Silvapro®

Instructions for the selection and preparation of wood for Silvapro® thermal modification and its processing

General

The Silvapro® thermal modification process increases the wood's resistance to biotic decomposition factors (fungi, insects), improves its dimensional stability (less swelling and shrinking of the wood with changes in humidity) and increases its insulating capacity (reduced thermal conductivity). At the same time, the colour of the wood, the surface properties and some mechanical properties are also modified. As a result, the life span of the products is significantly extended allowing longer maintenance intervals. During modification, the dimensions and density of the wooden elements are slightly reduced. The higher the degree of modification (the higher the temperature and the longer the modification time), the more significant the above-mentioned changes.

Wood to be used outdoors needs to be modified to a higher degree than wood to be used indoors. Therefore, the modified wood shall be used in accordance with the intended use specified when the raw material was delivered. It is not recommended to use thermally modified wood in direct contact with soil or in constant contact with water, or in places where the wood never dries. In this case, it needs to be additionally protected with Silvaroyal® preservative.

During the modification process, the density of the wood can be reduced by up to 15% and its flexural strength by up to 25%. For this reason, care should be taken when using thermally modified wood for the construction of structures expected to carry high loads.

Although the processing of Silvapro® modified wood is similar to that of natural, unprocessed wood, there are certain specificities that need to be taken into account in some of the processes. Most of these are a consequence of the lower equilibrium moisture content (higher hydrophobicity) and the slightly higher fragility of thermally modified wood.

Wood quality and moisture content

Healthy wood of good quality is suitable for thermal modification, containing only mature ('healthy') gnarls, preferably as few as possible. For thermal modification, the radial and semi-radial orientation of the wooden planks is the most suitable (so that the widest surface is in the radial plane - Figure 1). Tangentially oriented wooden planks are less suitable for the modification process (the phenomenon of delamination by growth rings is more frequent).

In principle, most softwood and hardwood species can be modified. The appropriate wood species is selected on the basis of its basic characteristics, aesthetic appearance and price. For example, harder and denser wood species should be selected for surfaces where higher mechanical loads are expected than for surfaces where lower loads are expected.

The quality of Silvapro® thermally modified wood is closely linked to the quality of the raw material, i.e., the quality of the wood selected before processing. For larger projects, the quality of the selected wood (raw material) shall be in accordance with the project documentation and other norms.

Our experts will gladly help you choose the right wood and carry out the optimum modification process for your intended use.

Figure 1: Graphical illustration of the anatomical planes of wood

The moisture content of the wood to be thermally modified shall be mandatorily lower than that specified for each wood species group, i.e., between 12 % and 14 % for the simpler conifers (spruce, fir), between 10 % and 12 % for the more complex conifers (pine, larch), and between 8 % and 10 % for most deciduous trees (hardwoods). Particular attention should be paid to deciduous wood, as some species (oak, ash, robinia) show a very rapid onset of wood case hardening already in the drying phase, if the correct drying parameters are not observed during the drying process (in particular, too high drying speed or too aggressive drying process - temperature, rate of temperature rise, pressure, etc. are problematic.) If wood is thermally modified which has been previously hardened due to incorrect drying (an additional complicating factor is that wood case hardening cannot be detected visually), there is a high probability of defects occurring after the thermal modification process (mainly in the form of micro-cracks) appearing during mechanical treatment. Therefore, the correct drying process is even more important than the moisture content of the wood (8 to 10 %) when thermally modifying hardwoods. To avoid inconveniences, we therefore advise you to choose wood suppliers that have sufficient experience in drying specific types of wood. Before carrying out the thermal modification process, you should therefore consult our experts who will be happy to advise you on the particular wood species.

Preparing the wood stack

If you intend to deliver a large quantity of wood for thermal modification, it makes sense to prepare the wood stack in advance, in order to speed up the processing and make the process more economical. When doing so, please note:

Stack dimensions:

• stack length: max. 500 cm
• stack width: max. 120 cm
• stack hight: max. 120 cm.
• The wood should be slatted at least every 35 cm of its length with slats at least 20 mm thick for the usual (Figure 2).
• We thermally modify planks with thickness of 50 to 60 mm, either in a single elemebt thickness, or a s a combined thickness of two elements (one row of 50-60 mm thick planks, or two rows of 25-30 mm planks). The wood in the stack shall be of the same species and of similar thickness (the maximum recommended variation in thickness of the wood elements is 10 mm).
• The width of the wooden elements is not important, and the elements can be stacked close together in each row.
• There shall be no other materials (string, plastic, paper, etc.) in the stack.
• The maximum capacity of the thermal modification chamber is about 25 m3. For this reason smaller quantities of delivered wood can only be thermally modified according to our standard procedure appropriate for outdoor exposure. In case key modification factors of the process are required (lower temperature, lighter/darker colour, …), the full capacity of the chamber must be filled (at least 20 m3).

Figure 2: Preparation of the stack; slat thickness 20 mm every 35 cm length, planks with maximum thickness of 60 mm (one row of 50/60 mm planks, or two rows of 25/30 mm planks - as shown on the photograph).

After the takeover

After modification, Silvapro® wood needs to be inspected and conditioned before use - balanced to a moisture content of between 4 and 8%. We therefore recommend that you start finishing the wooden elements (decimation, gluing, assembly, surface treatment) only 14-28 days after the modification process (depending on the type and thickness of the wooden elements). Attention: thermally modified wood shall have a minimum moisture content of 6% before being installed outdoors or exposed to weather conditions.

Mechanical processing

Due to the slightly higher fragility, the use of sharp blades is recommended when mechanically processing Silvapro® wood. To avoid chipping of the edges, we recommend using finely serrated saw blades. When using blades made of high-quality materials (e.g., carbide hardness blades), the sharpening intervals can be longer.

As the internal (growth) tensions in the wood are released during the modification process, there are no deformations of the wood elements when sawing Silvapro® wood. Silvapro® wood does not contain resins, so the machine power can be lower, providing a longer service life of the machine.

The wood dust produced when Silvapro® wood is processed is fine and dry. For this reason, the extraction system has to be suitably efficient and well-sealed.

Gluing

Due to the higher hydrophobicity of Silvapro® wood (slower water absorption), the water content should be minimised and/or the curing time of the glue joint should be extended when using certain water-based glues, e.g., PVAc (especially glues whose curing is based on water absorption).

Polyurethane (PU) glues require water (either from the wood or from the surrounding atmosphere) for it’s curing. We therefore recommend (especially if the ambient air is very dry) that the surface of Silvapro® wood is moistened with a water mist before gluing with PU glues (the equilibrium moisture content of Silvapro® wood is approximately 50% lower than the equilibrium moisture content of natural, unprocessed wood).

When gluing, also follow the general instructions of the glue manufacturer.

Nailing and screwing

Especially for Silvapro® wood types with lower density, the use of countersunk holes or self-drilling screws is recommended to avoid splitting. Pneumatic nailing achieves good results, the only thing that needs to be taken into account is the appropriate pressure and the appropriate depth of penetration of the nails. It is also recommended to avoid nailing close to the edges (at least 2 cm from the edge). For fixing Silvapro® wood elements for use in damp areas and outdoors (exposed to weather conditions), we recommend the use of stainless steel (so-called inox) nails and screws.

Colour and surface treatment

Silvapro® wood has a dark brown colour that is uniform throughout the entire cross-section of the wood element. The colour tone depends mainly on the degree of modification and the type of wood, but also to a lesser extent on the origin of the wood (growing area), possible discolorations (e.g., red heart in beech), etc., which may lead to slight variations in the colour tones of the elements.

When exposed to direct sunlight - just as with natural, unprocessed wood - the surface of Silvapro® wood starts to change its colour over time, and when sunlight exposure is combiuned with precipitation events, thermally modified will gain its natural grey patina of (aged) wood, but this does not affect the quality and functionality of the product itself. The influence of abiotic factors on the surface of Silvapro® wood can be mitigated or prevented by using pigmented, UV-resistant coatings. If you would like to additionally colour the modified wood or prevent the surface from greying, we recommend the application of Silvalazur or Silvacera® natural wax.

It is good to now

By confirming the order, the customer/buyer declares that he/she is fully aware of the characteristics and properties of Silvapro® thermally modified wood.

Silvaprodukt shall not be held liable for any deformation of the elements during the modification process caused by growth tensions and/or anisotropic shrinkage of the wood (entered raw material). We also do not take responsibility for cracks, fallen out gnarls and other defects resulting from poor quality, damage or hidden defects in/on the raw material.

The quality of the final product depends on the quality of the raw material.

Use the wood in accordance with its declared use.