Replies

1. 
   SgianDubh | Nov 01, 2008 01:03pm | #1

   "Am I out to lunch, or what?"

   Yes and no. The preferred moisture content of wood varies with its planned end use, but to summarise, about 19% MC suits outdoor wooden artefacts, and wood for internal furniture should range between about 7% to 12% MC. Wood at various moisture contents between 12% and 18% is suitable for a variety of construction work and joinery in heated sheltered locations, sheltered unheated locations, and for exterior joinery.

   In previous centuries all wood used for the construction of buildings, forts, ships, joinery, furniture, turning, carving, etc., would have been either dried, partially air dried or used green. Mankind learnt long ago that using green wood for internal furniture resulted in the problems of shrinkage and splitting.

   It has long been the case that woodworkers have found ways to condition wood prior to using it. In times past if dry wood was required this meant planking it, stacking it with spacers (stickers) between the planks, and allowing it to air dry in roofed open sided sheds or similar. Not all cabinet making businesses in the 18th century had a wood store near or attached to the workshop with timber available for selection; but of those businesses that did have such a store, the Master and the journeymen working for the business knew to bring air dried wood into the warmer workshop for final conditioning before turning it into furniture.

   These craftspeople didn’t have access to kiln drying techniques as we do now, nor were moisture meters available to them, but highly skilled woodworkers knew when wood was too wet or dry for the planned project. Without a moisture meter I can usually tell whether a piece of wood is too wet for a job just by the way a plane cuts a shaving or a chisel pares end grain. 18th Century Masters and journeymen were surely just as sensitive, if not more sensitive than I am to the condition of the wood they were working with.

   Another factor to consider are the typical housing conditions or interior environments that furniture makers have built for over the generations. Today modern houses are generally well insulated and heated (or cooled) according to local climate conditions and seasonal norms. These conditions present particular challenges to us as designers and builders of wooden furniture.

   In earlier times, certainly here in Great Britain, dwellings and workplaces had poor insulation, and were draughty and quite cold and damp, particularly in the winter. This would apply to the better houses available to the moneyed classes and wealthy. Poor people often lived in very basic accommodation probably little better than an outdoor shelter in the fields for cattle and sheep, albeit with heating of some sort such as an open fire.

   Many people today choose to live in old houses. Old houses that have somehow escaped the developer’s upgrading efforts exhibit seasonal internal conditions much the same as they’ve done since the day they were built. Insulation is quite poor and winter heating is expensive. Single glazed windows in ill fitting frames let draughts in which brings in moist air. It is my experience that many old houses that have not been modernised cause wood furniture in them to exhibit a moisture content range between about 10% and 14%. In the table on the following page this equates with the description, “Buildings with intermittent heating giving room temperatures between 12- 21ºC.”

   If we assume that cabinetmakers used wood that was air dried to make furniture then the case is that air drying would bring the timber down to a moisture content close to the top of the range that it would exhibit in well built old houses. Today, if I buy thoroughly air dried timber, I normally find that testing the moisture content shows that it’s between about 18% at the low end and about 22% at the upper end. Cabinetmakers of earlier centuries would be able to reduce this moisture content by four or five percentage points easily through bringing the wood indoors into their heated workshops prior to making furniture with it.

   The inference is that the furniture they produced used wood quite well matched to the final in-service conditions. Antiques in modern homes, well insulated and heated (or cooled) as they are suffer quite badly. Joints become loose, veneers buckle, and solid wood table tops crack, especially if these have clamped ends (breadboards.) These pieces simply weren’t designed for modern conditions and suffer because of the unexpected stresses they must cope with.

   Modern furniture makers have to contend with similar problems—but we have kiln drying facilities available and modern dry houses to build for. Slainte.

    

   Richard Jones Furniture

2. 
   ring | Nov 01, 2008 01:06pm | #2

   Derek, what's important is not any given MC values that you mention. The true aim is to have wood that has reached equilibrium with its surroundings. Until it has come to that point of rest, it is still in the process of drying and contracting. We commonly use figures like 8-10% as benchmarks to aim for, but that is just a useful rule-of-thumb assumption about climate, winter heating, etc. Modern central heating has changed our assumptions and requires dryer wood. If you live in the tropics and do not heat in winter, I'd guess that 15% MC would be just fine.

   David Ring

   http://www.touchwood.co.il/?id=1&lang=e