What the Workshop
can do for you.
Custom Built Apparatus
Quartz, Soda or Lead.
Solvent Still Heads-
Schlenk Tubes & Flasks
-Glass to Metal Seals
-Quartz to Pyrex Seals-
or any idea you come up with.
|Silicon greases have one major wonderful noteworthy feature--they
are cheap! Secondly, they are mostly impervious to hydrocarbon solvents.
Unfortunately, they are not perfect. The biggest problem is that they are
composed of two materials, a volatile and a non-volatile component, and
the volatile is very volatile. In fact, once purchased, silicon grease
should not be kept in the lab over 18 months as it ages in the tube. Once
applied on a joint or stopcock, it should be completely removed and replaced
every month or two. Even if you do not rotate the plug, a stopcock or joint
may leak if left alone for several months. In addition, although silicon
grease has a low vapor pressure (<10-5), it does creep under vacuum
and will spread from a joint into the apparatus itself laying a film throughout.
While silicon grease is very inert and not likely to cause any problems,
complications do arise if you want the items repaired and/or altered. The
problem is that it is not easy to remove the non-volatile component of
silicon grease: the silicon. Under high heat (>400°C), the silicon
component will fuse onto the surface of the glass and if a torch is applied
to the glass, the silicon will burn into the glass destroying the glass
that was heated. The best way to remove silicon grease is to remove
the bulk of it with methalene chloride followed by a soaking for about
20-30 minutes in a base bath.
Also items that will be used for long term storage, e.g., desiccator
jars for long term storage, should also never use silicon grease because
of all of the aforementioned issues.
Gary Coyne is the author of
from Wiley Scientific
This book can be obtained by such vendors as Amazon.com
Glassblowing Workshop, School of Chemistry, North Haugh, St Andrews,
Colin F. M. Smith
Tel: 01334 463890
Fax: 01334 463890
The workshop is always open
for anyone to come and discuss
or design work projects.
Please let me know as early
as possible any projects you
have planned for the future as
soon as possible as the
materials needed may have to
be ordered and deliveries can
take a long time.