Urine works well to but it takes much longer to produce the desired effect. The strangest of all was used by a good friend of mine to age the brass handles he uses in his Cabinet making business Pig slurry it works extremely well except for the smell.
Those techniques work because of the production of ammonia by the animal waste. Often used to "fume" oak to get a color that replicates antique wood for repairs or reproductions on furniture and trim. A cotton ball with a little liquid ammonia is a lot cleaner for small bits.
