UNESCO resource kit - science and technology educa...

Problems with humidity
UNESCO Module 12: Wood as a structural material
When a tree is alive, the hollow fibres in the wood are filled with a watery
solution called sap. There is usually about the same amount of water in the
wood as there is solid material. When a tree is chopped down for timber, most of
the water slowly evaporates until only about 10-15% is left. The wood is then
seasoned and is ready to be used for making things.
After the seasoned wood has been used to make things, the amount of water in
the wood does not always stay the same. This is because the amount of water
vapour in the air - the humidity - changes.
When the surrounding air is dry, the water content of wood can fall as low as 5%.
When the surrounding air is very humid (damp) the water content of wood can
rise as high as 25%. The table shows what happens to a piece of wood when the
humidity changes.
14 (a) What happens to both the length and the width of the piece of wood
when it is moved from a dry place to a damp place?
(b) What, difference is there in the changes to the length and to the width of
the wood? How can this difference be explained?
15 A new wooden door is fitted to a house during dry weather in the summer.
The door fits perfectly. A few months later during cold, wet weather, the
door won't close properly. Explain why.
16 Wooden floors are usually made from long narrow boards about 15 cm wide.
(a) Why aren't wider boards (e.g. 50 cm wide) used"?
(b) Why can long boards be used without causing any problems*?
[*You could use figures from the table in your answers.]
Compare the percentage change, along and across the grain, when:
• a dry piece of tropical hardwood is made very damp;
• a dry piece of softwood is made very damp.
(a) The dry pieces of wood should have been kept in a warm place for a few
daysbefore they are measured.
(b) Pieces of wood can quickly be made very damp by putting them in a
pressure cooker for a few minutes.
(c) Small pieces of wood will only change in length by a few millimetres.
Try to think of some way of measuring this, or of magnifying the change.
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