Coke is coal heated with too little air to burn, which drives off its gas and tar and leaves a hard, porous, mostly carbon fuel that burns hot and with little smoke. Only certain bituminous coals that soften and swell make strong coke, so test a lump in a covered pot first; lignite, sub-bituminous coal and anthracite will not.
Part of Coal Surface Mining
Making coke from coal for the forge: which coals work, a pot test, a covered-mound method, and the gas and tar hazards.
Why Smiths Want Coke
Raw bituminous coal burns with a long smoky flame because it contains volatile gas and tar. The parent article says coke is nearly pure carbon and burns very hot with almost no smoke, ideal for a forge and for smelting iron. Coal tar and coal gas are the by-products (Coal Tar).
Sulphur matters for metal. Sulphur in the fuel makes hot iron crack when it is worked (red-short). Coking does not remove all of it: a large share of the coal’s sulphur stays in the coke, so choose a low-sulphur coal to begin with.
Which Coals Coke
A coking coal softens as it heats, swells, and fuses into a hard porous lump. Many bituminous coals do this. Lignite and sub-bituminous coals crumble to powder, and anthracite does not soften at all.
Pot test. Put a lump of crushed coal in a small lidded iron pot with a pinhole in the lid. Heat it in a fire until fumes stop coming out of the pinhole, then cool it before opening. A coal that comes out as a fused, silvery, swollen cake is a coking coal. Loose powder is not.
What You Need
- Bituminous coal that has passed the pot test, in lumps
- Earth for covering and brushwood for lighting
- An open, level place away from buildings and downwind from them
- Water and a long rake for quenching from a distance
Method: Covered Mound
This follows the parent’s method, which is the same as charcoal burning (Charcoal Production).
Step 1: Pile the coal in a large mound with a chimney hole at the top and air holes at the base.
Step 2: Cover with earth, leaving the holes open.
Step 3: Light from the bottom and watch the smoke. Start by closing air holes if the mound burns too freely.
Step 4: Run it for 24-72 hours, depending on the size of the mound (parent figure).
Step 5: Close all openings and let the mound cool fully before opening. Smother with earth if it reignites.
Alternative: Coke In The Forge Itself
A smith can coke small quantities at the edge of the forge fire. Push fresh coal beside the fire, so that it heats and gives off its gas, which burns as a flame. After it has cooked, rake the coked mass into the fire.
Common Mistakes
- Using lignite or anthracite. They will not coke.
- Opening the mound too early. The fresh air ignites the hot coke and it burns to ash.
- Quenching a hot mound with a lot of water inside. It releases steam and can scatter hot material.
Safety
Coking gives off flammable gas, carbon monoxide and tar
The coal gas is flammable and poisonous. Coke outdoors, upwind of any building, with nobody sleeping nearby. Never coke under a roof or in a shed. Stay out of the fume plume, and keep away from the base during lighting where flames can flash. Treat the mound as a fire risk for hours after it is closed.