Date: Thu, 27 Sep 2001 15:07:54 -0400
Reply-To: Conchologists of America List <CONCH-L@LISTSERV.UGA.EDU>
Sender: Conchologists of America List <CONCH-L@LISTSERV.UGA.EDU>
From: "Monfils, Paul" <PMonfils@LIFESPAN.ORG>
Subject: Re: Fluid to suspend shell in pendant?
Finding a vacuum pump on the road is a challenge I have not yet been
faced with. For the simple task you are contemplating, a powerful,
motor-driven vacuum pump is not necessary. There are small hand-operated
vacuum pumps, which produce a pretty good vacuum when you squeeze a sort of
pistol-grip handle. Some of them are even made of plastic. I think auto
mechanics use them, (though I am not knowledgeable enough about car repairs
to say just what for). So, they might be available from an auto parts
distributor, or a hand tool company.
Actually, a small glass vial can usually be evacuated safely.
Implosion results from major inequality between the total pressure on the
outside surface of a container, and the total pressure inside. When you
evacuate a container, the inside pressure is reduced to near zero, while
outside pressure remains unaffected. The maximum psi differential you can
achieve at sea level by evacuation is 15 psi. 15 pounds doesn't sound like
much - but it is that "psi" part - PER SQUARE INCH - that is the problem.
If you have a lot of square inches, the TOTAL pressure resulting from 15 psi
can be large. A small vial may have a total surface area of only 3 or 4
square inches, and the maximum total pressure differential of 40 to 60
pounds is not likely to crush the vial. However, something like a large
mayonnaise jar may have a surface area of 100 square inches, which means
that evacuating the jar can create a total pressure differential of about
1500 pounds, and if the jar fails under that kind of pressure, you don't
want to be standing anywhere nearby!
The surface tension of mineral oil is considerably less than that of
water, but the viscosity is much greater, which prevents the oil from moving
easily into small apertures. Vacuum will remove the air not only from the
interior of the shell, but also from other areas, like an open umbilicus,
tubular spines, and pits or depressions in the external sculpture of the
shell. You are right about transferring the shell from one container to
another. Once the air has been replaced by oil, the shell can easily be
transferred from one oil-filled container to another, without air getting
back in. Another way to accomplish this would be to submerge the uncovered,
oil-filled vial, containing the shell, in a larger oil-filled container,
then evacuate the larger container. Once the air has been removed from the
shell, just remove the vial, put on the cover, and wash the oil off the
exterior. But again, if you use a larger container, you would have to start
thinking in terms of container strength. The whole process would also be
facilitated by warming the oil, which temporarily decreases its viscosity.
I anxiously await a peek at the finished product!