The inclusion of even small amounts of some elements can have profound effects on the behavioral characteristics of a batch of iron or the operation of a smelter. These effects can be both good and bad, some catastrophically bad. Some chemicals are deliberately added such as flux which makes a blast furnace more efficient. Others are added because they make the iron more fluid, harder, or give it some other desirable quality. The choice of ore, fuel, and flux determine how the slag behaves and the operational characteristics of the iron produced. Ideally iron ore contains only iron and oxygen. In reality this is rarely the case.
Silica (SiO2)is almost always present in iron ore. Most of it is slagged off during the smelting process. At temperatures above 1300 °C some will be reduced and form an alloy with the iron. The hotter the furnace, the more silicon will be present in the iron.
The major effect of silicon is to promote the formation of grey iron. Grey iron is less brittle and easier to finish than white iron. It is preferred for casting purposes for this reason. Turner (1900, pp. 192–197) reported that silicon also reduces shrinkage and the formation of blowholes, lowering the number of bad castings.
Phosphorus (P) has four major effects on iron: increased hardness and strength, lower solidus temperature, increased fluidity, and cold shortness. Depending on the use intended for the iron, these effects are either good or bad.
Small amounts of aluminium (Al) are present in many ores including iron ore, sand and some limestones. The former can be removed by washing the ore prior to smelting.
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 can be removed from ores by roasting and washing. Roasting oxidizes sulfur to form sulfur dioxide which either escapes into the atmosphere or can be washed out.