Cherry, Baked Well?

Nic Westermann explores the issue of green wood shrinkage

The main rule I have as a toolmaker is not to make any tool I can’t use myself. I have been slowly developing a double-edged twca cam – Welsh for bent knife, the literal translation though is ‘knife, bent’. It has been a slow process, but over six years I have finally homed in on a design and aesthetic that I am really pleased with. However, although online tool sales are often driven by looks, it is still essential that the performance matches those looks and the only way to really know is to use the tool extensively. It’s a tool that excels in deep hollows, forms you would normally need a left and right-handed bent knife to cut with the grain in all quarters of the hollow. I started carving with one of these ‘final’ prototypes at the Harrogate show last year. I hollowed out a small pot and then finished the outside with a straight knife.

I was pleased with how the tool performed but it quickly became apparent that very subtle changes to the edge geometry affected the way it cut and so another, new range of final prototypes was made. At this point I made a decision that rather than endlessly hollowing and discarding blanks I would finish them all.

This proved to be a really interesting diversion over the long winter evenings and gave me a chance to use more of the tools I make, namely axes for roughing the outside and then a variety of straight blades for refining this shape. I enjoyed putting a tiny rolled rim on the later versions. I know this is uncommon within the pages of this magazine, but I was always carving green, or at least wet, wood. I find it is so much easier to work with tools that are pushed rather than struck, so it is much more enjoyable than battling with dry wood.

However, I then came up against the issue of shrinkage when the wood dried and this is really what I am going to talk about. I’ll limit this to the pots but obviously it can be related to anything you carve green – any design will change shape as it dries. Although I have a background that touched on plant biology, I’m not really going to talk about the wood’s cellular structure (i.e. why it shrinks the directions it does) but will look at it in a more experiential way.

Wood, like all organic materials, shrinks as it dries, but it is maddeningly non-linear, in both the rate and direction it moves.

When wood is very wet it has free water and removing this has virtually no effect on the dimensions, so no movement is noticed until the fibre saturation point is reached, at approximately 25% then it will start to shrink. The water that is left at this point is chemically bound to the wood and it takes increasingly more energy to remove – over 200% as much is needed at very low percentages than at 25 %. This means that although wood does actually shrink at a fairly uniform rate with decreasing moisture content, the moisture content itself will not decrease linearly. Wood is also hydroscopic so very dry wood will slowly regain moisture if kept in a room at a higher humidity.

Wood will shrink in three directions typically – lengthways -0.5%, radially -5% and tangentially -8% in hardwoods. In all woods, tangential shrinkage is greater than radial – this is why wood left in the round, a branch or trunk of tree, will tend to split. It also means that even if you are carving into end grain, your carefully scribed circle will end up oval, the degree varying from species to species. If you are carving across the grain, the effect is even more pronounced as you are getting effectively zero shrinkage in one direction and up to 10% in the other, as opposed to the difference between radial and tangential shrinkage.

Other factors which affect shrinkage include the state of the wood. Wood which is spalted although wet seems to have lost quite a lot of structural integrity and tends to shrink much more evenly. There is also some anecdotal evidence that if you soak wood in oil it will shrink less as you dry it as you are now using the oil to keep the cells inflated rather than water.

The way you dry wood is also important, leaving it on a radiator or a sunny windowsill will often be asking for trouble. It’s undoubtedly best to dry it very slowly. If you dry quickly in a warm room with a rapid turnover of air the piece will tend to dry unevenly, with the thinner sections drying and shrinking first, setting up further stresses which can lead to cracks. Generally, it is sufficient to leave in a cool, dry space to dry, but if you are having issues with cracks or just really want to be safe then try the following method to allow the moisture content of the wood to continually stabilise as it dries. Wrap your carving in dry kitchen roll and put it in a plastic bag. The kitchen roll will absorb the moisture but not let the wood dry further. Every few days you can swap the paper for a dry sheet and set aside the wet sheet to dry. The number of cycles will vary on size and how wet the wood is, but eventually you will notice the paper coming out as not noticeably damp, and then it’s safe to leave it in a cool, but dry room to fully air dry. It’s long-winded but the safest method I’ve found.

In terms of pots, an even but thin wall thickness definitely seems to help with allowing the wood to move as it changes shape – 3-4mm is often cited as the optimum, but my pots tend to be thicker at the base, up to 8mm, and thinner at the rim, down to 1mm. I suspect this accounts for some of the uneven warping I’m getting on drying, but I like the stability and feel this gives them in the hand. If the worst happens and a crack appears you can often save this by re-wetting the pot, which will tend to close up the crack, then dry it slowly until the crack is just starting to become visible, then run thin superglue in to seal it and dry again very slowly – I’ve used this technique with larger bowls but not had to with any of these smaller ones.

On one hand I was just trying to test and acquaint myself with these tools, but the form of the pots I was creating was important. I wanted to produce round forms. If I carve a round pot it will end up oval as it dries. When I first started on these forms, I was working with spalted alder and worked so slowly that as I refined the shape as it dried and moved I could keep it round. I recut all the surfaces numerous times. Eventually, as I got quicker, this grew tiresome and I started trying to do the finishing cuts while the wood was still wet.

It became increasingly important for me to carve ovals to end up with the form I wanted. If I had been patient and did a different job, this would have been relatively easy. However, with my day job as a tool maker my hands were never clean enough not to mark the wood, especially as it was still green.

By the time I had recut the outside I had marked the inside and vice versa. Gloves were, of course, an option but I don’t feel I work as well with them, and this is a hobby activity not work, so I’m happy not to take the most practical route.

In despair at one grubby pot, I decided to bake it to see if that would hide the grime. It really did and I find it an amazing technique as it provides highlights in a way that a dye or stain wouldn’t. The heat also burnt the thinner rim more and gave the impression of age.

I make sure the pot is as air dry as I can get it first–being impatient, I put it in the microwave once drying has slowed (remember the increased energy needed to get to low moisture contents, below 10%?). I put the pot in the microwave on a medium heat and give it 15 to 30-second cycles – wearing gloves to take it out and let it cool slightly. You will see and hear steam being given off as the bound water boils out of it. This steam permeates the wood and softens it, making it less likely to crack. If you overdo it, you will quickly smell the wood toasting (to me this is reminiscent of chestnuts roasting) – this is the time to stop, well, probably past the time to stop but in practice it’s still OK. While I have never set fire to wood in a conventional oven, I have seen it done in a microwave. I wouldn’t really recommend a microwave for this reason – it does nothing that slow, careful drying won’t. It also seems to make the pots dry rather unevenly. They may not crack as they are softened by the steam but they do move, often in unexpected ways.

Oven baking, though, is another matter – the colour changes are, for me, magical. A very thin layer of oil seems to help with this – sometimes I get purple out of cherry. This I have been roasting at temperatures from 200°C up to maximum of 250°C. If you have just air-dried the piece I would start at a much lower temperature and bring it up slowly, but I have been going straight from the microwave into 200°C. If you do oil the pot before baking – I’m using tung currently – it tends to exude the oil as it heats up, so it needs wiping off before it dries fully and mars the surface. This can take quite a lot of force and I have twice cracked pots wiping them down. In the next article I will explain the process I use to repair these cracks.

I put the pot in baking tray on a sheet of kitchen paper so it isn’t directly exposed to the flame-heated air. I am using a gas oven, and these produce a little bit of water as they burn so you may find that in an electric oven, which is intrinsically drier, you need to start at a lower heat. The pots will darken most where they’re in contact with the baking tray, so I tend to have them upside down so that the rims darken more. All I do is slowly increase the temperature, by 10° every five minutes or so and observe the colour change as I take the pot out to wipe it down. I just turn the oven off when I am done, put the pot back and let it cool slowly in there. I have yet to crack a pot in the oven – yet I have cracked a few, this is largely a function of how thin I carve the walls of the pots. They are 1-2mm thick and very fragile – research shows that woods can become four times more brittle when heat-treated at these temperatures. This is thought to be due to the sugars in the wood degrading and is, I think, also largely what causes the colour change. These sugars are caramelising, although at the highest temperatures I think I am more into a range where burning is taking place – like the microwave, not something I would recommend, so I would start with a light toasting around 200°C.

There is, of course, the question of how dry the wood needs to be – 6-9% is generally taken to be the norm for woodwork indoors. You can measure this directly, moisture meters are relatively inexpensive now, but that generally involves sticking two spikes into your carefully carved work and is not something I relish. On larger carvings I weigh them, and it is fairly easy to watch the weight loss drop and stabilise. You will need accurate scales for smaller carvings, although in practice smaller carvings will dry so quickly it’s rarely a problem. Some experienced carvers say they can feel when the wood is dry; it will be warm to the touch. This makes sense as the insulating properties of wood are much better when it is dry – high water content will conduct the heat away from your hand. It’s not the most accurate method though for certain.

When microwaving, it’s obvious when you first take a pot out you can often feel a ‘wet’ heat radiating from it. Use common sense with this – don’t poke a finger in a plume of steam, you can assume it is still wet. If you are putting it in the oven, you can also be certain it will come out dry.

If we know that the pot is going to shrink and we know we can mitigate this by carving an oval, how do we actually do it? Well, one way is to look up the shrinkage rate of the wood you are using – there are lots of online resources with this information

expressed as a percentage. Often longitudinal shrinkage isn’t given as it’s so negligible. In the case of the cherry, I’m using theory that says 7.1% tangential shrinkage – 0.5% longitudinal = 6.6% extra needs to be added to a circle – if it was 100mm, then the side across the grain (tangentially) will be 106.6mm, the side along the grain (longitudinally) 100.5mm.

In practice this is only a starting point, so keep a record of what you have actually done and adjust the next one, try to have enough wood from the same tree and grain direction or you will find yourself starting from scratch each time. There are various ways of producing these ovals, but the easiest for me was a glorious mix of high and low tech. I have an expensive CADCAM program so it was easy to produce a set of correctly proportioned ovals. I make three – the inside of the rim, the outside and also the final circumference – to make sure the blank is the right size. However, I didn’t have a printer at home, so I turned up the brightness as much as possible then taped a thin piece of paper to the screen and traced around it, cut out with scissors, and drew around these to set my overall dimensions of the pot. I’m sure you can think of a better method.

Air drying is much more forgiving than microwaving and baking and it’s not perhaps as hard as I make out, and I would certainly recommend this before jumping into baking, but it is such an interesting effect I can’t resist and will embrace the inevitable cracks when they occur. I think it’s important to be aware of shrinkage and I’m surprised that more people don’t take account of it when designing a green wood piece. There is a huge amount of effort involved in getting the perfect form for your work, so it makes sense to factor in shrinkage, even if it is still our best, but now informed, guess.

PHOTOGRAPHY BY NIC WESTERMANN