level, the moveable covers alone projecting above the surface.

The pots for melting steel are made of a mixture of different clays, burnt-clay, and coke-dust. In Sheffield they are composed of a mixture of Burton and Stannington clays, to which is sometimes added Stourbridge clay, and if the pots have to withstand excessive heat, china-clay is also added. This mixture is incorporated with about a third of its quantity of coke-dust and old pots, the whole being ground very fine and sifted. The pots are moulded in an iron frame by means of a wooden plug, then slowly dried, and the night previous to being used, they are gradually raised to a dull red-heat; this is termed annealing. Each pot is about 18 inches high, 7 inches in diameter at the mouth, and lasts one day, during which time three charges are melted of about 50 lbs., 44 lbs., and 38 lbs., respectively.

The bars of blister-steel are carefully selected, according to the temper required, all "flushed" or "aired" bars being rejected. They are broken up into small pieces, weighed, and introduced into the pot, and the lid carefully adjusted. When the melting is completed, the crucible is removed and the metal skimmed, then poured or "teemed," as it is termed, into a mould. Should a piece of coke accidentally get into the pot, it makes the steel red-short, and produces a bright glittering fracture. If the steel does not remain a sufficient time in the fire,

or if poured at too high a temperature, it will teem "fiery," and the ingot will be honeycombed. If the metal remain too long in the fire it will teem "dead,” the fracture will appear scorched, and though sound, it will be brittle if hard, and wanting in tenacity if mild. If the metal be cooled too much before teeming, the fracture will be dull in colour and full of pin-holes. Mild steel rises in the mould after teeming, and to make the ingot sound a cast-iron stopper is inserted in the top of the mould. Hard steel, containing from 2 per cent. of carbon and upwards, teems "dead," that is, settles down 3 or 4 inches at top of the ingot. A pot of cast steel, when removed from the fire, has the appearance of a boiling liquid from the liberation of gases, which are accompanied by scintillations, and if poured in such a condition the metal would boil over in the mould, and produce a spongy structure; it is therefore allowed to stand some time to be "killed," as it is termed. For large ingots, the contents of several crucibles are poured simultaneously, by gangs of men, into channels which convey the steel to the mould.

For the best class of cast steel, the iron obtained from Dannemora ore, which is a magnetite containing manganese, is used; good steel cannot be made from inferior iron, containing sulphur, phosphorus, silicon, etc. in notable quantity, as these elements are very hurtful. All elements present in addition to carbon increase the hardness, brittleness, and liability of the steel to crack in hardening, but small quantities of each confer special properties, which may be useful in specific cases.

In 1801 Mushet introduced the method of making cast steel, by melting malleable iron with carbon and oxide of manganese in crucibles, which method has since been largely practised, but such steel is only of secondary quality. Another plan is to melt bar-iron with charcoal, and add a little spiegel-eisen at the end of the melting. The celebrated Heath process consists

of adding to the crucible, with the steel, a carbide, (obtained by reducing oxide of manganese with carbon, or a mixture of oxide of manganese with carbonised pitch) to improve the steel in melting. Sherman introduced a little potassium iodide into the pot along with the steel, with the object of removing sulphur and phosphorus, in virtue of their strong affinity for iodine. Many other physics have been tried from time to time, such as salammoniac, common salt, yellow prussiate of potash, potassium chromate, etc., with the object of making high quality cast-steel from inferior bar-iron, but without

success.

Indian or Wootz' steel is made in crucibles by melting 1 lb. of malleable iron with 10 per cent. of the wood and leaves of the "Cassia auriculata" and the surface covered with green leaves. A lid is luted on, or the mouth stopped up with clay. The charge, at the conclusion of the heating, is allowed to cool in the crucible, which is then broken, the iron re-heated, and drawn into a bar.

Malleable cast-iron.-This variety of iron is produced by an "oxidising" cementation. For this purpose, the cast articles should be made of a superior quality of iron, such as that produced from Cumberland hæmatite smelted with charcoal, white pig-iron being preferred, especially for large castings.

The articles are embedded in powdered red hæmatite, in cast-iron or wrought-iron cases, which are arranged in a furnace or annealing oven as it is termed, heated with gas or with the flames from solid fuel. At each operation, a certain portion of fresh ore or iron scale from rolling mills is added, the used ore being mixed with it, which, being somewhat clotted, renders the mass more permeable to the gases generated inside. About three days are required for annealing small articles, and five to six. days for large work, the temperature being gradually raised at the commencement, and allowed to cool slowly at the conclusion.

The oxide of iron Fe,O,, which is used as the cement powder, gives up oxygen, itself being reduced to the magnetic oxide Fe3O4. This oxygen unites with carbon and other impurities forming oxides, but only the gaseous oxides, such as carbonic oxide CO, are removed from the iron, hence silicon and phosphorus remain, which explains why only pure cast-iron can be employed. Some chemists consider that the change in physical properties is not due to the removal of carbon by oxidation, but to the separation of amorphous carbon within the metal, without which the metal would remain brittle, notwithstanding the oxidation of carbon on the surface. The iron, by this annealing process, is changed from very brittle white iron to malleable iron, of a greyish white colour when broken, resembling mild steel; it is only with very great care that it can be welded.

QUESTIONS.

1. How is steel produced in the finery? What iron is most suitable for this process?

2. What is the difference in the processes employed for producing puddled-iron and puddled-steel?

3. Describe the process of converting wrought-iron into steel by the system of cementation.

4. How is iron case-hardened?

5. What is shear-steel and how is it made?

6. What is blister-steel? Describe the character of its fractured surface. For what purposes is it used?

7. How is blister-steel made into cast-steel? What is the general character of cast-steel?

8. On comparing cast-steel with cast-iron, what points of resemblance and difference respectively would be observed? 9. Describe the chemical changes which occur during the conversion of a bar of iron into blister-steel.

10. How do you account for the change which occurs in bar iron by the process of converting it into blister-steel?

11. Describe some form of crucible-furnace in which gas is used as fuel.

12. What are the nature and composition of the pots used in melting steel?

13. Explain the use of manganese in making cast-steel.

CHAPTER XIII.

THE BESSEMER PROCESS FOR STEEL.

THIS process consists of blowing air through molten pigiron, in a vessel called a converter, whereby the carbon, silicon, and some of the iron are oxidised; the oxidation produces a very high temperature, keeping the mass in a liquid state, without the aid of ordinary fuel, and leaving the iron commercially pure.

Two distinct modes of working, employing totally different classes of pig-iron are now adopted. In one, the converter is lined with an acid material, termed ganister, and the method may be designated the "Acid" Bessemerprocess; in the other, the vessel is lined with calcined dolomite, which is a basic material. This method is called the "Basic" Bessemer-process. In the acid process, the iron employed must be free from sulphur and phosphorus, as these elements are not removed from the iron, so that only the purer classes of pig-iron can be used, such as those obtained from Cumberland hæmatite.

The converter is sometimes fixed, as in the ClappGriffiths process, but more generally it is so arranged as to be moveable through an angle of 180°, thus enabling the metal and slag to be poured from the mouth. Figs. 63 and 64 represent the most usual form of vessel; it consists of a shell of wrought-iron plates riveted together, the neck being inclined at an angle of 30° to the axis of the body. The centre of the body is enclosed with a stout band of iron, upon which are fixed two arms, termed trunnions, by which the vessel is suspended on iron standards.