Cracking glass - additional demonstrations
Practical Activity for 14-16
Two different demonstrations on how to crack glass through thermal conduction. See also this experiment:
Apparatus and Materials
- Glass bottle
- Glass cutter (e.g. wheel or diamond)
- Unused car-engine oil
- Large glass pipe
- Non-electric soldering iron or poker
- Nichrome wire
Health & Safety and Technical Notes
Whenever a demonstration is performed with glass which might shatter and hurt someone, you must use a pair of large safety screens of Perspex. The screen between the teacher and the apparatus should be approximately 1 m high by 60 cm wide. This is narrow enough so that you can reach round and manipulate the apparatus and high enough to shield the face. The screen between the apparatus and the class should be approximately 1 m square.
These screens (of 3 mm Perspex or thicker) should not be framed. That would spoil the feeling of full transparency. They can be supported by pairs of slotted bases.
The fractured glass tube must be disposed of safely.
Used engine oil contains carcinogens and must NOT be brought into a school laboratory. New and unused oil is safe to use.
Demonstration 1 An entertaining (as well as useful) technique is to half fill a bottle with unused car engine oil.
- Heat a large non-electric soldering iron or poker in a Bunsen flame.
- Plunge it into the oil.
- The bottle will crack cleanly at the oil level. This is because the glass above the oil level is not being heated. Therefore it is not expanding as is the glass below the oil level.
- Turning the bottle into a 'beaker and a funnel' is entertaining.
Demonstration 2 A further demonstration is to use a Nichrome wire heated electrically to crack a glass pipe.
- Start the crack in your glass pipe with a wheel or diamond.
- Wrap the wire over the crack and heat it to 'red heat'. The glass pipe will break cleanly at the wire.
- This is useful technique for cutting glass tubes.
This experiment was safety-tested in March 2006