Whether Si is reduced from liquid or gaseous, high temperatures are required.  The higher the temperature of the hearth, the more Si is reduced into pig iron. 

Conversely, the less Si in pig iron. Production statistics show that the hearth temperature (slag iron temperature) is linear with the amount of iron in the pig iron. 

Therefore, [Si] is commonly used in blast furnace production to indicate the furnace temperature, and [Si] is synonymous with the hearth temperature. 

Of course, sometimes there is a phenomenon that is not completely consistent, which indicates that the hearth is not working properly and only appears in very few cases.