Greg Relaford on sun 15 apr 12
A couple of places to look for Guidance:
I attended the recent Steven Hill workshop in Seattle. He uses spray-on
glaze layers to create some great effects. From a distance, it looks like
wood-fire. At cone 6 electric ...
He has some specific techniques that could help you a great deal. He uses
'zones' of glaze sprays, with specific characteristics for each zone. I
say, take a riff off what he is doing, and make your own 'system'.
http://stevenhillpottery.com/StevenHillPottery/Home.html
There are a lot of ionic and electro-chemical processes in glaze chemistry,
so looking at the basic chemistry will almost certainly help, too. Strong
negatives attract to strong positives.....And migrate toward each other in
the liquid and semi-liquid glazes.
Finally, take a look at the research these guys have done. Very
interesting.
http://www.mattanddavesclays.com/Science/ScienceHome.html
-GregR
On Sun, Apr 15, 2012 at 6:00 PM, Randy McC wrote:
> I have been working on layering glazes for a number of years. I have
> found I can get really special effects.One way is to just layer one glaze
> over another. With this method it seems that the bottom glazewill absorb
> some of the top glaze and you will get one type of effect. You can also
> layer a glaze overa previously fired glaze and refire and even though the
> two glazes are the same as I mentioned before youwill get a totally
> different effect. My question is this.....is there any medium I can use
> between the glazes to prevent absorption of the topglaze and it still
> adhere to the bottom glaze? I hope this makes some sense............
> Randy
Randy McC on sun 15 apr 12
I have been working on layering glazes for a number of years. I have found=
=3D
I can get really special effects.One way is to just layer one glaze over a=
=3D
nother. With this method it seems that the bottom glazewill absorb some of=
=3D
the top glaze and you will get one type of effect. You can also layer a g=
=3D
laze overa previously fired glaze and refire and even though the two glazes=
=3D
are the same as I mentioned before youwill get a totally different effect.=
=3D
My question is this.....is there any medium I can use between the glazes t=
=3D
o prevent absorption of the topglaze and it still adhere to the bottom glaz=
=3D
e? I hope this makes some sense............
Randy =3D
James Freeman on mon 16 apr 12
On Sun, Apr 15, 2012 at 9:00 PM, Randy McC wrote:
I have been working on layering glazes for a number of years. I have found
I can get really special effects.One way is to just layer one glaze over
another. With this method it seems that the bottom glazewill absorb some
of the top glaze and you will get one type of effect. You can also layer a
glaze overa previously fired glaze and refire and even though the two
glazes are the same as I mentioned before youwill get a totally different
effect. My question is this.....is there any medium I can use between the
glazes to prevent absorption of the topglaze and it still adhere to the
bottom glaze?
Randy...
When you apply a glaze slurry to your pot, it dries to a coating of
individual molecules (tiny chunks of molecules and compounds, actually) of
your various ingredients. These do not all melt at the same time in the
heat of the kiln. Some of your ingredients will melt and become active at
relatively low temperatures, and these in turn act to "dissolve" the higher
melting point ingredients. Other molecules may combine to form eutectics,
or groups of molecules that melt at a much lower temperature together than
any of them would in isolation (calcium, alumina, plus silica, in a precise
ratio, for instance). Note that things are becoming active within your
glaze coat, and within the contrasting glaze coat atop it, at a point far
earlier in the firing than that at which the entire glaze becomes molten
(the maturing temperature). While they are active, they may of course
interact, and they have plenty of time to do so.
If one looks at a fired glaze, it can be thought of as a single new
compound (molecule) which you have created (again, greatly simplified).
For instance, rather than having calcium carbonate, diboron trioxide, and
silicon dioxide, each with their own unique melting points and preferred
chemical interactions, you now have a single substance, a calcium
borosilicate, say, that will have a single melting point near the maturity
temperature of the glaze (this is greatly simplified, and the actual
chemistry is beyond me). There would thus be far less time and opportunity
for a re-melted glaze coat to interact with an new unfired glaze coat
placed atop it. There will still be some interaction, as the early-melting
fluxes and eutectics within the new topcoat will attack the surface of the
underlying fired glaze, but I think you can now see why the interaction
could easily be much reduced from that occurring between two unfired glaze
coats.
In terms of a barrier between two unfired glazes, you can likely see how
this would be impractical at the very least. Even supposing such a
compound could be formulated, it would in itself interact with and alter
the character of the two glazes. Thus, you would likely merely be
substituting one interaction for another.
An easier course of action is to simply employ two glazes that interact in
the manner you wish, rather than in ways you find undesirable. You have
made many observations of how this glaze interacts with that, as have the
rest of us, and there are some very knowledgeable glaze folks on the list
who can provide you with more or less definitive rules. From my own
observations, I have seen that when a relatively stiff glaze is applied
atop a relatively active and fluid one, the fluid glaze tends to bubble
through, creating streaking, spotting, and other such localized
interactions. When the active glaze is applied atop the stiff glaze, I
tend to see more of a sheeting action, with long sweeps of mutant
"interacted" glaze. With two stiff glazes (or low-fired glaze atop a
higher-fired glaze), I often find a quite defined demarcation between the
two, while with two active glazes, I often find the necessity to scrape and
grind my kiln shelves.
There are a couple of real live chemists on the list, as well as several
glaze experts, so perhaps they will have a better explanation for you, or
better rules of thumb. However, the explanation above, while simplified,
should get you close.
Good luck in your endeavors and exploration.
...James
James Freeman
"Talk sense to a fool, and he calls you foolish."
-Euripides
http://www.jamesfreemanstudio.com
http://www.flickr.com/photos/jamesfreemanstudio/
http://www.jamesfreemanstudio.com/resources
ivor and olive lewis on tue 17 apr 12
Those who seek a scientific model for the behaviour of ceramic materials as
they are processed in the kiln might gain a better insight if they were to
carefully study Chapter 10, "Grain Growth, Sintering and Vitrification" in
"Introduction to Ceramics" 2nd Edition by W. D. Kingery, H. K. Bowen and D.
R Uhlmann. ISBN 0-471-47860-1, 1976.
It is not difficult to design a test that explores the melting behaviour o=
f
mixtures based on the 1170 deg C Eutectic based on Lime, Silica and Alumina=
.
All you need is a measure each of Whiting, Silica sand and Aluminium oxide.
The percentage proportions are given by Michael Cardew in "Pioneer Pottery
", Ch 5, Page 51. Anticipation of fusion below Orton Cone 6 puts this
mixture well within the grasp of most potters.
I would be interested in reading the experiences of those who wish to get
first hand knowledge by testing the Cardew Recipe.
Best regards,
Ivor Lewis,
REDHILL,
South Australia
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