Editing Iron ore (section)

====Sulfur====
[[Sulfur]] (S) is a frequent contaminant in coal. It is also present in small quantities in many ores, but can be removed by [[calcining]]. Sulfur dissolves readily in both liquid and solid iron at the temperatures present in iron smelting. The effects of even small amounts of sulfur are immediate and serious. They were one of the first worked out by iron makers. Sulfur causes iron to be red or hot short.{{sfn|Gordon|1996|p=7}}

Hot short iron is brittle when hot. This was a serious problem as most iron used during the 17th and 18th centuries was bar or wrought iron. Wrought iron is shaped by repeated blows with a hammer while hot. A piece of hot short iron will crack if worked with a hammer. When a piece of hot iron or steel cracks, the exposed surface immediately oxidizes. This layer of oxide prevents the mending of the crack by welding. Large cracks cause the iron or steel to break up. Smaller cracks can cause the object to fail during use. The degree of hot shortness is in direct proportion to the amount of sulfur present. Today, iron with over 0.03% sulfur is avoided.

Hot short iron can be worked, but it must be worked at low temperatures. Working at lower temperatures requires more physical effort from the smith or forgeman. The metal must be struck more often and harder to achieve the same result. A mildly sulfur-contaminated bar can be worked, but it requires a great deal more time and effort.

In cast iron, sulfur promotes the formation of white iron. As little as 0.5% can counteract the effects of slow cooling and a high silicon content.{{sfn|Rostoker|Bronson|1990|p=21}} White cast iron is more brittle, but also harder. It is generally avoided, because it is difficult to work, except in China where high-sulfur cast iron, some as high as 0.57%, made with coal and coke, was used to make bells and chimes.{{sfn|Rostoker|Bronson|Dvorak|1984|p=760}} According to {{Harvard citation text|Turner|1900|pp=200}}, good foundry iron should have less than 0.15% sulfur. In the rest of the world, a high-sulfur cast iron can be used for making castings, but will make poor wrought iron.

There are a number of remedies for sulfur contamination. The first, and the one most used in historic and prehistoric operations, is avoidance. Coal was not used in Europe (unlike China) as a fuel for smelting because it contains sulfur and therefore causes hot short iron. If an ore resulted in hot short metal, [[ironmaster]]s looked for another ore. When mineral coal was first used in European blast furnaces in 1709 (or perhaps earlier), it was [[coke (fuel)|coked]]. Only with the introduction of [[hot blast]] from 1829 was raw coal used.