Breaking Bimetallic Beads
Most recent answer: 12/19/2013
- esther (age 23)
Brooklyn,ny USA
Your general idea makes sense, but I think there must be some other complication in this case. The reason is that silver and brass have very nearly the same thermal expansion coefficients: 18.7x10-6/K for brass,
19.5x10-6/K for silver.
It's possible that the thermal strain is caused by the outside cooling down quickly while the inside is still near room temperature. That could cause the outside plating to shrink and crack.
Another possibility is that the brass undergoes a phase transition from one crystal form to another, which can happen a bit below room temperature for certain compositions of brass. I can't say exactly what temperature because it depends on the copper/zinc ratio of your brass. Also, the phase diagrams I've found don't go below room temperature, although from the part I could find it looks like these phase transitions would be present below room temperature. There's a story (nobody seems to be sure if it's true) that Napoleon's retreat from Moscow turned into a catastrophe largely because the tin buttons on the French winter uniforms disintegrated when they underwent a phase transition caused by the cold.
It's also conceivable that there are some fumes in the luggagae compartment that get into the brass and cause the "season cracking" described in the Wikipedia article on brass ().
How could you test these ideas?
If it's the first (thermal strain due to a temperature difference between the outside and the inside), then you could get the beads very cold (maybe to dry ice temperatures) without cracking them, so long as the cooling was very slow. Even modest cooling (say to ice temperature) might crack them if it's fast enough, say by immersion in ice water.
If ithe cracking is caused by a brass phase transition, then the rate of cooling shouldn't matter much, only how cold they get.
If it's a chemical effect, you could probably get it to happen by exposing the beads to ammonia fumes at room temperature. Don't breath the fumes, ammonia is bad for you.
Mike W.
(published on 12/19/2013)