Today I took a bit of time and I have chosen and cut to size some wood for the handle scales. Among the species that I have chosen for this experiment are: Black locust, Cherry, Jatoba, Black elder, Larch, Oak and some unknown semi-rotten wood, probably birch.
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Because the purpose of this project is to gather information, I have taken all these various pieces of wood and I have given them into a jar with a mixture of ammonia and alcohol. I have written about this process before as “ammonia fuming”. The ammonia reacts with acidic lignin compounds in the wood and thus artificially ages it. In some woods the effect is really subtle – and I already know that using it for maple is a waste of resources – but on some woods, it can be really profound by giving the wood significantly darker and richer color. Oak should get almost ebony black after a few days. The rule of thumb is that if the wood has differently colored heartwood, then it is worth a try.
Some color will leech out into the solution, as you can see already (it was not a fresh solution), but that should not be a problem, it will not seep into the wood itself any more than any other pigment would. What is important here is the chemical reaction, if the wood does not react with the ammonia, it won’t change color significantly no matter what.
Ideally, only ammonia fumes would be used, with the sitting wood above the solution. But I cannot do that comfortably yet, for that I will have to make a grit of sorts that I can put into the jar. If I will ever bother, because whilst that process is a bit safer, it is a lot slower.
That is why I have added alcohol to the solution. It reduces the swelling of the wood during the soaking and subsequently reduces shrinkage and risk of cracks when drying. Plus it makes the subsequent drying a bit faster. That is something that I have tested already on two pieces of fresh birch which I have subsequently put away somewhere in my wood stash and now I cannot find them.
I will take the pieces out of the solution after a few days and let them dry outside for a bit (they stink like hell as you can probably imagine). Then we shall see what has happened to which wood. Some effects can already be seen after a few hours.
Put some iron in the mix with the oak and it’ll blacken beautifully. The bad news is it takes a few thousand years and lots of pressure to fully perfuse.
@Marcus I know about the effect of iron hydroxides on the process, and I shall do that for some future batches. Allegedly it also colors basswood into almost ebony-black. But it takes about a year at room temperature for 1 cm thickness, so for that I will probably have to get an extra jar, fill it, forget it and then discover it a few years later.
I have been thinking that a partial vacuum system could do it
The darkening colour is probably due to same kind of chemical reactions that make kraft pulp brown, that is reactions of alkaline compounds with the chromophores (colour-forming functional groups) of lignin. And according to my old pulping technology compendia, they have tried using ammonia as a catalyst in ethanol based chemical pulping (which hasn’t AFAIL ever become a commercial process because the pressure needed during cooking is way too high among other reasons).
@Ice Swimmer, thanks for the info. I think that all basic solutions react with lignin and they also act as plasticizers(?) I have experimented with strong lye solution in the past and it makes the wood not only significantly darker, but it also compacts it. I tested it on a piece of hazelwood, it went from white to chocolate-brown, shrunk about 40% volume increasing its specific gravity as compared with water from about 0,6 to nearly 1 and its hardness and e-module went through the roof (yes, the wood got harder and stronger). I have only tested a very small piece though, but I will test it on a knife-scale sized chunk sometime too.
I have also read somewhere that ammonia fumes under high pressure can make wood pliable at room temperature even better than steam does.