Skip to main content

Ground-Glass Joint Glassware

For many years, manufacturers have been fabricating lab glassware with ground-glass fittings, or "joints", having standard dimensions and designed to fit each other perfectly. The joints are sometimes referred to as "standard taper" joints, because the joint is designated by a size (or taper), and any piece of standard taper glassware will fit any other piece of glassware with a similar standard taper.

The size used in the O-Chem labs is "19/22." Other common sizes include 14/20 and 24/40. The first number indicates the approximate diameter of the larger tube in millimeters; the second indicates the length of the ground surface. Generally speaking, the larger the numbers, the larger the joint size, and the larger the glassware.

There are several advantages to using ground-glassware, including avoidance of contamination of chemicals due to cork, rubber, or plastic connections, efficiency in setting up due to the quick assembly of apparatus, etc. About the only disadvantage of ground-glassware is the higher cost of the individual units: the items in your equipment drawer with ground joints are among the highest-priced items you will be using in your lab course.

Glass 1 Glass 2 Glass 3 Glass 4 Glass 5 Glass 6 Glass 7

Care of Ground-Glass Surfaces

Ground-Glass Joint glassware is precision-ground to a specific taper, and this allows the snug fit when two corresponding pieces are joined. But proper care must be taken of the ground surface to ensure that the equipment will function properly when used.

A.   Lubrication:

Although ground-glass joints usually seal quite well without the use of lubricants, it is generally a good idea to lubricate them to prevent "freezing" and therefore breakage. Under most conditions in the O-Chem teaching labs, a lubricant should be used. This makes it easier to separate ground-joint ware and prevents leakage. Ground-glass joints must be kept clean and must be cleaned prior to lubrication. Dust, dirt, chemical residue, and/or particulate matter may score the surface and cause leakage.

Stopcock grease is the joint lubricant used in the Chem labs. It is kept in a small jar near the instructor's station in the teaching lab, with the rest of the stock reagents and supplies. Note that the grease is hydrocarbon (petrolatum) based, and therefore is soluble in most organic solvents. This means that you will need to re-apply fresh grease every time you carry out a reaction, since it may dissolve somewhat during use.

   1. Lubricating Ground-Glass Joints:

a.   Use a boiling stick to apply a small amount of grease to the upper part of the inner joint, taking care not to grease any part of the joint which may come in contact with vapor or liquid and cause contamination. A properly lubricated joint appears completely transparent, with no striations.

   2.  Lubricating Stopcocks:

a.   The following applies only to glass stopcocks; Teflon stopcocks require no lubrication.

b.   Use a boiling stick to apply two circular bands of grease around the stopcock, midway between the bore and the ends. Insert the stopcock into the barrel (of the separatory funnel) and twist several times. The grease will spread out and the joint will be completely transparent.

c.   Avoid using too much grease; it could plug the bore of the stopcock. If this happens, come to the Chemistry Stockroom window and request a pipe cleaner to unplug the bore or ask Dan to help.

B.   Storage of Glassware

At the end of your lab period, you should take apart and clean all ground-glass joints. The grease is hydrocarbon-based, so it is soluble in most of the organic solvents stocked in the teaching lab rooms. Leaving a joint sticky with grease provides a surface to which dirt and particulate matter can adhere, possibly scoring the surface when the apparatus is used again. Leaving a joint assembled can result in the joint "freezing" together. Similarly, take apart (glass) stopcocks and store the piece unassembled.

Assembling of Ground-Glass Joint Glassware

Proper assembly of apparatus is necessary to assure a successful experiment. It also helps to prevent glassware breakage, and therefore might save you from having to pay for a few costly items. Follow these guidelines:

  1. Plan your assembly so the working area will be uncluttered, with easy access to all components.
  2. Use as few clamps as possible to support the apparatus firmly. The precision of the ground-glass joints allows little room for misalignment, and the joints themselves provide mechanical support and rigidity.
  3. Support all flasks with support rings for stability, even though clamps may support the neck of the flask.
  4. Always assemble the apparatus from the bottom up:

a.   Fasten all clamps loosely at first, except the bottom clamp.

b.   Gradually tighten all clamps as the apparatus goes into complete assembly.

c.   Be sure alignment is correct, then finally tighten all clamps.

d.   Do not position your apparatus at an angle.

e.   Never force alignment of ground-glass apparatus. This causes breakage, leakage, and improper function of the apparatus.


Frozen Joints

Laboratory distillation glassware, separatory funnels, and reagent bottles with glass-to-glass connections sometimes become "frozen," or stuck. To loosen stopcocks, stoppers, or ground-glass joints, you may use the following techniques at your own risk.


Caution: Remember glass is fragile, and broken glass can cause cuts and serious injury. Use care and handle gently! Do not apply too much force or subject to rough treatment. If you would rather not attempt any of the following procedures, simply bring your frozen joint to the Chemistry Stockroom window and request that CS personnel assist you.

To loosen a frozen joint:

  1. Gentle tapping: Gently tap the frozen stopcock or stopper with the wooden handle of a spatula (or with the wooden test tube rack in your drawer). Do not tap with another piece of glassware. Tap so the direction of force will cause the stopper to come out. If you tap too hard, you can break the stopper. Always work directly over a benchtop covered with soft cushioning material to prevent the falling stopper from breaking.
  2. Heating: Heat causes the housing to expand. Try immersing the frozen connection in hot water. When it is hot, gently tap with the wooden handle of a spatula. Repeat several times, allowing the housing to expand and contract to break the frozen seal.
  3. Soaking: Soak the frozen assembly in hot dilute glycerine and water. The penetration of the liquid is evident by the appearance of liquid between the joints. Allow to soak long enough to allow fullest penetration, then remove and rinse thoroughly with tap water and try gently twisting the joint, or tapping as described above.
  4. The "I Give Up" Approach: Often the most popular method of dealing with a frozen connection is to simply ask Stockroom Dan to take care of it for you. Keep in mind, though, that we have been dealing with this stuff for years, so if we hand back your piece of equipment after loosening it with about 2 seconds of effort, don't feel bad, as if your problem was trivial or a waste of time: we've just had more practice! Of course, sometimes we will simply exchange your frozen piece of equipment for another (non-frozen) one.