On April 2, 1889, Charles M. Hall received U.S. Patent No. 400664 on a Process of Reducing Alumina from its Fluoride Salts by Electrolysis:

before Hall’s invention, aluminum was very expensive to produce — so much so that its price was near that of precious metals such as silver and gold. When the Washington Monument was completed at the capping ceremony of December 6, 1884, a pyramid of solid aluminum 22.6 cm high, 3.9 cm at its base, weighing 2.85 kg, and costing $225 was installed at the top, forming part of the monument’s lighting protection system.