The pig-iron used should be dark grey, containing a large proportion of free carbon, and a small percentage of silicon and manganese. It should be almost free from sulphur and phosphorus.
A blast of air is then blown through the metal in the converter for about twenty minutes.
This removes all the carbon, after which from 5 to 10 per cent of spiegeleisen 4 (a variety of cast iron rich in carbon and manganese) is added.
The blowing may then be resumed for a short time, in order to thoroughly incorporate the two metals, the steel is run off into a ladle, and thence into moulds.
The colour of the flame issuing from the mouth of the converter indicates the moment at which all the carbon has been removed, or this may be accurately ascertained by examining the flame with a spectroscope.
1 Bloxam's Metals.
2 In some works the melted metal is carried direct from the blast furnace to the converter.
3 A sandstone from the coal measures much used in a powdered state for this and similar purposes.
4 Mirror-iron; so called from its shining appearance.
The ingots produced contain air-holes, and are not sufficiently dense. They are therefore kept hot and rendered compact by the blows of a steam hammer, after which they may be rolled or worked as required for the purpose for which they are intended.
Bessemer steel is used chiefly for rails and tyres for the wheels of railway carriages, also for common cutlery and tools, such as hatchets, hammers, etc.
The Basic Process, by Messrs. Thomas and Gilchrist, resembles the Bessemer process, but that the converters into which the fluid pig-iron is run are lined with basic material, generally magnesian limestone or some refractory substance as free as possible from silica. By this process the less pure ores of the Cleveland district, containing a large proportion of phosphorous, can be converted into steel. Lime having been added, the blowing in the converter commences, the silicon passes off first, then the carbon, and then the phosphorus. When the operation is nearly completed, a small sample ingot is cast, cooled, and broken, and by the fracture the amount of phosphorous still remaining is estimated.