Cleavage sheets are frequently disfigured and rendered of little value by brown, red or black spots and stains, often with a dendritic arrangement of iron oxides.
The advantage of this method rests chiefly on the small amount of iron required; but its disadvantages are that any silver present in the ores goes into solution, the formation of basic salts, and the difficulty of filtering from the iron oxides.
In this process (" new Douglas-Hunt ") there are no iron oxides formed, the silver is not dissolved, and the quantity of iron necessary is relatively small, since all the copper is in the cuprous condition.
The flint artifacts found in the ancient river gravels are frequently rolled, battered, and stained brown by iron oxides.
In geology, the cement of breccias and conglomerates is usually silica, iron oxides or calcite (mineral calcium carbonate ).