Wood charring
Charring wood surfaces by burning is an ancient wood protection method that shields the surface from moisture and decomposers while improving its weather resistance. Hiil has harnessed this age-old technique and updated it for the modern era, transforming it into a fast, energy-efficient and waste-free means of protecting wood.
Wood charring: a brief history
The tradition of charring wood surfaces dates back centuries. Wood surfaces have been burned to enhance the weather resistance of cladding and for posts embedded in the ground.
The protective quality of charred surfaces lies in the absence of nutrients that would attract decomposers. The charred surface also repels water and prevents it from seeping into the wood’s structure. Charring not only enhances the wood’s properties – such as weather and rot resistance – but also creates a natural, beautiful surface that is popular in contemporary architecture and interior design.
This method was named in Japan, where it is known as yakisugi, which translates to “burning Japanese cedar”. Another globally recognised term for this technique is shou sugi ban, which partially uses the same Japanese characters as yakisugi but is not an authentic Japanese term. Instead, it appears to be a mistranslation originating in the West. Before the invention of chemical wood preservatives, charring was also used in Europe to protect wood from pests and the elements.
The effect of charring on wood surfaces
In essence, charring wood is a chemical transformation that occurs at the cellular level. When the wood is heated, its cellular structure begins to break down and change. Burning removes hemicellulose from the wood’s surface – a nutrient attractive to decay organisms – and leaves behind primarily charcoal, which is unsuitable as nourishment for decomposers.
Charring forms a charcoal layer on the wood’s surface, protecting it from various decomposers. It also reduces the wood’s ability to absorb water, making it less prone to rot and weathering. Simultaneously, the wood’s appearance transforms into a deep black or charcoal grey, and the surface texture may become more pronounced, highlighting the wood’s unique character and grain.
The process of wood charring
The basic principle of charring wood has remained the same for centuries. The surface must ignite and burn a few millimetres deep. However, it should not burn deeper; the goal is specifically to protect the surface layer. This ensures that the plank, board or panel retains its durability and ease of installation while its surface is protected. Charring wood should be considered a natural and non-toxic alternative for wood surface treatment.
Charring methods have evolved, however. Hiil’s modern wood surface charring line ignites the wood, maintains an even burn using the wood’s own energy, and extinguishes the flame at the desired stage, resulting in a consistently charred product with lower energy consumption than other charring processes.
Key factors to control during charring include the wood’s moisture content, burning temperature and burn time. Different wood species also require different settings. The speed of an industrial production line is significantly faster than manual burning.
Hiil uses biogas made from food waste as the ignition fuel, after which the combustion is sustained by the wood’s own energy.
Surface treatment of charred wood
After charring, the wood surface is coated with soot that adheres to the wood.
The charcoal layer on the wood’s surface can also be washed off with a pressure washer. This process removes the charred layer from the wood, leaving behind a dark, natural-looking wood surface that accentuates the beautiful grain patterns and knots.
After washing, the charred wood can also be brushed. Brushing softens and smooths the surface, making the product particularly suitable for applications such as patio and decking boards or sauna benches.
All charred, washed and brushed products are finished with a colourless, specially refined linseed oil at the end of the process. Oiling binds the fine soot on the wood surface, preventing it from rubbing off upon contact.
Charred, washed, or brushed surfaces can also be stained to achieve the desired colour tone, and fire retardant can be added for fire protection.
Applications for charred wood
Charred wood is suitable for a variety of applications both outdoors and indoors, in private and public construction.
For exterior cladding, charred wood is a weather-resistant and long-lasting solution that adds character to buildings. It is also well suited for patios, fences and noise barriers. Charred wood is particularly fitting for nature-based structures due to its natural appearance and non-toxic properties.
Indoors, the products can be used for panelling, interior design, sauna benches and on walls and ceilings. The charred surface can be treated to ensure the soot adheres firmly and does not rub off upon contact.
It must be remembered that charred surfaces are not resistant to hard impacts and repeated mechanical stress in the same area. For such high-wear applications (such as patios, decking, railings and sauna benches), washed or brushed products are recommended instead of charred surfaces.
Environmental impact of wood charring
We are currently conducting more detailed calculations on the environmental impact of our new factory and will validate these results during 2025. We calculated the environmental impact of our pilot line in 2024, and we will be happy to provide the results upon request.
It is well known that wood construction produces significantly fewer carbon dioxide emissions compared to concrete, brick and steel. Wood construction inherently acts as a carbon store, with one cubic metre of wood binding approximately 1,000 kg of carbon dioxide (source: Finnish Forest Industries). Hiil’s goal is to process wood using as little external energy and labour as possible, ensuring this natural carbon store is preserved to the greatest extent.
The choice of wood raw material also affects our environmental footprint. We aim to reduce the amount of waste wood and wood residue globally, which is why the majority of our production consists of CircuWood ™ products – manufactured using by-products from the wood industry.
Beyond the wood raw material, the charring process itself is a key stage that generates emissions. Here too, we have developed our technology to operate almost entirely on the wood’s own energy without external power sources. We use a small amount of ignition material at the start of the process, selecting biogas produced from food waste in the same region as our factory.
Wood charring and the circular economy
We currently manufacture most of our products using by-products from the sawmill and planing industries, as well as wood batches that would not otherwise be used for construction. We call this product CircuWood ™.
If this material were not used in long-term construction, some of it would be burned for energy, releasing the stored carbon into the atmosphere. Some would also be used for temporary applications in the packaging industry, where, after a few years of circulation, the wood is typically chipped and burned for energy. In construction, wood remains in the structure’s walls for decades, thus acting as a carbon store.
We actively participate in construction networks and strive to obtain as much used wood from the construction industry as possible for further processing.
