I had a commenter in an earlier post ask what was the problem with sulphur and steel. In specific this was in reference to high-sulphur coal being used in forging. I mentioned before that sulphur is added to some steel in carefully controlled amounts to make it easier to machine, but some of these steels, if forged, can actually crumble under the hammer.
I should have mentioned one of the better examples of too much sulphur in steel: the Titanic.
One of the things they found out after testing samples of the hull steel was that it contained a LOT of sulphur; so much that when the steel became cold- say, from water in the North Atlantic- it became brittle. So when the ship scraped against the iceberg, instead of the steel deforming- denting- and maybe some seams springing, the steel itself cracked and split.
I remember reading about a test a few years back. They did a standard test using 'cigarettes', pieces of steel cut to an exact dimensions, one from standard steel used in ship hulls and one from recovered hull plating from the Titanic. They cut a notch of exact size in one side of the cigarette, lock it in a vise, and swing a weighted pendulum against it, then measuring how far it deforms(bends). They chilled them to the same temperature as the water in the North Atlantic at the time the Titanic went down and ran the test. The standard steel bent; the cigarette from the Titanic steel snapped.
So unless very carefully controlled in amount, and for certain uses, sulphur in steel tends to be a bad thing.