NOTE: this page was created before I created my extensive illustrated glossary, which contains these terms and many more. I've left this page up, however, because it gives a more extensive set of examples (pics) of the terms that it does include.
Sometimes, in a few different woods, medullary rays will, for some reason, clump together to form what appears to be an excessively wide ray, quite unlike the other rays in the particular wood.
When viewed at a high magnification (10X won't do it) the "clump" actually resolves to several uniseriate rays, usually slightly separated from each other by other tissue. Sometimes the uniseriate cells intertwine with each other and other tissue to form a spaghetti-like mix.
This phenomenon occurs most often in Alder but is occasionally seen in other woods such as American hornbeam.
For quite a bit more detail about aggregate rays, including numerous examples, click here: AGGREGATE RAYS
There is a category of beetle that is NOT a taxonomic group but rather a diverse range of beetles that share the characteristic that they have a symbiotic relationship with a particular fungus, called ambrosia fungus. When the beetles burrow holes in a tree, in which they bury their larvae (they don't eat the wood of the tree, they just excavate it) This fungus causes discoloration that is carried up and down the tree in the sap. These beetles do not normally kill the trees they bore into, but if a sufficiently large number of them use one tree, they can kill it because the fungal growth clogs up the flow of tree nutrients. Ambrosia markings happen most often in maple, and ambrosia maple is quite popular among wood turners, but it also happens in other woods. The density of marking can vary by a huge amount, with some planks having only a hole or two and others being just full of them.
Cutting across stump and butt sections of trees sometimes produces a staircase-like, somewhat curly figure, that is commonly called "angel step". This occurs most frequently in walnut, but may occur in other woods such as ash and maple.
Bear claw figure is a series of fairly straight lines that occur on flat cut surfaces because of one or the other of the two kinds of INDENTED GRAIN. In maple, the bear claw is caused by the spiky kind of indentations and in Ponderosa pine it is cause by the normal kind of indentations. I have been told, but do not have confirmation, that bear claw figure in spruce is caused by the normal kind of indentations. In spruce, the figure on flat cut surfaces is significantly different than that on maple and pine in that the lines do not follow the direction up and down the tree but rather wander all over the place. In maple and pine, the bear claw lines are almost exactly aligned with the direction up and down the tree.
A small-scale, very tight, mottle figure is sometimes referred to as "bee's wing" figure due to the similarity with what the wing of a bee looks like. East Indian satinwood is extremely well known for having this figure, and it also occurs occasionally in narra, mahogany and eucalyptus. So when is a figure "block mottle" and when is it "bee's wing" ... well, pretty much whenever a particular dealer decides that's what they want to call it.
A few woods, most notably maple but also anigre and a few others, can exist with large numbers of small round "defects" that do indeed resemble the eyes of birds. The density of the eyes ranges from sparse to dense, and the definition of "dense" frequently depends on the greed and honesty of a seller, so this is not a good figure to buy sight unseen. A good, truly dense, bird's eye maple board can make a spectacular addition to a project; it is very popular for jewlery boxes.
When cut into veneers, the logs are most often rotary cut or half-round sliced (in an arc) to produce the most uniform distribution of nice round eyes.
There are a few woods that are sold as "bird's eye" with a density of eye figure so low as to make the term a joke. Zebrawood in particular comes to mind for his. I have seen zebrawood veneer sheets that re sold as "bird's eye" due to having literally 8 to 10 eyes in an area of 5 or 6 square feet.
Blister is a form of figure that has fairly short, fairly straight raised sections. If they were more oval, it would be a quilt figure. If they were longer, it might be a fiddleback figure. If they were shorter, it would be pomelle figure. BUT ... it isn't any of these, so needed its own name. It occurs in several woods but is particularly available in makore and sapele.
Blue stain (aka sap stain) is caused by a fungus that under certain conditions will spread into wood once it is removed from the stump. The fungus is frequently, although not necessarily, deposited on the tree by insects. The pine beetle, for example, frequently deposits the fungus on the tree just inside the bark. The beetle goes no further but the fungus can penetrate all of the sapwood. Since the fungus eats nutrients found primarily in the sap, it usually only occurs in the sapwood and not the heartwood. Once present, it cannot be removed. It does not occur in live trees because live sapwood does not contain enough oxygen to sustain the fungus. It is common in pine and maple but also occurs in many other woods such as anigre and cedar. The color is usually blue but may also be gray and or even dark gray verging on black. The color is the fungus itself rather than a chemically induced change in the color of the sapwood. There are extensive articles on the internet discussing it and how to avoid it and even how to reduce its effect (but once it's there, you can't get rid of it totally --- you need to avoid it during drying). It does not normally start to affect logs for many days, sometimes weeks or more, and of course, sometimes not at all, but it CAN start immediately after cutting and have a pronounced affect within just a few days.
A burl is a wartlike, deformed growth on the trunk or root and sometimes even the branches of a tree, caused by (1) an injury to, or (2) and infection in, the tree just under the bark, or (3) the existance of an unformed bud which has all the genetic material necessary to grow a full branch, or even a whole tree, but which for some reason did not grow properly. In any case, the result is that the tree cells divide and grow excessively and unevenly in a process somewhat analogous to cancer cells in a mammal. Burls are sometimes called tumors on wood, although I'm not aware of their ever being fatal. Trees with burls continue to grow otherwise normally.
Continued growth follows the contour of the original deformity, producing all manner of twists, swirls and knots in the wood fiber. Usually, this results in wood that has a spectacular pattern that can be used to great effect in woodworking, and sometimes it is also accompanied by the creation in the burl of dormant buds which create "eyes" that make the burl even more spectacular when worked.
Burl wood is usually darker than the rest of the tree and in some cases (Paela comes to mind) may be a significantly different color altogether. Because of the diverse grain direction, burl wood cannot be relied on for strength, but that's of little consequence since burls are prized for beauty, not strength.
Burl wood can be difficult to dry without cracking. Sometimes there are bark inclusions in burls, and also sometimes gum pockets, either of which can cause surface defects when the burl is worked. In some species of wood, gum pockets are common in any burl found on the tree.
Burls come in all sizes and shapes from golf-ball and smaller to thousands of pounds of massive growth on the side of a large tree or in the root system. Burls as large as an SUV and weighing 10,000 pounds have been reported as have trees with hundreds of small burls. On really large trees, such as the redwood, burls commonly exist that are large enough to be used to create veneer and even smaller burls are sometimes sliced into veneer for use in craft items such as jewelry boxes. Burl veneer frequently does not stay flat after cutting and has to be moistened and clampled flat before and/or during application.
"Cat's Paw" and "cluster burl" are a couple of commonly identified types of burl figure. Cat's paw is frequently found in cherry and cluster burls are found in a number of species. Most often, burls have no sub-designation and occur in a large number of species. Common burl species include redwood, oak, ash, maple, madrone, elm and walnut. Some exotics with very popular burls are mappa (poplar burl), thuya and imbuya, and there are MANY more.
Cat's paw is a form of "pippy" wood or perhaps burl wood where the little dots occur in a pattern that is strongly similar to the pattern of a cat's paw (see how obvious the name is?). I've only seen it in cherry and oak but it may occur in other woods. It is, in my experience, a very sparse pattern and an almost meaningless term as far as useful woodworking goes.
compression wood is a portion of a tree where the wood fibers have been compressed due to stress in the tree. This can be caused by irreglar growth such as a tree that grows on a riverbank and then tilts as the bank slides into the water, causing the tree to have compressed wood in the side next to the water because the tree is bent over in that direction.
Crotches are an extreme form of compression wood. They are caused by the forces exerted within the tree to support a main branch where it joins the trunk, and of course the bigger the branch, the more the compression. The compression decreases as one moves away from the point where the branch meets the trunk, so crotch wood frequently exhibits an extreme degree of grain variation. The compression process that strengthens the tree so it can support the branch causes the wood fibers to twist and compress, creating various figures and grains that can be very beautiful. Unlike burls, crotches have grain that, while quite distorted, is basically the same grain as the other wood in the tree and does not tend to the extreme swirls and eyes of burl wood, but even so, crotch wood can be wonderful to behold.
Crotch wood is typically harder and more dense than a straightgrained portion of the same tree. Depending on the apprearance, a crotch may be called a "flame crotch" or a "feather crotch" (and less frequently as "plume", "roostertail" or "burning bush") and frequently the crotch area is somewhat symetrical on both sides of the branch so that a crotch piece cut parallel to the bole of the tree will produce a look similar to that of book matching. Terms such as "feather" and "flame" should not be relied on if you haven't actually seen the wood, as they are used VERY freely. I've seen one gun-stock maker who states on his web site that he always calls all crotches flame crotches because "it sounds more impressive".
In veneers, crotch sheets are seldom found in larger sizes (although I have seen some HUGE mahogany crotch sheets --- I'm talking 3 feet by 5 feet), and mahogany and walnut species dominate the field of crotch veneers because they are the main trees that consistently produce large crotch areas. In mahogany, enough veneer has been produced to be able to establish grain pattern types. Thus one can select a swirl, a feather, a rat-tail, and others. Mahogany has always been the classic crotch because of consistency, size, and soundness. The price range is moderate to expensive.
Contortions in grain direction sometimes reflect light differently as one moves down the grain and this creates an appearance of undulating waves known as curly figure. It is frequently described as looking like a wheat field in a mild wind, and can be so strong an effect that your eyes will swear that a flat piece of wood has a wavy surface. Many species develop this figure, maple being a very common example. Stump and butt sections of trees often produce a diagonal, staircase-like curl referred to as "angel steps", and a rolling curl figure that is called "cross-fire". An extreme form of curly figure is called "fiddleback". The amount of curl in a wood sold as "curly" can range from almost none to truly spectacular, so this is not a term to be trusted via mail order purchasing.
Trees that grow in temporate zones (that is geographic regions where there is little change in climate throughout the year) grow pretty much the same amount every day and consequently have little variation in texture. Trees that grow in regions with seasonal climatic changes, however, grow at different rates during the seasons, ranging from exhuberant "early growth" in the spring and early summer, to a slower "late growth" in the late summer and fall, to essentially none in the winter. The early growth is typically wider, less dense, weaker, and more porous than the late growth and in many cases is significantly lighter in color. These early/late growth sections are what make up the growth "rings" ("annual rings") in trees. One tree that particularly shows this characteristic in a very striking way is Douglas fir. Since there is one growth ring per year, the number of rings in a cross section show the age of a tree. Using overlapping informaton from both old dead trees and old live trees, scientists have dated events back for thousands of years based on tree rings.
To see some samples of early/late growth and rings, click here: TREE RINGS
Fatwood is not a species of wood, it is ANY wood (but almost always pine) that is utterly soaked in resin. When gymnosperms, especially pine trees, die, the resin in the branches tends to pool in the part next to the bole. Also, the area just above the roots often continues to get nutrient-laden sap up from the ground but since the tree is dead there is nowhere for it to go and it just pools in the lower area, plus the fact that the sap from the higher areas of the bole has nowhere to go in a dead tree so tends to flow slowly back to the lower area.
SO ... the portion of such tree branches near the bole, and the stumps of such trees, tend to have areas that are just saturated with resin. Wood cut from these areas is called fatwood and it is a great fire starter. There are many Internet videos on how to locate such and get shavings that will act as fire starters with just a spark. Small sticks of it can be lit with a match and will keep burning far longer than the match would have and so can be used for a fire starter if a match might not otherwise be strong enough to get your kindling started.
Figure is the form of the grain and color patterns in wood that give it a unique appearance. There are many factors or characteristics that go into making up the figure, and some of the terms that are associated with figure are fiddleback, curly, wavy, tiger stripe, marbled, spalting, feather, flame, bee's wing, bird's eye, and more. The figure of woods is heavily influenced by how the wood is cut. Cutting terms to see include quartersawn, flat cut, rift cut, etc. Veneers have additional figure capabilities since they have an additional cutting style (rotary) that is not available to lumber.
FOR AN EXTENDED DISCUSSION OF FIGURE AND GRAIN, CLICK HERE
Curly figure in wood (and fiddleback is just a variation of curly) is caused by contortions in grain direction such that light is reflected differently at different portions of the grain, creating an appearance of undulating waves, also called a "washboard" effect because it looks like an old corrugated-steel washboard. "Fiddleback" figure is a form of curly figure where the curls are very tight and fairly uniform, generally running perpendicular to the grain and across the entire width of a board. The name comes from the fact that such wood became popular to use on the backs of violins (fiddles), and nowadays guitars, because the figure is frequently very lively and attractive and such wood generally has good resonance properties. Logs for fiddleback veneers are quartersawn to produce very straight grain with curls running perpendicular to the grain and uninterrupted from edge to edge of the sheet.
Some reports claim that a tree which buttresses itself against north winds will have compressed annular growth rings in the area facing north and expanded rings facing south and that the stress in the compressed rings is believed to cause the fiddle back figure. I have no idea whether this is true.
Many species develop this figure, but the most common ones are maple, makore, anigre, and "English Sycamore" (which is actually a form of maple). Some of the prettiest versions occur in claro walnut and koa. There are woods (laurel comes to mind) that have a figure that is technically a true fiddleback figure, but which is so light as to be almost indiscernable, and there are others that have fiddleback figure that only runs for a few inches of width in a plank although it may run the full length. None of these marginal figures would actually be used on the back of a fiddle, so application of that name to them is purely technical and/or a marketing ploy and should not be taken seriously. Also, plain curly is sometimes mislabeled fiddleback and true fiddleback is sometimes labeled as just curly, depending on the whims of the vendor.
Figure is the form of the grain and color patterns in wood that give it a unique apperance. There are many factors or characteristics that go into making up the figure, and some of the terms that are associated with figure are fiddleback, curly, wavy, tiger stripe, marbled, spalting, feather, flame, bee's wing, bird's eye, and more. The figure of woods is heavily influenced by how the wood is cut. Cutting terms to see include quartersawn, flat cut, rift cut, etc. Veneers have additional figure capabilites since they have an additional cutting style (rotary) that is not available to lumber.
FOR AN EXTENDED DISCUSSION OF FIGURE AND GRAIN, CLICK HERE
On a few woods, most notably butternut and basswood, the growth rings sometimes regularly "dent" outwards from the center, as shown on the basswood end grain shots displayed in the link below. This is known as fluted grain and it causes a distinctive figure in face grain pieces, as also shown below. It is sometimes a way to help distinguish butternut from walnut since although walnut CAN have it as well, it is much less often seen in walnut than in butternut. It is clearly visible on end grain and flat cut face grain but has no notable effect on rift cut or quartersawn surfaces.
Grain is the stripes in the wood created by growth rings which may be tight, indicating slow growth, widely spaced, indicating quick growth or any variation in between. Different woods have distinct grain patterns that help identify them. The grain is caused by the fact that trees in non-temperate zones of the world grow at different rates in the summer (late growth) than they do in the spring (early growth), and the density and coloration of the early and late growth can vary signifcantly. For a very graphic example of this, check out the coat rack shown in the pictures under Douglas Fir on this site. In temporate zones, there is no early and late growth, so the wood tends to have a uniform grain but may still have significant color variations. Growth rings tell how old a tree is, since there is one per year. This fact has been used with combinations of live trees and tree stumps to determine dates going back several thousand years.
The appearance of the grain pattern in woods can be changed dramatically by making a different kind of cut in the wood. The main cut types are quartersawn and flat cut. In a flat cut, the cut is parallel to the pith but does not intersect it, and the resulting grain pattern sometimes has what is called a "cathedral" pattern that looks like a series of stacked "V" shapes, although it can be just a series of parallel lines that vary in spacing depending on their distance from the pith. In quartersawn, the cut is radial (that is, all cuts intersect the pith of the tree for the entire length of the cut) and the resulting pattern tends to be a series of totally parallel grain lines with spacing that is determined entirely by the amount of yearly growth.
FOR AN EXTENDED DISCUSSION OF FIGURE AND GRAIN, CLICK HERE
Indented grain refers to exactly what it sounds like ... a section of grain that is indented from the grain line of which it is a part. There really are two notably different forms of indented grain. The first is what I personally refer to as "normal" indented grain, as discussed in Bruce Hoadley's "Identifying Wood" on page 65 where he specifically states that it is the cause of "bear claw" figure. This type of indented grain occurs in softwoods such as spruce and pine. Then there is the kind of indented grain that I call, for obvious reasons, "spiky" indented grain. It has spikes running through the grain lines, which are moved off of the straight and narrow by the spikes, but in a more triangular way than the "normal" indentations which are rectangular. This kind of indented grain occurs in hardwoods such as maple. I do not have absolute confirmation (only anecdotal and unconfirmed statements) that bear claw in spruce is cause by "normal" grain indentations but since it is a softwood, I tend to believe the unconfirmed reports.
Normally, the fibers in a tree trunk run parallel to the centerline of the tree, but under some conditions the tree will twist during growth and the result is a spiral orientation of the fibers up the trunk. When this spiral changes direction from year to year, the result is what is called "interlocked" grain and it is the primary reason for such figures as "ribbon stripe" (in quartersawn stock), "mottled", "curly", "fiddleback" (an extreme case of curly) and others. It also is frequently accompanied by a more shiny surface than is otherwise the case for non-interlocked samples of the same species.
There are numerous kinds of defects that lumber can have. Some are inherent in the wood (eg. gum deposits) and some arise from the processing. So far, I have included a description, with graphics, of the major type of warping defects that lumber is subject to.
To see examples of wood defects and discussions, click here: LUMBER DEFECTS
The term "mineral stain" was given, apparently, because of a mistake belief that the cause of the phenomenon was minerals brought up into a tree from the soil through the sap. This is NOT necessarily correct, although it sometimes is. The stain can be blotchy or fairly well demarcated or even sharply demarcated (although that's rare). Colors are generally gray, brown, or greenish, but can also include purple and other colors (see rainbow poplar in particular). I've seen mineral stain in maple, poplar, cherry, box elder, and other woods. I've seen this mentioned with hickory and basswood but I haven't experience it myself.
Sometimes, wavy grain in a wood combines with spiral, interlocked grain to produce a wrinkled, blotchy figure known as mottle, which would be called "curly" if the curl lines were not so broken up. If the mottled figure is scattered randomly it is tecnically called "broken mottle" but if it appears as a regular checkerboard pattern it is called "block mottle", and a very regular, sharp, block mottle is called "razor mottle". I have never seen the term "broken mottle" used. Perhaps vendors think it carries a negative connotation, but in any event, the plain "mottle" is used instead. Anigre, makore, and sapele frequently exhibit all kinds of mottle figure, and it also occurs in mahogany, koa, bubinga, African satinwood and some other species. The various "mottle" terms are, like many terms regarding wood figure, used very loosely and should not be trusted sight unseen.
Wood is a porous, fibrous, material and when the air around it is very humid, it absorbs some of the moisture in the air, and when the air around it dries, the some of the moisture in the wood evaporates back out into the air around it. When the wood absorbs moisture, it expands slightly and when moisture leaves the wood, the wood contracts slightly. Different woods absorb and retain moisture to different amounts and so they "move" (expand and contract) by different amounts. This is the so-called "movement in service".
The reason door panels are done in a "floating" construction technique is because in many woods the movement in service is so large that a solid door would warp and/or crack due to it but the floating contruction technique, wherein the panels are not actually attached to the rest of the door but instead "float" inside the rails and stiles, allows doors made of such woods to suffer large changes in atomospheric moisture without adverse effects.
For a further discussion of the stress separations that can occur in joint due to movement in service, see STRESS SEPARATION below
There is a form of blistered or pomelle figure found fairly often in Tamo (Japanese Ash) and less frequently in a few other woods, where the grain pattern forms into the classic shape of a peanut shell (the standard, common, kind with two peanuts in one shell). It has a very "busy" look but can be quite attractive
A form of "defect" or "character" (depending on how you look at it) in wood where it looks as though the wood has a bad (or sometimes mild) case of the measles, with little spots dotting throughout the grain. Occurs in yew and a few other woods. In England, there is a form of oak burl that is called "pippy oak" in which the burl characteristics are pretty much the same as what we Americans call pippy, but more dense.
A form of "defect" or "character" (depending on how you look at it) in wood where there are numerous spots or elongated areas throughout the wood caused by localized decay.
Pecky can also look a lot like wormy;, in that it shows long fingers of hollow areas instead of just spots and those look just like worm track. This is particularly true of pecky cypress.
In hickory and pecan, the pecky figure is generally in the form of spots that look as though they could have been caused by a bird's pecking at the tree. In birch the wood is not generally called "pecky" but rather has its own designation of "Karelian" or "Masur" birch and is a burl form. "Karelian" just refers to a region between Finland and Russia.
For more details and some examples, click here: PECKY
Pomelle is a type of wood figure that resembles a puddle surface during a light rain: a dense pattern of small rings enveloping one another. Some say this has a "suede" or "furry" look. It's usually found in extremely large trees of African species like sapele, bubinga and makore. Some domestic species with a sparser, larger figure are referred to as "blistered". The term is not used totally reliably and you may encounter some confusion among the terms "blistered", "pomelle", and "quilted" from different vendors. The name Pomelle comes from the French word for "quilted", so it's not too surprising to find this confusion. In sapele, there are even more confusing names used for variations among figure types including pomelle, so you may see "pomelle pebble", "pomelle swirl", "pomelle quilted" and other combination terms that are not necessarily used consistently among vendors.
Spelling variations include "pommele", "pommelle", "pomelle", and "pomele"
Quilted figure somewhat resembles a larger and exaggerated version of pommele or blister figure but has bulges that are elongated and closely crowded. Quilted grain looks three-dimensional when seen at its billowy best. Most commonly found in maple, it also occurs in mahogany, moabi, myrtle, and sapele, and less often in other species.
Trees have ribbons of parenchyma cells that carry nutrients laterally through the bole (that is, they go from the pith out to the bark). Technically, if they start at the pith they are called "medullary ray cells" but in actual practice all rays are called medullary rays or just rays.
In softwoods these tend to be very thin and invisible even at 10X in hardwoods they tend to be wide enough to see at 10X and in some woods such as oak at lacewood they are easily visible to the naked eye. They are what cause the "ray flake" figure in quartersawn planks.
Rays are ribbons of parenchyma cells that carry nutrients laterally through the bole in hardwoods. In some woods, these are very pronounced and a quartersawn board from such a tree will look very different from a flat cut board from the same tree because in the quartersawn board the surface of the board will contain significant swaths of ray cells. These are formally called "ray flecks" or "ray flakes" but more often are just called flakes or flecks. "Flecks" is the preferred term but a lot of us learned it as "flakes" and that is an acceptable alternate term. You will occasionally see the terms "button figure" or "snowflake figure" for heavily flaked woods but the most common term is simply "flaky".
A resin canal is a characteristic of wood that occurs only in 4 genera in the pine family, PINACEAE. These are the pine (Pinus spp.), spruce (Picea spp.), larch (Larix spp.) and Douglas-fir (the single species Pseudotsuga menziesii) genera. Resin canals consist of tubular canals (like little straws) lined with living parenchyma cells which exude resin (aka pitch) into the canals. The sizes of resin canals varies among species and this is used for wood identification. They tend to be particularly large in the Pinus species and smaller in the other. The frequency and density of such canals varies widely even for different ares of a given piece of wood.
They can cause a gummy mess on freshly cut surfaces. Kiln drying can be used to "set" the resin and greatly reduce the amount of bleed out. Trauma to trees can also cause resin canals to form at the injury site. Some trees that have resin canals develop blobs of resin called pitch pockets at various places in the wood.
Spalting is a generic term for various forms of discoloration that can occur in wood due to invasion of the wood fibers by fungal spores, which then form colonies and continue to grow in the wood. Since spalting is a form of rot, the discoloration is usually accompanied by a degradation in the strength of the wood fibers and the wood can become quite punky and eventually just rot out entirely. Spalting, if caught at the right time and stabilized, can result in some beautiful effects that can be use to great effect in various wood objects, especially (but not limited to) turned objects such as bowls. Some spalting results in what are formally known as "zone lines". The can be very sharply defined black lines in which case it is sometimes called "black-line spalting" --- in some trees, oak for example, spalting is rarely black-line and in fact can be VERY vague, amorphous, blotchy black or dark-gray areas. In some woods, spalting causes some color changes other than black. I've seen some spalted woods that are very colorful.
Zone lines may appear as single or double lines. This is the result of the spore colonies creating a protective barrier either around themselves (single-line spalting) or in what is basically a "war" with a neighboring colony (double-line spalting). Both are shown clearly in the link below.
Spalting can be encouraged by keeping a dead tree moist. Spalting is a form of decay and if spalted wood isn't stabilized at the right time, it will just rot. Wood that is really heavily spalted and still completely solid is rare, since advanced spalting is generally accompanied by enough decay to soften at least some areas of the wood.
One form of spalting is called white rot and consists of very light colored areas which may be surrounded by black lines (see discussion directly above) or may have very vaguely demarcated boundaries with no black lines.
Wood is an organic material and there can still be activity in the wood long after it is dead, cut down, and worked into a product. This happens because of the way in which moisture is taken in and given off by the wood as the humidity of the surrounding air changes.
Wood is either air dried or kiln dried before being worked (an exception is turned bowls, which are sometimes partially turned while the wood is still unseasoned and full of water). Either way, and depending on the type of wood, the moisture content will be brought down from a large amount to a relatively small amount. There are a few woods that actually have 300% moisture content when unseasoned. That means that a 100lb chunk of such wood if totally dried out would weight 25lbs.(the percentages are always based on the weight when totally dry).
Once dried, the moisture content will range from about 6% to about 20%, depending on the species and the intended use. This is a very significant change in wood moisture content and has to be done in a controlled way or the wood will develop cracks and/or warping. Once it is stable, it is worked into a final product, but the story's not over yet (and by the way, wood moisture is a complex topic and I have only touched the surface here).
Even after being worked into a final product, wood changes size when the moisture content changes as the air moisture content around it (the "ambient" moisture content" as it is known) changes. This is very slight along the direction of the grain (that is, oriented as it would be up and down the tree when the wood was still in the live tree) but it is considerably larger across the grain. Because of this, pieces of wood that are glued up against each other will sometimes develop what are known as stress separations in the joint because one piece of wood moves a different amount than the other. This happens most often when the woods' grains are not oriented parallel to each other, and is most likely to be pronounced when one is at right angles to the other (but it CAN happen even when they are parallel, it's just less likely).
Swirl in wood can be caused by several things. Two common reasons are near-crotch areas, where the figure is also sometimes called "crotch swirl", and another is one that occurs most often in rotary cut veneer. In both cases, as the name implies, the grain meanders and swirls around, often seeming to convolute and fold in upon itself. The densest portions of the swirl may show up darker or shaded compared to the lighter surrounding wood. Crotch swirl occurs in several species including walnut, mahogany, cherry and maple and swirl due to rotary cut veneer can occur in any wood that is rotary cut, as is often the case of sapele for example.
In crotch areas and possibly other tension wood, it is sometimes the case that normal rays become very short compared to those in the rest of the tree. I have not found any literature on this but I have numerous examples so have created my own article here.
Tyloses is a waterproof, foam-like substance that forms in the heartwood pores of some species of wood and blocks them, thus impeding or even totally preventing the movement of moisture through the pores. It is an extension of parenchyma cells. It is common in white oak and makes that wood excellent for barrels to hold liquids (tight/wet cooperage) because in the case of white oak, the tyloses completely close the pores and make the wood impermeable even though it is porous. When wood is examined in transverse section, tyloses usually glistens. Woods high in tyloses dry very slowly and are difficult to treat with preservatives for outdoor use.
A form of grain in which the wood fibers run up/down the tree in a wavy pattern rather than the normal straight up/down lines. This may or may not (but usually does) result in curly figure. Curly figure is generally NOT caused by wavy grain but rather is caused by interlocked grain or other variations in grain and/or alternating directions of wood fibers. Wavy grain is considerably more rare than curly figure.
A type of spalting, this particular form of biodegrade is caused by certain wood-destroying fungi that attack both cellulose and lignin in wood, producing a spongy and stringy mass that is usually whitish but which may assume various shades of yellow, tan, and light brown. Wood affected by white rot normally does not crack across the grain and will only shrink and collapse when severely degraded. Infested wood will gradually lose its strength and become spongy to the touch. Boards can become severely damaged in 6-12 months, and even faster under hot and moist conditions. It can be found in exterior columns, steps, porches, and decks, as well as in doors, windows, and door and window frames. Occasionally, it occurs in wood trim, rafters, joists, sheathing, siding, sills, joists, and sub-flooring.
NOTE: my own experience is that very often areas of white rot are NOT weak or spongy, despite reports to the contrary. Probably this is because the areas in my experience were stopped before the white rot progressed far enough to really eat into the wood.
Although for most woods, "spalted" means black-line spalting, for some woods (apple and alder come to mind) "spalted" most often just means "has white rot" rather than "has black-line spalting".
For some woods (apple, alder, padauk, and bloodwood all come to mind), the white rot, when present, stands alone; there is no black line spalting, although those woods can have black line spalting in other areas. In other woods (black walnut in particular), the areas of white rot are delineated by black line spalting. The black lines are where the different colonies of white rot fungi have literally fought to the death, leaving lots of little tiny black corpses.
A form of "defect" or "character" (depending on how you look at it) in wood where there are numerous elongated "spots" throughout the wood where it has been eaten away by various boring agents (generally beetles). Sometimes the eaten away area is filled in by some kind of natural process so that there are no voids but just discolored areas. This is usually in the form of elongated worm-shaped areas, but may also occur as spots (much like "pecky") depending on the cut of the wood and other factors. NOTE: worm holes are quite common in wood and the presence of a few worm holes does not generally trigger the designation "wormy" which, as I said at the beginning of this paragraph, is usually reserved for wood with NUMEROUS worm holes. HOWEVER, not all wood vendors agree with or follow what I just said, so you may find "wormy" wood that doesn't have many worm holes.
In many trees, wormholes are far more likely in the sapwood than in the heartwood.