This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
A good way to remove the black deposit left on bathtubs by the water is to use a strong solution of sulphuric acid. The acid should be poured on the discolored enameled surface and washed around with a cloth. The acid should not touch the hands so it is best to use a stick to move the cloth. Neither should it be allowed to touch the metal parts, but if this happens, no harm will result if it is quickly rubbed off with a cloth and water then applied.
In using this acid never pour water into the acid, but always pour the acid into the water. - Contributed by Loren Ward, Des Moines, Iowa.