Ivor and Olive Lewis on sat 5 feb 05
Dear Candice Roeder,
Substances do several things as they are heated and cooled. Those that
are solids at room temperature melt first and most, when heated
further begin to boil. This is vaporisation, and the material turns
into a gas at its "Boiling Point". But not all things follow this
route. Some decompose while they are still solid and become new
mixtures of different compounds. Whiting, Calcium Carbonate does this,
decomposing to give Calcium oxide which has a very high melting point
and Carbon dioxide which is a gas at ordinary temperatures.
So, to effectively "Fume" the glazed surface of a pot you need to use
materials that will change into vapours and then react in some way
with the surface of your glazes.
The second important characteristic of a compound is called its
"Vapour Pressure" This is a measure of the ease or difficulty atoms
and molecules have of shaking themselves free from the binding forces
which hold them together in a solid or liquid mass. This may be
thought of as the way these minuscule bits of matter muscle themselves
away from their neighbours and fight their way out through the oxygen
and nitrogen molecules of our atmosphere. As we warm things up they
get more power to break free. The physical forces which they overcome
in breaking free is called Vapour Pressure.
For example Water boils at sea level at round about 100 deg C or 212
deg F. Its Vapour pressure is "One Atmosphere". But twelve thousand
feet up in the mountains it may be able to boil at (And here I guess)
90 deg C because air pressure has fallen to less than "One
Atmosphere". Not such a hard fight to break free. If you reduce
pressure with a vacuum pump you can get water to evaporate freely at
room temperature.
Now to answer your question about specific compounds.
The best materials to use are Metallic Chlorides.
Copper Chloride (Poison), Cobalt Chloride (Probably poison), Iron
Chloride (corrosive !!!), Chromium Chloride (Poison ! ! !) and
Manganese Chloride (Poison ! ! ! ) will work.These have relatively low
melting points, should fume and are not prone, as are other salts, to
decomposing into a gas and an oxide which would not work as you wish.
Some have boiling points well below Cone 6. Do not use Nitrates. They
are strong oxidising compounds and decompose.
Information about Chemical Properties of Compounds used by Potters and
studio ceramists can be found in Kaye and Laby, "Tables of Physical
and Chemical Constants". the C.R.C "Handbook of Chemistry and Physics"
When you are doing fuming work treat the process with great respect.
Regard it as hazardous and wear a suitable respirator that will give
protection to your eyes as well as your bronchial passages and lungs.
Vapours can damage mucous membranes.
Sorry to be long winded but there is a lot of information to consider
and not a lot of knowledge floating around.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
-- Original Message -----
From: "Candice Roeder"
To:
Sent: Friday, 4 February 2005 12:43
Subject: Fuming point of chemicals?
> Hi all,
>
> I've been doing pit firing for years, and am now beginning saggar
firing. In reading about saggar firing methods, I see that people are
using various chemicals within the saggars for fuming.
>
> I am familiar with using Copper Carb and Copper Sulfate, and know
they fume well, particularly when combined with salt (I do not know
why the salt helps the fuming).
>
> I see many are using Iron Oxide, Copper Carbonate & Copper Oxide and
Yellow Ochre in their firings.
> I thought these chemicals didn't fume (volatilize?) at low temps
(cone 012-010).
>
> Wouldn't the Sulfate forms of cobalt and iron be more apt to fume?
> Would combining salt with the iron oxide, cobalt carbonate cause
them to fume at lower temperatures?
>
> I can find online data listing the melting points of these
colorants, but how do I find the fuming temperature (or are they the
same)?
>
> I plan on soaking combustible materials in the above, but prefer not
to waste my time, chemicals & money if they're just not going to
fume.
>
> Thanks for any insight.
>
> Candice Roeder
>
>
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