Tap the outer member with a sharp light blow at a glancing angle.

Fig. 3.35 Lightly tapping stuck (ungreased) joint or stopcock members with wood can sometimes separate the pieces.

very difficult. Trying to grab onto remaining glass with a pair of pliers often is futile as the glass breaks and chips off. If there is access to the opposite end, it may be possible to tap, or push, the piece out. Alternatively, if the plug or joint is hollow and is closed like a cup, it may be possible to pour melted lead or melted electrical solder into the item like a screwdriver tip. Then when the melted metal hardens, you have a good handle with which to remove the remaining portion of the plug. The solder may then be removed and reused.7

WARNING: Although the materials used in these unsticking techniques are, for the most part, inert or nonreactive, practice safe chemistry. If you have a chemical (that is trapped by a stuck stopcock or joint) that may react with one of the materials or solutions mentioned in this subsection, select a different approach.

3.3.8 Leaking Stopcocks and Joints

There are three reasons that a joint or stopcock might leak: Either the item is not being used in the conditions, or manner, for which it was designed, the stopcock or joint is worn, or the item was poorly manufactured. Although extra grease may be used for short-term emergency situations, it is not a solution because extra grease is not fixing the problem. Similarly, adding extra stopcock grease to a regular stopcock will neither make it act like a high-vacuum stopcock nor help a defective stopcock or joint.

Surprisingly, many assumed defects of a joint or stopcock are caused by using an item improperly. By trying to "make do" with what you have, you either may jeopardize the quality of your work or may subject yourself and others around you to potential dangers. Thus, adding extra grease onto a standard stopcock to that it can be used for vacuum work will be a losing battle.

Assuming that the joint or stopcock selected is proper for the application and there is evidence of leakage, you should look for potential human errors:

1.Did you apply the stopcock grease properly, or is the current grease old?

2.Did you properly clean the items before assembly? Paniculate matter

left in a joint or stopcock can not only cause a leak, but can also cause the stopcock to jam.

3. Are the pieces of a Teflon stopcock assembled in the correct order? For example, on a Teflon stopcock, if the black O-ring is placed next to

the stopcock barrel, counterclockwise rotation may cause the plug to loosen.

4. If you have a vacuum system with glass stopcocks, do the numbers on the plugs and barrels match (see Sec. 3.3.1 and Fig. 3.30)?

Even if failure to prevent the above errors has not caused a leak outright, prolonged misuse eventually causes wear on a joint or stopcock, which can then cause permanent damage. Not all defects caused by wear can be distinguished from manufacturing defects by appearance alone. However, if a new item leaks from structural defects, it probably is a manufacturing imperfection.

Although the quality of joint and stopcock construction from the glass manufacturing industry is generally excellent, mistakes do happen. It is possible to have received hundreds, or thousands, of excellent joints and stopcocks from the same manufacturer, plus one that leaks. Fortunately, manufacturers will promptly replace defective materials.

The Rock or Jiggle Test. It is possible for a joint or stopcock to be so badly made that a mated pair can rock, or jiggle, when a little lateral motion is applied. This test should be done without any stopcock grease on the pair to prevent artificial damping of the motion. Do not force or twist any joint without grease as it is likely to jam. The biggest problem with this test is that it is impossible to know which of the pair is the bad one. To use a pair with this problem will only make both pieces bad as the bad one will wear the good one unevenly, thus making it unusable.

The Pencil Test. Before performing the pencil test, completely clean the entire apparatus in question. Remove all traces of any oil or grease (if the grease is hydrocarbon-based, you can heat the glass item in an oven to approximately 400°C to burn off any remaining grease). Next, take a standard no. 2 pencil and scribble a line all over the inside surface of the outer piece and the outside surface of the inner piece, as shown at the left of Fig. 3.36. Then, wet the joint (or stopcock) members with water (for lubrication) and rotate back and forth five to ten times (do not bear down, let the water be a lubricant). If the ground sections of both faces of the joint (or stopcock) are perfectly flat, the pencil will have been

Fig. 3.36 Sample of using the pencil test to examine the quality of a joint.

also be used on glass stopcocks or joints. By rotating the two pieces you can note with which member the dark spot travels. If you hold the outer piece still, rotate the inner piece, and the spot travels, you know the depression is on the inner piece. Similar logic is used to identify depressions on the nonrotated piece.

3.3.9 What to Do About Leaks in Stopcocks and Joints

Unfortunately, there are not many options available when you have a defective stopcock or joint. The best solution is to replace the defective part of the apparatus, or the entire apparatus if necessary. If the item is new, return it to your supplier and have it replaced.

If the defective part is the plug of a Teflon stopcock, you can try to simply replace the plug. If the plug was damaged by abrasion (or other abuse), replacement offers a simple resolution. The possibility that a distorted Teflon plug is causing a leak is very unlikely. If the glass barrel of a Teflon stopcock is distorted (showing the problems seen in Fig. 3.37), you have no recourse but replacement.

If the item is all glass, it may be reground. However, do not regrind a glass stopcock if:

1.You have any other recourse (see Point 5).

2.You are not experienced in grinding stopcocks or joints.

3.You are unable to completely remove all traces of oil or grease from the item. Any oil or grease will retain grinding compound onto the part, and subsequent use of the item with the grinding compound on it will cause it to jam or wear unevenly, leaving you back where you started. The best technique for removing organic greases is to burn them off in a high-heat oven.

4.If the item is a stopcock and the overlap of the holes on the plug and barrel are such that any grinding will close the hole passageway (see Fig. 3.17).

5.You can accept that the piece in question is expendable.

Grinding should be done with 120-grit Carborundum and plenty of available water. Adding a little dishwater soap will help lubricate of the pieces as they are ground.

After removing all grease, wet both glass members with water and place a small bit of grinding compound on one of the members. Insert the inner member into the outer member and, while applying a light amount of pressure, rotate the members around and around, not back and forth (this point is very important because back and forth rotation with grinding compound will cause uneven wear). Remember, incomplete rotation will mean that you are making a depression in only one region