Now I am completely confused. How exactly does one "damage" metal
by heating it. We control the working properties of silver or gold
with heat and working... It is one of those things we learn from
experience..... Guided by a firm understanding of basic theory.
Fusing changes that?? Really I am confused and not being
disrespectful. Metal is metal...
I have addressed what happens if metal heated too much, so let me
try to answer if metal is changed by heating, even if solidus point
is never crossed.
It is true that quality of crystalline structure of a substance
significantly affects it’s properties. A cliche example is diamond
and graphite. Same composition, different crystalline structure and
vastly different properties.
The same is true of metals, precious metals in particular. It is
goldsmith responsibility to create crystalline structure of the
finished piece. It starts with casting an ingot. Casting has
irregular, coarse crystalline structure. That is true irrespective
of the claims of casting community. Gravity cast ingots are worse
than those cast with centrifuges and vacuums pumps, but all of them
is pretty bad. That structure is improved by forging and subsequent
annealing cycles. Forging with cross-peen hammer breaks large
crystals into smaller fragments. Annealing re-crystallizes structure
into more uniform, smaller crystals entity. Repeating process
several times improves metal significantly.
Rolling ingot into sheet or wire is the next step. Changes to
crystalline structure is different. Rolling mill flattens the
structure. Crystals slide past each other forming different kind of
structure. It gives ‘grain effect’ to metal. When metal fully work
hardens, it must be annealed. Pay attention I said FULLY. If metal
under-worked, the annealing can be detrimental. How detrimental will
depends on the degree. Annealing after 90% work hardening is probably
fine, when annealing after 20% of work hardening should be avoided
at any cost.
This can be more difficult than it appears. Let’s say you finished
forming and in assembly phase. If there are several joints to solder
one after another, it could be trouble. First soldering acts like
annealing, but second soldering we annealing at 0% work hardening. So
you see, planning fabrication steps is crucial.
Even the best planning fails from time to time and we must 'anneal’
at low percent work hardening. Then the character of flame and
heating pattern becomes important as well. Anybody who has my DVDs
knows that I use very deliberate pattern of heating. This is because
of one must damage the structure, at the very least the damage must
be uniform. The very important point to keep in mind is that there
are no ideal situation. Once one is in assembly phase, there will be
damage to metal crystalline structure. What separates expert from
beginner is the degree of damage that one inflicts. The better
original ingot is prepared, the more control goldsmith has over final
This is very tiny introduction to a very complex subject. I want to
encourage reading authors like Brepohl, and try to plan fabrication
steps taking into account annealing and soldering milestones.
Fusing can be considered extreme form of soldering. Because metal
actually flows, the resulting structure is like after casting. If
forging is possible afterward, the damage can be repaired, but if
not than one has to consider carefully if it the best way to