From Wikapedia
Modern pewters must contain at least 90% tin and be alloyed with copper, antimony, or bismuth to be considered a pewter. Lead is no longer permitted to be an alloying element. Older pewters with higher lead content are heavier, tarnish faster, and oxidation gives them a darker silver-grey color.
A typical European casting alloy would contain 94% tin, 1% copper, and 5% antimony. A European pewter sheet would contain 92% tin, 2% copper, and 6% antimony. Asian pewter, produced mostly in Malaysia, Singapore, and Thailand, contains a higher percentage of tin, usually 97.5% tin, 1% copper, and 1.5% antimony. This makes the alloy slightly softer.