Silver Birch Bark Dye - Fresh versus Dried Bark

My companion, Elinor Gotland and I climbed over a huge silver birch fallen right across our path through the woods.
"Isn't it odd how trees withstand great storms coming the same way as the prevailing wind, then topple at a few gusts blowing in the opposite direction?" 
"There's a metaphor in there somewhere, Beaut." My companion leaned against a branch and lit up while the dog headed off after a squirrel and I got my penknife out. "Planning to peel the whole tree?"
"One percent of this bark would be far more than I could fit in a pot. It's a shame really, when you think of all the times I've gone hunting for birch to dye with and found nothing freshly broken."
"What makes you so sure the bark has to be fresh? Couldn't you just come back here when you need some more? Or bring a bag of peelings home to store? I remember seeing dried wood dye stuff on sale at Wonderwool." 
"That was logwood and fustic, not silver birch. I think bark does need to be fresh."
Elinor stubbed out her fag.
"Oh, I shan't argue. You know best."
Naturally, her unwonted complaisance quite toppled my convictions.

On the way home, we passed a silver birch tree that fell last winter and I stopped to examine the part I had peeled to dye a great pile of chunky merino wool and knit the Betula Jacket. Over the intervening months, the exposed layer between the core of hard wood and the silvery outer skin of the bark had dried, changing colour from pale green to a pinkish brown. A dozen tiny beetles scuttled for cover when I prised up a section to get a better look.

The dye bearing underlayer had turned russet brown and friable. It was easy to flake off a few big chunks. Back in the kitchen, I put 100g of this old bark in one pot and 400g of the fresh bark into another, added water and left them to ferment for a week. On day eight, both pots were simmered for an hour and left to stand overnight.

Testing with indicator paper showed both dye baths had fermented equally, dropping to pH 5. Once I had added enough soda ash to raise the pH back to neutral, there was an obvious difference between the two samples, the fresh bark dye being a warm orange while the old bark dye looked pale yellow. 

Here are the results of dyeing two 50g skeins of unmordanted Blue Faced Leicester yarn by simmering them almost at the boil, one in each bath for over an hour. The old bark gave no pink at all, just a hint of tannin beige. Perhaps over time as they turn the bark itself visibly russet brown, the dye molecules in silver birch bark oxidise or otherwise change their state. Presumably the dye becomes insoluble once you can see it in the wood, since no pink was extracted into the dye bath from the old russet bark, while plenty came out of the pale, fresh peelings..

To make it a fair trial, in each bath I dyed fibre totalling half the weight of the bark. One hundred grammes of old bark dyed (or didn't dye) 50g Blue faced Leicester wool. The 400g of fresh bark dyed 50g Blue Faced Leicester and also 150g 'British Wool' - tough old carpet yarn of unspecified species - another of my Wonderwool bulk buy bargains in three base colours, cream and two shades of pale grey. I think overdyeing grey robbed the pink of some of its warmth, giving it a faintly mauve caste.

All this dyeing was done with the bark still floating at the bottom of the dye pot. I've noticed that after repeated heating with new batches of fibre, the dye bath becomes a stronger orange/pink rather than appearing paler and weaker. I guess that more dye is being released from the bark with more time and warmth. While fibres don't need mordanting, they do need to be taken almost to the boil before the dye latches on. Silver birch bark is both curious and generous dye stuff. I put another 150g carpet wool in the pot next day and it dyed at least as deeply as the first lot. Felted wool blanket weighing 300g was turned a peachy pink the day after that.

I turned to my companion.
"See I was right, silver birch bark does have to be fresh to give pink dye."
"If you recall, Beaut, I said as much myself. Still, credit where it's due, that's a considerable result from 400g bark."
"I noticed today the fresh bark has turned dark red in the dye pot. Funny how the initial orange dye bath turned wool deep pinky brown, and now the bark and the bath both look dark red but don't dye things half as strongly. This morning I took out this big skein of two ply merino and Tussah silk blend and the colour on the yarn is much paler now, but I think a soft pink shawl might be rather lovely. There's a cotton Tee shirt I might dye too. It takes a lot of fibre to exhaust a silver birch bark dye pot and I do like the paler pinks."
"No need to push your luck, though. That whole tree is still lying in the woods and the bark can't have dried out in just two weeks. Get your penknife, put the dog on her lead and let's see if we can beat the beetles to it."