Step 4 - Prepare a Fireproof Base

Before operating the pumpkiln, ensure it is placed on a stable, fireproof base.

The surface beneath the pumpkin must be resistant to heat, sparks, and embers. Suitable options include metal trays, fireproof tiles, stone slabs, or other non-combustible materials.

Support and Ventilation

If your pumpkin has ventilation holes in the underside, it must be placed on a metal screen, grill, or mesh during operation. This keeps the holes unobstructed and allows air to circulate from below.

Because hot sparks, embers, or ash may fall through these holes, the setup must include an additional fireproof layer beneath the screen.

Recommended Setup (Enhanced Safety)

For added safety, the screen can be placed over a fireproof metal dish or pan that collects falling sparks.

The metal dish may be partially filled with water to further reduce the risk of ignition from embers.

This setup offers an additional practical benefit:

At the end of the process, the dish can also be used to collect quenching water. This helps prevent uncontrolled runoff. The collected water, enriched with dissolved minerals and fine char particles, can later be used as a fertiliser.

Step 5 - Fill with Dry Biomass

Select clean, dry biomass for the pumpkiln. Dry kindling wood is generally well-suited because it ignites reliably and allows for stable combustion.

Do not leave the biomass inside the pumpkin for extended periods before ignition. Biomass can absorb moisture from the pumpkin walls or ambient air, which may make ignition more difficult and increase smoke formation.

Ideally, fill the pumpkin only when you are ready to begin the process and ignite the biomass within the next few minutes.

Arrange for Ventilation

When filling the pumpkin:

• Keep the biomass loosely arranged.

• Allow space between pieces to support airflow.

If the material is packed too tightly, oxygen supply and heat transfer will be restricted. This may lead to incomplete conversion and increased smoke.

Proper spacing supports a more stable and cleaner process.

Fire Starter

Waxed wood wool is recommended as a fire starter. The wax coating makes it hydrophobic, improving ignition reliability.

Place the waxed wood wool at the bottom of the kiln before adding the main biomass.

Alternative Biomass

Participants may also experiment with other dry, clean biomass, such as:

• Walnut shells

• Wood chips

• Chopped straws

Lighter biomass and smaller particles may ignite more quickly but will typically burn for a shorter duration. Adjust expectations and monitoring accordingly.

Step 6 - Ignition

Before ignition, pause and check conditions once again.

Ensure that:

• The pumpkiln is positioned on a stable, fireproof base

• Ventilation holes and top opening are unobstructed

• Weather conditions are suitable (no strong wind)

• Water or other extinguishing means are readily available

Safety should always determine whether ignition proceeds. 

Lighting the Pumpkiln

Ignition can be carried out using a long match or an extended lighter, inserted carefully through one of the carved cut-outs.

Light the fire starter (e.g., waxed wood wool) at the bottom of the kiln. Avoid placing hands directly above the top opening during ignition.

Once lit, allow the fire to establish gradually. Do not add additional biomass at this stage unless necessary.

Step 7 - Monitor and Manage the Burn

Once ignition is established, remain present and attentive throughout the process.

The pumpkiln should never be left unattended. Observe flame behaviour, smoke levels, and airflow regularly.

As the biomass begins to convert, additional dry material may be added through the top opening if needed.

Maintain a Stable Burn

Add small amounts at a time and avoid overfilling. Keep the biomass loosely arranged to maintain airflow and prevent excessive smoke.

If smoke increases or the flame becomes unstable, check that:

• The material is dry

• The biomass is not packed too tightly

• Ventilation holes remain unobstructed

A steady, controlled burn is preferable to a large or erratic flame.

Heat Use

The heat produced may be used in a controlled and safe manner, for example:

• Roasting marshmallows

• Baking bread on a stick

• Preparing a simple s’more

Food preparation should remain secondary to safe kiln operation. 

Step 8 - Quenching

When the biomass has largely converted and visible flames decrease, carefully begin quenching the process by adding water through the top opening.

Pour slowly and in controlled amounts. Stand back and avoid leaning over the kiln. Steam will form immediately when water contacts hot material and can rise quickly.

If the pumpkin has bottom ventilation holes, hot water may drain from the underside during quenching. Ensure the pumpkiln is positioned on a fireproof surface and, ideally, over a metal dish or tray to collect runoff and prevent uncontrolled flow.

If the pumpkin does not have bottom holes, water will most likely exit through the carved facial openings (typically the mouth). Be prepared for this runoff and ensure that the surrounding surface can safely handle hot water, especially at the beginning of quenching.

Continue adding water gradually until all active combustion has stopped.

After the Burn - Using the Outputs 

Once the biochar has fully cooled and dried (if necessary), it can be prepared for use.

Biochar

Biochar may be used as a soil improver in gardens or mixed in small quantities into potting soil. It can also support composting processes by improving aeration and helping retain nutrients.

Before application, biochar should be appropriately prepared (e.g., combined with compost or nutrients) according to best practice.

Quenching Water

The water used for quenching may contain dissolved minerals and fine char particles. It can be reused as a fertilising input.

Be aware that the water will typically be alkaline. Use cautiously and dilute if necessary, particularly for pH-sensitive plants.

The Used Pumpkin (Pumpkiln)

If the pumpkin remains structurally intact, it may be reused for another burn. Once no longer usable, the pumpkin remnants can be composted or disposed of as green (kitchen) waste in accordance with local regulations. 

The used pumpkin should not be fed to animals.