Actually, to quote wikipedia:
"Native aluminium metal can only be found as a minor phase in low oxygen fugacity environments, such as the interiors of certain volcanoes.[17] Native aluminium has been reported in cold seeps in the northeastern continental slope of the South China Sea and Chen et al. (2011)[18] have proposed a theory of its origin as resulting by reduction from tetrahydroxoaluminate Al(OH)4– to metallic aluminium by bacteria.[18]"
"Bars of aluminium were exhibited at the Exposition Universelle of 1855.[54] Napoleon III of France is reputed to have given a banquet where the most honoured guests were given aluminium utensils, while the others made do with gold.[55][56]"
"Aluminium was selected as the material to use for the 100 ounces (2.8 kg) capstone of the Washington Monument in 1884, a time when one ounce (30 grams) cost the daily wage of a common worker on the project.[57] The capstone, which was set in place on 6 December 1884, in an elaborate dedication ceremony, was the largest single piece of aluminium cast at the time, when aluminium was as expensive as silver.[57]
So it was certainly around.
and dammit the whole 19th century=1800s thing got me again.
Dwarves can refine aluminum, though, (or at least find iseable deposits) so it's only reasonable that they can alloy it.