excerpt by sanmarzano:

A sharp penknife is the crucial tool. You need to peel branches along the plane between the hard inner core and the softer outer wood, long strips come away quite readily. The outer silver surface could be picked off and discarded (though I don't bother), it is the middle layer that has the dye, it turns reddish when exposed to air. So long as the wood is fairly freshly fallen, the peeling is easy, except around knots. Then I soak the peelings for a week or two in water before boiling them for an hour or so and leaving overnight. I add the wool with the bark still in the pot and have found that a high temperature simmer for several hours gives the best colour. That was just one broken branch, which gave about 200g bark. Adding copper really brings up the pink/purple colour. It takes better on cotton and linen than wool. Although the initial yarn is not a strong pink, I have used the same dye pot of bark to dye a cotton bundle, which was a second hand summer dress and I just took out a linen shirt bundle this morning, expecting the colour to be pale by now, but the bark is still going strong.