Laboratory Glass-Working for Scientists - Sciencemadness Dot Org

RUBBER CAPS
a cork or stopper, is to employ rubber caps. These are made in a
variety of sizes to fit different diameter tubing, ranging up to about
30 mm, and when of good quality rubber can easily be slipped over
the open end. They create a compression rather than a tensile strain
in the glass tube and are therefore useful for closing tubing with
ragged ends when it is desirable to avoid fire polishing. These caps
are usually supplied made from rubber approximately 1 mm thick.
Thin Rubber Sheet
Pieces of wry thin sheet rubber, of the order of 0-1 mm thick and of
very elastic quality, as, for example, pieces cut from toy rubber
balloons, can be very useful in closing ragged ends of large tubing.
They are of particular use with tubing larger than 20 mm diameter.
A small piece, of about the same diameter as the glass tube itself, can
be stretched over a ragged end of the tube and will be found readily
to stay in place by means of its own tension—provided excessive
pressure is not built up when glass blowing. For detailed use of
rubber caps and pieces of sheet rubber see Chapter 9.
Rubber Stoppers
Rubber stoppers are used extensively for making a closed system for
blowing out. A complete overlapping range is a necessity to the glassworker.
The rubber should be of good elastic quality since poor
quality stoppers often will not *give' sufficiently to be squeezed easily
into the ends of glass tubing. The stress on the glass is less and an airtight
seal is more easily obtained when good rubber is employed.
Two sets of stoppers are very useful, one set unbored and a second
set with a single hole bored in them to carry blowing-tube connectors.
A longer piece of glass tubing inserted into the hole will also form a
very adequate handle when working small pieces of apparatus.
An assortment of small corks for closing small bore tubing can, in
addition, be very useful. If the flame is to come near to a cork or a
rubber stopper it is best to wrap either in thin asbestos paper before
inserting into the tube. When pyrolysis of the cork or rubber
stopper seems probable it is best to use a cork and not a rubber
stopper, because the pyrolysis products from a cork can be cleaned
off more easily than those from rubber. Sometimes a plug made from
damp asbestos paper is adequate for closing a tube of small bore, and
this plug can be heated quite strongly.
Blowing Tubes
Rubber tubing is necessary for blowing when the piece being worked
cannot be brought to the mouth. A length of about 80 cm is convenient
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