Fig. 408.—Coarse wire-drawing.
A brief summary of the Bessemer process may be interesting. If a bar of steel as soft as iron be made red-hot and plunged into cold water, it will become very hard. If it be then gently heated it will become less hard, and is then fitted for surgical instruments. The various shades of steel are carefully watched,—the change of colour being due to the varying thickness of the oxide; for we know that when light falls upon very thin films of a substance,—soap-bubbles, for instance,—the light reflected from the under and upper surfaces interfere, and cause colour, which varies with the thickness of the film. These colours in steel correspond to different temperatures, and the “temper” of the steel depends upon the temperature it has reached. The following table extracted from Haydn’s “Dictionary of Science” gives the “temper,” the colour, and the uses of the various kinds of steel.
Fig. 409.—Fine wire-drawing.
| Temperature Cent. Fahr. | Colour. | Uses of Steel. |
|---|---|---|
| 220° = 430° | Faint yellow | Lancets. |
| 232° = 450° | Pale straw | Best razors and surgical instruments. |
| 243° = 470° | Yellow | Ordinary razors, pen-knives, etc. |
| 254° = 490° | Brown | Small shears, scissors, cold chisels, etc. |
| 265° = 510° | Brown and purple spots | Axes, pocket-knives, plane-irons, etc. |
| 277° = 530° | Purple | Table-knives, etc. |
| 288° = 550° | Light blue | Swords, watch-springs, etc. |
| 293° = 560° | Full blue | Fine saws, daggers, etc. |
| 316° = 600° | Dark blue | Hand and pit saws. |
The Bessemer process transfers the metal into a vessel in which there are tubes, through which air is forced, which produces a much greater heat than a bellows does. Thus in the process the carbon of the iron acts as fuel to maintain the fusion, and at the same time by the bubbling of the carbonic acid mixes the molten iron thoroughly.
During the bubbling up of the whole mass of iron, and the extreme elevation of temperature caused by the union of the carbon of the impure iron with the oxygen of the air, the oxide of iron is formed, and as fast as it forms fuses into a sort of glass; this unites with the earthy matters of the “impure” iron, and floats on the upper part as a flux, thus ridding the “cast iron” of all its impurities, with no other fuel than that contained in the metal itself, and in the air used. When the flame issuing from the “converter” contracts and changes its colour, then the time is known to have arrived when the iron is “ de-carbonized.” The amount of carbon necessary is artificially added, ebullition takes place, a flame of carbonic oxide comes out, and the metal is then run into ingots.
The compounds of iron which are soluble in water have a peculiar taste called chalybeate (like ink). Many mineral springs are so flavoured, and taste, as the immortal Samuel Weller put it, “like warm flat-irons.” Iron is frequently used as a medicine to renew the blood globules.
Protoxide of Iron is known only in combination.
Sesqui-Oxide of Iron is “red ironstone.” Powdered it is called English rouge, a pigment not altogether foreign to our use. In a pure state it is a remedy for arsenical poisoning, and is really the “rust” upon iron.