Annealing as cast silver will not soften it. ...
But this hammering or rolling is not generally what you want to do with the piece you just cast. Unfortunately there is not really a way to make as cast metal less brittle.
Does barrel-polishing have this effect? I know that it reduces
the softness of the metal. Does it also reduce brittleness?
Dauvit Alexander,
Glasgow, Scotland.
I’d really like to dispell some misconceptions or perhaps that
have been noted here.
The grain structure of castings do not have to be large. As-cast
grain size is a function of many variables not the least of which
are composition and metal/mold temperature differences. Spaces
between crystals are no larger because of cooling rate. Grain
boundary “spaces” are of a size approximating that of the largest
crystal in the alloy. You are talking about atomic dimensions here
guys.
There might be debris/inclusion material at the grain boundary’s
that make them look large but what you are seeing is an area that
was probably the last to solidify. There was insufficient liquid
metal to fill the voids during solidification. That BTW is part of
the definition of shrinkage. Which is probably the most prevalent
defect to be found under a sprue. You can cause this defect by
quenching a casting to early. However, if you wait 5-10 minutes in
most castings you have a completely solidified casting. The tree
may be liquid but most castings are solid after 5 minutes.
Varying rates of solidification do cause stresses. A good anneal
of as cast product does prepare it for subsequent hammering without
cracking. However. this varies with the alloy. Silver alloys are
available that are soft and “stress free” if cast properly.
The grain structure of castings do not have to be large. As-cast
grain size is a function of many variables not the least of which
are composition and metal/mold temperature differences. Spaces
between crystals are no larger because of cooling rate. Grain
boundary “spaces” are of a size approximating that of the largest
crystal in the alloy. You are talking about atomic dimensions here
guys.
If that were only true. The grain structure of as cast silver is
visible to the naked eye if you etch with the proper reagent.
Cooling rate is one of the major influences on the vacancies in the
crystal structure that accumulate to form the dislocations in the
crystal lattice that break up the symmetry of the cast metal
structure. If you were to cool the metal at a very slow rate the
crystals would be more nearly perfect in shape. The discontinuities
in the lattice are what allow the metal to be worked with a hammer,
if it had a more perfect crystal structure the metal would not be
malleable the perfect crystal structure would make it more rigid.
The large crystal structure provides larger discontinuities
between the grain boundary regions which make for easier formation
of fractures along those boundaries. This is why a forged tool is
better than a cast one. The smaller size of the crystals and
discontinuities produced by hammering forms a denser compact
lattice that is less likely to fracture. You are right in that the
crystal structure does not have to be large but to keep the grain
size down you must cool the sterling fast which produces a more
highly stressed casting. In the extreme this will shatter the
metal.
There might be debris/inclusion material at the grain boundary’s
that make them look large but what you are seeing is an area that
was probably the last to solidify.
Sterling silver in the cast condition is hetrogeneous. The outer
parts of the crystals are richer in copper than the inner. At the
crystal boundary is where the copper oxides form as the silver
releases the oxygen it absorbed while molten ( up to 10 times its
volume in oxygen can be absorbed by molten silver ) the copper will
not release the oxygen it absorbs and this is the dreaded “fire
stain” it forms on the outer layer of the crystals. The combination
of this oxide layer and the eutectic alloy which is the last part
to solidify are the things that can delineate the crystal
boundaries to the naked eye.
There was insufficient liquid
metal to fill the voids during solidification. That BTW is part of
the definition of shrinkage. Which is probably the most prevalent
defect to be found under a sprue.
Gas porosity is the more likely culprit in sterling castings.
Examine the voids if they are round then they are gas if they are
elongated and look torn then they are shrinkage.
You can cause this defect by quenching a casting to early.
You can shatter it due to the high thermal shock shrinkage is
generally a slower phenomenon.
However, if you wait 5-10 minutes in most castings you have a completely solidified casting. The tree may be liquid but most castings are solid after 5 minutes. > Varying rates of solidification do cause stresses. A good anneal of as cast product does prepare it for subsequent hammering without cracking.
This anneal will only help if the flask was cooled too rapidly
and unevenly after pouring, otherwise it makes no difference to the
malleability of the metal. And the metal will still be more libel
to fracture due to the large discontinuities of as cast metal.
However. this varies with the alloy. Silver alloys are available that are soft and "stress free" if cast properly.
Yes but they will not be sterling which is only 92.5 % silver and
7.5% copper the addition of other metals as grain refiners and
de-oxidizers create a different type of silver alloy.
Jim
@jbin
James Binnion Metal Arts
2916 Chapman St
Oakland, CA 94601
510-436-3552
Jim Binnion,
Since you seem rather knowledgeable about matters metallurgical
I’d like to pose a question. Seems there was some talk a couple
years back about producing a sterling casting grain alloyed with a
metal other than copper, to finally rid us all of that annoying
firescale. I somehow recall that the alloy metal to be substituted
for copper was germanium. Any truth to this?
David
Yes but they will not be sterling which is only 92.5 % silver and 7.5% copper the addition of other metals as grain refiners and de-oxidizers create a different type of silver alloy.
Not to nit-pick, but as long as the alloy contains 92.5% silver it
is called sterling. Traditionally sterling has been Ag-Cu but for
close to 100 years sterling has been made with the addition of
various alloys of Zn, Si, Ge, Sn, Cd (and probably others that I
don’t know of). When disscussing grain size and intergranular
composition it makes a big difference what the composition of the
alloy is. What is true of Ag-Cu or Au-Ag-Cu is not at all true if
you are using Ag-Cu-X or Au-Ag-Cu-X.
I know that an alloy that is 92.5% silver may be sold/stamped as
sterling in this country (USA) but it is not Sterling Silver which
is the Silver 92.5 Copper 7.5. You are right many metal refiners
have added various other metals and other elements like Si and Ge
to silver alloys to deal with the fire stain and other issues and
this does provide for different and more complex behavior in the
formation of the grain structure of the alloy. But the majority of
suppliers still provide Silver 92.5 Copper 7.5 as their Sterling
alloy and if they offer different alloys they list them as being a
proprietary alloy. My experience with these alloys is that they do
not look like Sterling and they pour differently and when solid
they have a reduced malleability. Also the use of Si or Ge in the
metal reduces the solderability of the metal due to the formation
of Silicon and Germanium oxides on the surface of the metal that
are not dissolved by standard soldering fluxes. They also patina
differently from Sterling.
Jim
@jbin
James Binnion Metal Arts
2916 Chapman St
Oakland, CA 94601
510-436-3552
Quite a bit has been written about this alloy. See the 1997 Santa
Fe Symposium articles.
Okay, let’s clarify a few things…
The grain structure of as cast silver is visible to the naked eye if you etch with the proper reagent.
The grain structure of all metals are visible to the naked eye if
you etch them and may be visible if you don’t etch. That is to
a large degree an indication of the casting conditions.
Sterling silver in the cast condition is hetrogeneous. The outer parts of the crystals are richer in copper than the inner. At the crystal boundary is where the copper oxides form as the silver releases the oxygen it absorbed while molten ( up to 10 times its volume in oxygen can be absorbed by molten silver ) the copper will not release the oxygen it absorbs and this is the dreaded "fire stain" it forms on the outer layer of the crystals. The combination of this oxide layer and the eutectic alloy which is the last part to solidify are the things that can delineate the crystal boundaries to the naked eye.
You have accurately described the solidification considerations
for equilibrium or near equilibrium solidification. However, if you
want to avoid firescale melt and cast fresh silver quickly. I’ve
manufactured silver castings of various sizes that contain little
if any oxide and no fire-scale because they were “handled” with the
entire casting system in mind.
There is no specification for the balance of a sterling silver
alloy. The silver content must be at least 92.5% Ag but the balance
can be any other element. I can’t think of a standard alloy that is
7.5% Cu unless it’s made up in someone’s shop from “pure” metals.
Every supplier will add SOMETHING to their alloys to insure
commercial viability.
We can go on and on about these issues, Debate and jaw-bone all we
want. The point{s} that should be made to all practioner’s out
there is 1. metals are very sensitive to what you do to them but
not in the same way. 2. there is an enormous amount of good
out there. Learn as much as possible about the specific
metals you deal with. 3. everything we participate in is a system.
Even the way we work metals. You need to understand all the parts
of the system to understand why they work together and why they
don’t. What works for you in your shop may not work for others in
theirs.