To understand why and how to dry wood, it helps to know how it is made up and how it behaves. Let’s take a look.

Unlike materials such as concrete or steel, whose particle arrangement tends to be uniform, wood is a highly heterogeneous material that comes from something that was alive and dynamic. Plant growth is directly linked to the distribution and movement of water between roots and leaves, because water carries the nutrients the plant needs to grow. The figure below shows examples of the homogeneous microstructures of concrete and steel and the heterogeneous microstructure, or anatomy, of wood.

Why wood moisture needs to be controlled

The amount of water in wood can vary widely depending on the species, the age, the climate in which the tree grew and the conditions where the piece is stored, among other factors. Drying is a way to standardize wood moisture and therefore guarantee the quality of the material. It makes processing easier, helps control the mass transported and sold, and prevents biodegradation and stability problems in later use, whether the final product is solid wood, panels, paper, biomass or composites with other materials.

7 reasons to dry wood

In short, these are the main reasons why drying is essential:

1. Dimensional stability

Once cut, wood starts drying naturally until it reaches equilibrium with its surroundings. During drying, the material first loses its free water, stored in the channels and empty spaces of the cells. Beyond a certain point (the fibre saturation point, around 28% to 30% moisture), the wood also starts losing the water held in the cell walls, and this is accompanied by shrinkage of the piece. The purpose of drying is to control this movement. Knowing the environmental conditions where the piece will be used, the shrinkage is brought forward in the industrial process, ensuring that movement at ends, in furniture, joints and fittings is negligible and does not compromise the final project.

Without the dimensional stabilization that drying provides, defects such as cracks and warping are common and often mean the processed pieces have to be discarded.

2. Mechanical strength

Besides preventing defects such as cracks and warping, drying increases the mechanical strength of wood and its nail-holding capacity.

3. Finishes

Finishes such as paints and varnishes also depend on proper drying. Paint and varnish manufacturers commonly advise against applying them to damp surfaces.

4. Biodegrading agents

Many types of bacteria and of staining and decay fungi grow better and faster in wood with higher moisture content. Drying protects the wood against these biodegrading agents.

5. Transport costs

Drying avoids transporting water unnecessarily and cuts logistics costs, since loads become lighter and more cost-effective.

6. Phytosanitary control

Drying also kills fungi and insects that may have settled in the wood. The process can even serve as a phytosanitary treatment and meet the requirements of the international timber trade, provided it meets structural and process requirements (mainly calibrated equipment and controlled temperature and time).

7. Environmental and international requirements

With environmental requirements becoming ever stricter, it is important to add value to a biodegradable material like wood. Native forests are shrinking, so the use and yield of wood from planted forests must be optimized, avoiding waste as much as possible. Drying is one of the processes that make this possible.

It is also essential to plan the drying of wood intended for export, since Brazil is a major exporter of the material. Wood quality, largely determined by drying, is decisive for the continuity, safety and success of timber exports.

How to dry wood

That leaves one last question: how should wood be dried? Marrari offers a wide range of drying kiln controllers and management systems that help run the process with precision and quality.

Author: Elisa Pizzaia Goltz, Industrial Wood Engineer (UFPR, 2014), MSc in Forest Engineering (UFPR, 2018), quality and technical manager at Marrari Automação’s Umilab laboratory

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