to air-space is as 4: 7, which allows for varying amounts

of air while the gas remains the same, and an oxidising or reducing flame as required.

The bed of the furnace is made of cast-iron boxes, cooled by circulation of water, and provided with similar water-bridges at each end. It is lined with oxide of iron, slag, etc., like an ordinary puddling-furnace. A heating chamber is arranged at each end of the furnace, in which the pig-iron is heated to redness before charging. The heated gas is brought into the furnace by a narrow rectangular chamber, opening into a slit in the body of the fire-bridge. The hot air enters through a parallel flue behind the gas-flue, but at a higher level, so that by its greater density it falls and mingles with the gas. At this point combustion takes place.

The gas-producer Fig. 41, is

Fig. 41.

a nearly rectangular chamber lined with fire-brick.

The side A is formed of iron plates lined with fire-bricks, and has a step grate B, with wrought iron bars C. The fuel is charged through the hopper D. The gas passes up the pipe E, which is cased with iron, and into a horizontal wrought iron pipe which conveys it to the regenerator. The combustible portion of the gas consists chiefly of carbonic oxide CO, called air gas. When a jet of steam is introduced into the regenerator the gas liberated contains hydrogen and carburetted hydrogen; it is then called water gas. The gases generated in gas-producers for metallurgical purposes contain from 25 to 34 per cent. carbonic oxide, 55 to 60 per cent. nitrogen, the remaining portion consisting of carbonic acid, hydrogen, and hydrocarbons.

The effect of water vapour or steam on incandescent carbon is to form hydrogen, marsh-gas, and carbonic oxide, thus:

The effect of oxygen is to produce carbonic oxide. The amount of steam admitted should never exceed 10 per cent. of the air, otherwise the water vapour will tend to stop the process by diminishing the combustion. By the use of superheated steam a slightly larger amount than 10 per cent. may be advantageously admitted.

Waste heat.-The waste heat of furnaces used for refining iron has been utilised in a variety of ways, but chiefly for heating steam-boilers. The commonest form of boiler is a vertical cylinder with hemispherical dome, placed in the chimney. The flame passes round it, or through a central flue connected with the outer shell by horizontal tubes. Two furnaces are generally connected with one boiler.

At Rhonitz a combination of puddling and re-heating furnaces is adopted. An ordinary gas-producer using

wood fuel is combined with three hearths in one straight series. The first is used for re-heating, and receives the hot gases without air. The second is the puddlingfurnace where the gases are burned by admitting air at the fire-bridge. The third is heated by the waste heat from the puddling-furnace, and used for heating the pigs previously to puddling. Lastly, a hot-blast stove is placed in the base of the chimney, by which means the air employed in the puddling-furnace for burning the gases is raised to 200° C.

The method of using the waste heat in Siemens' furnace has been already described. A somewhat analogous plan is employed by Ponsard (see Fig. 53).

The loss in puddling varies, according to the impurity of the pig-iron employed, from 5 to 10 per cent.; and the fuel consumed is from 20 to 25 cwt. per ton of puddled bars produced. The charge of pig-iron for an ordinary puddling-furnace is 4 to 4 cwts., and in Staffordshire 5 to 7 heats are worked off in 12 hours.

QUESTIONS.

1. In what respects does the modern differ from the original process of puddling?

2. In puddling pig-iron containing phosphorus, sulphur, silicon, and manganese, in what way are these bodies removed? 3. Roughly sketch and describe some form of mechanical puddling-tool, and state its advantages and disadvantages as compared with hand-tools.

4. Describe some form of mechanical puddling-furnace, and state how far it has been successful.

5. Describe the chemical changes which take place during the puddling of pig-iron in the ordinary manner.

6. Describe the method of refining iron in the "finery." 7. Contrast the nature of the chemical changes taking place in the puddling-furnace with those occurring in the blastfurnace.

8. Sketch and describe a modern puddling-furnace.

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9. Sketch and describe some form of rotating furnace used for puddling iron. State its advantages and disadvantages. 10. Describe some method of puddling iron in which gas used instead of solid fuel.

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11. How is the waste heat of puddling-furnaces utilised? 12. From puddling a ton of pig-iron of a given quality, how many cwts. of rails would you get?

CHAPTER IX.

TREATMENT OF PUDDLED-IRON.

THE operations which are conducted in that part of an iron-works known as the "forge," include puddling, shingling or hammering, rolling, etc. The oldest classes of hammers for shingling are the "tilt," where the axis is between the point of application of the cam and the head; and the "helve-," or lift-hammer, where the hammerblock and lifting-cam are on the same side of the fulcrum. The former is used for light work, the latter for heavier work, such as shingling puddled-balls, blooming piles, etc. The modern form is the steam-hammer, employed both for shingling and welding.

Various forms of squeezers are also used, instead of the hammer, for the treatment of puddled balls.

The rolling mill is generally used for merchant iron, the rolls being of two kinds-" roughing" rolls, and "finishing" rolls of several forms, according to the shape of the bar required.

The white-hot balls of iron formed in the puddlingfurnace are taken to a hammer or squeezer, in order to expel the enclosed slag and weld the particles of iron into a compact mass, and also to confer upon them a shape suitable for passing through the rolls.

The tilt-hammer Fig. 42 rarely exceeds 5 cwts., and is not much used

quantities are treated. It generally consists of a wooden shaft hooped

Fig. 42.

with wrought iron; at one end is the head in the form of a heavy hammer, and the other end is depressed by the projecting teeth of a revolving cam, falling by its own weight as each tooth passes. fulcrum is placed between the head and the cam, but nearer to the latter.

The

The helve-hammer consists of a heavy mass of castiron weighing from 2 to 10 tons, lifted by projecting pieces fixed in a cam-ring, and falls by its own gravity through a space of 16 to 48 inches. Fig. 43 represents a belly-helve" in

which the cam is placed below the surface of the ground, and acts on a projection termed a "bray,"

Fig. 43.

which can be lengthened or shortened according to the size of the iron to be hammered. This form enables the workman to go all round the hammer to inspect the work. The head of the arm contains the hammer face, made of wrought iron and dovetailed in. The anvil is also of wrought iron attached to a heavy casting weighing from 12 to 15 tons.

The steam-hammer Fig. 44 is now largely employed both for shingling and welding. It is a simple direct-acting machine, and takes up little room compared with the cumbrous wheel-work of the old helve. The force of the blow in shingling is generally required to be light at first, and with the steam-hammer the force can be varied with the work to be done. Another advantage