What density tells us about the behaviour of wood so we can choose the right one for our project

We can predict the behavior of wood in different situations using its properties. We have seen here how mechanical properties help us know whether or not a wood is suitable for the timber structure of a house or which species to choose if you are planning to make curved furniture. Acoustic properties tell us why some species are more suitable than others for musical instruments. Density is one of the important properties closely related to the subsequent behavior of wood. It tells us whether a wood is light or heavy, dense or loose, hard or soft.

Density and specific weight of lemons

When studying the properties of a wood species, we sometimes find the density mentioned, sometimes the specific weight, both expressed in kg/m³ (or g/cm³). We can put an equal sign between the two values even if each expresses something different. Density is the ratio between the mass (weight) and volume of a piece of wood, while specific weight is a simpler way of expressing the density of wood relative to the density of water. They have the same value because in the metric system of measurement water has a density of 1g/cm³ (1000 kg/m³).

There are several types of density because wood is not a uniform material. Inside a piece of wood there are cell walls, voids filled with air or water, bound water and other substances that make up the wood (colorants, tannins, resins, gums, salts). If only the substances which form the cell wall (cellulose, hemicellulose and lignin) are taken into account, the density value varies insignificantly from species to species, and is considered to be 1500 kg/m³ for all species. This value is also called the density of the woody material.

But for those who transport and store wood, in furniture making or construction, this value is not useful. In their case, the wood is used in its entirety, along with the voids, water and other interior elements. In this case we are talking about bulk density which is different depending on the species but also on the water content. Water, having a high density, influences wood density, which increases with increasing water content.

There are several types of bulk density. The most commonly used are the bulk density of anhydrous wood, the conventional bulk density and the bulk density of wood at a given moisture content. For simplicity, the specification of "apparent". Density of anhydrous wood is obtained in the laboratory by total drying of the wood (0% moisture content), it is the ratio between the mass and volume of anhydrous wood and is useful to see the difference between species.

Conventional density is the ratio of the mass of anhydrous wood to its volume at 30% moisture content (also called wet wood or green wood) and helps to determine the amount of chemically processed products or products processed by partial wood deterioration processes (pulp, paper, wood fiber) that can be obtained from wet wood. Density at a given humidity of wood is of interest to furniture manufacturers, builders or shippers, and the mass/volume ratio is usually calculated at a wood moisture content of 12%.

Factors on which wood density depends

We have already seen how water content influences density by changing its value. But the density is also very different depending on the species. It can vary from about 200 kg/m³, as in balsa wood, to about 200 kg/m³ in balsa wood. 1300 kg/m³ is the density of a species of African ebony. The big difference is due to wood structure, i.e. the proportion between wood mass and voids, on the one hand, and the content of other substances, on the other. That is why African wood, with its high content of tannins, colorants and salts, is usually very heavy, heavier than water, so it does not float but sinks. Wood from the temperate zone, which includes Romania, is generally between 300 and 990 kg/m³, making it lighter than water.

Density is also different within the same species and even between different elements of the same tree. Younger wood is lighter than older, and branch wood lighter than trunk wood. Sapwood is less dense than heartwood, and earlywood less dense than latewood. It's all to do with the inner structure of the wood. In general, the increase in annual ring width comes with a decrease in the proportion of latewood in softwood (resinous) and an increase in hardwood (broadleaved) with uniformly arranged pores. The presence of larger rings indicates a lower density in resinous wood and a higher density in hardwood with uniformly arranged pores. In hardwood with diffuse pores no clear correlation between ring width and density can be made.

How density information helps us choose the right wood

We've linked the structural elements of wood to density, but what does this tell us about how we choose wood? Is there a link between wood properties, strength and density? Yes, there is, and with this in mind we can choose the most appropriate wood for the project we are about to do. A higher density means a wood with more wood matter and fewer voids, so a denser, harder and stronger wood, but also a heavier wood.

For furniture production, a wood density of 400 to 800 kg/m³ is considered good. The wood is dense enough for a nail or screw to penetrate well and be county inside so that it is not easily torn out, but not so dense that the nail or screw can enter and cause the wood to crack. Also, wood with a density within this range has sufficient strength to withstand the various forces acting on it, but is not so heavy that furniture is unusable.

Density also gives us clues about how much water the wood can absorb, about shrinking and swelling it. The lower the density of wood, the looser it is and the more water it can absorb. In this case the dimensional variation is greater, and the swelling and shrinkage can affect the work it is used for. On the contrary, denser wood means more wood mass and fewer voids, less room to absorb water, and therefore more stability.

With density we can know more about the calorific value of wood. In general, the denser the wood, the better the fuel, because it burns more slowly, the longer the char lasts and the more heat it gives off.

In general, softwood has a lower density than hardwood, but there are many exceptions. There are species of pin having density comparable to a frasin, just as there are low-density deciduous species with low density and loose wood (plop, willow). Below are examples of the densities of some wood species, although these are approximate values. The exact density of an item can be determined by the ratio of mass to volume in the case of regular pieces, or by determining its specific weight by immersion in water in the case of irregular shapes. Find here how the determination is made.

• carpen - 980 kg/m³
• oak - 600-800 kg/m³
• fag - 700-750 kg/m³
• ash - 750-850 kg/m³
• acacia - 750 kg/m³
• nuc - 700-750 kg/m³
• cherry - 600 kg/m³
• tee - 520 kg/m³
• plop - 300-400 kg/m³
• pine - 450-850 kg/m³
• larch - 500-550 kg/m³
• molid - 520-550 kg/m³
• fir - 460-550 kg/m³
• juniper - 550 kg/m³

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