Sulphur is of most trouble to rolling and forging operations when conducted at a red heat. It makes steel tender and brittle at that temperature--a condition known to the workmen as "red-short." It seems to have little or no effect upon the physical properties of cold steel--at least as revealed by the ordinary testing machines--consequently many specifications do not set any limit on sulphur, resting on the idea that if sulphur is low enough not to cause trouble to the manufacturer during rolling, it will not cause the user any trouble. Tool steel and other fine steels should be very low in sulphur, preferably not higher than 0.03 per cent. Higher sulphur steels (0.06 per cent, and even up to 0.10 per cent) have given very good service for machine parts, but in general a high sulphur steel is a suspicious steel. Screw stock is purposely made with up to 0.12 per cent sulphur and a like amount of phosphorus so it will cut freely. Manganese counteracts the detrimental effect of sulphur when present in the steel to an amount at least five times the sulphur content. PHOSPHORUS is an element (symbol P) which enters the metal from the ore. It remains in the steel when made by the so-called acid process, but it can be easily eliminated down to 0.06 per cent in the basic process. In fact the discovery of the basic process was necessary before the huge iron deposits of Belgium and the Franco-German border could be used. These ores contain several per cent phosphorus, and made a very brittle steel ("cold short") until basic furnaces were used. Basic furnaces allow the formation of a slag high in lime, which takes practically all the phosphorus out of the metal. Not only is the resulting metal usable, but the slag makes a very excellent fertilizer, and is in good demand. SILICON is a very widespread element (symbol Si), being an essential constituent of nearly all the rocks of the earth. It is similar to carbon in many of its chemical properties; for instance it burns very readily in oxygen, and consequently native silicon is unknown--it is always found in combination with one or more other elements. When it bums, each atom of silicon unites with two atoms of oxygen to form a compound known to chemists as silica (SiO2), and to the small boy as "sand" and "agate." Iron ore (an oxide of iron) contains more or less sand and dirt mixed in it when it is mined, and not only the iron oxide but also some of the silicon oxide is robbed of its oxygen by the smelting process. Pig iron--the product of the blast furnace--therefore contains from 1 to 3 per cent of silicon, and some silicon remains in the metal after it has been purified and converted into steel.