While forges or fires are of many shapes and sizes, the principles of their construction remain the same. An ordinary blacksmith forge is a fireplace in the bottom of which there is a tuyere for admitting a blast of air to blow the fire. Where the air blast is furnished by a hand bellows, the pipe leading therefrom to the tuyere is open throughout. Where a power-driven blower furnishes the blast, there is a valve in the pipe for regulating it.
The usual form of tuyere consists of a single blast pipe, opening into the bottom of the fire pit. This may be a simple nozzle as in Fig. 1, with the blast regulated by a damper in the pipe; or, it may have a regulator at the mouth of the tuyere as shown. Sometimes the tuyere has several openings, and is then in the form of a grate. Whatever its form, it should be possible to clean it from below, in order that coal and clinkers falling into it may be removed.
A modern type of forge is shown in Fig. 2. This is provided with a hood for carrying off the smoke. The pipe connected to the hood extends downward to an underground flue leading to an exhaust fan which draws out the air. The blast pipe is also underground, and a small pipe leads upward to the tuyere, the amount of blast admitted to the fire being regulated by a slide in this pipe. This system of underground piping is known as the down-draft system.
In some shops no provision is made for carrying off the smoke, while in others hoods are placed above the forges and connected to overhead pipes, which may be either connected to an exhaust fan or led directly to the roof. The down-draft system is the more modern and generally the best.
The blast is furnished to the fires of a blacksmith shop by blowers of various kinds. For many years the ordinary bellows was used. This has been superseded by the fan blower which is now almost universally used, even for hand power.
Such a fan blower is shown in Fig. 3. It is formed of a thin cast-iron shell in which there are a set of rapidly revolving blades.
These blades set up a current of air which presses against the side of the shell and escapes through the tangential opening. The pressure of the blast used for an open blacksmith fire varies from about 2 to 7 ounces per square inch. The lower pressure is used for a light fire and light work. The higher pressure is suitable for heavy classes of work.
The common fuel for small fires is soft or bituminous coal. coke for large fires and furnaces, and occasionally hard coal in small furnaces. The soft coal used is of a grade known as smithing coal. It should be very clean and free from impurities. A lump of good forge coal breaks easily with a crumbly looking fracture and the coal shows clean and bright on all faces. It will not break up into layers as "steaming" coal will, such seamy looking breaks being caused by the more or less earthy impurities. If forge coal splits and shows dull looking streaks or layers, it is poor coal. Good coal has little clinker and breaks easily. When used, the coal is dampened and kept wet before putting on the fire. It should be broken up fine before dampening, and not used in lumps.
The fire must be carefully watched. It is very important that it should be in first-class condition at all times for the work in hand. A certain depth of fire is always necessary. If the fire be too shallow, the cold blast will penetrate the fire in spots, making it impossible to heat the metal. There should be depth enough to the fire to prevent this. For small work there should be at least three or four inches of fire below the metal that is heating. There should also be thickness enough of fire above the work being heated to prevent the metal from losing heat to the outside air. The fire should be kept as small as possible to heat the work properly. As a general rule the fire will follow the blast. If the fire is wanted larger, it may be made so by loosening the edges of the fire by a bar, allowing the blast to come through around the sides, and causing the fire to spread. When a small fire is wanted the damp coal should be packed down tightly around the sides and the center of the fire loosened up slightly. For light work a small round fire is used. For heavier heating the fire is started by placing a large block on top of the tuyere, on each side of which green coal is packed down hard in the shape of an oblong mound. The block is then removed and the fire started in the hole left. These mounds are left undisturbed and fresh fuel is added to the fire in the shape of coke which has either been previously made by loosely banking a quantity of green coal over the fire and partially burning it to coke, or is bought ready made. With a small fire the fuel is constantly added around the sides where it is turned into coke. This coke is raked into the center of the fire as wanted and more coal added around the sides and patted down to keep the fire in shape.
When too much blast is blown through the fire all the oxygen is not burned out of the air. This attacks the iron, forming a heavy coat of oxide or scale (the black scale which falls from heated iron). This sort of fire is known as an oxidizing fire and should not be used when it is possible to avoid it. When just enough air is being admitted to keep the fire burning brightly and all of the oxygen is burned, the fire is in good condition for heating. Very little scale is formed and some of the scale already formed may even be turned back to iron. This sort of a fire is known as a reducing fire. In other words, when the fire is in condition to give oxygen to anything, it is an oxidizing fire. If in condition to take away oxygen, it is a reducing fire.
The fire may be kept for some time by placing a block of wood in the center and covering over with fresh coal.
In nearly all manufacturing work and in large work in the jobbing shop, the heating is done in furnaces. The heatis generally supplied by either hard coal, coke, oil, or gas - coke being more commonly employed in jobbing shops. Sometimes ordinary coal is used.
A furnace used for heating small work for manufacturing is shown in Fig. 4. This may be used with either ordinary coal or coke. Gas furnaces, a simple type of which for all around work is shown in Fig. 5, are used when an even heat is wanted, particularly for hardening and tempering. For manufacturing work the furnaces are sometimes fixed to do the heating automatically. The pieces to be hardened are carried through the furnace on an endless chain which moves at a speed so timed that the pieces have just time enough to be heated to the right temperature as they pass through the furnace. Such a furnace is shown in Fig. 6.