Bruce Girrell on thu 20 nov 03
Tony C wrote:
> i'll be damned if i can figure how the
> glaze melt can be altered [as a result of refiring].
> Is the glaze not stablized at this temperature?
And Hank Murrow wrote:
>I am convinced that the greatest improvements occurr to glazes which
>have an abundance of either flux or alumina.
>The highly silaceous ones are not much affected...
Mind if I have a go at it?
Melt is a function not only of temperature and materials, but also of
material size and mixing. Size matters because the interesting interactions
occur only at the surface of materials and material that is broken into
small pieces has a lot more surface area with which to interact. Consider
the result of putting a spoon full of finely ground sugar into your tea as
opposed to adding the same quantity of sugar as a single crystal. Now also
consider two cups of tea, each with an equal spoon full of sugar added, but
one is stirred and the other isn't.
Most glazes are not completely melted as a result of a single firing. If you
look at glazes under a microscope you will see lots of small chunks of glaze
material suspended in the glaze. Refiring gives these particles another
chance to melt and interact with other glaze components, changing the
appearance of the glaze.
Sticking with the sugar in tea analogy, we know that sugar dissolves much
better in hot tea than in iced tea. If we were to add sugar to iced tea,
warm the mixture and then take it back down to ice cold again the tea would
have a certain sweetness. But, unless the tea had been stirred while it was
hot, probably there would be sugar remaining in crystalline form. Heating
the tea a second time would cause more of the sugar to go into solution. In
the same way, refiring allows more of the glaze constituents to melt on a
second firing.
Every glaze that I have examined microscopically also shows signs of mixing
that occurred during firing. The mixing results from bubbling and from flow
that occur as the glaze constituents melt and interact with one another
(surface tension effects) and from gravity. Refiring allows this mixing
process to continue. Again, a more complete melt is accomplished. More,
smaller, particles are involved in the mix. The particles are necessarily
smaller than on the first firing because of the melting that occurred then.
Smaller particles interact with one another to a higher degree. All of this
results in a changed glaze appearance.
Another process, called diffusion, occurs at high temperature at the atomic
level. Without it we wouldn't have transistors. At high temperatures, atoms
of any given species are more free to move about on their own regardless of
mixing or flow. Every firing will result in more diffusion. I know that this
process occurs but I'm not sure how much it would affect the appearance of
the glaze, as I would expect its effect to be secondary to mixing and flow.
Now what about Hank's observation: "...the greatest improvements occur to
glazes which have an abundance of either flux or alumina. The highly
silaceous ones are not much affected..."?
Glazes that are high in alumina are refractory; they don't melt well. A
second firing gives them more time to melt. Fluxes, as indicated by their
name, assist in flow. The assist in melting. They help speed up the
interactions that occur at the particle surfaces. Again, refiring a glaze
with high flux would result in better melt and mixing.
Siliceous glazes are ones that form a good glass. They melt well the first
time. They also tend to be runny glazes; they flow well, which improves the
mixing on the first firing. It is consistent, then, that less change is
noted on refiring of siliceous glazes.
Finally, a hypothesis: Mixing can be improved prior to firing. I suggest
that if we take two samples of glaze from the same batch and ball mill one
of the samples, then apply the glazes to identical pots and fire them
identically, then 1) the appearance of the glaze on the two pots after the
firing will be different and 2) after a second firing the milled glaze will
show less difference than the unmilled glaze.
Bruce "one lump or two?" Girrell
John K Dellow on sat 22 nov 03
You can use a very thin piece of glass set in the spie hole . I have
done this ,the glass will not melt
but will crack. One crack each firing. I would change it after 3 firings
. Saved pulling out the spie to
look at the cones. I have another spie at the back of the kiln low down
to test for back pressure and flame
length during reduction.
By the way I reduce by a mix of damper and closing the primary air.
John
Mr. and Mrs. Overall wrote:
>Wouldn't it be neat to have a camera scope set up inside the kiln and monitor outside to observe a pot during the firing process? Too cool; if they had one reasonably priced, I'd drool!
>
>Kim Overall
>Overall Studio
>Houston, TX
>
>
>Bruce Girrell wrote:
>Tony C wrote:
>
>
>>i'll be damned if i can figure how the
>>glaze melt can be altered [as a result of refiring].
>>Is the glaze not stablized at this temperature?
>>
>>
>
>And Hank Murrow wrote:
>
>
>>I am convinced that the greatest improvements occurr to glazes which
>>have an abundance of either flux or alumina.
>>The highly silaceous ones are not much affected...
>>
>>
>
>Mind if I have a go at it?
>
>Melt is a function not only of temperature and materials, but also of
>material size and mixing. Size matters because the interesting interactions
>occur only at the surface of materials and material that is broken into
>small pieces has a lot more surface area with which to interact. Consider
>the result of putting a spoon full of finely ground sugar into your tea as
>opposed to adding the same quantity of sugar as a single crystal. Now also
>consider two cups of tea, each with an equal spoon full of sugar added, but
>one is stirred and the other isn't.
>
>Most glazes are not completely melted as a result of a single firing. If you
>look at glazes under a microscope you will see lots of small chunks of glaze
>material suspended in the glaze. Refiring gives these particles another
>chance to melt and interact with other glaze components, changing the
>appearance of the glaze.
>
>Sticking with the sugar in tea analogy, we know that sugar dissolves much
>better in hot tea than in iced tea. If we were to add sugar to iced tea,
>warm the mixture and then take it back down to ice cold again the tea would
>have a certain sweetness. But, unless the tea had been stirred while it was
>hot, probably there would be sugar remaining in crystalline form. Heating
>the tea a second time would cause more of the sugar to go into solution. In
>the same way, refiring allows more of the glaze constituents to melt on a
>second firing.
>
>Every glaze that I have examined microscopically also shows signs of mixing
>that occurred during firing. The mixing results from bubbling and from flow
>that occur as the glaze constituents melt and interact with one another
>(surface tension effects) and from gravity. Refiring allows this mixing
>process to continue. Again, a more complete melt is accomplished. More,
>smaller, particles are involved in the mix. The particles are necessarily
>smaller than on the first firing because of the melting that occurred then.
>Smaller particles interact with one another to a higher degree. All of this
>results in a changed glaze appearance.
>
>Another process, called diffusion, occurs at high temperature at the atomic
>level. Without it we wouldn't have transistors. At high temperatures, atoms
>of any given species are more free to move about on their own regardless of
>mixing or flow. Every firing will result in more diffusion. I know that this
>process occurs but I'm not sure how much it would affect the appearance of
>the glaze, as I would expect its effect to be secondary to mixing and flow.
>
>Now what about Hank's observation: "...the greatest improvements occur to
>glazes which have an abundance of either flux or alumina. The highly
>silaceous ones are not much affected..."?
>
>Glazes that are high in alumina are refractory; they don't melt well. A
>second firing gives them more time to melt. Fluxes, as indicated by their
>name, assist in flow. The assist in melting. They help speed up the
>interactions that occur at the particle surfaces. Again, refiring a glaze
>with high flux would result in better melt and mixing.
>
>Siliceous glazes are ones that form a good glass. They melt well the first
>time. They also tend to be runny glazes; they flow well, which improves the
>mixing on the first firing. It is consistent, then, that less change is
>noted on refiring of siliceous glazes.
>
>Finally, a hypothesis: Mixing can be improved prior to firing. I suggest
>that if we take two samples of glaze from the same batch and ball mill one
>of the samples, then apply the glazes to identical pots and fire them
>identically, then 1) the appearance of the glaze on the two pots after the
>firing will be different and 2) after a second firing the milled glaze will
>show less difference than the unmilled glaze.
>
>Bruce "one lump or two?" Girrell
>
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>
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>
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>
>______________________________________________________________________________
>Send postings to clayart@lsv.ceramics.org
>
>You may look at the archives for the list or change your subscription
>settings from http://www.ceramics.org/clayart/
>
>Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
>
>
>
--
John Dellow "the flower pot man"
From the land down under
Home Page http://www.welcome.to/jkdellow
http://digitalfire.com/education/people/dellow/
John K Dellow on sun 23 nov 03
no just ordinary window glass. I think the very fine glass used in
picture frames would last longer.
John
Mr. and Mrs. Overall wrote:
>Wow John! Would that be #3, 4 or 5 welding glass? How does it secure into the spy port? How thin? Thanks for the tip.
>
>Kim
>
>
>
>John K Dellow wrote:
>You can use a very thin piece of glass set in the spie hole . I have
>done this ,the glass will not melt
>but will crack. One crack each firing. I would change it after 3 firings
>. Saved pulling out the spie to
>look at the cones. I have another spie at the back of the kiln low down
>to test for back pressure and flame
>length during reduction.
>By the way I reduce by a mix of damper and closing the primary air.
>John
>
>
>
John Dellow "the flower pot man"
From the land down under
Home Page http://www.welcome.to/jkdellow
http://digitalfire.com/education/people/dellow/
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