Hardwoods & softwoods
The answer might surprise you. As with a lot of the English language, words don’t always mean the same thing in different circumstances. Normally, we would expect the term ‘hardwood’ to mean that the wood is hard to the touch, heavy, or dense, with the reverse being true of ‘softwood’. However, as it turns out, the terms hardwood and softwood really have nothing to do with the hardness or softness of the wood. These names, although misleading, refer to the type of growth or tree vegetation that they are taken from.
Softwood is taken from coniferous (cone bearing) evergreen trees with needles.
Hardwood is taken from deciduous trees that are leaf bearing and drop their leaves in winter.
The best examples of contradictions to the misleading names include balsa and pitch pine. Balsa has a weight of 8 lbs/cu.ft., versus pitch pine that weights 40 lbs/cu.ft. Balsa is classed as a hardwood because of its deciduous nature, but we all know it is commonly used for model aircrafts because it is light weight. Pitch pine, on the other hand, is a coniferous evergreen and therefore a softwood, but at 40 lbs/cu.ft. it is very dense. It is often used for high traffic piers and docks where extreme wear and exposure is expected.
In order to clearly understand the differences between these two categories, we have to look at the structure of the wood itself. Coniferous and deciduous trees produce two totally different types of wood, because the growth structure is completely different. This marked difference in the structure of the wood at the cellular level can be seen by cutting the end grain of the wood and viewing it with a magnifying glass. One essential difference is the presence of vessels in hardwoods. These hollow tubes run the length of the tree and serve as conduits for water and nutrients. In hardwoods, the cells are closed and cannot function as conduits. In softwoods, the cells have openings to other cells.
Another difference is that, in general, the density of wood depends on the thickness of the cell walls in relation to the size of the cells’ hollow center. So, softer woods tend to have cells with thinner walls and wider hollows, while harder woods have cells with thicker walls and narrow hollows.
Now you may be wondering how this affects your woodworking. Well, in a general sense, the structure of the wood determines not only which category it belongs to, it also affects the wood’s weathering ability.
For example, if you were going to make a piece of outdoor furniture, you would choose a softwood. We have all seen roof shingles, made from western red and eastern white cedar. This wood has been used for fence posts and split-rail fence rungs, and can last in excess of one hundred years. We are also familiar with cedar picnic tables, and pine barn boards. Those old barns, exposed to the weather 24/7, also last more than a hundred years. The Muskoka style outdoor chair is another example of softwood lasting well in the elements.
What is it about the structure of softwood that would account for its weathering ability? Remember that the softwoods are generally closed-cell structures while the hardwoods are generally open-cell, with pores and vessels. It makes sense that a closed cell softwood would resist the penetration of water better than an open cell hardwood. Since most of the deterioration of wood is caused by wetting and drying, it also makes sense that the less penetration the better. Therefore, softwood is best used for outdoor projects.
If we return to the barn example, we can see that our ancestors were well aware of the weather’s effect on woods. The threshing floor was supported by hardwood beams and joists, as was the interior superstructure, because it was protected from rain by the cedar shingle roof and the pine board cladding.
So, to get back to how this affects your woodworking. If you were making a chair for interior use, with four delicate legs, you would use a strong hardwood. Alternately, if you were making a chair for outside use, you would change the design from four delicate legs to wider, flat board legs (like those found in the Muskoka chair design) to allow the use of pine or other softwood, which would hold up better under the elements.
Having an understanding of the nature of hardwood and softwoods will give you better insight when it comes time to choosing the most appropriate wood for your next woodworking project.
This is the third of three articles that look at the general characteristics of wood. Next issue we get back to looking at specific trees and their properties.