaporous state, so it is left behind in and on the
outermost surface of the dry glaze layer.
If you were to scrape off a thin section of the outermost layer of glaze
and analyze it with very sophisticated equipment, you would find that the
content of sodium in that part is FAR greater than that you would find in
the layer of glaze powder just below it. That sodium will act as Na2O in
the melting glaze, a powerful flux. If you then plugged the high Na2O
sample analysis into a Seger formula, you would find a completely different
molecular formula than that calculated EITHER for the original glaze as a
whole, OR from that obtained from analysis of the more interior parts of
the dry glaze layer. You would find that when compared to typical limit
formulas, the different layers may and probably will fall into different
cone ranges from the original batch formula.
So the outermost layer is melting at a much lower temperature than the rest
of the glaze due to the high soda content. This layer is (like the
interface layer) not a distinct plane.... but a zone. It will react with
things like reduction in the manner predicted for a glaze of that cone
range and at that stage in the melting process. For example, this is why
true shino (sodium fluxed, not lithium fluxed) glazes are quite shiny in
the kiln at very low temperatures and why they =22carbon trap=22 so readily.
I know of no real way to accurately predict this or take it into account in
a caluclated formula. If you know you have soluble materials, you know
this will happen...... the exact extent can only be determined by testing,
as far as I know. It is complicated. For example, you also have to know
how much of the available soluble in the raw material will actually go into
solution at the time you use the glaze.......... this varies with the PH of
the water used to mix the glaze, the time it is mixed up, the particle size
and surface area of the soluble or partially soluble compound, and possible
interactions with other chemistry in the glaze or water.
In the case of my own shino glaze, I know the formula of the glaze with the
soda ash completly removed, and also the batch with the soda ash
concentrated into 1/4 of the remaining batch. This is just a guess.....
nothing accurate.
If you look at the fired glaze cross section again, you have the added
variable that the composition of the outermost skin is not only affected by
the cooling atmosphere (usually oxidized) but it also will have a slightly
different composition from the mid-layer of the glaze.
So...... hope this is of some help for you. Gotta go make pots =3Cg=3E.
Best,
.......................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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