This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
A very fine, grayish-black, lustreless powder, permanent in dry air, without odor or taste and insoluble in water or alcohol. When ignited in contact with air, it is converted into ferric oxide. When treated with diluted sulphuric acid, it causes the evolution of nearly odorless hydrogen gas, and, on being warmed, it is dissolved without leaving a residue. Reduced iron is very seldom employed in the manufacture of artificial mineral waters, and but then when the soluble ferrous salts cannot be applied. It is only in small quantities applicable, and slowly soluble in carbonated waters when under pressure. It is added in its powdered state to the fountain after the atmospheric air has been expelled, and a prolonged agitation by high pressure has to be kept up for some time to effect its solution, otherwise it would appear a mechanical impurity in the beverage.