From DT Online
Over half the weight of a growing tree is water or Sap. Freshly Felled or Green timber with such a high Moisture Content (MC) is susceptible to attack by Fungi and may crack, warp and twist if dried out too quickly.
Timber with greater than 20% MC is prone to decay and shrinkage starts as the timber dries below approximately 30% MC.
Seasoning is a controlled drying process to reduce the Moisture Content (MC) of the Converted timber until it approximates the surroundings or environment where it will be used. This Equilibrium Moisture Content (EMC) and ranges from10% to a 20% approximately, dependent on whether for interior or exterior use.
The most common method of checking Moisture Content is to take advantage of how moisture affects Electrical Resistance. An electrical resistance meter, or Moisture Meter, (specified in BS EN 13183-2: Moisture content of a piece of sawn timber. Estimation by electrical resistance method), has two probes which are pushed into the wood, about 300mm from the end, and a resistance reading taken which is automatically converted into Moisture Content. Other types of meter are available which avoid damage to the wood surface.
More traditionally, Moisture Content is measured by taking a small wood sample and weighing it. The sample is then baked in an oven set at 1000C and checked regularly until no further weight loss occurs. The final weighing gives the dry weight and Moisture Content is calculated as follows:
Vitruvius, a military engineer who served the Roman army under Julius Caesar, specialised in the construction of Siege Weapons for which he needed to source large timbers. He was concerned about what we would now call Moisture Content and Seasoning before use to reduce the risk of cracking and distortion. In his book De Architectura he states: In felling a tree we should cut into the trunk of it to the very heart, and then leave it standing so that the sap may drain out drop by drop throughout the whole of it. . . . . Then and not till then, the tree being drained dry and the sap no longer dripping, let it be felled and it will be in the highest state of usefulness. In other words, the tree is killed, but left standing until dry enough for use.
There is evidence (e.g. in De Re Aedificatoria written by Leon Battista Alberti in the mid-15th Century) that this method was practiced also in Medieval Europe but this is not to say that all timber was Seasoned before use. The large timbers needed for house construction would have taken years to Season naturally and Green timber is easier to work. A high proportion of the timbers in medieval buildings were riven or Cleaved - additionally, joints were not only easier to cut into Green timber but may shrink and tighten over time (ref: ‘Traditional Timber Framing - A Brief Introduction’ from University of the West of England).
Note: It is probable that the cracks, bends and twists we now see in the timbers of old buildings which have survived, are the result of the timbers ‘Seasoning in situ’ as it were, and have probably been a feature of the building since early in its life, requiring constant patching up and filling with plaster etc.
Natural or Air Seasoning is the traditional method of reducing the Moisture Content in wood. After Conversion, boards of timber are stacked on open-sided sheds which are protected from sun and rain but through which air can circulate freely.
The ends of boards are painted to avoid splitting by drying out too quickly, then stacked one above the other with thin strips of wood known as Stickers separating them to allow air to flow through. It is important the Stickers are placed vertically above each other to avoid undue warping or bending of the boards. Stickers are up to about 20-25mm thick and normally spaced about 700mm apart, but both their thickness and spacing can vary to control the air flow according to the timber species and sections. Boards are also separated from each other along the Stickers. Each timber stack is kept away from its neighbours and raised off the ground on bearers or piers.
Natural or Air Seasoning can reduce Moisture Content to 15-20% depending on local climatic conditions - but it takes time: 25mm thick Softwood boards stacked in Spring may reduce to 20% MC after 2-3 months and 50mm thick boards 3-4 months but these times are extended if the boards are stacked in Winter. Hardwood boards are in fact best stacked in Winter so that the initial drying is slower: 25mm boards could be ready the following summer but 50mm boards may take a full year or more.
Following Natural or Air Seasoning, any timber which is to be used for interior work should be stored inside the workshop for a period of time before being used to reduce the Moisture Content further.
Kiln Seasoning or Conditioning (aka Artificial Seasoning) is the method now most commonly used, especially for Softwood, because it is quicker and more economic. Care must be taken not to dry out timber too quickly otherwise the timber may split and crack or have severe stresses induced into it. Kiln Seasoning is often used following a shortened period of Natural Seasoning to help avoid this - especially for Hardwood.
Kilns are essentially brick-built chambers where temperature and humidity can be controlled using a combination of heating coils, fans and steam jets. Timber is stacked as for Natural Seasoning is placed in the kiln which is then closed and a drying programme suited to the particular timber species is initiated. Humidity is high at the start, to help avoid surface splits as a result of too rapid contraction, then is gradually reduced and temperature increased until at the end, there is a flow of hot dry air.
Kilns are either static Compartment Kilns where the timber stack stays still and is exposed to a changing humidity/temperature programme, or Progressive Kilns in which the timber stack slowly moves through a changing humidity/temperature environment.