The Moisture Content of the Wood

moisture content of the wood vs. humidity air graph

The moisture content of the wood is very critical in the wood product. The improper moisture content always leads to many problems in the wood product. The controlling of the wood moisture content of the wood in the woodworking industry is very crucial. Theoretically, the moisture content of the wood at the time of manufacturing and finishing should be approximately the average moisture content it will attain in use or slightly less. The humidity of the air surrounding the article in use should therefore govern the extent to which the wood is dried. And then conditions in the shop should be maintained at this moisture content level. This is particularly important in the manufacture of furniture, cabinets, or other wooden products in which shrinking and swelling must be reduced to a minimum. While the locality in which the finished article will eventually be used will have some bearing on the relative humidity of the atmosphere.

The effect of the finishing layer on the wood moisture content.

The variation in moisture content is reduced considerably by the finishing coat of paint, varnish, enamel, or lacquer. It is a common observation that when cracking or checking in of the finish takes place on wood products due to the change in moisture content, the cracking may start from the uncoated edge and become more severe near the edges toward the center. The finishing layer will wrap the wood, close the pores and grain, and make it more stable. An inexpensive coating can be applied to the backs of the furniture or other wooden product to make them equal in moisture resistance to the face surface coating. These coatings for the back can usually be applied in 1 or 2 coats depending upon the color and quality requested.

Any changes in the moisture content of the wood will result in a movement of the wood surface under finish, and if repeated frequently or if of sufficient magnitude, the shrinking and swelling movement will result in serious problems for the finish. That is why the controlling of the moisture content is very important in the woodworking process. Watch the moisture content of the wood before finishing, the relative humidity and temperature of the department during the process of manufacture, finishing, and storage, and the drying time allowed for the evaporation moisture added to the wood by gluing, sponging, and staining operations. Keep the wood moisture content to the proper level.

Wood that is too dry (when it is finished) is as bad as wood that is too wet. For either case, a change in moisture content will result in a movement of wood under the finish. Dry the wood to the proper moisture content for its ultimate uses and keep it at that moisture content during the construction, finishing, storage, and use. Then many of the finishing troubles now experienced will be reduced or eliminated entirely.

These are the recommended moisture content for some wood product.

Product                                                                  final MC %

Furniture                                                                     5 - 7

Interior woodwork                                                      6 – 8

Flooring                                                                      5 – 7

Vehicle stock, except wheel and box part                 15 – 18

Vehicle wheel and box parts                                         8

Gun stock                                                                    6 – 8

Aircraft                                                                         8

Army                                                                            12

Outdoor sporting good                                                10

Musical instrument                                                      5 – 7

Miscellaneous outdoor material                                   12