The process in detail consists in freeing the bath of melted pig-iron
from excess of carbon by adding broken lumps of pure hematite or
magnetite iron ore. This causes a violent boiling, which is kept up
until the metal becomes soft enough, when it is allowed to stand to let
the metal clear from the slag which floats in scum upon the top. The
separation of the slag and iron is facilitated by throwing in some lime
from time to time. Spiegel, or specular iron, is then added; about 1
per cent. more than in the scrap process. From 20 to 24 cwt. of ore are
used in a 5-ton charge, and about half the metal is reduced and turned
into steel, so that the yield in ingots is from 1 to 2 per cent. more
than the weight of pig and spiegel iron in the charge. The consumption
of coal is rather larger than in the scrap process, and is from 14 to 15
cwt. per ton of steel. The two processes of Siemens and Martin are
often combined, both scrap and ore being used in the same charge, the
latter being valuable as a tempering material.
At present there are several large works engaged in manufacturing the
Siemens-Martin steel in England, namely, the Landore, the Parkhead
Forge, those of the Steel Company of Scotland, of Messrs. Vickers & Co.
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