I've seen several postings here recently stating that a thief could easily shatter the cylinder of any lock using a can of freon. I go to wondering if this is true or just an urban legend. It just happens that I had an old Masterlock No. 1 padlock (the ubiquitous 'standard' laminarted steel lock) and a spare can of R134a coolant laying around so I decided to conduct an experiment for the benefit of the bikeforums readership.
Although R134a is the standard coolant used in automobiles today. It costs a couple bucks per 12 oz can at walmart and is probably the coolant most easily accessible by a thief. Although the old R12 is still available from ebay, it costs approximately $20 per can and is unlikely to be used by a thief for stealing a bike. R134a does not cool as well as R12 in an air conditioner so I would not expect it to freeze a lock quite as well as R12. nonetheless, that's what is available and will be referred to as freon for this discussion.
The idea behind breaking a lock with freon is to direct a stream of liquid freon into the keyhoe of the lock. The theory is that the evaporating freon will rapidly cool the cylinder so cold that the metal becomes brittle. A hard blow with a hammer will cause the cylinder to shatter and allow the shackle to release.
Before anyone attempts to duplicate this experiment, I should remind them to be very cautious when handling any freon-like product. Freon gets extremely cold when it changes from liquid to gas and the vapor can freeze you skin and cause frostbite. Frostbite is serious and can result in amputations, blindness, and surely other nasty conditions. You must wear proper safety equipement and know what you are doing. This doesn't imply that I knew what I was doing, so don't use my description as an example of the proper way to handle this stuff.
What I did was clamp the shackle of the lock in a pair of vice grips which I held using a cloth lined rubber glove. I connected a standard automobile recharge hose to the can of freon. Holding the lock with the keyhole straight up, I opened the valve on the can and held the can upside down so the liquid freon would flow through the hose. I held the end of the hose against the lock cylinder and allowed the entire can to drain out onto the lock.
While the can was draining, I was able to observe liquid running on the lock and into the cylinder. I could also see frost form on the outer surface of the lock.
When the can was empty, I quickly took a hammer and delivered several hard blows to the keyway area of the lock attempting to shatter the cylinder.
After repeated blows, including some with the lock held against the driveway, there was no evidence of any shattering or damage of any kind to the cylinder of the lock. The outside of the lock did exhibit some minor damage. The plastic 'bumper' arond the base of the lock broke off after several blows. One of the steel laminations comprising the body showed a slight outward bowing, and the rivets on the bottom of the lock were flattened by the impacts of the hammer.
The lock did not open. I attempted to turn the cylinder with a screwdriver without success.
Conclusion. The use of freon to defeat a lock is not a viable technique. My test was against a single lock. Other locks may or may not be susceptible to this attack. However, the lock I used was a low end lock and would likely be most susceptible to attack.
I recommend thieves continue to invest your money in bolt cutters and car jacks.