Editor’s note: The third edition of “Cut & Dried” should arrive in February. You can sign up to be notified when it arrives here. In this post, author Richard Jones explains his update to Chapter 6.
Rombald’s Moor: The opening image to Chapter 6.
In 2021, I decided I ought to update “Cut & Dried,” and the third reprint of it at the end of 2024 was a good opportunity to do so. For a long time I had been aware of two ways to determine wood moisture content, i.e., the “dry basis” (db) and the “wet basis” (wb). In Section 6.6 Measuring Wood Moisture Content in the already printed book, I emphasised we woodworkers use only wood’s dry weight as the base weight to assess wood moisture content. This dry basis methodology wasn’t actually named in the book and nor was the alternative wet basis methodology named or described except the wet basis was hinted at in an exchange I had with a furniture student at the end of page 76 and into page 77.
However, since the last printing of “Cut & Dried” in 2019, things have evolved and environmental issues are ever more pressing. The drive is on to reduce carbon emissions, reduce particulates and pollutants etc. I am not here to proselytise on these issues but burning biomass fuel in the form of logs, wood chips, pellets etc. is one potential source of particulates and pollution. Many people and organisations around the world burn biomass fuel for heating homes, cooking, industrial boilers etc., and burning wet fuel is both inefficient and pollutant. The U.K. government, for example, created legislation to regulate the supply of biomass fuel, including setting the maximum moisture content levels for biomass fuel suppliers, and putting in place organisations to verify that such suppliers meet required government standards.
Crucially the authorised method of determining wood moisture content in the biomass fuel sector is the wet basis. It’s the case that the biomass fuel sector might be considered peripheral to us woodworkers with our focus on making things out of wood, and where we want to know its moisture content, but the biomass fuel sector, like use, require felled trees, so there is an environmental impact which deserves some discussion in “Cut & Dried.”
To illustrate the difference between dry basis and wet basis calculations for wood moisture content I’m including some text from the latest iteration of section 6.6 of “Cut & Dried,” but modified slightly for this blog post’s purposes.
A learner approached me with the following figures for a piece of wood both before and after oven-drying:
Wet Weight = 20 grammes
Oven-Dry Weight = 15 grammes
This learner questioned the calculated moisture content result. Using the formula already provided she calculated: ((20 – 15) / 15) X 100 = 33.3%MCdb. This learner, in trying to grasp the basis of the calculation, changed the formula to calculate thus: ((WW – ODW) / WW) X 100 giving the sum ((20 – 15) / 20) X 100 = 25%wb. We discussed the different results, i.e., 33.3 percent and 25 percent, and it is easy to mentally visualise a 5 gramme weight loss is a quarter of the 20 gramme wet weight of the sample, i.e., 25 percent. Similarly, it’s quickly apparent that a 5 gramme weight gain is one third (33.3 percent) of 15 grammes, the sample’s oven-dry weight. As soon as the learner understood the base line for the dry basis calculation is the dry weight of the wood, not the pre-dried wet weight, all was clear to her. She was then able to comprehend how, using the dry basis methodology of assessing wood moisture content wood MC figures such as 100 percent or greater were possible, e.g., wet weight, 200 grammes and oven-dry weight of 100 grammes.
This learner’s confusion had led her to unknowingly stumble upon the methodology for assessing wood moisture content referred to earlier, i.e., the “wet basis” (Forestry Commission, 2011). To calculate the wood moisture content percentage on the wet basis (wb) the formula given by The Forestry Commission (2011, p5) is:
“The MCwb = (the weight of water in a sample/ total initial weight of the sample) X 100.” MCwb as indicated earlier, means Moisture Content Wet Basis. Results are expressed as a percentage.
Further reading, if so desired, can be found at the following links:
Late this year, we sold out of “Cut & Dried”. Author Richard Jones had some changes he wanted to make in the third edition. This week, we sent the book, with these changes, to press.
Some of the changes were small corrections, such as moving an illustration up a bit to better match the text and a degree mark slightly lower and larger than the rest.
Other changes were more significant.
This includes an extensive rewrite of section 6.6: Measuring Wood Moisture Content. Here, Richard adds new information on how biomass fuel moisture content is assessed, which differs from the methodology used for assessing the moisture content of wood used by woodworkers.
Richard wanted to add this because it’s particularly relevant to environmental considerations, such as reducing pollution from wood smoke.
This addition added a few pages to the book, which may not seem like a big deal until you consider the table of contents, text in chapters directing readers to a particular page number, and the index. We were lucky enough to once again work with Rachel, who created the original index and was familiar with the book, to make the necessary index updates.
Another significant change is the cover. Since the book’s second printing in 2019, paper and printing costs have skyrocketed. When we received the quote for the third printing, we had two choices: Increase the retail price (by a lot) or ditch the dust jacket and switch to paper over boards for the cover. We chose to keep the retail price the same. This means the design on the dust jacket will be printed directly on the hardback cover. The 9” x 12” book will still be printed on heavy #80 matte-coated paper.
When reviewing ‘Cut & Dried’ back in 2019, J. Norman Reid of Highland Woodworking wrote: “‘Cut & Dried’ is one of the most complete and detailed works on wood and wood technology available to non-specialist cabinetmakers. For this reason, it merits a place on the reference shelves of all serious woodworkers. I highly recommend this important book.”
“Cut & Dried” should be back in stock in early 2025.
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.
Figure 14.19. The elasticity of two types of wood, one stiff wood represented by blue lines, and a more flexible one represented by red lines. Generally, stiffer materials are stronger than more flexible ones. Up to the proportional limit, increasing stress (X axis) results in a proportionate increase in strain (Y axis) from which each of the two pieces of wood in this illustration can recover. The slope of the straight-line portion of the graph represents the modulus of elasticity. A steeper line indicates a higher modulus. Stresses above the proportional limit result in greater proportional strain, permanent deformation of the material and permanent set. For instance, a bookshelf loaded beyond its proportional limit takes on a permanent bend. Beyond the proportional limit, the greater the load, the greater is the permanent distortion until the point of failure. In most wood species the proportional limit is generally between 50 percent and 65 percent of load leading to complete failure.
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.
Figure 14.23. A stress/strain graph derived from readings taken during the experiment shown in figures 14.20, 14.21 and 14.22.
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.
Figure 14.27. Normal wood, left, breaks across the grain with ragged tearing of the longitudinal fibres; this piece of oak was very difficult to break requiring great physical effort, clamps, benches and bearers to clamp against. At right, the brashy oak snapped like a “carrot” in my hands with little effort.
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.
Cross shakes. A piece of sapele with transverse shakes. These may be the result of improper drying resulting in honeycombing, but they might also be the result of natural cross shakes. In this case, I wasn’t able to come to a definite decision when the fault revealed itself during normal wood machining. Whatever the cause, the whole board went into the scrap pile and became firewood.
The following is excerpted from “Cut & Dried,” by Richard Jones.
Richard 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, 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.
The main drying faults in planks or boards are: distortion or warping that are the result of shrinkage in the grain; plus the internal checking, surface checking and end splitting caused by shrinkage where all these faults may be exacerbated by drying processes. The following faults are entirely drying faults: collapse (aka core collapse in North America), shell set in oversize condition, honeycombing, case-hardening and the very rare reverse case-hardening.
Another drying fault sometimes apparent is discolouration of the wood. One discolouration, sticker stain, has already been discussed in section 8.3. Additional drying-induced discolouration of wood is discussed in section 9.2.
The causes of distortion or warping are discussed in section 7.4, but the natural warping of wood due to moisture loss and aggressive drying, whether in a kiln or air dried, may magnify the distortion.
With reference to figure 9.1, at the beginning of the drying process wet wood is not under undue stress. It is only as it dries that stresses begin to develop. At the beginning of the drying process all the cell lumen are full of liquid, or at least partially filled and, most importantly, the cell walls show no significant sign of stress-inducing shrinkage. It’s not until free water in any cell in the wood has gone and the bound water in the cell walls and the cavities begins to leave that shrinkage starts. It’s counterintuitive but drying faults such as surface checking and honeycombing develop at high wood moisture content, but the following discussion explains this phenomenon.
At the beginning of the drying process water is first lost through the ends of a board where the end grain is exposed, and from the fibres near the board’s surface. The 12″ to 16″ (300 mm to 400 mm) at each end of a board exchange water vapour faster through the relatively porous end grain than the board edges and faces. As wood dries, a moisture gradient develops. If the wood is dried quickly with high heat and fast-moving air, a steep moisture gradient forms. If we take as an example wet wood, e.g., at an average 50 percent MC, and subject it to high heat, this causes moisture at the surface to rapidly evaporate out of the cavities and the cellular structure. The tissue below the surface or shell is still at an average 50 percent MC and also still cool. But the situation changes quickly as the now drier and warm shell transmits heat toward the centre of the wood through the intermediate zone. The additional warmth affecting the intermediate zone encourages moisture movement toward the now drier shell. In turn, the intermediate zone transmits heat toward the core of the wood and moisture starts moving from the core to the intermediate zone, and on outward toward the shell and out of the wood. It’s not difficult to see, having just described the mechanics of drying how, for example, surface checking develops whilst wood still has a high average moisture content.
All these different zones at different moisture contents create the moisture gradient within the wood. A steep moisture gradient means the wood is drying very quickly. For instance, extremely rapid drying occurs in the oven-drying test used to determine moisture content. In this case the samples are small and there is a large surface area (particularly end grain exposure) to volume ratio, letting the moisture out relatively easily. But you could put a piece of green wood 20″ long x 8″ wide x 4″ thick (500 mm x 210 mm x 105 mm) in a large-enough oven and start drying it in the same way. Now the surface-area-to-volume ratio is small compared to the small samples used in oven drying to determine moisture content. The rapid drying of a large piece of wood causes a steep moisture gradient that puts large stresses on it. The surface dries quickly, but the moisture in the cells in the intermediate zone and the core can’t escape fast enough to prevent tension and compression stresses developing in the board.
Figure 9.1. Flow chart of drying stages and potential drying faults.
On the other hand, if you put the same piece of green 20″ x 8″ x 4″ wood in a sealed plastic bag it will barely dry at all. Even keeping the bagged piece of wood in a warm room where heat transfers to the wood and causes the moisture in the shell to evaporate, there’s nowhere for the moisture to go once the air in the bag reaches 100 percent RH. In all likelihood leaving a piece of wood encased in a plastic bag like this for a couple of weeks in warm conditions would result in a fuzz of mould developing. But, importantly, from our point of view of discussing moisture gradients, this piece of wood would exhibit a shallow moisture gradient. Shallow moisture gradients don’t put much stress on the wood, but the problem from a timber or lumber dryer’s point of view with shallow moisture gradients and slow drying is twofold: firstly, stock turning over too slowly to make any profit; secondly, serious disfiguring mould development, which is less likely when wood is dried faster.
Tension stresses are “ripping apart” forces. Compression stresses are “crushing forces.” To dry wood quickly in a kiln requires getting the balance right between tension and compression forces induced by the movement of moisture out of the wood. Get the balance right and the wood comes out of the kiln stress free, or near-enough stress free. Get them wrong and the faults depicted in figure 9.1 reveal themselves.
