Introduction
Oak (Quercus spp.) is one of the most popular wood species in engineered flooring for its wide range of benefits and excellent decorative properties. Colour is one of the most important indicators for lamella or veneer grade, and also an important indicator to evaluate the flooring quality and price.
The colour of oak lamella vary from one species to another, and from one origin to another. This is affected by factors such as illumination, temperature and humidity. Some oaks have a significant and strong natural colour difference, while others have a uniform colour tone. Customers in different countries have different preferences on the colour of oak flooring.
In addition to its instinctive colour, oak flooring can come with a variety of intrinsic colour defects in production. Unlike surface stains, these are difficult to repair by sanding.
In this article, wood colour and colour defect formation of oak engineering flooring are introduced from the point of view of wood processing. This will help customers have a better understanding of the defects. Let’s get started by taking a closer look at the wood colour.
Wood colour
The main chemical constituents that make up the wood cell wall contain cellulose, hemicellulose and lignin. There are various colour-emitting and colour-assisting groups on these polymer compounds but there are no obvious colour differences between these groups. As a result of various pigments, tannins, resins and gums in wood lumina and cell walls, the wood presents with a beautiful range of diverse colours.
An obvious bicolour distinction can be easily found on the lamella of some species. An example of this is sapwood being light in colour, while the heartwood looks dark. The reason for this wide colour variation is greatly related to the tannin content and cellular activity.
The longitudinal parenchyma (longitudinal axial tissue with thin primary walls) in sapwood is active and works in the mechanical support, transport and storage of water and nutrients. There is an active stage in the longitudinal parenchyma of sapwood before dying, when starch is consumed and tannins increase.
Then the longitudinal parenchyma dies and transfers into heartwood with the tannins spreading into the cell wall to make the heartwood darker. This distinctive colour differentiation of the flooring reflects a sense of authentic nature, making it a hit in the European and American marketplace.
As with other biomass materials, wood is susceptible to colour changes from external environmental factors. The main factors that can cause colour changes include sunlight, oxidation, physical effects, microbial effects, chemical reagents, temperature, and humidity changes.
For example, teak flooring needs sunbathing before painting to achieve a more aesthetically pleasant colour by photo-oxidization (WHY ENGINEERED TEAK FLOORING HAS TO BE OXIDIZED IN BRIGHT SUNLIGHT). However, some colour changes not only affect the appearance quality and processing of the flooring but are also accompanied by surface roughness, cracking and other degradation. Therefore these colour defects call for special attention.
Oak engineered flooring colour defects
- Oxidation
If the unfinished oak engineered flooring or oak lamella in the warehouse is not well-protected, we may find the colour at the edges becomes dark yellow to reddish-brown, which is more common in the flooring with PF resin plywood as the core material. This is due to the destruction of phenolic compounds such as ellagitannins by photo-oxidation in oak wood.
Although the tannins that discolour the flooring are water-soluble, other discolouring substances are inevitably introduced when the discolouration is treated with water or other chemical reagents. The oxidation colour can not be processed by sanding, either. The coloured unfinished flooring or veneer has to be cut, resulting in a waste of raw materials.
- Water stains
When there are water drops on the oak surface, water stains will be formed on the wood. This is due to the hydrolysis of the tannins in the wood, resulting in compounds with colour-emitting groups. Again, as tannins are widely distributed in the cell walls and lumen, the water stains remain after sanding.
- Yellow stains
The oak lamella needs to be dried to a low moisture content after being sawn or planed. Yellow stains are likely to occur if the moisture content is not uniformly distributed after drying.
Yellow stain is a typical fungal discolouration caused by coloured fungi and soluble pigments. It usually appears as spots or small streaks. The pigments have penetrated into the cell walls and lumen, which cannot be removed by sanding.
- Grid marks
The grid mark is also a processing defect that occurs in the lamella drying process. It is usually visible as a lighter colour mark on the wood surface, which cannot be removed by sanding either.
The drying of thick oak lamella, either sawn or planed, is carried out in a drying kiln rather than the traditional dryers used for thin veneer drying. In the kiln, the veneers are separated from each other by grids or spacers for ventilation, allowing the moisture and hot air to diffuse evenly.
The contact between the conventional wooden spacers and the lamella surface is not conducive to the diffusion and the migration of colour-emitting substances to the surface of the veneer. This results in a light colour mark on the contact surface. To avoid grid marks, factories are now using plastic spacers instead of traditional wooden spacers.
Summary
Colour and colour defects in wood are caused by the chemical composition in the cell walls and cell lumen. Colour defects found in lamella or flooring cannot, therefore, be removed by sanding but only by resizing or dark painting.
The colour defects introduced above are not only found on oak floorings but on other species too. The above information should help you avoid possible colour defects when selecting floorings.
