Kiln drying below 10% moisture content is a pivotal part of manufacturing process for the timber destined for internal use, essential if you want to enjoy high quality, long lasting wooden surfaces.
Our timber vacuum kiln drying plant is a unique, patented technology, water evaporation and air circulation system, which is responsible for the most gentle yet the most effective method of wood drying. We produce expertly dried, with uniform moisture content English hardwood that is stress relived. This process guarantees that our high quality wooden tables and worksurfaces are dimensionally stable and free of defects associated with the drying process (such as longitudinal distortion, cupping, splitting or staining).
Quality of wood drying cannot be underestimated as it determines the quality of the final product. It is the most significant, requiring skill, care and understanding timber properties, element of the wood manufacturing process.
We also offer vacuum kiln drying as a part of the ‘fallen tree to the table’ projects.
We specialise in vacuum kiln drying homegrown timbers and European hardwoods such as but not limited to the following species: maple, birch, wild pear, pear, sweet chestnut, alder, ash, European ash, English ash, spruce, European spruce, cherry, larch, limba, lime, walnut, English walnut, European walnut, European elm, English elm, silver fir, hornbeam, cedar, London plane, acacia, douglas fir, yew tree, oak, white oak, English oak, European oak, red oak, willow, plum tree, Siberian larch, Siberian oak.
The planks for all our projects are carefully kiln dried to the desired moisture content 10% or less. For each completed process of drying the certificate is issued confirming the moisture content at the beginning and the end of the process.
Being precursors of this patented technology not previously accessible on UK market we are indeed proud of our knowledge, experience and value we add to the timber from English woodlands
– We will ask if you followed Tree Preservation & Forestry Commission regulations and had permission for felling or pruning the tree
– We will consider when we can dry your wood species (please note different species cannot be mixed in the kiln, unless they have very similar properties. If they are varied, they may have to be dried in separate batches)
– The timber thickness (please note different thicknesses cannot be mixed. If they are varied, they may have to be dried in separate batches)
– The initial and required moisture content. Have the boards been sawn during the same season or have you got a mix from different years, if so the moisture content varies significantly from board to board. If you haven’t got an accurate moisture reading than the estimated time the tree was fallen and the time it was cut and stacked for air drying will be useful. We will take the precise measurements once the timber is delivered to our site. Please keep the boards cut during different seasons separated.
– The detailed list of all lengths and widths of the wood planks or wood slabs.
It is only after full consideration is given to the above that we can calculate the cost and most effective options for kiln drying requirements of your own wood planks. Please note that if the timber already has defects due to incorrect storage or incorrect air drying process the problem areas may deteriorate further during the kiln drying course.
Beautiful English hardwoods can be used for making bespoke furniture as well as joinery and construction purposes. However, in order to use the English wood to its full potential the timber must be adequately dried before it is further processed. Correctly seasoned and kiln dried timber can be used in many more settings than freshly sawn timber. Kiln dried timber will also command a substantially higher price. Fresh cut timber contains up to 90% of water. If the water is not removed , the wood cannot be used to produce a high quality finished product. When timber is correctly dried, it machines, glues, stains and finishes better. Drying also maximises the strength of the wood, as the technical properties of timber increase as it dries below 25 % to 30 % moisture content, preventing the unacceptable shrinkage after installation. The correct wood drying kills wood infections, reduces vulnerability to fungal decay as the wood reduced to 20 % moisture content is unlikely to be attacked by wood decaying fungi. Kiln drying increases the effectiveness of preservative treatments as most of the preservatives should only be applied when the moisture content of the timber has been reduced. Kiln drying is also preventing the corrosion of metal fixings in timber. The moisture content of the timber used for construction purposes has to meet legal requirements relating to the use of structural timber in buildings.
Please bear in mind, the timber that is not dried under controlled conditions is prone to warping, staining, and other degradation that diminishes its application and workability.
You can use this guide to successfully take control over the first step of wood drying and invest in the emotional and future financial value of your timber planks. Please remember to follow the Forestry Commission and Tree Preservation Order regulations and seek permission for felling or pruning before you commit to any work on your tree.
Air drying is a low cost and low energy method of drying timber. However, conditions must allow the timber to dry slowly and evenly to avoid degradation. If dried too fast, cracking, collapse and deformation may happen. The initial stages of drying are the most important to avoid degradation of green sawn timber.
Green sawn timber should not be left block stacked, especially when its warm. Fungal discolouration of the timber can occur very quickly, especially if sapwood is present. Tree trunks should be protected from sun prior to cutting into planks.
Stacking the timber for air drying requires great care and attention to detail for good results. For removable foundations a perfectly level base of bearers must be prepared. There must be sufficient space between the ground and the top surface of the bearers to allow for plenty of air movement under the stack (200 mm will be adequate; 300 mm is ideal). Weeds should be removed. There should be a minimal ground moisture and good surface drainage. The two end bearers should be levelled.
The ground must be firm and compacted so that it does not sink under the weight of the timber stack. Spacing between bearers should reflect the type of timber being stacked, but is usually around 1000 mm, with bearers under every line of planks. When the timber is stacked the planks must be lined up vertically above each other from the bearer below. Attention needs to be paid to keeping the width of the stack consistent and without any lean. Outer boards should be placed first and lined up carefully.
Timber should be covered during air drying. If the stack is not covered, the water will penetrate into the stack and cause plank stains and discolouration of sapwood leading to decay. Often sheets of corrugated iron are used but these must not leak. Drying sheds are expensive but effective. What must be prevented is water penetrating into the centre of the stack where air-movement is minimal and the timber will stay wet. Covers will need to be restrained to prevent uplift.
Covering the stack also allows the timber to dry evenly. Timber nearer the top of the stack is more likely to dry too quickly and thus requires a higher level of protection from sun and wind. Covering the sides with hessian or paraffin will help to slow the airflow. Building a group of stacks in close proximity also reduces airflow.
Weighting the stacks helps reduce distortion in the top layers of timber. Weighting must hold down the timber evenly over the top surface of the stack. Waste slab wood can be used or special concrete blocks can be made.
Seasonal weather differences should also be taken into account when drying timber. Drying can take place too fast in summer and too slow in winter. In summer heat can build up under the cover and dry the top layers of timber too fast causing warping and splitting. It is advisable to weight the stack well and use a layer of buffer material such as slab wood on top of the stack and under the iron. The top two or three layers of boards should be lower quality seconds. Overhead shading is beneficial for summer drying, such as that provided under trees. Shadecloth can be used for the sides of the stack to slow airflow and drying. During winter drying of timber is slowed, but provided the stack is covered well so that rain does not penetrate and there is reasonable air-flow through the stack, the timber will not deteriorate.
Some durable timbers can be left stacked in the weather and in fact can be conditioned this way in order to slow the drying process. In general, however, sun and rain are not conducive to drying of timber.
Please note the moisture content should be measured in the deeper layers of the plank (half way through). Surface moisture meters (pinless moisture readers) will not give you a valid reading! This is a very important fact to consider especially with very thick or/and very wide timber slabs. So please take this basic care at the beginning of the process and buy your wood from a reliable source if you want to avoid disappointment of the table or breakfast bar moving uncontrollably (incl but not limited to warping, twisting, cracks, splits and potential damages to the neighbouring furniture) in your kitchen or dining room!
Timber loses it’s moisture naturally until it reaches an equilibrium with the environment around it (for the timber air dried under the roof maximum 17-18%), at which point it neither gains nor loses moisture. This is called the equilibrium moisture content. The EMC will change as the relative humidity and temperature of the surrounding environment changes. Timber dried to 17-18% moisture content is suitable for construction industry and external furniture.
Once the moisture is below 30% the timber can be kiln-dried. If air dried to 20% MC the timber can be block-stacked indoors without risk of decay. For accurate readings wood must be clean, with no decay. A general rule of thumb for air drying is one year for every cm of thickness.
Lower density wood generally dries faster than wood with higher density.
During the lifecycle of the tree the liquid water responsible for exchanging moisture with its surroundings moves through the cells of the wood. This water is called “free water” because it exists in water form and can be removed relatively easily from the wood.
“Bound water” is water that becomes part of the wood fibre itself, and is more difficult to remove. When wood is dried, the first thing that happens is that the free water evaporates until the timber moisture content drops to what is called fibre saturation point. Fibre saturation is generally reached when the moisture content gets to around 28%. At that point, all the free water is gone and only bound water remains. Wood does not shrink until it is below fibre saturation and the bound water begins to be removed from the cells of the wood.
Kiln drying below 10% moisture content is always essential for wood intended for interior uses, with the special attention given to super wide and super thick boards as they are the ones that can cause the most serious problems if the process has not been executed, or not sufficiently executed including but no limited to wooden slab warping, twisting, swelling, encouraging the infestation of woodworm. Also the surfaces and cabinets in direct contact with timber regardless of composition (mdf, timber plywood etc.) could potentially be affected and damaged. Unfortunately these problems can take weeks to manifest themselves, by this time your warranty for the product may expire.
Thicker and wider boards (slabs) should be dried slower and require individual approach, each drying procedure requires very close supervision and frequent adjustments so the process is time consuming and rather demanding. The rate at which timber is dried in a kiln must be carefully controlled by air temperature, humidity and air flow. To avoid excessive evaporation from board surfaces the air in the kiln must be kept relatively humid.
Generally speaking, difficult species and thick sections should be dried slowly because higher rates of drying will develop greater stresses in the timber which may cause cracking and distortion. Moderate temperatures and adequate humidity levels allow timber to dry slower. Higher temperatures and higher levels of moisture extracted mean faster drying but with a higher risk of degradation.
Once below 30% MC, wood shrinks as it dries. The rate at which moisture is evaporated from the surface of the wood must be controlled so that it equalises with the rate at which moisture moves outward from the inner wood. The drying rate is controlled by three crucial factors: temperature, relative humidity and the rate of air circulation.
Conditioning at the end of vacuum kiln drying
We always condition our wooden planks at the end of the vacuum drying process to offset stresses caused by moisture gradients in drying. A high temperature, high humidity treatment period at the end of drying will relieve such stresses.
Desired moisture content
Kiln drying allows for a final predetermined moisture content, usually the equilibrium moisture content (EMC) for the intended end use. This is to minimise dimensional changes in the wood product in service. The equilibrium moisture content will vary depending on use. For example, the in-service average EMC is between 10-12% for interior uses in England. However, this depends on temperature and humidity which may vary, such as in dry centrally heated houses or permanently air-conditioned buildings.
The rate of drying is influenced by different species, the way the timber is sawn (e.g. quarter sawn or flat sawn) and different thicknesses. It must be kept in mind that timber does not dry at a linear rate over time. More energy is needed to remove each additional unit of water as the moisture content falls.