Since then I never ran into difficulties of making
some hollow bodies but now I am and I’m wondering if something
like this really could happen. What is happening when you’re
closing that small hole? Why should the danger of exploding not
be present then?
Sabine, when you’re soldering shut that small hole, you’re sort
of reversing what happens when you heat a closed body. The trick
is that until the hole is closed, there is no pressure problem,
due, of course, to the open hole. And in soldering it shut,
you’re sealing the hole while the item is hot, so the air is
already somewhat expanded. If you were then to take the
now-sealed piece, and solder somewhere else, much more strongly
heating the thing, you might get the inside air hotter than when
you sealed the hole, and then it might indeed burst, but aside
from that, when you seal the hole, that’s the hottest it’s
supposed to get, so as it cools, the now sealed form develops not
pressure, but a partial vacuum inside. This might slightly
collapse a sufficiently thin form, but it won’t burst it. And you
could even reheat the thing somewhat, which would only equalize
the original pressure, so long as you don’t exceed the original
temp it had when you sealed the hole. But heating a sealed form,
especially if, in sealing it, you accidentally got some liquid
inside, forming steam, can create pressures much higher inside
than the partial vacuum you can develop in sealing the hole. In
initially soldering shut a hollow form, you usually also have a
similar situation, but often, you’re soldering large seams, with
more heat, etc. And what happens is that sometimes that partial
vacuum that forms as the piece is cooling, sucks just a trace of
the solder away from the seam while the solder is still sludgy
and not quite solid. This can create very tiny microscopic
openings that nevertheless equalize most of the air pressure.
But the piece is still hot, right? So innocently, you dunk it in
water or pickle… Looks fine. But this last bit of cooling and
the additional partial vacuum it causes just sucked a bit of
liquid into the pore and inside the piece. Now in polishing and
finishing, maybe you burnish that pore down a bit, or clog it
with polish compound, etc. Or maybe a bit of the pickle, later
leaking out just a tad, dries/crystalizes, plugging the pore.
Anyway, it’s easy to end up with a sealed form with normal air
pressure inside it, no real means of letting air in or out
quickly, and yet a bit of moisture inside. If you then heat
this up again, in another assembly step, when the moisture boils
to steam, it quickly can build up to quite high pressures. And
THAT is what can give you rather dramatic results. Even without
trapped moisture, heated air can compress to significantly high
pressures. I’ve seen pieces pop with enough force to pretty
much destroy them, sometimes even creating a safety concern for
the person doing it. Other times, it just opens up a previous
solder seam again. Either way, it’s best to avoid this
happening. If you’ve made a hollow form, be sure there is at
least some, clear way for pressure to equalize before heating it
again. Doesn’t take a large hole. sometimes, it doesn’t even
need to be soldered shut. Can leave open, or just burnish it
shut if you plan it right. Often the open hole can be hidden
inside some finding or stone setting, or mechanism, or the like.
You don’t always need to actually drill it either. If you
simply file a small notch, or the start of a saw cut, in an edge
before soldering the thing shut, that tiny notch will form a tiny
gap in the solder seam. That’s often enough, not then needing an
actual drilled hole.
Hope this helps.
Peter Rowe
