Wood features

Natural gloss of wood - why it occurs, what influences it, species with high gloss

The shine of furniture or flooring is given by special finishing materials which, when applied correctly, give the object a unique glow. However, there are species of wood that have a sheen even if they are not coated with glossy varnishes or paints. This is the natural sheen that depends on the structure and the way the wood has been cut and sanded/painted. The sheen can be uniform or intermittent, forming waters. It can also have a silky appearance with silvery or golden iridescence. Why this natural sheen occurs and which species are known to have this characteristic, below.

Why some species have natural lustre

Light appears on surfaces that have the ability to reflect light. This is how it appears on wood. Even if the species is one with a natural sheen, we will only see it if the surface can reflect light. Both the structure of the wood and the inlays and the way it has been cut and smoothed are important in reflecting it.

The structural elements that give natural lustre are medullary rays, popularly called and mirrors. They are made up of woody cells that join the bark of the tree to its pith or to one of the annual rings. Sometimes, to distinguish them from those that run through the wood to the centre, those that stop at a ring are called rays. The woody cells that form the medullary rays may be arranged in a single row, forming a thin string invisible to the naked eye, or in several rows, in which case they become visible. This is why some species have natural lustre and others do not, as medullary rays are a characteristic of all wood species. The cells that form the medullary ray are very compact, the resulting surface is dense and the colour slightly different from the surrounding wood.

In cross-section through the trunk, the medullary rays are seen as lines joining the bark to the medulla. In the radial section, however, they look like bands of different lengths and widths. The wider they are, the better they reflect light. This is why the natural lustre of the wood is most visible in radial cuts. In tangential section, the medullary rays are seen as straight longitudinal lines that can be longer or shorter, uniform or with thicker areas.

The width of medullary rays can sometimes be misleading. There are species (carpen, hazel) that appear to have wide rays but no lustre. These are the so-called matt medullary rays which are made up of several very narrow rays joined together. They do not form a continuous surface and therefore do not reflect light correctly to form the natural glow.

The medullary beams not only give the wood its gloss but also its ability to split easily. Wood with wide and long medullary beams splits very easily.

Cutting, polishing surface area and fibre fineness influence how natural gloss is perceived

The medullary rays, which are wider in radial section, cause the pronounced natural gloss when the wood is cut radially. Tangential cutting follows, and when cross-cutting, the gloss disappears almost completely, even if the species has large and many medullary rays. In cross-cutting we are also dealing with a lot of fibre ends which make the surface very porous and do not reflect light.

It is also the fibre ends that give roughness to surfaces with a different type of cut. This roughness will not allow light to reflect off and despite the presence of medullary rays, the wood will not have a pleasant sheen. This is why it needs to be sanded or polished. The natural sheen is best brought out when the wood is planed with a very sharp edge. Sometimes the term geluit for this operation. The reindeer precisely cuts the fibres leaving them smooth while straightening the surface. It remains much smoother than sanding, even very fine sanding, reflecting light much better. Fine sanding does not have the ability to straighten the surface, and the resulting fine dust can affect the way light is reflected.

The fineness of the fibre is also important. Not all species have fine fibre and even if there are medullary rays, coarse fibre can reduce surface gloss. This is the case oak. Despite the highly visible medullary rays, the surface is less shiny than that of the cherry or hairwhose rays are much less visible. If the smoothness of the fibre is matched by a veined appearance, the surface gloss will be much more spectacular. Paltinul can give such an effect, very well highlighted by oil or a very transparent varnish.

Species with natural lustre

Palint is, in fact, the best-known example of a native species with a natural lustre. It has a silky, satiny lustre, just like sycamore, ulmul or hair. The Mastic has a slight silvery sheen, and acacia sometimes has golden iridescence. Well smoothed oak surfaces, fag and hornbeam have a pleasant gloss, much less pronounced than would be expected from the width of the medullary rays. Instead teepee, with narrow medullary rays, thanks to the fine fibre often has a pleasant sheen. Spruce, despite its very narrow medullary rays, still has a slight natural sheen that differentiates it from brad, that has no sheen at all. It is also a how the wood of the two species can be differentiatedotherwise very similar.

Exotic species often have a natural lustre. Abanosul, the different types of mahon, palisander (rosewood) are known for their high natural brilliance. The fact that the wood is very dense also helps in this case. In some, however, the presence of gums and salts causes the natural lustre to diminish.

What problems can medullary rays cause when finishing

Whether or not the wood has a natural sheen, a more or less shiny appearance can be kept with the help of finishing materials. Varnish, oil, wax can add gloss to surfaces while enhancing the natural wood grain. The more transparent the materials, the more pleasing the effect will be. In the case of wax, polishing is needed to give it that silky sheen. Wax sits much better on a varnished or at least sanded surface than directly on wood. Varnish/gloss has the ability to make the surface much straighter and more even after drying.

However, medullary rays can cause problems if we want to stain the wood. Their very dense surface means that absorption in that area is very low and they do not stain. The effect is very visible when staining oak. In this case, staining by wiping or staining will not give very good results. Wood is much more evenly stained by spraying, either manually or mechanically. Very fast drying stains or pigment-based staining solutions are recommended.

Because of the reduced absorption, in areas with wide medullary radii (in radially cut wood) problems of adhesion of varnish to wood. In this case, the recommendation is that the wood should not be sanded with very fine paper (120, max.150) before applying the primer.

I hope you find the above information useful. As usual, additions are welcome. And if you have any questions or queries, please leave them in the space below. I'm sure I'll reply.

About the author

Mihaela Radu

Mihaela Radu is a chemical engineer but has a great passion for wood. She has been working in the field for more than 20 years, wood finishing being what defined her during this period. She gained experience working in a research institute, in her own company, as well as in a multinational. She wants to continuously share her experience with those who have the same passion - and more.

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