Tools
The forge fire: coal, coke and gas
The fire is a tool, not a heat source, and it has to be shaped as deliberately as the work sitting in it.
A forge is two things at once: a hearth built for heating metal, and the shop around it, the smithy. The fire has one narrow job, carrying the metal to the temperature where it yields to a hammer, and the piece makes the trip in tongs. Everything else here exists to keep that window open.
What the smith can adjust
However it is fueled, a solid-fuel forge answers to three settings and only three: how much air enters, how much fuel sits in it, and what shape the fire is kept in. The hardware is short. A tuyere to carry the air in. A bellows or a blower to drive it. A hearth to hold burning fuel over the tuyere opening, once built of mud brick, fired brick or stone, later of iron.
With more air the fire eats fuel faster, gets hotter and smokes less, smoke being fuel that leaves the fire unburnt. The smith balances the two to suit the job, mostly by reshaping the fire.
What the coal is doing in there
In a typical coal forge a firepot sits centered in a flat hearth, the tuyere entering from below. The hot core is a ball of burning coke in and above the pot. Around it, coke that is hot but not burning, then coal halfway cooked into coke by the fire’s heat. Outside everything, a bank of raw coal, dampened and rammed tight so it holds the heart in shape and cooks before it catches. A bigger job gets more air and a deeper coke heart.
Coke is coal cooked out of contact with air until its volatile matter is gone: grey, hard, porous, concentrated in carbon. Industry makes it in airless ovens usually near 1,000 to 1,100 °C (1,800 to 2,000 °F); a coal forge does the same cooking in miniature, at the heart of the fire. Coke and charcoal go into the same forges, and since nothing needs converting there, the fire is handled differently.
From firepot to backdraft
The firepot has one major variation, the backdraft, with no pot at all: the tuyere enters horizontally through the back wall. At the far end it shrinks to a hole in the ground fed by a pipe, still a forge. In between, smiths and specialized work have pushed the design everywhere, and the forge article on Wikipedia collects the family, including the finery forge, a water-powered mill that turned pig iron into wrought iron.
What does gas change?
A gas forge burns propane or natural gas. One common design is a cylindrical chamber with a burner tube entering at a right angle, lined with a refractory: hard castable ceramic, or a soft blanket such as mineral wool. The burner mixes fuel and air, lights at the tip, and the heat follows the air pressure, raised by a blower or by the Venturi effect. Sizes run from industrial chambers to a coffee can on a cheap propane torch, or one carved from a single soft firebrick.
The trade is convenience against control. Gas is simple to operate next to coal, clean and consistent, which is why it suits a novice. It is also less versatile: the fire cannot be rebuilt around a large or unusually shaped piece, and heating one small section is awkward. The claim that gas never reaches a welding heat is a misconception; a well-designed gas forge is hot enough for any task.
How hot is hot enough?
Welding asks the most of a fire, and the heat it asks for moves with the carbon in the steel.
| Metal | Color at the weld | Range |
|---|---|---|
| Pure iron | nearly white | 2,500 to 2,700 °F (1,400 to 1,500 °C) |
| Steel with 2.0 percent carbon | orangish yellow | 1,700 to 2,000 °F (900 to 1,100 °C) |
| Common steel, 0.2 to 0.8 percent carbon | bright yellow | given by color only |
Steel at a welding heat grabs: a thin rod or nail at the same temperature sticks at first touch and must be twisted free. Sparks mean rapid oxidation, plain burning, and a weld made on burning metal comes out poor and brittle. Both faces of the joint must reach the same heat and be joined before they cool too much. The hammer work that follows belongs to welding two bars into one.
The tub of water beside the fire
A slack tub, a large container of water, earns its floor space: it cools parts of the work mid-forging, to protect them or keep neighboring blows from deforming a section; it hardens steel; it tends a coal or charcoal fire. In bladesmithing and tool making the same vessel is a quench tank, oil or brine replacing the water, many metals needing more than plain water to harden. The name is traced to slake, as in slaking the heat.
Sparks, scale and hot metal are also where a page stops and a shop begins. OSHA standard 1910.133 obliges employers to put eye and face protection on workers exposed to flying particles or molten metal. What to wear, and how close to stand, is workshop training.
Reading a fire from the outside
The system can be read with the naked eye. At a demonstration or in a good photograph, count the zones from the center out: burning heart, hot dark coke, coal turning, damp bank holding the shape. Then watch the metal, not the flames, and catch the moment the color says the window is open. Inside that window the work begins with the first two moves on a hot bar, and both go back to the first smelted iron, most likely Anatolia, around 1800 BC.
en.wikipedia.org is a free online encyclopedia; the page behind this piece is its article titled Forge: hearth and smithy, the coal, coke, charcoal and gas fires, the equipment from anvil to slack tub, and the gods of smithing, Brigid, Ogun, Hephaestus and Vulcan. The page carries a banner asking for additional references.