Wood charring

The method of charring the surface of wood by burning is an age-old wood protection technique. It shields the wood’s surface from moisture and decay-causing organisms, enhancing its weather resistance.

The History of Wood Charring

The tradition of charring wood surfaces dates back centuries. Wood surfaces have been burned to increase the weather resistance of building exteriors and for the preservation of fence posts buried in the ground.

The protective quality of the charred, or carbonized, surface stems from the fact that the charred surface no longer contains nutrients that attract decay organisms. Additionally, the charred surface repels water, preventing its absorption into the wood’s structure. Charring not only enhances the wood’s properties, such as its resistance to weather and rot, but also creates a natural, beautiful surface that is popular in modern architecture and interior design.

In Japan, this method is known as yakisugi, which directly translates to English as “heating cypress with fire.” Another globally recognized term for this method is shou sugi ban, which partially uses the same Japanese characters as yakisugi. However, it is apparently not an original Japanese expression but a translation error made in the Western world. Before the invention of wood preservatives, charring was also used in Europe to protect wood from pests and the elements.

Puun hiiltäminen- Marttakahvion seinät ovat hiillettyä puuta

Effect of Charring on Wood Surface

In practice, charring wood is a chemical change occurring at the cellular level of the wood. When heated, the cellular structure of the wood begins to break down and transform. Burning removes hemi-cellulose, which attracts decay organisms, from the wood surface, leaving behind mostly carbon, unsuitable for consumption by these organisms.

In the charring process, a layer of carbon forms on the wood surface, protecting it against various decay agents. Charring also reduces the wood’s ability to absorb water, making it less prone to rot and weather-related stresses. Concurrently, the wood’s appearance changes to a deep black or charcoal gray, and the surface texture may become more pronounced, highlighting the wood’s unique character and texture.

The Process of Wood Charring

The fundamental principle of charring wood has remained the same for centuries. The goal is to ignite and burn the wood surface to a few millimeters deep. The wood should not burn through; the aim is specifically to protect the surface layer. This way, the wood plank, board, or panel retains its durability and ease of installation, while its surface gets protected. Charring wood should be considered a natural and non-toxic alternative to wood surface treatments.

However, methods of charring wood have evolved. Hiil Oy’s modern wood surface charring line, which ignites and extinguishes the wood at a desired stage, ensures a consistent quality of the surface-charred product. Key controllable factors in charring are the wood’s moisture, burning temperature, and burning duration. Different wood types also require different settings. The speed of an industrial production line is significantly faster than manual burning.

As an ignition source, Hiil uses biogas formed from food waste.

Surface Treatment of Charred Wood

After the charring process, the wood surface is covered in soot that remains firmly attached to the wood. This product can then be further surface treated with a protective colorless or tinted oil or a flame retardant. Oiling binds the fine soot on the wood’s surface, preventing the charred wood from sooting upon touch.

After charring, the carbon layer on the wood’s surface can also be washed off with a pressure washer. This process removes the carbon layer, leaving behind a darkly toasted, natural-looking wood surface that highlights the wood’s beautiful grain and knots.

After washing, the charred wood can also be brushed. Brushing after washing softens and smooths the surface, making the product especially suitable for uses like deck planks or sauna benches.

Charred, washed, or brushed surfaces can also be stained to achieve the desired color tone.

Applications of Charred Wood

Charred wood is versatile, suitable for various applications both outdoors and indoors, in private and public construction.

As exterior cladding, charred wood offers a weather-resistant and long-lasting solution, lending character and a distinctive charm to buildings. It is also ideal for decks, fences, and noise barriers. Charred timber is particularly suitable for nature structures due to its natural surface.

Indoors, charred wood can be used in paneling, interior design, sauna benches, and for walls and ceilings. The charred surface can be treated to bind the carbon more firmly, preventing soot from rubbing off upon touch. Alternatively, the product can be left untreated, allowing it to naturally and gracefully patina over time.

It is important to note that charred surfaces are not resistant to mechanical stress. For areas subject to such stress (like decks, railings, and sauna benches), washed or brushed products are recommended instead of those with a charred surface.

Environmental Impact of Wood Charring

It is well-known that wood construction generates significantly lower carbon dioxide emissions compared to concrete, brick, and steel. Fundamentally, wood construction acts as a carbon sink, with one cubic meter of wood sequestering about 1000 kg of carbon dioxide. (Source: The Finnish Forest Industries Federation).

The choice of wood raw material also affects our environmental footprint. We aim to reduce the amount of waste wood globally.

Wood Charring and the Circular Economy

Currently, we manufacture most of our products using the by-products of sawmill and planing industries, as well as wood product batches that would otherwise not be used in construction. We have branded this product as Kiertopuu®️.

If this wood were not used in long-term construction, some of it would be burned for energy, releasing the carbon stored within it into the air. A portion might also be used temporarily in packaging, where after a few years of circulation, it is typically chipped and burned for energy. In construction, wood serves as a structural element for decades, acting as a carbon store.

We are also actively involved in construction networks, striving to repurpose as much wood used in the construction industry as possible for our further processing. However, at present, we only have access to by-products from the sawmill and planing industries.

In addition to the raw material of wood, a significant emission-producing phase in wood charring is the surface burning. Even here, we have chosen the most ecological and environmentally friendly method to char the wood surface. We use biogas made from food waste as the ignition source, produced in the same region as our factory.