this topic keeps rearing its ugly head, so Imma gonna blog it!
We all know that Water makes wood swell up. The wood shrinks in the first place as water leaves it, and water can make it swell up again, usually causing some sort of damage to it.
So if you do not think too much about it you might conclude that any liquid will swell the wood up. liquids like oils or petroleum solvents… And you might think that since oils don’t “dry out” then the wood will never shrink afterwards.
and you would be wrong.
Here is a conceptual visualisation to help you understand why oil doesn’t swell up wood:
Wood is composed of Cellulose fibers held together by Lignin (a gummy non directional binding protein), both are hydrophilic (they love water).
Wood is a bundle of tubes so:
Imagine that wood is a bundle of papers straws held together with Elmer’s glue. While the wood is wet and Green, the glue is wet and the straws are full of sap.
when you cut the tree down the sap starts drying out first, the tree can lose over 50% of its weight (a few species are over 90% water by weight when they are felled) just to drying the sap out, during this time the wood does not shrink. (think, you have poured the water out of the straws)
Once the sap is mostly gone the glue dries, the lignin which contains over 25% water (by weight) starts to dry out, this is when the wood shrinks. the cellulose cannot shrink and the cellulose fibers are oriented mostly vertically in the tree. which is why the wood shrinks across its width but hardly at all in it’s length.
So now you have nice dry wood and you make something out of it, and you soak it in BLO.
Remember when you learned that oil and water don’t mix? something that goes along with that is that hydrophilic (water loving) materials (salt , sugar, elmers glue) will take up water or dissolve, hydrophobic (water fearing) materials (tar, shellac flakes, lacquer) do not, but might be dissolvable or miscible in oil like products (oils, lacquer thinner, paint thinner).
The BLO coats the outside of the wood and penetrates up the empty tubes (straws) but it cannot get into the lignin or in between the cellulose bundles because they like water but not oil. Since most of the shrinkage was from the lignin drying out, you get no swelling from the oil.
Our Proof is an easy experiment that you can do yourself,
take 3 sticks of the same dry wood (preferably all cut out of the same board). They don’t need to be bigger than 1″ x 1″ x 6″, and a couple of empty cans. Carefully measure and record the precise measurements on the sticks (a vernier caliper or micrometer is good for this).
Put one so that it’s end is submerged in BLO about 1″, take another and do the same with water, and put the third in an empty container, wait 3 days or a week, measure the top (dry ends) and measure the bottom (wet) ends. If your humidity changes a lot then the sample in the empty container will show you how that affects the wood. The piece in the water will swell up in less than a day. the piece in the BLO should change about as much as the dry piece.
please alert me if I’m wrong.
for more information on this subject please refer to:
“Understanding Wood” by Bruce Hoadley
“the Woodwright’s Shop” by Roy Underhill
both of them outline this experiment and its results, but you don’t have to believe us when it is so easy to do the experiment yourself.
The wooden hygrometer that Roy makes is interesting too.
The folks who talk about this the most, seem to think that the tradition of sticking a tool with a new handle, head down in a container of BLO is for the purpose of swelling the handle to fit the head tighter. This is an error.
Wood Constrained by metal gets slightly crushed if it takes in moisture and swells. You do not “see” this damage until it dries back out and comes loose.
What you ARE doing is making sure that no moisture ever gets into the part of the handle inside the head so that it never swell up.
[ an aside; H. D. Thoreau really angered me when I read his “On Walden Pond”. He stuck his axe head in the pond overnight to tighten up the handle… He thought he was so clever ruining someone else’s axe (he had borrowed it). well, my take-away from that book is nearly exactly the opposite of nearly everyone else’s. Everyone else seems to see an independent spirit, Carving out his space on his own. I see a spoiled twit who relied on his generous neighbors and his sister to survive.]
Some older references to hanging handles and similar situations tell the reader to fit the handle, then without the head mounted, soak the handle head end overnight in BLO. Then put the head on and drive the wedge home! since in doing this the pores are not crushed or clamped closed it may be better than putting it in oil after… I wonder if the soaking in oil after is done because someone was in a hurry and couldn’t wait until the next day for the soak… and that became a tratition.
Some people also promote the use of motor oil for this job. Motor oil does not cure (BLO gets hard and provides a better moisture barrier), motor oil lets water vapor go past it (read up on semipermeable membranes). You know that water that inexplicably gets into the bottom of your gas can? That is from water vapor touching the surface and sinking straight through one molecule at a time. Motor oil does the same thing. And while motor oil will seem to work OK in an emergency (IE you have no BLO and you need to get back to work with the tool NOW etc), you might not want to use it as your regular treatment.
count your fingers