Category: Cut and Dried

The following in excerpted from “Cut & Dried: A Woodworker’s Guide to Timber Technology,” by Richard Jones.

The author has spent his entire life as a professional woodworker and has dedicated himself to researching the technical details of wood in great depth, this material being the woodworker’s most important resource. The result is this book, in which Richard explores every aspect of the tree and its wood, from how it grows to how it is then cut, dried and delivered to your workshop.

Richard explores many of the things that can go right or wrong in the delicate process of felling trees, converting them into boards, and drying those boards ready to make fine furniture and other wooden structures. He helps you identify problems you might be having with your lumber and – when possible – the ways to fix the problem or avoid it in the future.

“Cut & Dried” is a massive text that covers the big picture (is forestry good?) and the tiniest details (what is that fungus attacking my stock?). And Richard offers precise descriptions throughout that demanding woodworkers need to know in order to do demanding work.

In order to design successful structures we furniture makers and other woodworkers need to develop some understanding of wood’s strength. It is common knowledge amongst experienced woodworkers that some woods are stronger than others; we quickly learn both European oak or American white oak are stronger than balsa wood, or ash is a better material for hammer shafts than European red pine, i.e., Scots pine. But the question to pose is, “What determines the strength of wood?” The answer lies in the material’s ability to resist stress, and the strain or deformation resulting from the stress along with the material’s ability, or inability, to recover its original form when, or if, the stress is removed. Both stress and strain are definable and measurable.

Stress, more precisely described as unit force, is the amount of force acting on a defined area; strength is the ability of a material to resist unit force. Stronger materials resist unit force better. It’s relatively easy to work out the unit force a bookshelf must resist. To do so, weigh the books carried by a shelf to establish the load (L) and calculate the shelf’s surface area (A). The numbers for the following sample calculations came from a convenient load of books on a shelf in my home.

• 42 books weighed on domestic scales = 32 kg (or 71 lbs). Shelf dimensions: 870 mm x 295 mm = 0.26 M² (or 34.25″ x 11.61″ = 2.76 ft²).
• To calculate the unit force (UF) applied to the shelf, divide the load (L) by the area (A) thus: L / A: therefore 32 kg / 0.26 sq m = 123.01 kg per sq m UF.
• Working in pounds and feet calculate: L / A: therefore 71 lb / 2.76 sq ft = 25.72 lb per sq ft UF. This can be converted to pounds per square inch (PSI) thus: 25.72 / 144 sq in = 0.18 PSI.

Engineers and scientists seek greater accuracy than the methodology used here of weighing with bathroom scales and rounding results to two decimal places, but the methodology and values used illustrate the principle. Additional calculations using the source data shows the shelf carries approximately 11.04 kg per 300 mm length, or approximately 24.69 lb per foot length. My experience is these numbers are typical; for many years I have used 25 lb per foot length or 11 kg per 300 mm length as standard bookshelf loading. There are exceptions furniture makers have to design for, but those exceptions are generally readily spotted, e.g., a request to create shelving for a collection of large-format art books immediately triggers a reaction that the shelving should be stronger. For example, you might use 18 mm thick solid oak instead of 18 mm thick oak veneered MDF, or extra reinforcement is necessary, or the shelf span should be shortened, or a combination of all three measures may the right solution.

It is possible, where necessary, to calculate the load beams are likely to experience in use, then to design for and build in enough strength for the intended load, plus an additional safety margin. Situations where woodworkers are most likely to recognise the necessity for such calculations are in the building or construction industry, e.g., safe loading of wooden floors and roof truss design. Indeed, there are calculations, formulae and standard load tables used by structural engineers to account for the load-bearing requirements of such structures.

Posts, such as music stands, easels, benches and table legs, chair legs, parasols and umbrellas, cabinet sides etc., all experience loads or stress. In many cases each individual leg in a chair is more than strong enough to carry the weight of a person; the design challenge for a one-legged pedestal chair is finding a way of supporting the pedestal so it doesn’t fall over when applying a downward load and, further, making it strong enough to cope with any torsional (twisting or rotational stress) and horizontal forces a pedestal chair leg must endure.

Stressed parts, i.e., loaded parts, experience strain and strained parts deform; strain is defined as unit deformation. If you lightly tap the surface of a piece of 50 mm- (2″-) thick wood with a hammer the wood directly under the hammer head compresses, i.e., the thickness reduces and this illustrates unit deformation. After a very gentle tap with a hammer, the wood will regain its original shape and form showing the wood is elastic and it can recover if not unduly stressed. Without controlled laboratory conditions it is hard to measure the amount of compression but under a light load as just described let us assume, for the purpose of an example, the unit deformation is 0.2 mm (0.00787 inches).

Calculating the unit deformation caused by the impact of the hammer head requires the sum: Dimensional Change / Original Dimension

Using the figures given in the hammer-tapping example, i.e., original plank thickness = 50 mm and the amount of compression = 0.2 mm the calculation is: 0.2 mm / 50 mm = 0.004 millimetre per millimetre (mm/mm). The end result is expressed here as millimetre per millimetre, meaning 0.004 millimetre (unit deformation) per millimetre (of thickness), the same proportion as 0.2 / 50. In reality the expression “millimetre per millimetre” is not necessary from an engineer’s perspective because the proportion of deformation, i.e., 0.004 to the original thickness of the piece of wood is the key information. The same rule applies when you work in any other unit of measure as long as the same units are used on both sides of the equation, e.g., inches divided by inches, metres divided by metres, miles divided by miles etc. The following sum uses inches but note the end result is still 0.004.

After converting the metric measurements used in the previous paragraph to three decimal places in inches, the sum and the result are: 0.008 in / 2 in = 0.004 inches per inch (in/in). Dimensional change is 0.004 inch per inch.
Returning now to hitting the wood with a hammer, tapping the surface of the wood harder and harder with the hammer will eventually lead to one of those blows leaving a noticeable and permanent dent in the wood. This rough and ready experiment demonstrates Hooke’s Law.

“Hooke’s Law states that the strain is proportional to the stress” (Kollman and Côté Jr., 1968, p 292). Further clarification of Hooke’s Law leads to saying in wood, in common with other materials, stress and strain are proportional up to a particular point. Specifically, that point is the proportional limit. Beyond the proportional limit of the material, increased stress leads to disproportionate strain, i.e., greater deformation, until the material reaches a stage where further stress leads to failure.

Another way of describing this phenomenon is, up to its proportional limit, a material exhibits elastic properties whereby applying a load causes it to deform, and on removing the load the material completely recovers. Beyond the proportional limit of a material, adding bigger loads causes the material to become plastic rather than elastic, and it cannot recover completely after removing the stress and eventually additional load causes the material to fail.

Within the elastic range of a material (up to its proportional limit) the ratio between applied stress and the resultant strain is a constant with this ratio being the modulus of elasticity (MOE), also known as Young’s Modulus. “[It] is a measure of … stiffness or rigidity. For a beam, the modulus of elasticity is a measure of its resistance to deflection” (Forest Products Laboratory, 1955, p 68). Figures 14.18 and 14.19 illustrate the proportional nature of strain in response to added stress where incrementally greater loads act on the centre point of a shelf. This kind of load is a static load.

A rubber band is another item illustrating Hooke’s Law. The law, in the following description, is demonstrated visually rather than measured scientifically. If you hold a rubber band between your fingers and stretch it gently followed by releasing the stress, it will recover its original shape. Successively increasing the strain stretches the band further, and a common visual sign the band is approaching its recoverable limit is increased whitening of the stretched rubber. As the band has to cope with increasing stress it loses the ability to recover and return to its original shape, and further stretching eventually causes the band to break.
The elastic band experienced a tension force that stretched it whereas the previous example, a plank of wood, experienced a compression force through being hit with a hammer head. In both cases the important point is the material experienced a stress (loading) resulting in strain. And in both cases the stress and strain are proportional up to a specific point; beyond that point increased stress leads to greater strain. Stress is a force that can act in more than one direction – stress may in fact occur in multiple directions at the same time, e.g., a part could simultaneously experience compression, tension, and shear stresses (see figure 14.24).

The strength of a material determines its ability to resist stress: an 18 mm- (3/4″-) thick oak book shelf 610 mm (24″) long is significantly stiffer than an MDF shelf of exactly the same dimensions. As a consequence, when both shelves are stressed by loading the same weight at their midpoint, the oak shelf exhibits less strain indicated by less deformation, i.e., it does not bend as much. In addition, the oak shelf is able to carry significantly more weight than the MDF shelf before it fails completely.

A simple static bending load experiment to demonstrate Hooke’s Law.
Static bending occurs under a constant load or when a load gradually increases. The set-up is a rudimentary partially fixed end beam with a knot-free softwood fence paling (picket) screwed down at both ends to span between the two sawhorses. The distance between the bottom of the paling and the ground was measured and noted. Concrete blocks, each weighing approximately 10 kg, were loaded onto the paling, and the distance between the paling and the ground measured. This was followed by removing the blocks, and a note of the distance between the ground and the paling taken again. The sequence was: Add one block, measure, remove the block and measure again; next, load two blocks, measure, remove the blocks, measure again, etc. The paling recovered to its original condition up to the point where adding and subsequently removing 9 blocks (~90 kg); this was the “proportional limit” of the material. Loading additional blocks led to greater bending of the paling under the load, and ever greater permanent distortion (permanent set) of the paling after removing the load. Complete failure of the paling occurred with a load of 13 blocks (~130 kg). This experiment did not represent true scientific testing; it is evident, for example, the outermost feet of the sawhorses had lifted off the ground in the middle image, which compromises the accuracy of measurements gathered. However, the accompanying graph, figure 14.23 derived from the experiment, illustrates Hooke’s Law reasonably effectively.

The following is excerpted from “Cut & Dried: A Woodworker’s Guide to Timber Technology,” by Richard Jones.

Jones has spent his entire life as a professional woodworker and has dedicated himself to researching the technical details of wood in great depth, this material being the woodworker’s most important resource. The result is “Cut & Dried: A Woodworker’s Guide to Timber Technology.” In this book, Jones explores every aspect of the tree and its wood, from how it grows to how it is then cut, dried and delivered to your workshop.

Jones also explores many of the things that can go right or wrong in the delicate process of felling trees, converting them into boards, and drying those boards ready to make fine furniture and other wooden structures. He helps you identify problems you might be having with your lumber and – when possible – the ways to fix the problem or avoid it in the future.

“Cut & Dried” is a massive text that covers the big picture (is forestry good?) and the tiniest details (what is that fungus attacking my stock?). And Jones offers precise descriptions throughout that demanding woodworkers need to know in order to do demanding work.

For the first year or two of working wood in the 1970s, I didn’t come across the term brash wood because the craftsmen I worked with called the condition “carroty” or “carrot wood” and I assumed, being young and naïve, this was the normal name. The woodworkers around me, on finding some particularly weak stick would say things like, “It’s rubbish; the stuff just carrots off in your hands.” It was an apt description because a brash break in wood is visually slightly similar to a carrot broken into two half-lengths.

Brash wood has a variety of related names including brashy, brashness and brashiness. Other names for this condition are brittle heart, carrot heart, spongy heart, brash heart and soft heart. Natural brashness or brittleness develops in the living tree caused by the way a tree grows and the stresses it experiences in life. In every case brash wood is weak wood and it unexpectedly snaps across the grain under a load normal wood of the same species would carry with ease.

Brashness often develops in association with cross shakes discussed in section 13.3.3. In another instance, it develops in exceptionally slow-grown ring-porous species where the tree lays down a high proportion of soft spongy and weak spring growth, and a low proportion of denser stronger summer-growth wood. Ring-porous species with unusually narrow year-on-year growth rings are one possible feature to look for to identify brashness; the result of this growth pattern is the wood is also likely to be exceptionally light for its species, and this may indicate potential brashness. Fast-grown conifers tend to lay down a much greater proportion than normal of weaker, lighter spring wood than they lay down in denser and stronger summer wood, and this, too, is brashy. Juvenile wood is frequently brashy, especially if it has grown fast with widely spaced growth rings. Unusually dense reaction wood in coniferous trees, known as compression wood, is often brash, and this type of wood should not be used in furniture, but carvers and turners may find uses for it (Hoadley1, 2000, p 99-100). Shield (2005, p 133) discusses brittle heart or brashness being the result of growing stresses within plantation-grown Eucalypts. He notes that growth increments develop tensile stresses in their length with each successive new growth increment developing slightly more tensile stress than the previous year’s growth. To compensate for this the tree develops longitudinal compression stresses toward the tree’s core. Finally, an artificial cause of brashness is induced when wooden artefacts are subjected over time to high heat “such as wood ladders used in boiler rooms.” (Rossnagel, Higgins and MacDonald, 1988, pp, 43-44.)

The lesson for woodworkers is brash or brittle wood is not appropriate for load-bearing structures, e.g., floor joists, floorboards, table or chair legs and rails etc. The safest thing is to not use it at all except perhaps for purely decorative items such as small carvings or other non-critical parts. Secondly, materials other than wood might be better choices for shelving, steps, ladders and so on in high-heat environments including forges, boiler rooms, certain areas within commercial kitchens, glass-blowing workshops etc.

Richard Jones has spent his entire life as a professional woodworker and has dedicated himself to researching the technical details of wood in great depth, this material being the woodworker’s most important resource. The result is “Cut & Dried: A Woodworker’s Guide to Timber Technology” (from which the information below is an excerpt). In this book, Richard explores every aspect of the tree and its wood, from how it grows to how it is then cut, dried and delivered to your workshop.

In section 6.4 the drying and rewetting of wood was illustrated by using a sponge or towel to represent wood. An extension of this analogy serves as a preliminary introduction to terminology about the wood-seasoning process.
Let’s say, for the sake of discussion, that you soak a very large and thick bath towel in a water bath. Lift up the sopping towel and wring it out as thoroughly as possible. Let us also assume you have the unlikely physical ability to wring out every drop of loose (free) water in the towel so the only water left is that bound within its fibres. This towel now stands for wood commonly and erroneously described as being at fibre saturation point (FSP), although the comments on FSP made in section 6.5 should be borne in mind. Fold the towel up three or four times into a long large sausage and hang it over a washing line. It’s a cool, dull, still, overcast day with, perhaps, intermittent, very light drizzle.

The towel will barely dry any further in the described weather conditions until either a breeze starts, the sun comes out or both changes happen together. It’s common knowledge that even if the sun doesn’t come out but a breeze starts the bundled-up towel will dry. Similarly, if there’s no breeze but the sun comes out the additional warmth causes water to evaporate from the towel’s fibres. In both cases described, the towel will eventually dry through. Put the two factors together, i.e., warmth and moving air, and the towel dries more rapidly than it will with either just a breeze or just extra warmth. Within the bundled-up drying towel there is a moisture gradient: As the towel dries it remains wetter in the middle of the bundle than near the surface. Assuming drying continues, the moisture content within the towel gradually evens out until it has an equal moisture content all through.

Without really knowing any science or terminology we know how to dry clothes quickly. Options include hanging them on a washing line on a warm, lightly breezy, sunny day, putting them out on a dull but dry and windy day, or hanging them over a warm radiator, and so on. Clothes fully opened and pegged on a line dry much quicker than clothes bunched up tightly.

What applies to drying clothes has similarities to the conditions that will dry wood. To dry wood quicker, heat air and move the hot air over it, although with wood, when it has dried to approximately 20 percent MC, the primary drivers for further drying are air temperature and humidity, not the speed at which the air passes over the wood. Thin boards dry faster than thick boards, which is analogous to opening clothes out to dry rather than leaving them bunched up. Fast drying of wood with very hot dry air will certainly accomplish the task, but it usually comes with an unacceptable price, i.e., degradation of one sort or another such as splitting, surface checking, case-hardening18, collapse (aka core collapse), honeycombing etc., making the wood unusable and unsellable. It’s imperative to control the speed at which wood dries in order to produce an acceptable end product.

The air’s RH must be low enough to absorb more water vapour. Air at 100 percent RH cannot absorb any more water vapour. Wet wood in RH conditions like this is comparable to my earlier description of hanging washing out to dry on a cool, damp, intermittently drizzly day – the clothes dry very slowly.

Warm air transfers heat to the wood causing the moisture in it to evaporate into the air. Again, the RH of the air must be low enough to absorb the water vapour given off by the wood. Drying kilns add warm air to the drying chamber, which transfers heat energy to both the wood and the water within it. The difference in temperature between the introduced dry air and the wet wood is often, but not always, quite small at the beginning of the kilning process. Water in the wood converts to vapour and evaporates through the wood surface into the air introduced into the drying chamber. The air temperature within a kiln is high, e.g., at stages in the wood drying process temperatures of 65.5° C (150° F) or more are used. At this temperature if the air stays at 70 percent RH it will eventually dry wood to approximately 10.5 percent MC (see figure 8.2).

If the air becomes too humid to dry the wood effectively, one of two things must happen for the wood to continue drying. First, further raising the temperature of the air in the kiln reduces its RH. Hotter air is capable of holding additional moisture released from the wood. Second, moving the humid air out of the drying chamber and replacing it with drier air will continue the drying process. Raising the temperature of the air already in the chamber is the cheapest option, but too high a temperature may lead to faults in the wood, particularly in some species more than others, e.g., surface checking as described earlier.

As timber dries, a moisture gradient develops inside the wood much like the earlier-described folded-up towel hanging over a washing line. In a wood-drying kiln where air temperatures are artificially high, generally the greater the temperature of the air acting on the wood, the steeper the moisture gradient within it, and moisture moves out of the wood faster. This also leads to faster evaporation of moisture from the surface of wood. Conversely, when wood is air dried and therefore experiences normal weather conditions, or if the wood is in service in a typical environment found in habitable buildings, RH is the primary controller of the steepness of the wood’s moisture gradient – air temperature in these circumstances has only a small effect.

For green wet wood to dry, as freshly milled boards or planks, for example, air must be moving to carry moisture away from the wood’s surface; this creates a place for the water deeper in the wood to migrate to, where it will also be carried away by the flow of air. If only a small volume of stagnant dry air surrounds wet wood, that air quickly becomes fully saturated with evaporated water. At that point no further drying can occur until that pocket of air moves away and is replaced by drier air.

Moving air carries moisture away from the wood’s exterior, thus drying the wood. But the air molecules adjacent to the wood surface stick to it. Air molecules just above the surface collide with the stuck air molecules and their movement is disrupted and slowed down. In turn, these air molecules impede the flow of air molecules just above them. As distance from the wood surface increases, the collisions diminish until air movement is unimpeded and becomes free flowing. In effect there is a thin layer of viscous “fluid” near the surface where velocity changes from zero at the surface to free flowing some distance away from it. “Engineers call this layer the boundary layer because it occurs on the boundary of the fluid.”19 (Benson, 2009, p 1) Within the boundary layer next to the wood the air is wetter (because it’s picked up moisture from the wood) and travels slower than the air above the boundary layer – it tends to hold the moisture taken from the wood close to the wood’s surface. A faster-moving air stream reduces the effect of the boundary layer and it sweeps away the damp air with its high-vapour pressure. The damp air is replaced with new drier air, i.e. air with a lower vapour pressure better able to absorb further moisture from the wood.

Whether wood is air dried or kiln dried the air entering the wood stack from one end has a lower RH than the air leaving the stack at the far end. Moving air leaving a stack of drying wood is cooler than the air entering it. The air cools as it transfers heat to the wood, thus enabling the drying process. If the air continuously passes through a stack of wood in one direction, the wood at the “upwind” end of the stack always dries faster than the wood at the “downwind” end. This results in unevenly dried planks of wood where the downwind end of a stack might be 3 percent or 4 percent wetter than the upwind side. In more extreme cases, the difference in moisture content between the upwind and downwind side of a stack may be 8 percent to 10 percent MC if the wood is very wet at the start of the drying process – in this case one possible result is the stack of wood may lean toward the drier side. This effect is more evident in wide stacks of wood, e.g., greater than about 2 metres (~6′), than in narrow stacks. Natural changes in wind direction and speed cancel out this effect in stacks of air-dried wood. It is only if a kiln operator is drying a wide stack of wood, or some particularly difficult to dry woods, that there is a real need to regularly alternate the air flow direction within the chamber. To achieve this, the fan blade rotation is reversed at evenly spaced intervals, e.g., every two hours, four hours, 12 hours etc. This upwind and downwind disparity in the drying ability of moving air in a stack of wood limits the size of a stickered pile of planks. This is especially the case with air drying where the yard owner really has less control over temperature, wind speed or wind direction. However, it should be noted that air velocity in either a kiln or in an air-drying wood pile is most important at the initial drying stage of wet wood because of its role in carrying away moisture from the wood surface. As the wood dries the significance of air movement gradually diminishes until the wood reaches about 20 percent MC. At this MC the primary critical factors for further drying are humidity and temperature, with the importance of air movement reducing significantly the drier the wood becomes.