I annealed a piece of flat silver about 3mm thick and a big bubble
formed on the surface. I figured I could just put the piece through
the mill, but when I did, the bubble flattened out but left a copper
coloured mark on the rollers of the mill (bubble on silver
corresponds to copper coloured mark on mill).
My questions are:
Why does the metal bubble up when annealing and what caused the
copper coloured mark on the mill’s rollers?
The bubble is a hole under the surface, rolled flat during the making
of the sheet. Avoid using that piece of metal or return that part of
the sheet for a replacement.
If you pickled the metal after the bubble ‘formed’ then acid got
inside. and wasn’t adequately washed out.
Rolling it put some acid on your rollers (copper blush happens with
acid on steel).
I used to have this problem when I first started alloying my own
sterling. It can be caused if the copper used is contaminated or
dirty. Also if the copper and silver are not completely mixed during
the alloying process, parts of the resulting metal can have
inclusions of foreign or unalloyed material within them. When heated
these form the bubbles that you have found. At the jewelers where I
did my initial training, the local Venezuelan bench jewelers would
toss their ingots onto the cement floor after casting them. The
resultant “ringing sound” (or lack of) that the ingot would make
when striking the floor would indicate whether it had anyof these
inclusions (I think this is the origin of the phrase to"ring true").
I now do the same myself both with 18k gold and sterling. A clear
metallic ringing indicates a good useable ingot, a dull thud means I
have to do some repair work - usually re-melting and mixing more
thoroughly.