Tar Kiln Build

Part of Petroleum and Tar

Building a dedicated tar kiln for efficient, repeatable production of wood tar through destructive distillation.

Why This Matters

Wood tar is one of the most versatile substances a rebuilding community can produce. It waterproofs boats and roofs, preserves rope and timber, serves as an adhesive, and provides the base material for pitch, turpentine, and various chemical derivatives. While tar can be produced in simple pit methods, a purpose-built tar kiln dramatically improves efficiency, yield, and consistency.

A well-constructed tar kiln converts roughly 15-25% of the wood’s weight into tar, compared to 5-10% for crude pit methods. The kiln also allows you to collect the volatile byproducts — wood vinegar (pyroligneous acid), light oils, and wood gas — that would otherwise be lost. These byproducts have their own valuable uses in chemistry, agriculture, and medicine.

The investment in building a proper kiln is substantial — perhaps a week of labor for two to three people — but the structure lasts for years and can be fired repeatedly. Communities across Scandinavia, the Baltic, and North America operated tar kilns for centuries, producing the vast quantities of tar needed for shipbuilding and timber preservation. Their designs, refined over generations, form the basis of what follows.

Kiln Design Principles

How Tar Kilns Work

A tar kiln operates on the principle of destructive distillation: wood is heated in the absence of air (or with very limited air), causing it to decompose into tar, charcoal, gases, and wood vinegar. The key design requirements are:

1. Sealed chamber — limits oxygen to prevent the wood from burning to ash
2. External heat source — heats the wood without direct combustion (or a carefully controlled internal burn)
3. Collection system — channels liquid tar away from the heat source before it can be consumed
4. Drainage slope — the kiln floor must slope toward an outlet so tar flows out by gravity

Two Main Designs

Feature	Pit Kiln	Mound Kiln
Construction	Excavated into a hillside	Built on sloped ground surface
Difficulty	Moderate	Higher
Capacity	1-5 cubic meters of wood	2-10 cubic meters
Yield	10-15% of wood weight	15-25% of wood weight
Reusability	Limited — walls erode	Excellent — structure improves with use
Best for	First attempts, small quantities	Regular production

Building a Pit Kiln

The pit kiln is the easier design and a good starting point. Choose a hillside with a 15-30 degree slope.

Materials Needed

• Digging tools (shovels, picks)
• Clay (approximately 200-300 kg for a medium kiln)
• Flat stones for the floor (enough to cover ~2 square meters)
• A metal pipe or hollowed log (10-15 cm diameter, 1 meter long) for the tar outlet
• A collection vessel (ceramic crock or metal pot, 20-50 liters)
• Straw or grass for insulation

Step-by-Step Construction

1. Excavate the pit

Dig into the hillside to create a chamber approximately 1.5 meters wide, 2 meters long, and 1.2 meters deep. The long axis should run uphill-downhill. Keep the excavated earth nearby — you will need it for sealing.

2. Shape the floor

The floor must slope consistently toward the downhill end. Use a straight board and a plumb weight to verify the slope — aim for a 5-10 degree grade. Smooth the floor surface and compact it firmly.

3. Lay the stone floor

Cover the floor with flat stones fitted tightly together. Seal the joints between stones with puddled clay, pressing it firmly into every crack. The floor must be watertight — liquid tar will flow across this surface.

4. Install the outlet

At the lowest point of the floor (the downhill end), create a channel through the hillside wall. Insert a metal pipe or hollowed hardwood log that extends from inside the kiln to outside the hillside. This pipe should slope downward at least 5 degrees. Seal around it with clay.

5. Build the walls

If the excavated earth holds its shape well, the pit walls may need minimal reinforcement. For loose or sandy soil, line the walls with a 5-10 cm layer of clay, pressed firmly and smoothed. Allow this clay lining to dry for several days before first use.

6. Create the cover

The kiln needs a removable cover:

• Lay green (unseasoned) poles across the top of the pit, spaced 10-15 cm apart
• Cover with a thick layer of straw or grass (15-20 cm)
• Cover the straw with a 10-15 cm layer of clay-rich earth, packed down firmly
• Leave a small smoke hole (10-15 cm diameter) at the uphill end for initial lighting and draft control

First Firing

The first firing of a new kiln will produce less tar than subsequent ones. The heat cures and hardens the clay lining, improving the kiln’s seal and insulation. Consider the first firing a conditioning run — use lower-quality wood scraps.

Building a Mound Kiln

The mound kiln is more efficient and better suited for regular production. It is built on the surface of sloped ground.

Foundation

1. Select a site with natural slope of 10-20 degrees, good drainage, and clear of trees for 10 meters in every direction
2. Level a platform at the base of the slope, approximately 3 meters in diameter, to hold the collection vessel
3. Build a stone base — lay a circular floor of flat stones, 2-3 meters in diameter, on the slope above the platform. Seal stone joints with clay.
4. Create drainage channels — carve shallow grooves (2-3 cm deep, 5 cm wide) in the stone floor, all radiating from the center outward and converging at the downhill edge into a single outlet channel

The Central Chimney

The mound kiln uses a central chimney stack that serves as both the fire starter and the draft regulator:

1. Stack three or four straight poles (8-10 cm diameter) vertically at the center of the stone floor, tied together at the top
2. Pack kindling and easily-ignitable material (birch bark, resinous pine shavings) around the base of these poles
3. This chimney will be lit from the top after the mound is built and sealed

Loading the Wood

Stack the wood to be processed around the central chimney:

1. Inner ring — place split billets vertically, leaning against the chimney poles, forming a cone shape
2. Outer rings — continue stacking billets in concentric rings, gradually increasing the angle from vertical to nearly horizontal at the outer edge
3. Fill gaps — pack smaller pieces, offcuts, and bark into any gaps between billets
4. Final size — the completed wood stack should form a dome 2-3 meters in diameter and 1.5-2 meters high

Sealing the Mound

This is the most critical step — the seal determines how much oxygen enters and therefore whether you get tar or ash:

1. Straw layer — cover the entire wood stack with a 10-15 cm layer of damp straw, grass, or fern fronds
2. Earth layer — cover the straw with a 15-20 cm layer of clay-rich earth, tamped down firmly
3. Smooth the surface — use the back of a shovel to smooth the earth layer, sealing any cracks
4. Vent holes — poke 3-4 small holes (5 cm diameter) around the base of the mound with a stick. These are your draft control vents.
5. Leave the chimney top open — the central chimney must remain accessible for lighting

Operating the Kiln

Firing Sequence

1. Light the chimney — drop burning kindling down the central chimney. The fire starts at the top center of the wood stack.
2. Monitor smoke color:
   • White/gray smoke = water evaporating (drying phase) — keep all vents open
   • Yellow smoke = decomposition beginning (tar production starting) — begin closing vents
   • Blue/thin smoke = optimal tar production — maintain minimal venting
   • No smoke = fire has gone out or process is complete
3. Close vents progressively — as smoke transitions from white to yellow, begin plugging vents with clay, starting from the uphill side
4. Seal the chimney — once yellow smoke appears consistently, cap the chimney top with a flat stone and seal with clay. Leave one small vent (2-3 cm) for pressure relief.

Tar Collection

Tar begins flowing from the outlet 4-8 hours after lighting, depending on kiln size:

1. Place the collection vessel under the outlet pipe
2. Tar flows as a dark, viscous liquid, often mixed with wood vinegar (a thin, acidic liquid)
3. The two liquids separate naturally — tar sinks, wood vinegar floats. Pour off the wood vinegar into a separate container (it has its own uses).
4. A medium kiln firing typically runs for 24-48 hours
5. Collect tar throughout — check the collection vessel every few hours and replace when full

Hot Tar

Tar emerging from the kiln can exceed 80 C. Use long-handled tools and heat-resistant gloves when handling collection vessels. Hot tar sticks to skin and causes severe burns.

Timing and Yield

Wood Input	Firing Time	Expected Tar Yield	Charcoal Byproduct
0.5 cubic meters	12-18 hours	15-30 liters	40-60 kg
1 cubic meter	24-36 hours	30-60 liters	80-120 kg
2 cubic meters	36-48 hours	60-120 liters	160-240 kg

Maintenance and Improvement

After Each Firing

1. Allow full cooling — wait at least 24 hours after the last smoke before opening
2. Remove charcoal — the wood has converted to charcoal; remove and store it (valuable fuel)
3. Inspect the floor — check for cracks in the stone floor sealing; re-apply clay as needed
4. Check the outlet — clear any tar blockages from the outlet pipe
5. Repair the seal — for mound kilns, the earth covering is largely destroyed each firing and must be rebuilt

Improving Over Time

After several firings, you will learn your kiln’s characteristics:

• Draft adjustments — experiment with vent sizes and positions. Less air produces more tar but takes longer; too little air and the fire goes out.
• Wood selection — resinous softwoods (pine, spruce, fir) produce the most tar. Hardwoods produce more wood vinegar and less tar.
• Wood preparation — splitting wood into consistent-sized billets and air-drying for 2-3 months before firing significantly improves yield.
• Multiple outlets — adding a second outlet pipe at a higher level on the kiln wall captures lighter volatile oils that condense above the tar flow.

Common Problems and Solutions

Problem	Cause	Fix
No tar flows	Fire went out; insufficient heat	Re-light through a vent hole; add kindling
Tar catches fire at outlet	Too much draft; tar is too hot	Reduce vents; extend the outlet pipe to cool tar
Very low yield	Too much air; wood is burning, not distilling	Seal cracks in the mound; reduce vent sizes
Thick, unusable tar	Overheating; tar has been partially cracked	Reduce firing temperature; use slower burn
Kiln collapses	Weak earth covering; too-vigorous burn	Use thicker earth layer; moisten before firing

Permanent Kilns

After you have experience with mound kilns, consider building a permanent stone or brick kiln with a fixed dome, built-in vents with sliding covers, and a permanent outlet system. These produce the most consistent results and can operate for decades with minimal maintenance.