Ceramic Design Group on sun 18 feb 01
on 2/3/01 11:11 PM, iandol at iandol@tell.net.au wrote:
Dear Jonathon,
You made the following comment in a post recently, in reference to some of
your glazes:-
provides suitable eutectics for proper melt.>.
I, and I am certain other subscribers would, would appreciate an fuller
explanation of what is happening here. Or are you saying that Gersley borate
forms a fluid which has a greater solvent action on higher melting point
materials.
Best regards,
Ivor Lewis. Redhill, South Australia
I think that there are many things happening when glazes are layered and
there are some criteria:
1. Using 2 glazes, which one overlaps the other one? And what are the
results?
2. If item #1 is done in reverse, the results are usually quite different.
3. The specific gravity of each glaze
4. Maintaining the SG of the glaze but varying the thickness of the glaze on
glaze
5. The firings temperature
6. The atmosphere of the kiln
7. The rate of heating
8. The rate of cooling
9. The type of clay body that the glazes sit on
10. The type of glazes used and their component oxides
11. And I am sure many more that I have neglected and that others on the
list can add
While it is possible to observe phase diagrams and their resulting fired
results for any one glaze, I am sure that in layering multiple glaze layers,
phase diagrams are relatively useless. I would submit that really the only
way to get an accurate picture of the layering effects of glazes is to test
your 2 glazes, making sure that the SG of the glazes are the same for each
firing and that your application techniques are not only exact but
reproducible, an that your firing environment is also the same and also
reproducible.
I think that the chemistry of the reactions between layered glazes is
certainly explainable, but using either Seger formulae or any other
quantifiable method cannot predict with any degree of accuracy what really
is going on in the melts as well as between the melts. While glazes are
complex silicates, using multiple layers of glaze results in even more
complex silicates!
I think that in the case of the glazes that we use ( and these 2 blue glazes
at cone 10 reduction are available from me by sending a SASE) they work well
together because of several things. By themselves they are well balanced
glazes. They also contain many of the same ingredients, but also differ from
each other. The combination of rutile, iron oxide, and gerstley borate in a
reduction atmosphere that in my experience can produce beautiful colors and
surfaces that are loaded with visual activity as well as heartbreaking
pinholing, blistering, zitting, excessive running, and many more, work
successfully with its companion glaze bases. These are gloss glazes and when
they are layered, I know that I can always count on a very successful and
complete melt with no surface defects, fired in our kilns within the same
application techniques . I think that there are multiple eutectics
happening, all beneficial. There is an adequate environment for the
component oxides to exhibit crystallization, to increase surface activity,
as well as to enter into a successful and stable melt. And yes, I think that
the GB does indeed have some solvent effect in the melt. What this is
exactly and how it works eludes me. I also think that what you refer to as a
"solvent action" only a small part of the entire picture. And with GB, there
was no chemical predictability from batch to batch. We know that there was
some boron present, some clay type materials, lots of associated gangue (or
garbage material). Now that this material is no longer mined or available, I
know you probably have followed the trail of those companies that are
marketing substitutes. I also know that that layering effects can be quite
beautiful and successful with many other glazes, some with no GB at all.
Fortunately, I have a nice supply of GB from the same lot.
If I fire each of the glazes separately, the results are quite predictable.
They are both blue gloss glazes. If I layer glaze A over Glaze B, I get the
wonderful results that we need. In reverse, it does not happen. If I mix
equal parts of A and B together in a wet state at the same SG, the results
are different than the ones described earlier.
I have used layering techniques with matte glazes and glossy glazes and many
combinations between them. I have also tried combining them as above in a
50/50 mix, and the results are all together different.
Three glazes and be combined in a tri-axial in such a way also.
One of our most excellent glazes that we use of a number of clients is the
result of a 50/50 mix of a gloss iron saturate a a matte base containing a
small quantity of copper and cobalt fired to cone 4. The resulting fired
glaze is a glaze full of surface activity with blues and greens, no
pin-holing, and with a band of Barnard on the rim shows remarkable rivulets
of multicolored glaze. It is truly a spectacular glaze. Layering of these 2
glazes separately does not produce any of the before mentioned effects.
I would also suppose that one could spend a lifetime of work using just a
few glazes and blending them in various ways producing a palette of quite
useable glazes. Combining this technique with using a sprayed slip glaze
underneath or over, sprayed oxides or stains. the results can be quite
spectacular.
There are of course, so many exceptions to this and these techniques are
also an exception to the rule. And there are glazes that cannot be used in
this way. I think a way to start would be to find 2 glazes firing at the
same temperature and atmosphere, and test them in combinations with
themselves as in a bi-axial, then layer them using the bi-axial reults if
acceptable, and then perhaps using a slip glaze underneath, or over,
spraying pigments over or under, whatever.
The point being that there are no correct ways to work with such a
procedure. My results were solely the result of empirical testing. I think
we lucked out with the few glazes that we use. And yes, there have been more
failures then successes, but I am quite pleased with the several successes
that happened.
I hope this provided some explanation to your question. Thanks for your
inquiry!
Best
Jonathan
Jonathan Kaplan, president
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
voice and fax 970 879-9139
jdkaplan@cmn.net
info@ceramicdesigngroup.net
Plant Location:
1280 13th Street Unit 13
Steamboat Springs CO 80487
(please use this address for all deliveries via UPS, comman carrier, Fed Ex,
etc.)
"Custom design and manufacturing for the ceramic arts, giftware and pottery
industries. Molds, models, and tooling for slip casting, jiggering and
hydraulic pressing. Consultation on technical issues such as clay bodies
glazes, and kilns."
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