Pulling the end of a sample tube off using tweezers.

Fig. 8.10 Tipping off a sample not connected to a vacuum line.

and, with a pair of tweezers in the other hand, you must grab the open end of the sample tube. Now, alternately heat both sides of the tube end and pull off the end to seal the tube. The important point is not to dwell the flame on the tube and heat the solution, thereby raising the pressure inside the tube (see Fig. 8.10).

This "open-tube technique" should never be done on flammable solutions unless the sample is frozen!

Practice tipping off sample tubes that mimic the condition and environment of a real sample before you try with an important sample. In other words, try this procedure with the solvent that you plan to use (i.e., if water is the solvent of your material, practice with water), attached to the same vacuum line you plan to use, and in the state it will be in when you will be doing the tip-off (such as frozen in liquid nitrogen). Remember to keep the flame of the torch away from the walls of the Dewar containing the liquid nitrogen.

8.2.3 How to Fire-Polish the End of a Glass Tube

Chipped and broken ends of tubes are common sights in the average laboratory. Because it is easy to repair this dangerous and unnecessary situation, there is no reason not to take a torch in hand and repair the problem. However, there are a few limitations involved:

1.If the glass is thicker than 2 to 3 mm (3/32 to !/8 in.), do not fire-polish the

glass because you are likely to cause more damage than already exists. The heat-induced strain caused by the gas-oxygen torch is likely to crack the glass after it cools. Some examples of glass that should not be fire-polished are bases of graduated cylinders or rims of funnels (see Fig. 8.11).

2. If there are crack lines radiating from a broken section (see Fig. 8.11), fire-polishing by an inexperienced person is not advised, and should not be done with great expectations. Under the heat of a flame, cracks tend to spread far beyond your wildest imagination or your worst fears.

A simple chip on the edge of a glass tube is simple to fire-polish.

The glass on the base of a graduated cylinder or on the rim of a funnel is very thick and very difficult to fire-polish. If it is fire-polished, it must be oven-annealed.

When there are crack lines radiating from a broken section of glass, it is very difficult to fire-polish and should be attempted with no expectations.

Fig. 8.11 Problem repairs of broken glassware.

Safety Note: This manual is not meant to teach glassblowing because there are already several excellent books for that purpose (see Appendix D).

If you have an oven that can obtain temperatures of 565°C (1049°F) or greater and is big enough to contain the entire piece that you have fire-polished, you should formally anneal any piece you repair. After placing the entire piece in the oven, bring the oven's temperature up to 565°C, hold that temperature for about 15 minutes, and turn the oven off. It is best to leave the piece in the oven until it reaches room temperature, at least three to four hours or simply overnight.

8.2.4 Brazing and Silver Soldering

The high heat that the gas-oxygen torch can deliver allows it to be used for both brazing and silver soldering. However, because you can vary the gas and oxygen content of the flame, you can provide either an oxidizing or reducing environment to your work. Because brazing and silver soldering cannot be done with oxide on the material's surface, using an oxidizing flame will prevent a good seal.

Normally a flux is used to remove oxides from surfaces when brazing or soldering, and a gas-oxide torch also requires the use of a flux. However, if the flame is oxygen-rich, as the flux is boiled or burnt off, the excess oxygen from the torch will oxidize the metal's surface before the solder can wet the surface. In a reducing flame, as the flux is boiled or burnt off, the reducing flame will maintain the pure metal until the solder can flow over and wet the surface.

To make a reducing flame on a gas-oxygen torch, simply cut back the oxygen flow until it has a soft or fluffy character. If you see a bit of yellow in the flame, you will need to increase the oxygen a tad. There may be a desire to increase the oxygen flow to obtain a hot 'hissy' flame to get the metal hotter so the solder will flow. Unfortunately, this flame is the oxidizing flame that will prevent the solder from wetting the metal and completing the brazing or soldering operation.