Furniture factors

aegistra offers you some interesting and useful information and factors about furniture.

 

  • Timber movement

     

  1. The basics

 

  • Timber expands and contracts with fluctuations of moisture (more than with variations of temperature)
  • For example, an oak table top 1m wide (if made when the moisture content is at 20% will shrink by 2% (or 2 whole centimeters) when the moisture content is reduced to 12%).
  • This process is most pronounced as a new or refurbished building dries out or, once settled, according to the season and the heating provisions, central heating in the winter months will cause severe shrinkage (as will air conditioning)
  • Timber chairs or timber tables in institutional buildings without air conditioning or heavy central heating (such as churches or college halls), will stabilize at between 10-12% moisture content; modern office buildings with air conditioning settle down at 7-9% moisture content
  • As a rough guide, timber stabilizes as follows:

 

Roof timbers:                                                         15-17%

 

External joinery:                                                     15-17%

 

Suspended floor:                                                   16-18%

 

Wood floors:                                                             9-12%

 

Wood floors on under-floor heating:                           6%

 

Internal joinery:                                                         8-10%

 

Joinery and furniture in modern offices:                  7-9%

 

  • Typical problems for office furniture chairs and tables (prevented by expert design treatment) include the opening of mitred joints, binding of doors, “telegraphing” of solid lips through veneered surfaces, veneer shrinkage cracks, cracks in solid panels.

 

  1.  When should these problems be most expected?

 

  • In new buildings where wet trades have been used (plastering or screeds)
  • In the upper part of the building which tend to retain a higher moisture level for longer than the bottom (since hot air rises)
  • When timber sections are unnecessarily wide (such as solid table tops or excessively wide boardroom table mouldings) in newly air-conditioned offices
  • When there has been inadequate measurement of moisture content when selecting timber for a particular job
  • When different types of finishing material are used on a top or its underside (or even different number of coats) or by action which allows moisture to penetrate or escape unevenly from each side (e.g. polythene wrapping on one side only); the less permeable side will shrink quicker than the other causing the timber to cup.

 

  1.  What are the solutions?

 

These problems are best avoided by:

  • Using the most appropriate “cut” of the timber by specification or selection 
  • Leaving adequate time in manufacture for both selection and extra drying
  • Minimizing wooden moving parts (e.g. doors and drawers)
  • Keeping solid sections to the smallest, feasible dimension
  • Specifying the best combination of solids and veneers
  • Through attention to design details (e.g. hiding likely movement behind shadow gaps or mouldings)

 

  1. Typical distortions?

 

The basic rule is that drying boards tend to cup away from the tree’s original heart. This means:

  • Growth rings at right angles to the surface cause minimum shrinkage and distortion
  • Perpendicular rings give optimum stability – a handy tip when selecting
  • Square sections with diagonal rings tend to go rhomboid

 

  • Other facts about solid timber

     

  1. log conversion

 

  • “through and through”

this produces one or two boards of quarter sawn wood but the majority of the boards show contour markings; these boards are more susceptible to distortion

  • “billet sawn”

a similar method for large diameter logs or where there is a large central shake in the heart of the tree

  • “plain sawn”

this boxes out the heart but does not provide a good yield of stable material

  • “quarter-sawn” (the traditional way)

this enables all the boards to have the growth rings perpendicular to the face giving the most stable timber; it is rarely used by large commercial mills because it is time consuming and wasteful

  • “quarter-sawn” (the modern way)

two through and through cuts are first made to box out the heart of the tree, then the remainder is cut into narrow boards which yields a reasonable percentage of true quartered boards.

 

  1. appearance of solid timber parts

 

The most common effects are known as “radial” or “tangential”.

  • a tangential or crown-cut board give the effect of a pointed loop in the centre of the board with straighter grain at the edges
  • a radial or quarter-cut board gives a straight grain, sometimes with a pronounced stripy effect; some timbers such as oak produce a specially attractive feature – medullary rays

 

  1. colour changes

 

All timbers change colour after they a re worked once the revealed faces are exposed to both light and air. Dark timbers tend to get lighter, light timbers tend to get darker. They all change with time. Exposure to direct sunlight is always likely to have a dramatic effect on the timber colour; they are all bleached by sunlight.

Some timbers (e.g. American cherry) are extremely photo-sensitive and will change colour rapidly in the first year or before settling to a warm, mellow shade. If exposed to direct sunlight and partly covered (say, by a magazine), the timber will reflect a distinctive outline within minutes. The cure is further exposure to the light source but the short-term effect can be unsettling.

There is no known prevention for this phenomenon though certain stains mask some of the effect.

 

  1. wastage

 

 Average percentage waste figures depend on the timber, the quality of judgment of those who select it, the type of cut (e.g. “through and through” boards yield more to the sawyer than to the end user).

Designing furniture to get the most economic return out of standard stock sizes can be an art form. This is especially the case for curved components. The same is true for joinery.