Safety Emporium for all your lab and safety needs
Safety Emporium for all your lab and safety needs

*The Glassware Gallery*

Glassware Joints



Keeping air and water out of a reaction is an important concern for Inorganic chemists. For this reason, a wide variety of different kinds of glassware joints have been invented. This document explains the three most common kind of joints that you will encounter.

Need joints? We sell standard taper, o-ring, and ball & socket joints through our retail web site, Safety Emporium.

Standard Taper Ground Glass Joints

These are the most widely used form of ground glass joint (they can also be made out of quartz, steel etc.). The frosted, precision-ground glass surfaces make a good seal when properly greased and are extremely versatile. On the left we see a "male" (or "inner") joint and on the right a "female" (or "outer") joint. When the joint is assembled, the inner one fits completely and snugly into the outer one as shown further down this page:

some joints

The joint size is described by a number of the form xx/yy. The first number denotes the outside diameter of the top of the tapered male joint (or inside diameter of the top of the female joint) and the second number denotes the length of the joint. For example, a 14/20 joint (pronounced "fourteen-twenty") is 14 mm wide at the top and is 20 mm long. 19/22 and 24/40 (shown above) are two other very common joint sizes.

As each joint has a standard taper, any two joints with the same outside diameter can fit into each other as long as the apparatus to which they are attached does not interfere. For example, a 14/35 joint on a condenser will fit into a 14/20 flask but the ground glass joint will protrude (35 - 20) 15 mm into the flask.


One drawback to standard taper joints is that they may sieze or "freeze". This usually happens when the joint is not properly greased (see below) or material is caught between the two halves of the joint. Your local glassblower can help [read these stuck joint tips if you don't have a glassblower].

Greasing Standard Taper Joints

Ball and Socket Joints

Ball and socket joints are commonly used when the pieces being joined need some "play" or flexibility. These are quite common on the traps of vacuum manifolds because of the difficulty of aligning the two components. On the left we see the socket on top and the ball on the bottom. On the right is a pinchclamp (available at Safety Emporium) that is used to hold the two components together.

some joints

The joint size is described by a number of the form xx/yy. The first number denotes the width of the ball and the second number denotes the interior bore of the joint. For example, a 12/2 joint has a 12 mm ball and with a 2 mm hole.

The instructions for greasing a ball and socket joint are the same as those listed for greasing a standard taper joint.

Yes, your leg and hip form a ball and socket joint, but hopefully yours are not made out of glass!

O-ring Joints

O-ring joints consist of two grooved flanges between which is placed a single rubber or Viton O-ring. O-ring joints are sized according to the interior bore of the joint (not the diameter of the groove or O-ring). For example, a #15 O-ring joint has an interior bore of 15mm. Like ball and socket joints, O-ring joints must be held together with a pinch clamp.

unassembled o-ring jointassembled o-ring joint

cover illustration
The Chemist's Companion:
A Handbook of Practical
Data, Techniques, and References

by Gordon & Ford
O-rings have distinct advantages over standard taper joints in that they are unlikely to leach grease into a reaction mixture and they are virtually incapable of siezing. However, O-ring materials can degrade when used with certain solvents or subjected to high temperatures (think about the Challenger disaster).

O-ring joints should not be greased, however the O-ring itself should be treated with an extremely thin coating of grease. A good way to do this is to simply rub your nose with your thumb and forefinger and then rub the O-ring (honest. No kidding). Alternatively, Apiezon M grease is a good choice. When assembling an O-ring joint, be careful that no hairs or fibers stick to the O-ring as these will cause a vacuum leak. Also make certain that the O-ring is correctly sized and fully seated in the groove. It never hurts to have spare o-rings on hand.