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Friday, 14 October 2011

Green Wood?

One of the first questions people ask me on courses is how green the wood needs to be, this seems such a strange question to me now. But as a child turning wood on an electric lathe i used to turn green wood in the round (wood that has not been split in half or quarters but has been left whole with the pith running down the centre)  beautifully long (40/50 foot long) and dripping wet shavings would stream out the end of my roughing gouge and make a heap on the floor, i would take whatever object i had made inside with the knowledge that over the next few days it would dry and split. But that was ok because for real projects i would order wood from craft supplies (the Home of Woodturning), they would send various shapes and sizes of different woods from European Cherry to African Pink Ivory. Their wood was magically square sided and have wax on the ends, unfortunately it took several years before i understood that shipping endangered species across the planet wasn't what i wanted, and that virtually all the wood growing in this country has a use in woodwork and is very simple to convert to useful wood.

When wood is living it is full of water, probably something like 50% of it's weight, as it is worked into some form of product such as a spoon or a plank water evapourates from it's surfaces and depending on the dryness of it's new environment the product will continue to lose water until it reaches some kind of equilibrium.

As the wood dries it skrinks, gets harder and is less readily split along the grain. Because wood is made up of tubes the water evaporates from the ends of the tubes more readily than the sides this is why the wood from Craft Supplies came with wax on the ends. The wax prevents moisture leaving the ends of the tubes and therefore the moisture has to leave through the sides this is a slower and more even process. If the moisture is allowed to escape from the ends of the tubes that wood will skrink faster and may cause splits to occur this is notable on the ends of logs and can be prevented by painting them with wax/pva.

Wood skrinks more tangentally than radially (radially is from the pith outwards) and virtually no skrinkage occurs longitudinally. Tangental skrinkage is often twice as much as radial and in some species as much as five times. This differential skrinkage causes wood left in the round to split. If the wood is cleft into quarters and then turned round when the wood skrinks it should not split but will skrink to an oval cross section.


Green Wood Pros
It is readily available (grows on trees)

It is easily cleft along the grain into usable blanks
It is softer so is faster to shape and is kinder to razor sharp tools

Green Wood Cons
The wood skrinks so that whatever is made green will be a slightly different shape when dry (this varies considerably between species)
Depending on the species the surface of the wood can oxidise (not sure whether it is actually oxidising) this changes the colour of the wood, if the product is then reworked when dry it will have a different colour making it patchy.

Seasoned Wood Pros
In theory no skrinkage though there will usually be some change this is much less than Green Wood.
Dry wood is hard and therefore a razor sharp tool can leave a smoother surface than on Green Wood.
No colour change.


Seasoned Wood Cons
Expensive
Not easily cleft
Harder to work
Blunts tools quicker


Fritiof made a big point that when doing fine knife work like signing a spoon it is much easier when the wood is bone dry. In fact all fine work is better done when dry this is because the wood has much less "give"/flex when you try to cut it. If the wood flexs half a millimetre when you are trying to cut a quarter of a millimetre wide incison it is not very good. Fritiof was not a fan of half dry wood, he liked the wood wet and to either rough shape with an axe and then finish when it it is bone dry or to start and finish all in one go.

When i sign my work i tend to just make 2 incisions but there are times when i  make 3. I'm not certain what Fritiof did but i'm fairly sure he was using 2 cuts. The incisions he was making seemed almost vertical, at some point i'll do a long post on my engraving techniques, but a top tip is to not angle the cuts too shallow as that makes the grain tear more easily when turning corners. He was using very sharp tools which were hollow ground on a Tormek then honed. The problem with using the tip of your normal knife for engraving or detailed work is that most people aren't very good at sharpening and getting the very tip (by this i mean the last quarter mm) of your tool razor sharp is not easy, i think it is better to have a couple of different knives unless you have reason to only have one knife. Incidentally Fritiof was using short knives for virtually all of his work, these knives are easier to maintain an edge on. I use a frosts 106 but this is mostly because i want to have as few knives as possible, i keep that razor sharp then i have an old frosts "bushcrafty" knife i use for rough work and non spoon work. next post will be a bit about knives. More on knives next.

1 comment:

  1. Barn,
    I enjoy following your blog in general (being a semi-avid spooner), so I am also excited to see Fritiof's teaching through your eyes in this series. Thanks for sharing it with us all.

    Simon

    ReplyDelete