The Technique of FURNITURE MAKING

Permissible moisture contents

Wood which has been stored in the open, even under the most favourable conditions, is hardly dry enough for use in heated interiors. While little shrinkage takes place in wood seasoned from green to fibre saturation point (30 per cent), from then on the shrinkage is appreciable with maximum movement within the range 20 per cent down to 10 per cent. The maximum moisture content of timber in use should, therefore, not exceed the amounts shown below if excessive shrinkage is to be avoided.

22 to 15 per cent Usual limits of moisture con­tent in thoroughly air dried wood.

20 per cent Dry rot safety line.

16 per cent Outdoor furniture.

15 per cent Joiners' work in new build­ings. Normal church-work in intermittently heated interiors.

12 to 14 per cent Bedroom furniture with only occasional heating.

11 to 13 per cent Living-room furniture with normal heating, including reasonable central heating.

9 to 11 per cent Furniture in flats, offices and public buildings, etc. with continuous high degree of central heating.

8 per cent Radiator shelves and casings, etc.

The upper values in each case are usually safe lough, dependent on the wood species, as there is always some air movement from open doors, windows, etc.

It should not be assumed that because a work­shop happens to be continuously heated throughout the winter months wood stored in it will automatically dry down to the lower levels. it might conceivably fall to 6 per cent in exceptional circumstances, but owing to the continual movement, opening of doors, etc. it is more likely to be within the range 10 to 12 per cent and even 14 per cent and it is the average value which counts, for wood moves slowly and is virtually unaffected by day to day changes. some method of conditioning the wood before use may have to be adopted, therefore; but while the old adage that it should be stored for at least six weeks in the room in which the completed furniture is to stand is very sensible, it is hardly practicable. However, it is possible to create a small warm store, or even a very simple kiln, built of insulating materials and with a gentle source of heat, for even electric light bulbs in a closed compartment will give sufficient for small sections, though care must be taken not to overdry and at too rapid a rate. on occasions the writer has even pressed a heated linen-cupboard into service, but a close witch should be kept for splitting and distortion, with the wood sections cut slightly oversize so they can be squared later.

Additionally, it is always wise to protect the completed furniture at the first possible opportunity, for oil-paint (three coats) will give 50 per cent protection, French polish and cellulose (two coats) 30 per cent, and wax - polish 3 per cent against moisture absorption over a period of one month, with much higher values over seven days.