The coloring matters are impurities, often Fe and mn, if red or brown.
"An Introduction to Chemical Science"
R.P. Williams
It contains as impurities, C, Si, S, P, mn, etc.
"An Introduction to Chemical Science"
R.P. Williams
mn removes part of the extra O, and, though it remains, does not injure the metal.
"An Introduction to Chemical Science"
R.P. Williams
