The acid I used is a cold patination treatment called Tourmaline. It is designed for copper, bronze and brass, but the black version works nicely on aluminium. It is a diluted mix of Nickel and Copper Sulphates. It etches into the surface, but can be easily removed with sandpaper or wire wool. You can build it up in layers and if you catch it right you get a nice rusty brown colour before it turns black. It burns itself out quite quickly so it will not continue to eat away at the surface, unlike concentrated copper sulphate. Stinks of eggs.
I'm not sure what the leather is, it was left over from the last Harry Potter.
Cheers
Neil