Brass is a metal alloy made of copper and zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties. It is a substitutional alloy: atoms of the two constituents may replace each other within the same crystal structure.
Why you shouldn't be wearing brass or bronze jewelry!!!& why I don't offer close fitting jewelry in anything but silver or gold or vermeil! Brass is an alloy of copper and zinc. Copper isn't terribly harmful for you, though it will turn you greenish and is a sensitizer in many people causing rashes, irritation and staining. Zinc commonly upsets skin with comparable results. For "castability", most brasses contain nickel, which is poisonous and an irritant. Brasses and bronzes alike will frequently contain lead at concentrations of around 1-2%. This is a ridiculously harmful metal to be wearing against the skin Highly damaging to the brain and organ tissues. Other alloying materials include arsenic and phosphorus, and if a jeweler's "Brass" or "bronze" are not formulated by the jeweler themselves, it likely that they don't have access to accurate information regarding any metal's composition. Brass and Bronze that are not made by a jeweler come from an industrial source where it is alloyed with any number of additive metals to serve an industrial purpose; rustproof, spark resistant, insulation, heat dispersion not for close human contact. An additional problem with ambiguous "alternative metals" comes down to the ethics of the overseas manufacturers that produce a large percentage of the jewelry you find in stores in the US. Maybe you remember a few years back when Chinese-made toys for US markets contained lead paint. Well, if lead and other toxic additives guarantee a cleaner cast, you better believe they will be used to cut costs or reduce loss.