1.3.4 Inserting Glass Tubing into Stoppers

Fig. 1.16 Preventing hot gases from "tunneling"up a tube with a cork in the end.

piece must be rotated while in the flame. Otherwise only one side will be softened and surface tension will cause it to sag in and distort. It may be possible to firepolish a small tube over a Bunsen burner, but it will take a long time for borosilicate glass to get sufficiently hot to melt. Tunneling (the transport of hot gases from the flame traveling down the tube, causing the tube to become excessively hot) can be prevented by placing a cork in the opposite end of the tube (see Fig. 1.16).

After fire-polishing a glass tube, let it cool before trying to place it in a stopper. Do not try to speed up the cooling process by placing the tube under running water, because the rapid temperature change is likely to cause a crack on the newly prepared end.

Use some lubrication on the tube and rubber stopper. This lubrication can be glycerin, soapy water, or even plain water. Hold the glass rod close to the rubber stopper and keep the glass rod as short as possible. The preferable distance should be no greater than about three diameters of the tubing away from the rubber stopper (see Fig. 1.17). The longer the distance, the greater the torque that can be created. As the tensile forces increase, the chances increase that you will break the glass tubing. Use a rotating motion to guide (not force) the tubing into the stopper. For safety's sake, wear leather gloves. Leather gloves are preferred because they provide a good amount of tactile control, a reasonable amount of friction, and excellent protection. A paper towel is not sufficient to meet any of these three criteria.

Fig. 1.17 Insertion of a tube into a rubber stopper.

1.3.5Removing Glass from Stoppers and Flexible Tubing

After a glass tube has been left in flexible tubing or a rubber stopper for a period of time, it is typically hard to remove. The easiest and safest way to remove flexible tubing from a glass tube or rod is to cut it off with a razor blade and discard the destroyed end of the flexible tubing. This method is not always possible with a rubber stopper because the stopper is too big to cut through. However, whenever cutting tubing or a stopper is possible, it is the recommended and safest procedure.

The reason for cutting off flexible tubing or a rubber stopper is because the force to remove the tubing or stopper may be greater than the tensile strength of the glass. Generally, it is much more difficult and costly to repair and/or replace glass items than it is to replace a stopper or flexible tube. When the hazard of sharp glass is considered, the flexible tubing and rubber stopper become clearly expendable.

There is a trick for removing a stopper from glass tubing or a rod that uses a cork borer (see Fig. 1.18). Select a borer size that is just greater than the size of your glass rod or tube. Then prelubricate the outside of the borer with glycerin (preferred), or soap and water, and ease the borer between the stopper and glass. When the borer is inserted all the way into the stopper, the glass tube or rod can be easily pulled out from the borer's hole. This trick can also be used to insert a tube or rod into a stopper by reversing the process.

1.3.6 Film Enclosures

Ground joint stoppers and snap-on plastic caps are commonplace, but many laboratory containers do not come with built in stoppers or seals. In the absence of a formal closure, or closures such as corks or rubber stoppers, there is the covering film, PARAFILM "M"®.*

PARAFILM "M"® is a thermoplastic film. Depending on its width, it can come in rolls of 50 to 250 feet in length. A portion of the plastic sheet is cut off the roll, laid over the clean dry top of a container, and stretched over and downward. The

Fig. 1.18 A cork borer can remove (or insert) a stopper from a glass tube or rod.

'PARAFILM "M"® is a product of American National Can, Greenwich, CT 06836.