Inadequate air humidity and insufficient material humidity result in the following:

• The joints at the corners of the frame loosen
• Delicate pieces of furniture and wood interiors bend
• The joints and lateral (cant) lines are separated
• Veneer joints break
• Unevenness and crevices appear on painted surfaces
• The tree swells when it is moist and bends when it is too dry
  
  

These dimensional changes do not occur evenly in all directions, since the tree swells and shrinks more tangentially than radially (in a ratio of approximately 2: 1).

Unequal changes in dimensions and extreme effects on hardwood in comparison to the soft surface caused by changes in the amount of water, lead to deformation and, often, cracking. Wood deformation naturally depends on the type of wood, the specific weight, the annual creation of the ring on the trunk and the direction of the fibers. By properly selecting cutting techniques and cutting schedules, shrinkage and swelling can be reduced.

The moisture content of the wood is equivalent to the moisture content of the air surrounding it, if it is not directly affected by moisture (for example, rain). The ideal moisture content in the wood during processing is 9-12%. These values are automatically maintained if the relative humidity is 50-60%. Therefore, there is an equilibrium in relation of moisture between air and wood. This relative humidity decreases or increases very little during the summer months, even indoors. The situation is different during the heating season. Relative air humidity drops regularly below 30 - 20% when air is heated. In situations where humidity is so low, especially if the humidity in the air drops rapidly, there is a constant risk of damage to wood (and wooden objects) during storage and processing.

Recommended values of relative humidity in the wood industry: