The safest way to insert glass tubing into a stopper is to not have to. Most of these assemblies can be bought ready-made, or replaced with a metal or plastic connection, and both Cornell and East Carolina University tell laboratories to substitute wherever they can. Where that is genuinely not an option, the technique below matters — and so does the part almost no guidance covers, which is what to do when the assembly has seized and needs to come apart.

At a glance

Substitute a metal or plastic connection where possible; that is the only measure that removes the hazard rather than managing it. If you must insert glass, fire-polish the end and let it cool, lubricate with water or glycerol — glycerol is better, and not oil — grip the glass no more than 5 cm from the end going in, keep both hands close together, and insert with a slight twist rather than pressure. A cork borer one size larger makes a useful sleeve. When a tube has seized in a stopper, do not force it: slit the stopper with a sharp knife and sacrifice it. Removal is the operation that causes the worst injuries, because by then the assembly is aged and the person is applying force to free it.

Why this particular task hurts people

SUNY Geneseo’s chemistry stockroom puts it bluntly: this procedure, while seemingly a simple one, is responsible for a majority of student injuries in their chemistry laboratories. It is worth understanding why a task this trivial has that record, because the reason points directly at the technique.

Thin-walled glass tubing is strong in compression along its axis and weak in bending. When a tube meets resistance in a stopper hole and the hand pushes harder, almost none of that extra force goes straight down the axis — hands are not that precise. It goes into bending. The tube becomes a lever, the stopper becomes the fulcrum, and the length of glass between your grip and the stopper is the lever arm. Grip six inches back and a modest push generates enough bending moment to snap the tube. The break is not the injury; the injury is what happens next, when a freshly fractured end with a jagged edge travels the remaining distance into a palm or wrist under the force you were already applying.

Two consequences follow, and they are the whole technique. Shorten the lever arm — Cornell specifies gripping no more than 5 cm from the end being inserted. And shorten the travel — East Carolina advises holding both hands close together, so a fracture has less distance to accelerate over. Everything else is lubrication and hand protection, which reduce the chance of a break and the consequences of one, but do not change the mechanics.

First: can you avoid the task entirely?

Work down this list before reaching for a stopper and a length of tubing. This is a hierarchy-of-controls question, and glass-into-rubber sits at the bottom of it.

A note on the last row of that table. The reason this topic generates so much instructional content is historical: it used to be unavoidable, and it was taught to every undergraduate. It is now largely avoidable, and most published guidance has not caught up. If you are writing a laboratory SOP today, the substitution question belongs above the technique.

The insertion procedure

1. Check the fit before anything else. The tube diameter and the hole diameter should be compatible — snug, not tight. A stopper with an irregular hole, a hole that does not fit the tube snugly, a crack, or a leak should be discarded rather than persevered with. Bore a new one with a sharp borer one size smaller than will just slip over the tube.
2. Fire-polish the end going in, and let it cool. A cut end has a sharp edge that bites into rubber, so it needs more force and it cuts you if it breaks. Hot glass and cold glass look identical, so assume it is hot until you have established otherwise. Our guide to safe Bunsen burner use covers the flame side.
3. Lubricate with water or glycerol. Glycerol is the better lubricant. Do not use oil: it degrades rubber over time and is difficult to remove cleanly from a joint that will contact your sample. Stopcock grease is acceptable where a trace of it will not matter.
4. Protect both hands. Heavy gloves, or layers of cloth wrapped around the glass — and protect the other hand too, by holding the stopper with a layered cloth pad rather than bare fingers. Our guide to glove selection and use covers the hand protection side; cut resistance matters more here than chemical resistance.
5. Grip the glass no more than 5 cm from the end being inserted. This is the step that prevents the injury. Everything else reduces probability; this reduces consequence.
6. Hold both hands close together, so that if the tube fractures the broken end has minimal distance to travel.
7. Insert with a slight twisting motion, avoiding both excessive pressure and excessive torque. If it is not going in, stop and reassess the fit or the lubrication. Force is the failure mode — there is no amount of determination that makes a wrong-sized hole right.
8. Consider a cork borer as a sleeve. Slide a borer one size larger than the tubing into the hole first, pass the tube through the borer, then withdraw the borer. The rubber is flexible enough to accept both, and the borer takes the friction instead of the glass.
9. Keep the palm and forearm out of the line the tube will emerge along, and keep your face away from the assembly.
10. Wipe off excess lubricant afterwards, so it does not reach the sample. As a guide, a rubber or cork stopper should sit one-third to one-half inserted into the joint it seals.

Removal, which is the more dangerous half

This is the part most guidance omits, and it is where the serious injuries happen. An assembly that went together easily six months ago has since aged. Rubber hardens, takes a set around the glass, and bonds to it. The person removing it is applying increasing force to a component that is now more brittle than it was, usually while gripping it further back than they should, and often while it is still attached to something else.

The answer is to sacrifice the stopper. Cornell’s laboratory safety manual states it in one line: remove stuck tubes by slitting the hose or stopper with a sharp knife. A stopper costs less than a stitch. Cut along the length of the stopper away from yourself, peel it off the glass, and discard it.

• Do not twist harder. Torque is what breaks the tube, and a seized joint concentrates it at exactly the point where the glass is supported.
• Do not use pliers or a wrench on the glass. Point loading on thin-walled glass does not end well.
• Do not soak it and hope. Water will not reverse a rubber-to-glass set, and a wet assembly is harder to grip.
• Do not hand it to someone stronger. This is the most common escalation and it changes nothing about the mechanics except the amount of force available.

The prevention is a decision, not a technique. Either separate the assembly promptly after use, while lubricant is still present and it comes apart with no force, or commit to leaving it assembled permanently and treat the stopper as part of the apparatus. What causes injuries is the middle option: leaving it for months and then deciding to reclaim the thermometer.

Thermometers, and one specific case

Thermometers concentrate the risk, because they are longer than a tube offcut, which means a longer lever arm and a natural temptation to grip further back, and because the stem is thinner walled than tubing at the bulb end.

If the thermometer contains mercury, the calculation changes entirely. A mercury thermometer snapping in a stopper is a laceration and a mercury spill at the same moment, on the same hands. Mercury-in-glass thermometers have been progressively withdrawn from laboratory use, and substituting a spirit-filled, thermocouple or digital thermometer eliminates that combination outright. If your laboratory still holds mercury thermometers, this task is a good reason to review whether it needs to — and if a mercury device must go into a stopper, buy the assembly pre-made rather than building it.

If someone is cut

• Control the bleeding. Direct pressure with a clean dressing. For a deep cut or a puncture, pressure and immediate medical assistance.
• Do not probe the wound and do not attempt to remove embedded glass. Retained fragments are common in glass lacerations and are a matter for clinical assessment, not for tweezers at the bench.
• Get it assessed even when it looks minor. A small entry wound from a tube end can be deeper than it appears, and tendon and nerve involvement in the palm is not always obvious immediately.
• Tell the clinician it was glass, and whether the glass had contacted anything. Chemical contamination of the wound changes management.
• Report it. This injury is preventable by substitution, so every occurrence is information about an assembly that should have been bought rather than built.

Our guide to medical emergencies in the laboratory covers the wider response, and working with laboratory glassware covers glass handling generally.

Preparing the tubing: cutting, polishing, bending

Included because you cannot insert a tube you have not prepared, and because a badly prepared end is a cause of the injury above.

Cutting. Lay the tubing on a flat bench and hold it firmly near the cut point. Make a single deep scratch with a triangular file — one stroke, not a sawing motion, which produces an uneven weakness rather than a clean line. Hold the tube with both hands, thumbs directly behind the scratch, and snap it by pushing with the thumbs while pulling with the fingers. Wrap the tube in cloth for this. Tubing above roughly 6 mm outside diameter is generally considered too thick to break safely by hand.

Fire polishing. Hold the cut end in the hottest part of the flame and rotate continuously until the sharp edge softens and rounds. Stop before the bore starts to close. Set it on wire gauze to cool. An emery cloth will take the edge off if a flame is not available, though it will not round the end as well.

Bending. Fit a wing top or flame spreader and set a uniform blue flame. Rotate the fire-polished tubing in the top of the blue region, heating about a 5 cm section evenly until it begins to sag under its own weight. Remove it and bend in one smooth motion — hesitating produces a kink and a thin wall at the outside of the curve. Cool on wire gauze. Assume it is hot for considerably longer than seems reasonable.

Bench card

Copy this section and post it in the stockroom or wherever tubing is prepared.

Resources and further reading

• Cornell EHS: insertion of glass tubes or rods into stoppers — the most complete short procedure I found, and the source of the 5 cm grip rule and the slit-the-stopper removal instruction.
• Cornell EHS: preparation of glass tubing and stoppers — boring technique, borer sizing, and the criteria for discarding a stopper.
• East Carolina University EHS: glassware — the substitution recommendation and the hands-close-together point, plus the warning that stoppers must never be relied on as pressure relief.
• SUNY Geneseo chemistry stockroom: glassware safety — the source of the observation that this procedure accounts for a majority of their student laboratory injuries.
• General lab safety rules and PPE selection — the baseline this assumes, and cleaning laboratory glassware for what happens to the tubing afterwards.

This article was produced under Lab Manager’s AI Editorial Guidelines