regarding annealing, a book I have talks about taking the metal to
cherry red to anneal it, whereas another source said not to take it
that far but rather a sort of yellow/brown colour change. Which
would you recommend?
There is a huge confusion about annealing temperatures. There is no
physically defined specific annealing temperature like there is a
melting temperature. Annealing is the re-growth of the crystals in
the metal or alloy.
When working metal the crystals are broken up into a smaller and
more stressed form as the metal is deformed by your hammer, rolling
mill, drawing dies,etc… As more work is put into the metal the
crystals get smaller and the lattice becomes more stressed,
eventually the lattice will contain more stress than it can handle
and fracture. A perfect example of this is the bending of a paperclip
back and forth till it breaks. A directional bias is also present
when the metal is stretched in one direction as in rolling or drawing
so the crystals take on a shape that follows the direction of
movement in the work.
So we need some way to remove the stress that has built up in the
lattice. By heating the crstals up to the point where they start to
grow again the stress is allowed to dissapate and the crystals
return to a normal equiaxed form. The question is “What temperature
is needed for this to occur?”. The answer unfortunately is “It
depends”.
Is it a pure metal or alloy?
Pure metals anneal at much lower temperatures than alloys.
How much work has been put into it?
The greater the deformation of the crystal lattice before
annealing the lower temperature where it begins to anneal.
How long are you heating it?
Annealing is a time/temperature phenomena the lower the
temperature the slower the re-growth. So you may be able to
anneal sterling at 900 F (482 C) but it would take hours.
Finally how hot are you getting it?
The higher the temperature the larger the resulting crystals
are. There is still a time element so a high temperature for a
short time can result in the same sized crystals as a longer time
at a lower temperature but there will be less uniformity of the
resulting crystal sizes You can over anneal by overheating the
metal resulting in oversized crystals that create a weakened
crystal lattice and the dreaded orange peal surfaces that show up
when bending or polishing.
And also how often should I anneal a piece?
A rule of thumb is 30 % - 50 % reduction in cross section is a good
time to anneal. If you anneal too soon or too often you will not
achieve any stress reduction in the matrix because there will not be
enough in it to get the kind of re-growth and re-alignment you
desire. You also increase the size of the crystals and start to
degrade the integrity of the crystal matrix by over annealing. So
work the metal till it is too difficult to continue and then you are
probably close to the right time to anneal.
Regarding quenching. My book says to quench in water, whereas
people mention quenching in hot pickle. The hot pickle sounds
better than cold water. Is this the case?
Never, never, never quench in pickle, it is dangerous (splashing hot
acid!) and it gets pickel into cracks and pinholes that you will not
be able to get it out of and will cause grief when doing further
soldering operations as it will contaminate the joint. Use room temp
After such fierce heat I have let it air cool rather than quenching
it.
good idea
The thing I had most trouble with regarding cracking was a ring I
made from much thicker sterling (1mm thick). I suppose it is
probably a case of me not annealing it enough and therefore the
silver being too stressed.
I seriously doubt this, it is more likely an uneven cooling rate or
just too hot when quenched caused he problem. Up to the point where
the metal actually breaks while working it the more stress the
better the anneal will be.
Jim
James Binnion
@James_Binnion
James Binnion Metal Arts
360-756-6550