12 seasoning

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WOOD MATERIALS TECHNOLOGY 4 th Edition

Transcript of 12 seasoning

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WOOD MATERIALS TECHNOLOGY

4th Edition

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Chapter 12Timber Seasoning

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Seasoning

Why is timber seasoned?− To allow drying process to be controlled− Helps to prevent splits in the wood− Fungi do not attack dry timber− Dry wood doesn't shrink as much− Dry wood is easier to work with− Dry wood is lighter, harder and stronger

− The controlled drying out of timber

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Moisture content

The amount of moisture present in wood is shown as a percentage (%) of the weight of the wood when dry.

While seasoning is taking place boards must be checked for moisture content at intervals to make sure that the wood is being dried properly. The boards to be checked are taken from the middle of the stack. A loose board is withdrawn so samples can be taken at intervals.

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Finding the MC

Find the percentage moisture content of the following sample of wood given the following information;

Wet weight = 224g

Dry weight = 200g

Wet weight – dry weight X 100 = %MC dry weight

224 – 200 X 100 = %MC

200

24 X 100 = %MC

200

0.12 X 100 = %MC

MC = 12%

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Finding the MC

A moisture meter is most commonly used to establish the MC of a particular batch of timber. These meters are usually attached to two probes which send an electrical signal through the wood. Water is a conductor of electricity and therefore – the more water present the higher the conductivity and this can be read from the display.

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The MC (moisture content) at this point is 30% and the aim of seasoning is to get the MC of the wood below 20% which is the safety line to prevent dry rot.

Moisture Content: Situation:

20-22% Limit of good air seasoned wood

20% Limit of dry rot occurring

16% Outdoor Furniture

12-14% Occasional heated areas/ bedroom furniture

11- 13% Reasonable heating /livingroom furniture

9-11% High degree of central heating

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There are two main ways of seasoning timber, 1. Natural (Air) and 2. Artificial (Kiln) drying.

Timber will dry naturally but placing it in a kiln allows us to control the process as well as speed it up. Both methods require the timber be stacked and separated to allow the air to flow around the stack.

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Natural seasoning

Air seasoning

Overhanging roofStack raised off the groundStickers give space for air to

flow throughEnds of boards protected

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How are the ends of boards protected from splitting?

− Cover with a sack cloth− Paint the ends of the

boards

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Advantages− No expensive equipment needed− Little labour cost − Not energy dependentDisadvantages− Slow drying process− Large area required − Depends on the weather− Cannot be controlled− Only dries to 20% – 22%

Air seasoning

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Kiln seasoningKiln is a like large ovenTimber is stacked on a trolley and

wheeled inSteam is pumped in to heat the

wood and maintain moisture levels

The steam heats the timber but does not dry it out.

Once heated, the relative humidity of the timber is reduced but the heat maintained.

Fans keep the air circulating .This allows moisture in wood to

evaporate until required MC is reached.

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Kiln seasoning

• Advantages− Dries more quickly− Final moisture content is lower− Can be properly controlled.− Defects associated with

seasoning are minimised• Disadvs.− Heating the kiln uses a lot of

energy− It is an expensive method− Requires a skilled operator

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Relative humidity

• The rate of drying of the wood depends on the relative humidity

• There is a certain amount of water vapour in the air at all times. The exact amount depends on the time of year.

• You cannot see this water vapour, but it effects things like the speed clothes dry on a line or the drying of wood.

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Equilibrium moisture content

EMC Equilibrium moisture content.How the mc of the wood will reach a

balance with the air, eg winter and summer drying.

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Exam question1 When air seasoning, the planks being

seasoned are raised up on bricks or blocks. Why is this important? (JC, OL, 2005)

2 Splitting may occur on the end grain of a piece of timber during seasoning. Suggest one method of preventing this from happening. (JC, HL, 2007)

3 (a) Name the two methods of seasoning, A and B, shown in the diagram and state two advantages and two disadvantages of each method.

(b) Explain what is meant by the term: Equilibrium Moisture Content (EMC). (JC, HL, 2006)

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Sample answer to exam question

1 The planks are raised on bricks to keep them off the ground. This keeps them clean and dry.

2 Splitting can be prevented by painting the end grain or by hanging sackcloth over the end of the stack of boards. This stops the ends drying too quickly.

3 (a) The two methods of seasoning are:A Air seasoning/natural seasoningB Kiln seasoning

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Sample answer to exam question

3 (a) Advantages of air seasoning• Low labour cost once the stack is made• Low energy method as it does not require any heating

Disadvantages of air seasoning• It takes a lot of time and so uses a lot of space to store the

stacks• It is dependent on weather conditions and it is difficult to

control the drying rate

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Sample answer to exam question

3 (a) Advantages of kiln seasoning• A quick method• Careful controlled drying is possible

Disadvantages of kiln seasoning• Requires a skilled operator• High energy cost so it is an expensive method

3 (b) Equilibrium Moisture ContentWood absorbs moisture and releases it through evaporation into the air all the time. As boards of wood dry out, they reach a balance (equilibrium) with the air and so are neither absorbing nor releasing moisture. This balance or equilibrium moisture content will change as the humidity of the air changes.