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Defects in timber

Grade 8

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objectives

• What are defects in timber

• Types of timber defectives

• knots

• Shakes

• Waney edge

• Dry rot

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knots

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Knots

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Knots

• Where a branch has grown out of a tree the grain in the trunk becomes very twisted. When this is cut through during conversion the result is a weaker board and one which is difficult to work. Dead knots occur in some softwoods

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knots

• These are distinguished from live knots because they show a black line round their circumference and they sometimes fall out of the wood leaving a hole.

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Shakes

• These are splits along the grain of the wood.

• Heart shakes and star shakes are very similar; they radiate from the centre or heart (see drawing)

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shakes

• Cup or ring shakes follow the annual rings.

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shakes

• End splits are caused by rapid drying from the end grain

• Shakes may occur during the felling or as a result of faulty seasoning.

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Waney edge

• When the outside of the tree is left as part of a board it is called the waney edge. .

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Dry rot

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Dry rot

• This is caused by a fungus growth which lives on the wood and thus destroys it. It attacks wood in damp, unventilated positions causing it to dry up and crumble into dust.

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Fungus Attack

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Fungus Attack

• There are some more or less harmless fungi which affect timber. The result is usually to stain the wood with grey streaks or patches. This kind of fungus spoils a decorative white wood like sycamore

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Insect Attack

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Insect Attack

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Insect Attack

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Insect Attack

• There are several vaneties of insect that attack wood. One that does serious damage to furniture is the common furniture beetle.

• The beetle lays its eggs in cracks and crevices of the wood. These hatch into small worms which burrow into the wood. They continue to eat their way through the wood for up to two years. The worms then change into beetles and come out leaving a woodworm hole.

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Insect Attack

• It is thus possible for wood to be affected by woodworm for a long time before the woodworm holes are seen. The beetles are very small-about the size of a pin head-and light brown in colour. They are active in the summer months and during this time they fly about and lay their eggs on furniture. They are very fond of the plywood backs of cupboards. Although little can be done about the worms in the wood, several preparations are on the market which poison the beetles as they come out and destroy any eggs laid" on the surface.

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Dutch Elm Beetle.

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Dutch Elm Beetle.

• This is a fungus which is spread very rapidly through elm trees by a small beetle. The fungus kills the tree but the timber, though not perfect, can be used.

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classwork

Question

• 1. Show ,by means of sketches what you understand by the following: star shake, knots, cup shake, waney edge

• 2. Write down what you know about the common furniture beetle

• 3. Name a fungus which destroys wood particular damp unventilated postion