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Learning to Dry Wood, and Still Learning!

An old furniture maker once told me, “Wood is a living thing. It never dies. Elm’s the worst; tables have  been known to walk around the room on their own. The best way to dry the wood for a piece of furniture is to leave it in the room for a few months, to acclimatize.”
“Mum will never put up with all that walnut behind the sofa,” I replied.

I had turned four coffee table legs from some wood I had cut from some old bedstead legs, bought from an auction for 50p. They were 2” x 2” oak, and I managed to dig out a lathe that dad had made years before, get it working, and we made some old chisels and ground several files into scrapers to put some shapes on the legs. I do not advocate all of these methods and techniques, but it got a young lad started. I jointed the legs together with hand cut mortise and tenons, and was now ready to glue it up, but I decided to get the top first. I was eleven.

Dad took me to the local timber merchants to buy a piece of oak just for my first coffee table top. It was slightly over size, and just before I handed over my money, I asked the man if the wood was dry enough. “It’ll be good enough for you,” he replied as he took my only £10 note, without change.

I was allowed to take it into the spare bedroom to ‘acclimatize’ as I was told. The room wasn’t warm, and it wasn’t particularly dry, but after a few days, I just had to go and have a look. I was horrified to find the wood split at both ends towards the middle, and the board was starting to resemble the shape of a propeller.  That was my Grandmother’s birthday money wasted. I found out from my woodwork teacher that the wood was still too wet, even though I had put it up on some sticks, so that the air could circulate around it evenly. As the wood lost moisture, it split at the ends where the moisture could ‘escape’ from the end grain more easily than the sides.

These were a few of my first lessons in the movement of timber due to shrinkage, caused by loss of water (or Sap), I was on a steep learning curve, and running out of birthday money.

The problem with a tree is that when it’s growing, like all living things, it needs water to survive. This water travels from the roots to the leaves via the trunk and branches, where it plays an active part in photosynthesis, making sugars (and oxygen as a by product, which is great for the animal kingdom) from water and carbon dioxide using the sun’s energy trapped by chlorophyll.

Anyway, when the tree is cut or blown down, the cells in its trunk are full of water, rather like a sponge. We can help minimise this to some extent by cutting trees down before the sap rises (up the trunk) in Spring, in the Northern Hemisphere. This is also good for the birds, as they won’t have started making a nest in a tree that will not be there for much longer.

The aim of the exercise is to remove the water from the timber so that it will not bend, split, warp, twist (or anything else that my first coffee table top did) after it becomes a piece of furniture. As a timber merchant, it would be better still, if the wood didn’t do any of those unpleasant things at all. The aim of the game is to get the moisture content of the timber at the same level as its surroundings, i.e. ambient. The process is called seasoning.

Water in the cell walls and the cell cavities can be removed without a great deal of effort. If you put a green (freshly sawn) board outside on a hot summer day, you will remove plenty of water from the cells. However, this will work too quickly, causing cracks, splits, surface checks or even honeycombing (internal defects or collapsing within the board). By doing this, what you have done is removed too much water from the outside of the board, before the water from the centre has had time to balance out.  Equally the cut cells at the end of boards will release moisture more easily than the cell walls on the surface. This is why the ends split, and the ends of boards used to be painted or dipped in wax to slow the process down. These days only exotic woods are cared for as well as this during seasoning.

The trick is to season the wood gradually, allowing the wood to give up its moisture, without putting to large an imbalance between the outside, and the centre of the board (6% is recommended as a maximum in a dehumidifying kiln), causing stresses in the board, that will lead to unsightly cracks.

To get the best results in seasoning our wood we must control its shrinking process. A dehumidifying kiln will not only speed the drying process up in our damp British climate, but it will slow it down in a hot climate.

These days I tend to air dry the oak and ash before it goes into a kiln, as seems to be more stable, the turn around of the kiln is faster and I have plenty of air-dried wood to finish in the kiln, whereas when I first started, I was only cutting enough for a kiln load or two at a time (a cash flow thing). This also helps the electricity bill. There is no point wasting our earth’s resources, when nature will do it for nothing. However, nature in this country will not take the oak down to a moisture content of 9% or 10%, but we are looking into the use of solar kilns at the moment!

However, we must not overlook the precautions that we can take even in the damp British climate whilst air drying in relative safety (for the wood that is).

We always stack the timber on stout bearers, usually 4” x 3”, making sure that they are all flat and level with each other. If the stack is twisted due to the bearers being uneven, then you’ll never have flat boards. We also sweep the majority of the sawdust off the boards as this encourages mould that will stain the timber. It is a time consuming job, but worth it if you need to use the wood in its natural colour.

When we cut a tree trunk into planks, we first cut down the log’s length four times, turning it into a square, and removing the majority of the sapwood. I see no point in kiln drying sapwood, as it is not used for furniture making anyway. We can then cut boards off one by one until we have planked the whole log. The top and bottom boards may still have some sapwood on, and these can be used to form the bottom and top layer of the stack as they are poorer quality.

Moisture from the top and bottom layers will evaporate at a faster rate due to the outside surface being exposed to a greater air flow than the rest. This will cause uneven drying and result in small surface splits (checks) that you don’t want to happen on your prime boards. We save the best boards for the middle of the stack.

Going back to when were cutting through the log after we had squared it out on the mill, if you imagine the annular rings on the end of the log, then as you cut down through the log, the first third of the boards will be plain sawn, the boards in the centre third will be quarter sawn (the outside planks in this third could be classed as rift sawn, as they are not truly on the quarter), and the last third will be the same as the first, plain sawn. Quarter sawn boards are considered the best quality as the figure or grain is prettier. The easy way to remember it, is ‘Q’ for Quarter and Quality, while Plain is Plain. Quarter sawn boards are generally saved for the prime spots in the furniture (table tops and fronts), and the plain sawn timber is sent to the back of cabinets where it will not be seen as much.

Before we can put the second layer on top we must place some spacers (‘stickers’) between the boards. These should be softwood (less staining), and I have read that the thickness should vary depending on the time of year that the timber has been milled, ½” in the summer months and ¾” in the winter. The reason being that the narrower the gap between the boards, the less air flow there is, and the less evaporation will occur, and the wider the gap, the greater the air flow, and the faster the boards will dry.

Gradually we build a stack up and stop before it becomes unsafe. I usually only have them about 6’ – 7’ high. Once you have finished the stack, with more poorer quality boards on the top layer, you could build yourself a little roof to keep the worst of the rain off if it is outside. Many timber merchants just leave the stacks exposed to the elements, but you many find marks on the boards where the stickers were. These can be planed out, but you may find it necessary to remove a couple of millimetres before they are no longer visible.

I have been storing timber in stacks under a poly tunnel frame, covered in a tarpalin, but the wind soon rips the sheet into rags. These days I am storing most of my oak and ash boards in packs, either outside, or in a large shed, away from the prevailing wing, so that the stacks don’t dry out too quickly and split but, I am using ¾” stickers all the year round. We have been cutting the oak and ash into 100” lengths and banding them up in packs so that we can move them easily with a forklift. This includes in and out of our dehumidifying kilns.

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