Timber Growth

Timber growth

Geoff' s Woodwork for Students of Woodwork

Timber growth

Cross section of a tree truck Annual or growth rings ~ in temperate climates there are two distinctive growth seasons, spring and summer ~ the spring growth is rapid and is shown as a broad band whereas the hotter, dryer summer growth shows up narrow. In tropical countries the growth rings are more even and difficult to distinguish.

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Timber growth

Bark ~ the outer layer, corklike and provides protection to the tree from knocks and other damage. Bast ~ the inner bark, carries enriched sap from the leaves to the cells where growth takes place. Cambium ~ layer of living cells between the bast and the sapwood. Crown ~ the branches and leaves that provides its typical summer shape. Heartwood ~ mature timber, no longer carries sap, the heart of the tree, provides the strength of the tree. Usually a distinctive darker colour than the sapwood.

Medulla ray ~ (rays) food storage cells radiating from the medulla ~ provides a decorative feature found in quarter cut timber. Pith or medulla ~ the centre of the tree, soft and pithy especially in the branches. Sapwood ~ new growth, carries the raw sap up to the leaves. Usually lighter in colour than the heartwood, especially in softwoods. Trunk ~ main structure of the tree, produces the commercial timber. Root structure ~ Absorbs water and minerals from the soil. It is the anchor of the treee.

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Timber growth

Hardwoods and Softwoods. There are two main groups of timber producing trees used commercially; softwoods and hardwoods. These terms immediately create contention because they do not accurately describe the timber correctly. Softwoods. Softwoods are coniferous trees and the timber is not necessarily 'soft'. They are 'evergreen'. (The larch is an exception) Their general characteristics are: Straight, round but slender, tapering trunk. The crown is narrow and rises to a point. It has needle like or scale-like shaped leaves and it's fruit, i.e. it's seeds are carried in cones. The bark is course and thick and softwoods are evergreen and as such do not shed their leaves in autumn. Hardwoods. Hardwood trees are broadleaf and generally deciduous. Their timber is not necessarily hard. For instance, balsa (the timber used for making model planes) is a hardwood. The general characteristics are: Stout base that scarcely tapers but divides into branches to form a wide, round crown. The leaves are broad and may have single or multi lobes. The bark may be smooth or course and varies in thickness and colours. Its fruit may be: nuts, winged fruits, pods, berries, or fleshy fruits.

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Timber growth

Other terms ( ** I am afraid there are often contradictory explanations in some terms. UK readers especially be aware of the n.american term of 'burl' in examinations) Branch ~ main bough from trunk Bole ~ The greater part of the trunk under the first branch. Boulle (log) ~ french term for through and through cutting of the trunk and retained complete in the seasoning process especially for matching grain. (included for interest only) Bur(r) (burl n.americ.) ~ produced by the tree to form over early growth of twigs known as epicormic shoots. Also formed to protect damaged timber. Provides beautiful figure prized by cabinet makers and for making veneers. Burl ** ~ (buttress) Briar pipes are made from this material nearer the root structure. I believe North American usage includes the description similar to burr. Butt (butt-swell) ~ The wider part of the trunk especially close to the ground. Veneers have some 'burry' circles but is characterised by a wild freakish pattern. Buttress ~ On some trees the widening of the bole providing interesting grain especially for veneers. The part of the trunk or bole as it enters the ground. Large tropical trees have 'butresses' reaching well over a metre from the ground. Crotch ~ the forked meeting of the trunk with a branch forming interesting growth ring formation. Trunk ~ stem ~ main part of tree ~ see bole text and graphics © G.Malthouse ~ all rights reserved 17th April 2004 search my site:

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Conversion of timber

Geoff's Woodwork for Students of Woodwork

Conversion of timber

(updated Apr 2004)

As soon as possible after felling the tree should be converted into usable timber. There are two main methods of converting timber: ● ●

Through and through (or Plain or Crown sawn) which produces tangential boards and Quarter Sawn which produces radial boards.

The Quarter sawn is far more expensive because of the need to double (or more) handle the log. There is also more wastage. It is however more decorative and less prone to cup or distort. Note also there are two ways of sawing the quarter. Through and through produces mostly tangentially sawn timber and some quarter sawn stuff. (see diagram) Tangential timber is the most economical to produce because of the relatively less repetitive production methods. is used extensively in the building industry.

It

There are other ways but they are all variations of tangential and radial cuts to obtain the best or most economical boards for the use it is to be put. These basic cuts are not always able or need to be, on the exact tangent or radius of the trunk. The cuts, that fall between, crown and quarter are called 'rift' and between 'rift' and 'quarter' are identified as 'figured' - see below for explanation. Boxing the heart refers to eliminating the heartwood from the boards that would otherwise produce shakes, juvenile wood or may even be rotten.

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Conversion of timber

Tangential boards (crown, plain or flat sawn) are used extensively for beams and joists. They are stronger when placed correctly edge up with the load in the tangential axis. These type of boards suffer from 'cupping' if not carefully converted, seasoned, and stored properly. Annual growth rings form an angle less than 45 degrees. Radial boards (radial, figured or quarter sawn) are typically cut on 'the quarter' and produce a pattern of the medullary rays especially in quartered oak. Such timber is expensive due to the multiple cuts required to convert this board. The radial face of the board is slightly stronger and stiffer than the tangentially face but the cross section and condition of the timber has more effect on strength. Annual growth rings form an angle greater than 45 degrees. Crown sawn is obtained by sawing tangentially to the annual rings. It is also referred to as 'Plain Sawn' or 'through and through'. Rift sawn is the cut which falls between crown and true quarter sawn. It is straight grained and in oak, does not reveal any 'silver ribbon' grain features. Quality floor boards are prepared from rift sawn timber because it wears well and shrinks less. Annual growth rings form an angle between 30 and 60 degrees. Quarter sawn boards are radial cut from the centre http://www.geoffswoodwork.co.uk/conversion.htm (2 of 3)7/28/2004 8:00:38 AM

Conversion of timber

of the tree. It produces the distinctive silver ribbon effect (in oak) across the whole board. Annual growth rings form an angle greater than 45 degrees. True quartered boards producing the best features will have the angle on or very much closer to 90 degrees. 'Figured' - is the cut between 'rift' and 'true quartered'. It has varying degrees of 'silver ribbon' (in oak) showing through but not the full figured effect found in true quarter sawn boards. Different species have their best features enhanced by choosing the best cut appropriate to their species.

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seasoning

Geoff's Woodwork for Students of Woodwork

Seasoning Timber

Seasoning is the controlled process of reducing the moisture content (MC) of the timber so that it is suitable for the environment and intended use. We need to reduce the MC of timber for the following reasons: ●

Every time the MC reduces the timber shrinks especially tangentially.

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Consequently it will show fewer tendencies to warp, split or shake.

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Seasoned timber although lighter will be stronger and more reliable.

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The sap in timber is a food for fungi and wood parasites. Remove the sap and the wood will be less attractive to these dangers. For construction grade timber the timber must be below 20% MC to reduce the chances of Dry Rot and other fungi infestations. Dry well seasoned timber is stronger.

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seasoning

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Dry well seasoned timber is easier to work with and consequently safer especially machine working. Timber with higher moisture content is difficult to finish i.e. paint, varnish, etc.

There are two main ways of seasoning timber, Natural (Air) and Artificial (Kiln) drying. Both methods require the timber be stacked and separated to allow the full circulation flow of air, etc. around the stack. Air Seasoning. Air seasoning is the method used with the timber stacked in the open air. It requires the following: ●

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Stacked stable and safely with horizontal spacing of at least 25 mm. Vertical spacing achieved by using timber battens (piling sticks) of the same or neutral species. Today some timber yards are using plastics. The piling sticks should be vertically aligned and spaced close enough to prevent bowing say 600 to 1200 mm max centres. Ends of boards sealed by using a suitable sealer or cover to prevent too rapid drying out via the end grain. The stack raised well clear of the ground, vegetation, etc to provide good air circulation and free from rising damp, frost, etc. Over head cover from effects of direct sunlight and driving weather.

The details depend on the size, quantity and species of the timber. You cannot however expect to obtain less than 16 - 17% mc in the UK. Further seasoning needs to be done inside, in heated and ventilated buildings. Kiln Seasoning. There are two main methods used in artificial seasoning, compartmental, and progressive. Both methods rely on the controlled environment to dry out the timber and require the following factors: ●

Forced air circulation by using large fans, blowers, etc.

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Heat of some form provided by piped steam.

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Humidity control provided by steam jets.

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seasoning

The amount and duration of air, heat and humidity again depends on species, size, quantity, etc. Schedules are published for the various species to enable operators to select an appropriate drying environment. In the UK they are usually provided by the Kiln Manufacturers and also published in the Handbook of Hardwoods and Handbook of Softwoods (BRE). Compartmental. A compartment kiln is a single enclosed container or building, etc. The timber is stacked as described above and the whole stack is seasoned using a programme of settings until the whole stack is reduced to the MC required. Progressive. A progressive kiln has the stack on trolleys that ‘progressively’ travel through chambers that change the conditions as it travels through the varying atmospheres. The advantage of this system, although much larger, has a continuous flow of seasoned timber coming off line. How to Season your own timber. This process is for small batches and is based on a one inch thick board, for other thicknesses and varying species you should adjust this time according to your experience and judgement. I have made notes in italics below to help you along the way. In the UK - generally ~ for a one-inch thick board such as oak will take about 12 months. Beech is much faster. Further rough guides for Air drying times for fresh felled timber in temperate areas similar to the UK: Softwoods ~ 25 mm thick, stacked in spring reduces to about 20 % in 1 1/2 to 3 months. Softwoods ~ 50 mm thick, stacked in spring reduces to about 20 % in 3 to 4 months. Hardwoods ~ 25 mm thick, stacked in autumn reduces to about 20 % the following summer. Hardwoods ~ 50 mm thick, stacked in autumn reduces to about 20 % the following autumn. However, note that these times show reduction to only 20 %. Further outdoor exposure may bring it down to 16 or 17 % but usually will require indoor drying to get to 12 % or less. Extreme care and good judgement is needed to get timber down to these levels without tension and stresses developing. Low heat, correct humidity and ventilation is what you must get right and certainly do not try to force the pace. Read R.B. Hoadley's book, Understanding Wood for a better guide to home seasoning. http://www.geoffswoodwork.co.uk/seasoning.htm (3 of 5)7/28/2004 8:02:56 AM

seasoning

These are rule of thumb for temperate climates and you must make checks, record conditions and use your judgement and make adjustments where necessary. Take samples from centre of stack, identify, weigh and record. Note the season of the year that you start the process and carefully note weather extremities to help you predict the duration and effects. After approximately 3 months for softwoods and 12 months for hardwoods – start weighing samples (note times above) and record together with notes on the weather in the immediate past cycle. You may well want to start weighing and recording before these times to get a better understanding of the different species you may have in the stack. Weigh and record every week thereafter and after three consecutive weighs showing no difference consider the stack Air Dry for external use. Test using the oven method to determine the %MC and whether it is fit for your use. Re-stack the timber in-doors. Take care that the environment is not too hot and there is sufficient ventilation. You need to control this environment and thus you need to take regular readings of temperature and humidity of the air and % mc of timber samples. Take samples from centre of stack, identify, weigh and record temperatures and humidity if you have access to an hydrometer. Continue to weigh and record every week and after three consecutive weighs showing no difference consider the stack Air Dry for internal use. Test using the oven method. Continue the process until you reach the % MC you require for your particular use. Keep your notes and they will help you to predict the seasoning process of future batches. Test using the oven method. Remove sample from centre of stack. Cut off a member 300 mm long. Cut off a small cube sample approx. 25 mm long. Weigh and record. Place in a warm oven (experience must judge - dangers of over heating is obvious) for 15 to twenty minutes or so and re-weigh and record. Continue until there are no discernible differences between readings. You will now have a wet weight and a dry weight. The %MC is obtained by the formulae – Wet weight – dry weight / dry weight X 100 = %MC determining wood %MC - not mine !) http://www.geoffswoodwork.co.uk/seasoning.htm (4 of 5)7/28/2004 8:02:56 AM

(this is a standard scientific formulae for

seasoning

A moisture meter is very handy but not quite so accurate as the oven method. Why not test your sample using the oven method and then see how much you can trust your meter? Top more information at 'Conversation on seasoning' - (timber, boards, sticks and PEG) text and graphics © G.Malthouse ~ all rights reserved search my site:

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Shrinkage

Geoffs Woodwork for Students of Woodwork

Timber defects Shrinkage When timber is seasoning and it's moisture content (MC) is reduced below the Fibre Saturated Point (FSP) continued drying will cause dramatic change such as increase in strength but also distortion and shrinkage. Shrinkage is the greatest tangentially over the radial direction with little loss along the length of the board, etc.

Cupping Because of this varying shrinkage rates tangential boards tend to cup because of the geometry of the annual rings shown on the end grain. It can be seen that some rings are much longer than the others close to the heart. Therefore they will be more shrinkage at these parts than the others ~ cupping is the result. In square section timber cut from the same place, diamonding is the result.

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Shrinkage

Knots Knots are the result of the trees attempt to make branches in the early growth of the tree. They are the residue of a small twig, shoot, etc. that died or was broken off by man or an animal in the wood or forest. The tree subsequently continued its growth over this wood. The knot may be live, sound, or tight or if it has become separated and is contained in residue of bark, dead. Dead knots become loose and downgrade the appearance and stability of the board. Most grading systems uses the amount of knot area as an indication of its quality. The more knots the less the quality. Dead loose knots are extremely dangerous to machinists. The cutters may pick these up and eject them rapidly towards the operator.

Splits A separation of the wood fibres along the grain forming a fissure that extends through the board from one side to the other. It is usual in end grain and is remedied by cutting away the defected area. All boards should have an allowance so that some end grain may be cut away because of possible shakes or splits.

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Shrinkage

Checks and end checking A separation of the fibres along the grain forming a fissure which shows up on one face or at the end grain but does not continue through to the other side.

Wind or twisting Spiral or corkscrew distortion in a longitudal direction of the board. Due to the board being cut close to the centre of the tree which has spiral grain. The board is of not much use but small cuttings may be obtained from it with careful selection.

Bow Bowing is concave/convex distortion along the length of the board. It is a seasoning and or storage defect caused by the failure to support the board with stickers at sufficient intervals. The boards own weight and probably those above it bears down and the resultant bow is inevitable. This defect can and should be avoided by careful use of stickers supporting the board at the correct width.

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Shrinkage

Spring Spring is concave/convex distortion along the length of the board again but this time the distortion is in the flat plane of the board. Boards with this defect may have been cut from near the heart of the board and is the result of growth stresses being released on conversion. Useable timber may be recovered from these boards by cutting a straight edge and re sawing. The grain direction however may not be satisfactory for aesthetics and care should be taken for placing the possible short grain figure where stability is required.

Shakes Shakes are separation of the fibres along the grain developed in the standing tree, in felling or in seasoning. They are caused by the development of high internal stresses probably caused by the maturity of the tree. The shake is the result of stress relief and in the first place results in a single longitudal crack from the heart and through the diameter of the tree. As the stress increases a second relief crack takes form at right angles to the first and is shown as a double heart shake. Further cracks are known as star shakes and show the familiar pattern shown. Ring or partial cup shakes in the form of longitudal tangential cracks occur as a result of high radial tension. It is often said that http://www.geoffswoodwork.co.uk/shrinkage.htm (4 of 5)7/28/2004 8:05:12 AM

Shrinkage

it is caused by an early frost freezing the rising sap or perhaps a heavy felling on hard ground. External radial cracks are caused by the tree laying too long before it is converted and seasoned. top reference sources: H.E. Desch & J.M. Dinwoodie ~ Timber

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