Chapter 7: Timber defects

Definition and purpose

The defects found in manufactured timber have various causes.

1) Defects found in natural logs are shakes, knots and pitch pockets.

2) Defects caused by deterioration are rot and dote.

3) Defects caused by imperfect manufacture are imperfect machining, wane, machine burn and checks and splits from imperfect drying.

Shrinkage of wood

Shrinkage occurs during the process of drying or seasoning of the timber. All kinds of timber shrinks, regardless of type and method used. Some shrinks more than other timbers.

Figure 7-1. Shrinkages on wood

Drying of green timber involves two stages which are removal of free water filled in cell cavities and removal of moisture from cell wall. The cells do not shrink in their length, hence there is negligible shrinkage in the length of timber after seasoning. Most shrinkage occurs in the direction of growth rings at right angle to the medullary rays which is known by tangential shrinkage. This shrinkage is about double of radial shrinkage and about hundred times of longitudinal shrinkage.

Figure 7-2. Cell cavities

Similarly, if cells absorb water after seasoning then it begins to swell which ultimately increases size.

Defects caused by shrinkage

Shrinkage can cause defects like split, check and shake which are given below.

1) Bowing

It is the curvature formation in the direction of the length of the timber.

2) Twisting

It is the distortion in which both ends do not lie on the same plane.

3) Cupping

It is the curvature formation in the transverse direction of the timber i.e. edges do not align with the center of the wood.

4) Checking

It is the crack which does not extend from one end to another but separated fibers.

5) Splitting

It is a special type of check or crack that extends from one end to another.

6) Case hardening

When outer surface dries faster than inner portion, stress developed in outer surface results in surface hardening. It is the condition where outer portion of wood is stressed in compression and the inner core is stressed in tension.

7) Radial shaking

It occurs when felled tree are exposed to sun for seasoning (i.e. it results if seasoning is carried out too quickly). The cracks extends radially from bark to the pith through annual rings.

8) Collapses

Rapid removal of free water from wood cell causes weaker cells to collapse. When cell collapses, it causes uneven thickness and wavy surfaces.

9) Springing

It is the board with flat in width but curves in length like a river going around a bend.

Figure 7-3. Different defects caused by shrinkage

Defects caused by nature (Natural defects)

1) Knots

When the bases of branches are cut off or broken from the tree. There forms a dark hard ring known as knots which receives nourishments from the stem for longer period of time. The continuity of wood fibres are broken by knots forming a weak point in a timber. There are two types of knots and they are:

a) Live knots

This kind of knots are fixed in wood and cannot be separate out from it. Live knots are free from cracks and decay which makes it suitable for carpentry uses. Tight knot is the initial stage of live knots.

b) Dead knots

Dead knots are separated from wood body. So, it is not suitable for carpentry purposes. Loose knot is the initial stage of dead knots.

Figure 7-4. Live Knot and Dead Knot

2) Shakes

Shakes are the longitudinal separations in wood between the annual rings. Those longitudinal separations or cracks separates the fibres of wood making undesirable appearance. Different types of shakes are given below:

Figure 7-5. Different types of shakes in wood

a) Heart shakes

This defect occurs mostly in overmatured trees due to shrinkage of interior part of tree or heartwood while approaching maturity. These cracks or splits are found at the centre of wood extending towards medullary rays or circumference of the wood.

b) Ring shakes

Ring shakes are formed due to winds. These cracks are formed as cavity on rings of the tree.

Figure 7-6. Ring shake

a) Cup shakes

Cup shakes are formed due to excessive frost on sap when tree was not matured. It separates annual rings from one another.

b) Star shakes

Star shakes defects are formed mostly due to severe frost and extreme heat of sun. It is usually confined to sapwood. These cracks are found widest at the circumference of wood extending towards centre of the tree.

Similarly, there are other shakes like peripheral shakes and twisted shakes.

3) Upsets or rupture

The upsets are crushed or compressed wood fibres which are formed due to improper felling of tree in its young age to wind.

Figure 7-7. Upset or Rupture in wood

4) Burrs or excrescences

Burls are formed due to injury of tree in its young age which makes it unable to form branches.

5) Rind galls

The rind means bark and the gall means abnormal growth. The peculiar curved swelling found on the trees as shown in figure is rind gall. It appears due to improper cut off of branches.

Figure 7-8. Rind galls in a tree

Defects caused by man made

1) Defects due to seasoning

1. Twist

2. Cup

3. Bow

4. Spring

5. Split

6. Check

7. Case hardening

8. Collapse

9. Radial shakes

10. Honey combing

Figure 7-9. Honeycombing defect in wood

Due to incorrect kiln seasoning, various radial and circular cracks are developed in the interior portion of timber. These cracks occur due stress developed on inner surface of the timber when inner portion dries prior to external portion of timer. This stress developed causes the separation of the inner tissues of timber. For more details refer in section 7.3.

2) Defects due to conversion

a) Chip mark

This defect is caused when unwanted marks or signs are formed on finished surface of timber usually by planning machines.

b) Wane

This defect is causes rounded edge (i.e. wane) in the timber on one side because of timber’s original rounded surface.

c) Diagonal grain

This defect is caused due to carelessness while sawing of timber.

d) Torn grain

This defect is caused when small depression is formed on finished surface upon falling of a timber or when heavy things are dropped on finished surface.

Defects caused by diseases

1) Dry rot

Some fungi feed on the timber and converts it into dry powder form which is known as dry rot. Dry rot occurs in environments where there is no free circulation of air like basements. To prevent timber from dry rot, well-seasoned timber (i.e. moisture content less than 20 percent) free from sap should be used in area with well air circulation. Also, if timber is already affected by dry rot, the damaged potion should be removed completely and remaining portion should be treated with solution of copper sulphate.

Figure 7-10. Dry rot defect in wood

2) Wet rot

Fungi causes chemical decomposition of timber and such decomposition converts timber into greyish brown powder. This process is known as wet rot. This kind of rot occurs in environments like damp, moisture and alternate exposure to wet and dry conditions. To prevent wet rot, well-seasoned timber covered with paint or preservatives should be used for both exterior and underground work.

Similarly, there are other kinds of fungi damage like brown rot, white rot and heart rot.

Figure 7-11. Wet rot defect in wood

Reasons of timber decaying

There are various reasons that are responsible for timber decay and some of them are as follows:

1. High moisture content

2. Imperfect seasoning

3. Attack from insects and worms

4. Back stacking or storage of timber

5. Alteration of dry and wet states

6. Working with unseasoned timber

7. Working with seasoned timber without suitable preservatives.

Fungi is the main responsible factor for decay of timber. Fungi is a low-order plant which thrives on moisture, air, proper temperature above freezing point and food. So, proper seasoning is required.

What’s next?

Now we will learn about Insects and Wood borers that destroys timber in next chapter.