How is a forged object different from one that is cast? For example,
Michael Good can turn a sheet of metal into an anticlastic cuff, but
somebody else might make the same shape from a sheet of wax
(including realistic tool marks) and create the cuff using lost-wax
casting. Are there tangible differences between the two end
products?
After the forged cuff is annealed, is its internal structure the
same as the cast cuff?
Janet
Janet, Unless the cast item is worked with hammer–forged-- it will
never be as dense and as strong as its forged counterpart. This will
manifest in the rigidity and durability of the object as well as any
"springiness" that is needed.
Andy
Janet, I’ve taken a few courses with Michael Good(very interesting
man and class) and the technique is not what I would call forging.
But that’s not your question! Anything cast does not have the same
tight molecular structure, in other words not nearly as dense.
Rolling and hammering metal compress and stretches it and you can
make it ‘springy’. Casting can produce porosity, which is akin to
being ‘sponge like’ and very hard to make ‘springy’.
The bracelets I make using the Good technique could not be done in a
cast. And when you put them on, one can twist ‘arms’ to put over
wrist, and they spring back into place.
There are many other differences in cast and fabricating gold, but
time limits my reply, hopefully there will be othere folks to
continue this. Happy Hammering
Thomas Blair
www.islandgoldworks.com
Are there tangible differences between the two end products?
Very much so.
forging, rolling, and otherwise extensively cold working the metal
does several things. through several working/annealing cycles, you
get a much more homogeneous mass of metal, with a much smaller and
tighter crystal structure. If the rolling and working was in a
particular direction, the grain structure may follow that to an
extent, increasing strength in those directions.
Cast items, even cast with alloys that contain grain refiners,
generally have a less uniform structure, with slower cooling areas
having coarser grains, and the overall structure usually being a much
coarse grain size overall. Also, there’s the question of density and
porosity. Castings ALWAYS have some porosity, even if only at a
microscopic level. Very good castings can have densities
approaching, say, 99 percent of the metals theoretical density, but
that leaves 1% in voids and defects. Once cold worked and annealed,
the metal easily reaches full, 100 percent density, with
significantly greater strength as a result.
Another difference is just the composition of the alloy from one
part to another. Castings cool sequentially from the thinnest areas
to the thickest, generally, or otherwise in some sequence from one
part to another. This sequential solidification leads to some
differences in composition from the first chilled areas to the last.
One practical example of how significant this can be is in casting
ingots of scrap metal to send to refining. If you wish to take a
sample of the ingot to assay, it’s important to do it by drilling
well into the ingot in several places, to obtain samples that
represent the whole thickness from surface to center, or your assay
will not be accurate.
While annealing does allow grain growth to an extent, in general the
reduction in grain size that’s caused by first cold working and then
properly annealing, is far more than the modest grain growth that
normal annealing will allow, so that if you were to compare the grain
size in, say, one of Michael Good’s earrings, with a cast knockoff,
you might easily find that you’d need a microscope to clearly resolve
the individual crystals in the forged and worked example, while the
cast one could easily have crystals in the millimeter or more range.
I’ve seen cast gold rings with grain sizes of as much as 4-5
millimeters. And there are visual differences that are obvious
too. Just look at the polish you can get on forged, rolled, and
otherwise heavily worked metal, compared to what you can get on a
casting. It’s easiest to see just when you’re actually trying to
polish the thing. The worked metal just polishes up so much more
easily, and to a higher level, than the casting, that the differences
are clear. The level of difference, of course, depends on which
metal and alloy, but I can’t, off hand, think of one where it’s
really hard to tell the difference, and some, like sterling silver,
or platinum, can be really obvious.
Peter
That must be why a blade for a samurai sword goes through several
cycles of working and annealing.
Janet
How is a forged object different from one that is cast?
Ahhh – this is one I can answer – after spending much of the last
2 years studying repousse!! There are at least two very significant
differences – the first is the weight of the item when it is
finished. In creating a repousse piece, I can achieve incredible
height and three-dimensional shape in a piece of metal that is 20,
22, or 24 gauge to start with. That same height and detail created
from a cast process would weigh a ton by comparison. The second is
very, very subtle. Underneath all of the planishing and polishing
that you do to finish a piece that is hand wrought, there is a depth
in the quality of the metal that comes from the light finding those
hundreds of barely discernable hammer and tool marks that cannot be
reproduced in wax no matter what you do to it.
Laura