The difference between silver flux and flux for copper is, most
noticeably, the color. Flux for silver has a blue tint, and flux for
copper has a yellow tint. Both can be fired over copper and, when
fired properly at the correct temperature and time, both will fire
clear. Due to the tints, they will appear different but they will
both be clear and both are usable over copper. Both work fine
beneath transparents. Test strips are always advisable to determine
which achieves your desired effect.
It is not recommended to mix leaded and unleaded enamels because of
the differences in expansion and contraction, and flow temperatures.
In my studio, I fire Thompson leaded enamels at 1400deg-1500 F,
depending on whether they are classified as “soft,” “medium,” or
"hard" enamels. In class, we have found Thompson unleaded enamels
flow at 1350 F to 1380 F. Some may need to go as high as 1400 F, but
we have found the unleaded firing temperatures to be much more
uniform than the leaded. We seldom go higher than that.
Additionally, unleaded enamels may have a larger coefficient of
thermal expansion than leaded enamels.
These differences mean that, if combined in one piece, different
layers are firing differently. One is flowing before the other. One
is fusing to the metal before the other. One is expanding more than
the other. One is contracting faster than the other. Logically, this
would indicate that the layers might not bond and would separate or
crack off entirely. I have never actually had this happen because
logic tells me it should and I cannot see any reason to spend time
creating a beautiful piece and possibly have this happen. So, I
don’t mix leaded and unleaded on one piece.
For this reason, I don’t even advise leaded on one side of the piece
and unleaded on the opposing, counter side. Not paying attention to
these properties even if you are uniform in your choice - only
leaded or only unleaded - can present problems. You must also be
uniform in choosing the expansion, contraction, and melting points
of the counter enamels.
As for not having problems with a mixture of leaded and unleaded
counter enamel, it would seem that mixing the grains of many
different enamels with many different qualities evidently works
because it averages out the effects. This would be the basis for
most counter enamels. (You can actually change the flow point/firing
temperature of flux by mixing two with different firing
temperatures). If you are choosing a specific color for the back of
your piece, you need to be more careful. For instance, I have found
that black, leaded or unleaded, does not work well unless enamel
already exists on the front of the piece. In today’s market, counter
enamel costs the same as any other enamel. You can make your own by
tossing the leftover enamel from each finished project into a common
container. Once contaminated, you don’t want to use it in another
project anyway. When doing so, it really isn’t difficult to maintain
one container for leaded counter enamel and one container for
unleaded. It is so much faster and easier to do things carefully
from the start instead of having to correct problems later.