Burnished metal is very much like rolled metal. Considerable
pitting can be cleaned out and closed up by a couple of
burnishing-annealing cycles.
Have to agree with Bruce here. The thing to keep in mind is
that there is no such thing as a completely porosity free
casting. Even the best possible casting has some degree of
porosity, even if you can’t see it. Engineers will tell you that
the density of a good casting can be in the high 90s percentage
of the actual theoretical density of the alloy, but you never get
100 percent density. A 97% density is pretty good, but what’s
the other 3%? Air, oxides, shirkage pores, etc. And it can be
much lower percentages than that in some cases. We all know that
rolled, milled, die struck jewelry or metal is “better” than
castings. This density, as well as the finer grain structure,
is the reason. when you burnish the surface of a casting, you’re
creating a surface skin that then has the same density and grain
structure that a die struck piece would have.
Also, catagorically calling all porosity thats visible enough to
see and need burnishing, a mistake, is a nice theory, but it’s
pretty impractical in the real world. Dealing with experienced
professional casters, you’ll still see a little bit of porosity
on many castings. Very few will have none anywhere visible. If
you insist on sending all of them back for redos if you can find
any pits or pores, you’re gonna be pretty unpopular with your
caster. If you do it yourself, then you’re gonna be pretty
frustrated. It’s just the way it works. Especially with higher
karat golds and platinum.
Yes, you want to be as rigorous and methodical and careful in
your casting techniques as possible to get the best castings
possible. But that done, you then also want to use the most
effective means possible to convert those castings into the best
piece of jewelry. Burnishing is a vital part of that process.
It’s also capable of some things that a buffing wheel is not.
You can burnish well, for example, right down into a square
corner. Try doing that with a buff. You can brighten and
sharpen an edge, cleaning it up, straightening it’s line if need
be, all without actually removing any metal. Again, try that
with a buff… Burnishing, like many skills, is something to be
learned. it’s not a cheap trick. It’s not always easy. Neither
is the use of a simple file or a saw frame, when done well.
handled properly, a burnisher can give you effects that are
unique to itself, as well as better metal.
You suggest, or someone did, for example, drilling out and
filling a pit with a soldered in plug when a defect in a casting
is found. When you do that, you’ve introduced the different
color and melting point of the solder, which is almost always
visible (unless you burnish it well). Burnishing a pit doesn’t
just cover it over, it closes the opening down, compressing the
metal around it to fill it in, and litterally closing the large
opening down to an invisible pore. That has a lot more integrety
than a soldered in plug. Won’t discolor, won’t flow when someone
else has to work on the ring later, and doesn’t use up expensive
solder. There are, of course, pits too big to burnish, where
some sort of filling is needed (actually welding/fusing the plug,
instead of soldering it in, is better, if you can do it) but
when you can work over the surface to clean it up, instead of
soldering and welding it, you’ll have a better product in many
cases.
By the way, dont forget that there are many ways to do this. A
rub with a burnisher (and there are many shapes, sizes, and types
of burnishers too) is only one. You can forge the metal with a
hammer, planish a surface with a hammer or hammer handpiece or
chasing tool, bead blast with glass beads (which have a
burnishing/hammering action in addition to creating a texture),
or tumble polish with steel shot. All will serve to compact a
castings surface.
Peter Rowe