Wood defects
In the movement of goods with raw wood , wood defects or growth defects are those characteristics of wood that impair the use of wood. Wood defects reduce the value of the wood by limiting its usability or reducing the yield (the amount of the end product). Wood defects are the basis of the classification into quality classes for raw wood. In individual cases, however, such errors or special wood features can also open up further possible uses and thus increase the price. For example, woods with burl bulbs are in demand for their appearance for veneer production or for turning. So-called crooked timber (strains with bent or arcuate growth) often find particular use in shipbuilding or for stair stringers used.
Uniform wood structure
Features of a normal and regular wooden structure are:
- circular growth rings
- approximately equal annual ring widths
- equal early and late wood parts
- The pith (heart) runs in the middle
- almost cylindrical trunk shape (woodiness, rolling shape)
- no damage from external influences
- Color typical of wood species
Only for some tree species are regular:
- Tree-topped (continuous trunk) - typical for softwood, hardwood is usually short-stemmed
Trunks of the highest purity of growth are called peeling wood , it is particularly suitable for veneer production.
List of wood defects
The following characteristics are considered to be defects in the wood:
Defect in the longitudinal wood
Curvature
This is usually associated with reaction wood formation and an extreme form of fiber deviation; Trunks with different degrees of curvature are created. A main reason for this is the game browsing , in which the animals eat the top buds and shoots, which is why a side shoot then takes over their function. However, unfavorable incidence of light, one-sided wind and snow loads and ground movements on slopes can also cause crookedness. A unilateral curvature as einschnürig and a multi-curvature than unschnürig referred. Forms of curvature are z. B. saber growth, bayonet growth or post horn growth .
Leanness
A trunk is cut down when the decrease in trunk diameter to one meter of trunk length is more than one centimeter. In the case of a log that is felled, many rinds are produced in the sawmill, and the wood has, above all, reduced flexural strength as more fibers are cut through.
Typical is the deforestation of the spruce in free standing (in contrast to the fir ), which is why it has to be expanded at the end of the association. The log utilization or the yield of a sawmill is influenced by the top diameter, the length and the felling.
Twist growth
Twisted growth is an internal twisting of the trunk. This twisted growth can improve the stability of a tree. Twisted growth is therefore mainly found in trees that are exposed to particularly intense windings. This is the case, for example, in coastal locations or in high mountain areas. Most woods are of course always slightly twisted to a certain extent, only abnormally strong twisted growth counts as a wood fault. The error manifests itself in throwing flat wood and rotating beams, because the internal tensions in the wood are released during drying. These can develop enormous forces and even burst clean wood connections.
The larch is typically highly twisted , which is why it is not very suitable as construction timber despite all its other advantages : it does not stop working even in old age.
Fiber deviation (alternating twist)
Alternating twist is a species-specific structure that occurs in various tropical woods . It is created by a stratification of the cells one to eight centimeters wide, spiraling around the stem axis ( pith ) in alternating directions . Since the direction of the cell axis changes or crosses from layer to layer, the tensions in the sawn timber are largely neutralized. With radial cut, surfaces are created from strips with differently directed cells. This creates an alternating reflection of the light, which causes an alternation of matt and shiny stripes. Alternating twist does not restrict the utilization of the wood. During processing, however, it can have a disadvantageous effect through a streaky resistance (tearing out) and through greater warping in the case of side boards with a small thickness.
Wimmerwuchs
Growing growth is called growth rings that do not run straight in the longitudinal direction without any other damage. They result in a rough grain pattern and lead to wood that is difficult to plan. They are only a wood flaw in the true sense of the word in some woods; in others, uncontrolled growth can also become fashionable and lead to higher prices in the short term.
Zwieselwuchs (forked growth), double core
In Zwieselung, two or more trunks of the same tree species or one tree are grown together: A double core is created , or a young tree has forked itself with single trunks that are incorrectly growing. The Zwieselung is considered a root form error . The sawn timber is often unusable for the carpenter, especially if the double core is imperfectly grown together. With some types of wood, e.g. B. with walnut and mahogany , processing of the Zwiesel into valuable pyramid grain veneer is possible. A distinction must be made between so-called push pommel and pull pommel.
Defect in the crossbar
Eccentric growth
The trunk shape is elliptical and the pith is usually not in the center of the trunk. In addition, groove-shaped indentations are created on the trunk jacket, which in some tropical tree species develop into buttress roots. Trees such as B. beech , birch or teak , which tend to be out-of-round, have a gross density that varies greatly over the trunk cross-section and therefore tend to crack when drying and are difficult to work with.
Backward tension
As spannrückig a tribe is called that is not approaching in its cross-section of a circular shape but evolved asteroid. This is particularly common at the lower end of the trunk and is caused by uneven division activity in the cambium. Tension back is genetic or arises as a result of injuries. Mostly it lies between two root approaches. It is often found in hornbeam , yew , juniper , hickory and black locust .
Reaction wood (pressure wood / tension wood)
The so-called reaction wood is an active alignment fabric of a tree. This is a piece of tree that is under high tension due to physical forces, such as branches or a part of the trunk, which has been moved from its original position and strives back to its previous position like a tense arch. This creates zones with increased growth. These can be found on the underside of the conifers and on the top of the branches in the hardwoods . The resulting reaction wood differs anatomically, chemically and physically and mechanically from normal wood. Usually the formation of reaction wood is accompanied by an eccentric growth in which the pith is shifted from the center of the trunk cross-section. If a tree is loaded on one side due to a slope, exposed wind, the pressure of snow masses or some other force (such as fallen trees, boulders, etc.), reaction wood forms.
Softwoods on mountain slopes can form pressure wood or boxwood on the side exposed to the valley . This differs from normal wood in that it has a darker reddish-brown color and is therefore also called redwood. Due to the formation of thick-walled early wood tracheids , a distinction between early and late wood within a tree ring is very difficult or even impossible. Since the pressure wood has increased lignin storage , it has an increased bulk density , hardness and increased shrinkage in the axial direction compared to normal wood. When viewed microscopically, pressure wood tracheids show typical wrinkling in the central secondary wall .
In contrast, there is the formation of tensile wood in the deciduous trees. It is formed on the top of crooked trunks or branches. The lignin content is lower, but the cellulose content is higher than that of normal wood. It appears lighter and is therefore also known as whitewood. The properties of the tensile wood are that it causes deformation phenomena as well as tensile stresses. The usual swelling and shrinking behavior of the wood is also strongly negatively influenced and mechanical processing is difficult due to a partly woolly surface.
Knots and dents
A tree trunk can be roughly divided into three zones, the plait , the central trunk and the earth trunk block. In the lowest part, the trunk block, there are no or only overgrown branches . The middle trunk contains dead branches, while the top piece, the plait piece, contains green branches. Branches have a higher proportion of lignin and are therefore darker and more brittle and shrink more than trunk wood. The use of wood is less due to the sorting out or cutting out of the branches and thus reduces the value. There are, however, exceptions such as B. the stone pine or the bird's eye maple . If a branch breaks off in a tree, the resulting wound will over time be covered, which also leads to a disturbed fiber flow (bumps).
Wound overburden
Old, healed wounds ( overburden ) are wood defects, both because they contain dead, often already rotten wood, and because of the discarded grain.
Burl bulbs and burl growth
The trunks of some tree species occasionally show conspicuous cell growths, which are usually called measles bulbs . They are usually caused by the bacterium Agrobacterium tumefaciens . This smuggles fragments of its own genome into the genome of cells of higher plants with plasmids and triggers uninhibited growth in the infested area.
Tuber bulbs consist of a mostly spherical thickening on the tree trunk, often found in the lower trunk section. The very swirling, lively texture of the wood, interspersed with so-called eyes, is often worked up into decorative veneer . The most sought-after are the burl bulbs of the walnut tree, which are traded as walnut burl and have shaped the interior fittings of classic British automobiles for many years. However, the processing and drying of burl wood can be difficult. The term burl wood, on the other hand, is based on an amateur mix of the terms burl wood and root wood; only in the rarest cases do trees develop burl bulbs at the roots at the same time.
Adhesion
If there are two adjacent wood areas, woody plants can recede the bark and allow the two areas to grow together. This is used to close the wound during wound healing and to build up a strong root stock. Adhesions can also occur in the branches, some trees such as hornbeam or oak are very prone to this. This can lead to later grain defects in the wood (additional ingrown core). The strangler figs of the tropics are extreme examples of intergrowth .
Cracking
Cracks appear on the wood as surface cracks or internal cracks. They can have different causes. Drying cracks , including shrinkage or shrinkage cracks , arise as a result of drying because wood shrinks more tangentially than radially. Examples of shrinkage cracks are the core crack (a crack running through the pith ) and the star crack (running from the core outwards), which arise in particular on the brain of the trunk, as well as the air crack (running from the forest edge inwards), which usually runs over the entire length of the wood . Twisted growth is also visible on this . Frost cracks, on the other hand, are growth damage that grows together to form a frost bar .
Ring or core shells, ring fissures
Ring peeling is a detachment along an annual ring , which can occur due to tensions in the wood, but also fungal attack, and can stretch over several meters. A board that cuts the ring peels falls or gapes apart.
Core brittleness and core rot
Core brittleness and core rot cause irregular zones in the cross-section and brittle wood parts as well as a strong tendency to transverse brittleness . This wood defect occurs in deciduous and coniferous trees. Core rot is caused by pathological, parasitic fungal infestation, e.g. B. by Hallimasch . These trees have a low bulk density, reduced strength and a rough surface. Therefore, processing is often no longer possible.
Moon ring
The concept of moon ringing is based on the fact that a non-core zone appears in the cross-section (brain section) of the tree similar to a crescent-shaped sickle. The moon ring is caused by late frost damage and is a core failure . The moon ring cannot core again later. The non-core zone formed by the moon ring remains throughout the life of the tree. After a moon ring, the trunk can continue to grow normally.
Moon-wrestling is a significant wood defect, because over a certain length in the trunk wood it usually restricts the usability of the trunk wood or the sawn timber to be produced from it for optical reasons. The position of the moon ring affects the usability of the wood and thus its quality. In the affected sapwood, for example, it could prevent the sorting into knot-free pages , since according to the standard, moon rings are not permitted in knot-free pages. If the moon ring is very far in the core, it would not hinder the extraction of knot-free pages. An assortment of square timber z. B. would usually still be possible.
Since this moon ring zone has no pigment deposits, the zone appears lighter in heartwood types (such as oak, cherries) than the surrounding core wood. In the case of heartwood, the light zone of the moon ring can also be seen well in the middle of the darker heartwood when it is not yet infested by fungi. Due to its sapwood character, the moon ring is much more susceptible to fungal attack than heartwood. In extreme cases, a hole is created at the location of the moon ring.
Moon ringing does not only develop in heartwood species, but also in mature wood species (such as red beech, linden, spruce, field maple) and heartwood species (such as elm, buckthorn). In the case of frost wood species, the moon ring can only be recognized when it is infested with fungi, because then the moon ring stands out as a dark zone from the surrounding frost wood.
Resin galls
Some conifers, especially spruce , create cavities in the latewood that are filled with tree resin , which serve to ward off pests. During processing, they become noticeable through the flaws, resin-bonded tools and problems with painting. From this wood defect comes the saying have bad luck - bad luck is a technical term for the resins of trees.
Cancerousness
Tree cancer manifests itself in various heavy growths.
Bark brandy
Bark burn occurs primarily on trees with thin, smooth bark. The cambium dies, the bark dries up and separates from the trunk. This can easily lead to fungal or insect infestation . In some federal states, wood with bark brandy is excluded from quality class A.
Cortical necrosis (color)
When there is necrosis under the bark, water accumulates, which causes discoloration of the wood. As it progresses, the bark will crack and possible infection from fungi and bacteria will cause the bark to fall off. Beeches, where this wood defect occurs frequently, react with premature dilation (closing of the water transport channels) in order to prevent deeper penetration of pollutants. This makes it impossible to use the wood e.g. B. to impregnate in the boiler pressure process.
Feeding tunnels
Feeding tunnels are a result of infestation by animal wood pests . The causes are in particular various beetles (e.g. bark beetles ) or beetle larvae (e.g. longhorn beetles ), but also wood wasp larvae , ants and termites .
Mistletoe infestation
Mistletoe ( Viscum album ), as well as the belt flower (oak mistletoe , Loranthus europaeus), are semi-parasites on trees. They grow through the wood with their sinker roots and cause holes with a diameter of 5 mm, which are close together and can render the wood worthless if it is heavily infested.
Lightning damage (lightning gutter)
In the event of a lightning strike in a tree, a so-called lightning canal goes from the point of impact or several points of impact under the bark in the damp wood to the roots. The water in the sapwood and cambium evaporates and the bark is literally blown off by the steam pressure. Depending on the strength of lightning, the type of tree or the moisture in the tree, the damage to the bark can range from a few centimeters to the entire trunk. Often trees can cover this damage well, sometimes a lightning strike also leads to a permanent rib that overgrows and freezes again in winter (frost bar).
Frost bar
In the event of sudden extreme cold, thermal tensions between the inside and outside of the trunk can cause tearing in the radial direction. If the cold only lasts briefly, the crack can close again. If the cold snap occurs again, the crack can tear open again with a loud bang. As a result of repeated cold snaps and increasing attempts by the tree to overwhelm the crack, an axially parallel frost bar up to several meters long forms on the trunk.
Ingrown metal, splinter attack
Serious defects are ingrown metallic foreign bodies such as B. nails or pieces of fence wire. They represent an enormous danger during felling and further processing. They can be torn from the wood, but they can also damage or destroy tools.
One speaks of a splinter attack when there are bullet splinters in the wood. In some parts of Alsace, for example, due to the lively war activities in the past, it is forbidden to use trees for timber, as the risk of accidents due to metal parts flying around during processing is too great. In other areas, the incision is only possible after a prior X-ray .
Felling and back damage
Felling damage can be caused to the remaining stand or to the felled wood: In the stand through damage to the bark (especially in summer, as the bark abrasion resistance is then lowest) or broken branches during the felling or backing of neighboring trees, on the felled wood through tears at the felling cut or during the back .
Browsing, peeling and sweeping damage
These types of damage are typical forest damage caused by game:
- Game bite is eroding the top bud on the Young tree that results in a turbulent growth to a complete hold-down ( cripple growth leads) - main pests in this sense are deer and chamois .
- Peeling is the gnawing off of the bark by animals in winter, caused by all grains, hares and other rodents , as well as beavers . Complete ringing causes the tree to die.
- Sweeping is the abrasion of the bast from the fully grown antlers of deer . If thered deer population is too dense, entire stretches of forest can be damaged.
Windthrow / storm damage
As a result of violent storms, trees are bent so much that the longitudinal fibers are torn across. However, the tree does not break, but stands up again so that the crack is not visible. Even when cutting, the cracks can hardly be seen with the naked eye. Damage often only occurs after installation and under load.
literature
- Dietrich Böhlmann: Why trees don't grow into the sky - An introduction to the life of our trees. Quelle & Meyer Verlag, Wiebelsheim 2009, ISBN 978-3-494-01420-3 .
Individual evidence
- ↑ Wolfgang Nutsch: wood technology specialist knowledge. Europa Lehrmittel, 2001, ISBN 3-8085-4018-4 .
- ↑ Böhlmann: Why trees don't grow into the sky. 2009, p. 15.
- ↑ Wolfgang Nutsch and others: Holztechnik Fachkunde , 22nd edition, Verlag Europa – Lehrmittel, Haan – Gruiten 2010, pages 43 and 45, ISBN 978-3-8085-4045-9
- ↑ Explanation of the "characteristics of wood" on Proholz.at
- ↑ Böhlmann: Why trees don't grow into the sky. 2009, p. 36.
- ↑ Core rot due to fungal attack. ( Memento of September 23, 2015 in the Internet Archive ) Retrieved November 20, 2013.
- ↑ H. Zycha: Hallimasch (Armillaria mellea Vahl ex Fr.) Kumm.) As core rot pathogen on spruce (Picea abies Karst.). In: Forstwissenschaftliches Centralblatt , Volume 89, No. 1, 1970, pp. 129-135.