In-house vacuum kiln drying is an important element of our wooden tops manufacturing process. We also offer vacuum kiln drying as a service.
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 very effective and faster than conventional methods wood drying.
In the vacuum kiln, the water boiling point is significantly lower due to the pressure reduction allowing for an even moisture extraction whilst achieving the highest quality of kiln dried timber. The central fans at the end of the chamber distribute the air evenly throughout the wood stack via lateral aeration slots with constant cross-ventilation throughout the entire chamber length.
Our goal has always been the production of defect free English hardwood that is dried to a low, uniform moisture content and is stress relieved so the timber used for manufacturing our wooden worktops and tables is 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.
Vacuum kiln drying service
We provide kiln drying services for the following: waney edge timber, square edge timber, slabs (one long continuous length of timber with no joints and/or very wide plank of wood) up to 1200mm width, construction timber and cladding. There is no limit on the thickness of the wood, however please note the thicker sections will require more time for vacuum drying.
We vacuum kiln dry, but are not limited to the following species: afzelia, maple, balsa, birch, wild pear, pear, Brazilian pine, beech, steamed beech, sweet chestnut, alder, ash, European ash, English ash, spruce, European spruce, American spruce, Scandinavian spruce, hemlock, redwood, cherry, cherry tree, kosipo, larch, lauan, limba, lime, walnut, English walnut, American walnut, European walnut, jacaranda Brazilian rosewood, Amarillo, pine, elm, European elm, English elm, whitewood, silver fir, teak, hornbeam, wenge, cedar, London plane, acacia, douglas fir, yew tree, oak, American oak, English oak, European oak, red oak, white oak, larch, pine, willow, plum tree, Siberian larch, Siberian oak, abachi, fir, teak, iroko, sapelli.
The timber can be vacuum kiln dried to any desired moisture content. For each completed process of drying the certificate is issued confirming the moisture content at the beginning and the end of the process. This certificate will add value and therefore make your product more attractive to potential clients and buyers.
Over the last few years our emphasis has been on researching the way of improving the value and quality of the timber from English woodlands which can be now achieved by using the patented vacuum drying technology not previously accessible on UK market.
For those customers seeking to use our vacuum kiln drying facility, consideration should be given and details provided for the following:
It is only after full consideration is given to the above that we can calculate the cost and most effective options for your kiln drying requirements. 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.
Reasons for wood drying
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, maximising the profit for the seller. 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 selling price and workability.
Maximise the value of your fallen tree – understanding air drying procedures
You can use this guide to successfully take control over the first step of wood drying and invest in the future value of your timber. 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.
Determining moisture level
Timber will continue to lose moisture until it reaches an equilibrium with the environment around it (for air dried timber 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. Please note only timber below 10% moisture content is suitable for internal use, otherwise it will crack, split and may cause damage to other furniture in the house because of its uncontrolled movement. 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.
The difference between free water and bound water.
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.
Lower density wood generally dries faster than wood with higher density. Thicker and wider boards should be dried slower.
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 factors: temperature, relative humidity and the rate of air circulation.
Conditioning at the end of vacuum kiln drying
Steam conditioning can be used 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.