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The Correct Way to Insert Glass Tubing or a Thermometer into a Stopper

Prevent cuts by using the correct technique for this procedure.

by Lab Manager
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Cuts from Glass

Cuts occur most often when thermometers or pieces of glass tubing are forced into rubber stoppers. Prevent cuts by using the correct technique for this procedure. The hole should be lubricated with glycerol or water to facilitate the movement of the glass tubing. The glass should not be gripped directly with the hands, but rather by means of cloth towels. The towels will protect your hands if the glass should break. Use a gentle twisting motion to move the tube smoothly into the stopper. Avoid cuts from other sources by discarding chipped and cracked glassware according to your teacher’s instructions. If you should receive a minor cut, allow it to bleed for a short time. Wash the injured area under cold running water, and notify your teacher. Serious cuts and deep puncture wounds require immediate medical help. Notify your teacher immediately. While waiting for assistance, control the bleeding by applying pressure with the fingertips or by firmly pressing with a clean towel or sterile gauze.

Related Article: Cut to the Bone: A Dozen Tips for Working Safely with Laboratory Glassware

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Inserting Glass Tubing

In many experimental procedures, you are required to insert a thermometer or a length of glass tubing into a hole in a rubber stopper. It is essential that you know the correct way to do this. Otherwise, serious injury may result.

  1. Lubricate the end of the glass tubing with a few drops of water, washing-up liquid, glycerol, or vegetable oil.
  2. Hold the glass tubing close to where it enters the hole in the rubber stopper. Protect your hands with work gloves or pieces of cloth (one method is to insert a cork borer (one size above the OD of the tubing) into the hole. The stopper is flexible enough to allow the cork borer to slide right in. Insert the glass tubing and position it appropriately. Then pull the borer back out. The glass tubing will stay in).
  3. Ease the tubing into the hole with a gentle twisting motion. Push the tubing through the hole as far as is required. Do not use force!
  4.  Wipe excess lubricating material from the tubing before continuing with the experiment.
  5. If the glass tubing is to be removed from the stopper, it should be done immediately after the experiment is completed. 

CAUTION: The end of the glass tubing should be fire-polished or smoothed with emery cloth before being inserted into a rubber stopper. Do not try to bend the glass tubing—it will break. Ensure that the palm of the hand holding the rubber stopper is not in line with the emerging glass tube.


Cutting and Fire Polishing 

  1. Place the glass tubing or glass rod on a flat surface (such as the laboratory bench).
  2. Hold the glass tightly with one hand close to the area to be cut.
  3. Using a firm stroke, make a single deep scratch with a triangular file. CAUTION: Do not use a sawing motion or repeated scratching.
  4.  Grasp the glass in both hands with the scratch facing away from you and both thumbs directly behind the scratch.
  5. Push firmly with the thumbs and pull with your fingers. The glass should snap with a clean break. CAUTION: Be careful with the cut ends of the glass. They may be sharp and jagged. Do not attempt to break glass tubing having an outside diameter greater than 6 mm.
  6. The cut ends of the glass tubing should be fire-polished to make the tubing safe to handle. Rotate one end of the glass tube in the hottest part of a burner flame, until the sharp edges have softened and become rounded. CAUTION: Do not hold the tubing in the flame too long. If you do, the hole in the tube will close.
  7. Place the hot glass on a wire gauze square to cool. CAUTION: Hot glass and cold glass look alike. Make sure one end of a piece of glass has cooled before you attempt to fire-polish the other end.

Bending Glass Tubing

  1. Put a wing top or flame spreader on your gas burner.
  2. Light the burner and adjust the flame to produce an even blue (hot) flame across the wing top.
  3. Grasp a length of glass tubing that has been fire-polished at both ends. Hold the center of it lengthwise in the flame, just at the top of the blue region. This is the hottest part of the flame.
  4. Rotate the tubing in the flame to heat approximately a 5-cm section uniformly, until it becomes soft and just begins to sag.
  5. Remove the tubing from the flame and bend it to the desired shape in one movement.
  6. When it has hardened, put the glass tubing on a wire gauze to cool. 

CAUTION: Hot and cold glass look alike.

Thanks to Zvi Blank for contributing to this tip.