Its not actually combined. An enchanter scribes the spell, then casts it, consuming one platinum bar from inventory, and creating an enchanted platinum bar on your cursor.
Obtainable From Felwithe Enchanter guild. Level 34 spell, from left handside room from Female High Elf in Blue. The spell combined with bar makes the Enchanted(not to state the obvious). The mass enchant spell is from EC tunnels.
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'The only thing I cannot resist is temptation.', Oscar Wilde.