[willyshop]

I've been attempting to make nickel aluminide by reacting US nickels (25% Ni, 75% Cu) in molten aluminum. It seamed to work quite well. I started to wet sand one of the more promising samples, when I noticed some bubbles. I filled a glass with tap water and set the sample in the bottom, sealing the top with saran wrap. a firm bubble developed, confirming that the substance was indeed off-gassing. The gas has a peculiar odor that I can't place. Any idea what could be going on?
edit: The gas is flammable, which suggests that it might be hydrogen, but that wouldn't explain the smell

[ajkoer]

I would suggest an impurity arising from the surface exposure of the coin to air.

Three possibilities (or more, depending on where you live, a farm, a city, near an industrial park,.. and what chemicals/cleaners you have leaching into the air of your home), sulfur forming sulfides, oxygen forming oxides and/or CO2 forming carbonates.

Solution: physically or chemically clean the surface of the coins before use.