Joseph Herbert on sat 22 sep 07
Mark Potter wrote: "Silver has a low melting point. . . is there a way to
'condense' silver onto the surfaces of pots??"
'A cloud of dust, a flash of light, and a hearty cry..." Was Clayton Moore
a potter?
Metal vapors in a hot kiln have "funny" properties, they condense on
(relatively) cool items. Both lead and arsenic present problems in this
respect. If you have some lead in a glaze formulation and fire it at a high
enough temperature to "cook" the lead out of the glaze - Where does the lead
go? Much of it goes up the flue (or in an electric out the vent system we
hope) but not all. There are cooler areas within the kiln, like the edge of
the bricks where the door closes that are below the vaporization temperature
of the metal. The metal condenses there, on the stuff that is cooler than
all the other stuff. IF you were to make a high temperature glaze that
contained lead (early melting help? Reduce surface tension? Who knows?) and
then counted on the high temperature to remove most of the lead from the
melt, will you have a lead problem? Yes! The lead would not be on the
ware, having mostly left the glaze, but on all of the cooler kiln surfaces -
some of which are in the most likely area for touching during loading and
unloading.
The presence of arsenic in coal fueled power plant furnaces is interesting
because coal does not contain much arsenic at all but furnaces end up with
lots inside. While the furnace is burning millions of pounds of coal, the
arsenic is vaporized and much of it goes out the flue or ash system
condensed on the surface of the ash once it has cooled below about 1500º F.
But there are some parts of the furnace that are cooler than the
condensation temperature of arsenic all of the time. Those parts are
constantly colleting arsenic being condensed out of the gasses from all
those million pounds of coal. It builds up to the point that some of the
ash coming from the furnace qualifies as hazardous waste and the people
working in the furnace to repair it have to be careful about ingesting
arsenic when working on furnace parts.
So, if you put silver metal in your kiln (a quarter is copper and nickel)
there will be a silver vapor pressure in the kiln. It will probably not be
particularly high until the boiling point of silver is reached (3925º F. ).
Any silver vapor in the kiln will condense on the cooler surfaces. Since
the ware is the second hottest thing in the kiln, not much will get onto the
ware surface.
Lusters, specially formulated liquid coating applied to already fired
ceramic surfaces, leave a fine iridescent film of metal (often silver or
gold) on a pot's surface. These are usually fired at relatively low
temperatures (Cone 020 or some such) and are use in multiple firing
decoration scenarios.
Joe
Joseph Herbert
Technical Writer
Irving, Texas
214-725-8305 (Cell)
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