Frøslev Heatwood is heat-treated wood - the wood goes through a process during which it is heated to a high temperature while steam is added. The result is an incredibly dimensionally stable end product that is resistant to rot and fungi. The process is 100% free of chemicals, and Frøslev Heatwood therefore carries the Nordic Swan Ecolabel.

The method behind heat-treating wood is nothing new - quite the contrary. Already back in the Viking Age, wood was treated with fire and water to make it more resilient and durable. The treatment is not to be confused with heat treatment of e.g. wooden pallets for the food industry where the temperature does not get anywhere near the same degrees.

Image: Graph of the process

The production process of Frøslev Heatwood is based on the use of high temperatures and water vapour preventing the wood from burning. The process removes sugary substances from the wood, and the cellular structure is altered. Rot and fungi can therefore not settle in the tree, and the finished product becomes very dimensionally stable. Frøslev Heatwood is very suitable for use in the Nordic climate, especially during humid seasons.

Frøslev Heatwood is a very sustainable product as no chemicals are used at any point during the process. Moreover, energy consumption during production is very low.

The process can be divided into three steps.

Phase 1 - Increase in temperature and kiln desiccation

The air temperature in the oven is rapidly increased by using heat and steam until it reaches a temperature of about 100 °C. The wood reaches the same temperature. The temperature is then gradually increased to 130 °C, and kiln drying takes place. Steam is used as a moisture barrier to prevent the wood from splitting. The vapour also causes cellular structure changes in the wood.

After this phase, the moisture content of the tree is reduced to almost zero.

Phase 2 - Intensive heat treatment

During the intensive heat treatment phase, the air and wood temperature is increased to between 185 and 225 °C. The highest final temperature depends on the requested use of the wood.

When the requested final temperature has been reached, the temperature is kept constant for 2-3 hours. Steam is used to prevent the wood from burning and splitting and continues to be part of altering the cellular structure of the wood.

Phase 3 - Cooling and adding moisture

During the final phase, the temperature is reduced by using a water spraying system. The moisture content of the wood is raised to just above 4%, so work can still be done on the wood.

Variants

Heat-treated wood is available in two versions: Class D for outdoor use where the wood is heated up to 212 °C and Class S for indoor use

Frøslev only sells Class D for outdoor cladding and terraces. The profiles are not processed till after the heat treatment.

Images: Frøslev Heatwood as new and after 3 years of patination

Wood is a natural material, and the wood structure will never be the same in two boards when it comes to knots, colour etc.

Quality

The wood goes through a general quality assessment. The impact of one defect is evaluated by assessing the overall quality of the product. The board will be rejected if its overall appearance differs significantly from the general quality requirements. Several maximum defects in a product are allowed if the overall appearance remains acceptable. Boards are generally not sorted after 8 cm from the ends. However, cut products are an exception.

Patination

Heat treatment changes the natural wood colour, and a warmer shade is created in which the structure and annual growth rings of the wood are highlighted, giving it a unique appearance. At the same time, heat-treated wood has a very special smell. Over time, the wood will patinate and have a beautiful, greyish shade. To begin with, patination will differ depending on the position of the wood in relation to the sun, wind and the weather in general.

Thermowood/heat-treated wood can be left without further treatment after installation.

Frøslev Heatwood must be surface-treated before or right after installation in order to preserve the beautiful, brown shade. This is done with coloured or pigmented wood preservatives containing UV filters. The wood preservative is applied in a thin layer. 

Over time, small cracks and splinters may appear in the wood when exposed to the elements. This can be minimised with a surface treatment that also extends the life of the wood.

To obtain the beautiful, grey patination, the wood can be left untreated and will simply become more beautiful over time.

Terraces are always built on joists to prevent the terrace boards from touching the ground and to allow air circulation under the finished terrace.