Smelting

Part of Metalworking

Smelting is the chemical process of extracting metal from ore using heat and carbon. It transforms useless rock into the raw material for every metal tool, fastener, and machine component your rebuilding community will need.

Why Smelting Matters

Ore is not metal. The iron in hematite is locked in iron oxide — a chemical compound that requires sustained high temperatures and a carbon-rich environment to break apart. Smelting is the bridge between a pile of rocks and a workable metal bloom. Without it, metalworking cannot begin.

The core chemistry is simple: carbon monoxide from burning charcoal strips oxygen from the metal oxide, leaving behind metallic iron (or copper, or tin). The challenge is engineering — building a furnace that reaches sufficient temperature and maintains a reducing atmosphere long enough for the reaction to complete.

The Bloomery Furnace

The bloomery is the simplest iron smelting furnace and was used worldwide for over 2,000 years. It produces a spongy mass of iron called a bloom.

Design and Dimensions

Component	Specification
Height	1.0-1.5 meters (taller = hotter)
Internal diameter	25-35 cm
Wall thickness	8-15 cm
Tuyere hole	3-5 cm diameter, 15-20 cm above base
Tuyere angle	15-20 degrees downward
Wall material	Clay mixed with sand and straw/grass

Building the Furnace

1. Choose a site — Level ground, protected from wind, near your charcoal supply. Clay soil is ideal for the base.
2. Build the base — Dig a shallow bowl (30 cm wide, 10 cm deep) to collect slag. Line with clay.
3. Form the shaft — Build up clay walls in coils, smoothing inside and out. Work in stages, letting each section firm before adding the next.
4. Install the tuyere — Insert a clay pipe or hollow reed through the wall about 15-20 cm above the slag bowl. Angle it downward so the air blast hits the charcoal bed.
5. Dry thoroughly — Let the furnace dry for several days. Then light a small fire inside to cure the clay. Rushing this step causes cracks during the smelt.

Furnace Cracking

A wet or poorly cured furnace will crack during operation, potentially collapsing and spilling molten slag. Dry for at least 3 days in warm weather, longer in cool or humid conditions. Cure with a slow fire for 4-6 hours before your first smelt.

Operating the Bloomery

Preparation (2-3 hours before the smelt):

1. Fill the furnace with charcoal and light it
2. Let the charcoal bed establish and the furnace walls reach operating temperature
3. Prepare your ore — crushed to 1-2 cm pieces, roasted if needed
4. Have 40-60 kg of charcoal ready (you will use it all)

The smelt (4-8 hours):

1. Begin adding alternating layers: one charge of charcoal, then one charge of ore
2. Each charge is roughly 1 kg ore to 1-1.5 kg charcoal
3. Maintain constant air blast with bellows — 600-800 strokes per hour
4. Add new charges as the previous ones sink down
5. Listen for the furnace — a steady roar means good airflow; sputtering means moisture or blockage
6. Watch slag flow from the tuyere — bright orange flowing slag means the furnace is at temperature

Bellows Operation

Operating bellows for 4-8 hours is exhausting. Organize relay teams of 2-3 people, switching every 15-20 minutes. Consistent air supply is more important than maximum blast — a steady rhythm beats sporadic hard pumping.

Bloom extraction:

1. After the final charge has burned down (usually 30-60 minutes), stop the bellows
2. Break open the furnace base or pull the bloom out through a pre-built access hole
3. The bloom is a glowing, spongy mass of iron mixed with slag
4. Immediately hammer the hot bloom on an anvil stone to consolidate it and squeeze out trapped slag
5. Reheat and hammer repeatedly — this is called “shingling”

Copper Smelting

Copper smelts at lower temperatures than iron and was historically the first metal smelted. The process is simpler but follows similar principles.

From Malachite

Malachite (green copper carbonate) is the easiest copper ore to smelt:

1. Crush malachite to pea-sized pieces
2. Mix with an equal volume of charcoal in a clay crucible
3. Place the crucible in a charcoal fire with bellows blast
4. Heat to approximately 1,100C (bright orange-yellow heat)
5. The malachite decomposes and the copper collects at the bottom
6. Pour or break the crucible to retrieve the copper button

Small-Scale Copper

For small quantities, you do not need a full furnace. A deep bed of charcoal in a fire pit with a bellows-fed tuyere can reach copper-smelting temperatures. Use a clay crucible to contain the ore and charcoal mixture.

From Sulfide Ores (Chalcopyrite)

Sulfide ores require an extra step — roasting:

1. Roast the crushed ore in an open fire for 4-8 hours to drive off sulfur
2. Smelt the roasted ore with charcoal as described above
3. The result is often “matte” — an impure copper-iron sulfide mixture
4. Re-smelt the matte with fresh charcoal to separate copper from iron

Bronze Making

Bronze (copper + tin alloy) is harder than pure copper and casts beautifully. The standard ratio is 88-90% copper to 10-12% tin.

Bronze Type	Tin Content	Properties	Best For
Low-tin	5-8%	Harder than copper, workable	Hammered tools
Standard	10-12%	Good hardness, excellent casting	Axes, chisels, bells
High-tin	15-20%	Very hard, brittle	Mirrors, bearings

Alloying Process

1. Melt copper in a crucible first (it has the higher melting point)
2. Add tin gradually, stirring with a green wood stick (the moisture helps mix)
3. Alternatively, smelt tin ore and copper ore together in the same crucible
4. Pour into molds while fully liquid — bronze solidifies quickly

Bellows Construction

No smelting works without forced air. Natural draft alone cannot reach iron-smelting temperatures.

Bag Bellows (Simplest)

1. Sew a goatskin or deerskin bag, leaving one open end
2. Insert a clay or wooden nozzle in the closed end
3. Open the bag mouth to fill with air, then squeeze to force air through the nozzle
4. Point the nozzle into the tuyere

Box Bellows (More Efficient)

1. Build a rectangular wooden box with a sliding piston inside
2. Cut air inlet holes with leather flap valves on top
3. Connect the outlet to a clay tuyere pipe
4. Push and pull the piston to force air in both directions

Double-Action Bellows (Best)

1. Build two bag or box bellows side by side
2. Connect both outlets to a single tuyere through a Y-junction
3. Operate alternately — one fills while the other blows
4. This provides constant, uninterrupted air flow

Air Supply is Everything

The difference between a successful smelt and a failed one is almost always air supply. Invest time in building good bellows before attempting your first smelt. A well-made double-action bellows can sustain the 1,200C+ needed for iron smelting indefinitely.

Fuel: Charcoal Production

Smelting requires charcoal, not raw wood. Wood does not burn hot enough and contains too much moisture. You will need roughly 5-8 kg of charcoal per kg of iron produced.

Quick Charcoal Summary

1. Stack dry hardwood in a mound
2. Cover with earth and turf, leaving air holes at the base and a chimney at the top
3. Light from the top and control air to allow slow, oxygen-starved burning
4. When smoke turns from white to thin blue-gray, seal all openings
5. Let cool completely (24-48 hours) before opening

See the parent article on Charcoal Production for full details.

Slag Management

Slag is the waste product of smelting — a glassy mixture of silica, iron oxide, and other impurities that melts and flows out of the furnace.

Reading Slag

Slag Appearance	What It Means
Bright orange, flows freely	Good temperature, furnace working well
Dark, thick, barely flows	Too cold — increase bellows speed
Very light, foamy	Too hot or too much flux
Green-tinted	Copper content in the ore

Tapping Slag

During a bloomery smelt, slag must be periodically tapped (drained) to prevent it from drowning the bloom:

1. Poke a stick through the tuyere hole to clear any slag blockage
2. If slag pools inside, it insulates the bloom from the charcoal and kills the reaction
3. Some furnace designs include a dedicated slag-tap hole at the base

Scaling Up

Once you have mastered the basic bloomery, consider improvements:

• Taller furnaces (1.5-2m) reach higher temperatures and produce better iron
• Preheating air by routing the tuyere pipe through the furnace wall above the combustion zone
• Water-powered bellows using a simple waterwheel and cam mechanism
• Multiple tuyeres for more even heat distribution
• Dedicated smelting sites near ore and charcoal sources to minimize transport

Common Mistakes

1. Using wood instead of charcoal — Wood cannot reach smelting temperatures and produces too much steam. Always use properly made charcoal.
2. Insufficient bellows work — The most common cause of failed smelts. Maintain constant, steady air flow for the entire 4-8 hour duration.
3. Wet ore — Moisture in the ore steals heat through evaporation. Dry ore thoroughly before charging, and roast if possible.
4. Impatience with the bloom — A raw bloom is fragile and full of slag. It must be hammered while hot immediately after extraction to consolidate the iron. Letting it cool first makes this nearly impossible.
5. Undersized furnace — A furnace shorter than 80 cm cannot develop enough heat. Build to at least 1 meter tall.

Summary

Smelting — At a Glance

• The bloomery furnace is the simplest iron smelting technology — a 1-1.5m clay shaft with a tuyere for forced air
• Key chemistry: charcoal (carbon) strips oxygen from ore at 1,200C+, leaving metallic iron
• Copper smelts at lower temperatures (~1,100C) and can be done in a crucible
• Bronze requires 10-12% tin alloyed with copper
• Bellows are essential — double-action bellows provide constant air for sustained high temperatures
• Budget 5-8 kg of charcoal per kg of iron produced
• The bloom must be hammered immediately while hot to consolidate and remove slag