Studio PMC - Rio Grande

Aluminum Contamination of Fine Silver
By Tim McCreight
Perhaps you've heard this warning: Don't
allow any form of aluminum to touch
your PMC! That's a bit overstated, but good
advice in principle. This article will explain
why you need to keep these metals apart.
There is a difference of about three volts
in the electric charge between aluminum
and fine silver (or fine gold). This means
that even at room temperature, these two
metals create a series of corrosive oxidation
reactions. The photo at right shows a small
circle of household aluminum foil pressed
onto a wet slab of metal clay. This happens
to be PMC3, but the effect would be the
same for any silver clay.
The photo at bottom left shows a detail of
the same piece. After an hour at room temperature,
you can see the formation of dark
spots. If you rubbed your fingernail across
this, you'd feel tiny hard bumps at each spot.
The bottom center photo shows the
same piece an hour later, after I removed
the aluminum foil. As you can see, the
problem is a bit worse, as if the corrosion
continues to grow. It does. The aluminum
foil has holes, and the metal that used to be
attached to the foil is now embedded in the
silver. This would happen eventually with
a sheet of conventional silver, but the
increased surface area of the silver flakes of
PMC accelerate this process.
I fired the sample at the recommended
schedule and the result can be seen in the
photo at bottom right. A brittle scaly material
is interspersed in the PMC. During firing,
PMC does not become fluid, but we
can think of it as "creamy." The aluminum/silver
mixture will take the form
of miniature bits of gravel in this otherwise
smooth material. These lumps will
not dissolve, and they will prevent the silver
around them from bonding.
Note that this contamination requires
exposure for more than a few seconds.
Using an aluminum tool to press a texture
or swipe a cut is fine. In addition, many
aluminum tools and utensils have an electrically
produced tough layer created in a
process called anodizing. If this layer is
intact, no contamination will occur. These
nuances are easy to forget or misunderstand,
which is why you often hear the
overstated shorthand: No aluminum in
the PMC studio.
Aluminum on metal clay.
Corrosion spots continue to grow
after aluminum is removed.
Dark spots form after an hour
at room temperature.
Aluminum contamination
on fired PMC.
Fall 2007 · 19