Angela Davis on sun 26 aug 07
Concerning vitalization of metallic glazes,
I am very interested in learning to control this phenomenon
in my Raku kiln.
I make art tiles using a combination of
low fire and raku glazes. Often volatile metals are deposited
on my colorful glazes, this is usually undesired.
Reading at this site they speak of the volatility being kept down by
a reduction firing. I find that interesting because I was thinking the
opposite from watching the atmosphere in the kiln towards the end of the
firing.
To lessen the volatiles I have been firing to a lower temp, usually below
1740,
and keep the kiln from going into reduction. I still get the metallic
flashes
where I don't want them but they are greatly lessened.
Do any of the Raku firers out there ever try to reduce the extraneous
metallic flashing in
your kiln and what has worked for you?
Angela Davis
In Homosassa
http://rbmason.ca/ceramics.html
LUSTRE-PAINT: Lustre-painting is a unique contribution of the Islamic world.
It became an important paint for ceramics from about 800AD, but was used on
glass in Egypt and Iraq before this date. Lustre pigment is a compound of
silver, copper and iron oxide in a refractory earth kept together by gum. It
is applied to the glazed surface of a previously fired vessel. The painted
object is then refired to a red heat in a reducing atmosphere in the small
specially constructed kiln used only for this purpose. In the second
"firing" the metals are bonded to the glaze as a thin layer with a strongly
metallic lustre, the refractory earths can then be brushed off. If any
oxygen gets into the kiln it will hopefully join with the iron-oxide which
it prefers. If too much oxygen gets in, the copper and silver will oxidize,
volatilize, and be driven off. If the temperature is too low, the metals
will not fuse to the surface; too high and the entire pigment will fuse,
forming a blackened paint.
----- Original Message -----
From: "claystevslat"
To:
Sent: Saturday, August 25, 2007 8:11 PM
Subject: Re: Diffusion through Clay Bodies? What's the right term
Ivor --
You say that you "have great difficulty accepting that
some of the compounds we use are able to volatilise so
readily when their melting points are in the same
temperature range as our firing temperatures and their
boiling points when they would freely give a vapour are
way off the scale that we can measure."
My immediate thought about that is a simple reply --
your difficulty accepting the idea appears to be
contradicted by the occasional appearance of copper
effects on pieces fired near, but not touching, copper
glazed work.
Best wishes -- Steve Slatin
--- In clayart@yahoogroups.com, Ivor and Olive Lewis
wrote:
>
> Dear Daniel Sommerfeld,
>
> Thank you for the information about your glaze.
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2:59 PM
Paul Haigh on mon 27 aug 07
Sublimation is the process of something turning directly from a solid to a gas. Your ice cubes get smaller in the tray- that's sublimation.
If it changes molecular form in the process- that's pyrolysis (no oxygen, organics) or thermal docmposition. We talk about greenware in a bisque fire or wood in a woodfire "boiling off" hydrocarbons that are then combined with oxygen- but the wood and organics aren't just a mess of simple hydrocarbons. They are more complex ones that undergo a conversion into the volatile products.
For copper carbonate- the reaction here would be: CuCO3 --> CuO + CO2
Copper Oxide is reduced to metalic copper by carbon monoxide: CuO + CO --> Cu + CO2
(raku chemistry in action, baby)
CuO melts at 1201 C.
Boiling point is reached when the pressure of gas being evolved in the liquid state reaches the pressure of the atmosphere on that liquid. Water can be made to boil by adding heat- or by lowering the pressure (think "bell jar").
Think of it this way (because it's accurate)- vaporization of water happens when molecules at the surface get enough energy to become gas molecules (yeah- we're free!"). Boiling is a bulk process, where molecules below the surface become gas.
If you mix substances, then the boiling point, melting points, etc are effected so that simply looking at the BP of the substance doesn't say that much. Ceramists start talking about "phase diagrams" and "eutectic points" and such. This is the reason that sugar solution, antifreeze, alcohol, etc all decrease the freezing point of water.
Of course liquids and solids emanate molecules below boiling point. Simple evaporation or sublimation. For stuff like water- this will happen until equilibrium is reached. As much water is condensing as is volatalizing. More water evaporates at higher temps.
Even steel has a vapor pressure. It aint much :)
OK, there's my rant for the day.
Angela Davis on tue 28 aug 07
Thank you Ivor, that tin oxide wash tip sounds very interesting.
It also leads to an endless list of testing, something that fascinates me.
Just imagine the endless combinations to try!
Angela Davis
----- Original Message -----
From: "Ivor and Olive Lewis"
To:
Sent: Tuesday, August 28, 2007 2:00 AM
Subject: Re: Diffusion through Clay Bodies? Volatilization
Dear Angela Davis,
I have had little experience of processing Raku but if do recall a workshop
where one person took from the smoking drum a very dirty black object which,
when washed, had a wondrous silver lustre suffused in patterns across her
pot. When asked about her glaze recipe she said it was the class glaze over
which she had painted Tin Oxide. I have made a resinate lustre using Iron
chloride that gave an orange metallic lustre on a white commercial tile.
Transmutation Lustre, as describe in the extract you give, has a popular
chemistry. But this ignores known properties of the basic ingredients,
especially Silver compounds.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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6:20 PM
Ivor and Olive Lewis on tue 28 aug 07
Dear Angela Davis,=20
I have had little experience of processing Raku but if do recall a =
workshop where one person took from the smoking drum a very dirty black =
object which, when washed, had a wondrous silver lustre suffused in =
patterns across her pot. When asked about her glaze recipe she said it =
was the class glaze over which she had painted Tin Oxide. I have made a =
resinate lustre using Iron chloride that gave an orange metallic lustre =
on a white commercial tile. Transmutation Lustre, as describe in the =
extract you give, has a popular chemistry. But this ignores known =
properties of the basic ingredients, especially Silver compounds.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
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