Hi there Capucine and anyone still interested in yashibushi!
Im actually in the process of doing some Yashibushi staining right now.
I have done it on a boxwood netsuke a few months ago and am now actually using it to stain an alder wood guitar body! I thought alder dye on alder wood could be an interesting project.
I have a couple of different alder trees around me. I have found one of the trees has smaller cones (im unsure of the species exactly but it is evergreen) the cones are a bit yellower in colour and I found produce a much yellower dye. I have also noticed that cones from the larger of two deciduous trees I have produce more dye. So I guess bigger cones are better. I came to the conclusion through testing that It doesnt really matter how long you immerse the wood, as in 10 hrs or 10mins wont make much difference. The dye is only really deposited on the surface and doesnt soak in much at all. Especially with box wood due to the density. I find 10mins is sufficient to get a good coating.
I have found that the cones release a kind of resin as well as the dye and when this cools/drys it creates a kind of protective resinous surface. so I found that the polish and luster of the netsuke was greater after yashibushi even if the colour was not much different. (I think Masatoshi talks about this effect...) So I think its a good idea to work with at least warm dye.
Also because its really only on the surface, whilst its wet its really easy to remove it with your fingers so I suspend the netsuke and make sure I dont touch it at all when its drying.
I have also found that allowing the netsuke to dry thoroghly then re-dye in the yashibushi and repeat as needed will lead to a darker netsuke.
After first trying a pretty weak solution of yashibushi on this guitar im working on, I doubled the number of alder cones that I was boiling. Ive just made a batch with about 30 cones in it and I reduced it down so that I only have a very small amount of liquid, no more than a centimeter in the bottom of a jar. Its very dark, ive uploaded a photo so you can see what it looks like.
I hope that helps anyone working with Yashibushi, I couldnt believe my luck when I moved into a house that had 3 alder trees nearby!