Though I know silver birch bark contains enough colour to dye an equal weight of fibre, I prefer to start with twice as much bark, so as to be sure of a deep pink result. After peeling 600g bark off a fallen birch branch, I planned to dye 300g yarn. The peelings were left to ferment for a week in a bucket of water, then simmered for an hour or so and left overnight. The process of fermentation had reduced the pH of the dye bath to a fairly acidic pH 5, so I added enough dissolved soda ash to bring it up to neutral pH 7 before simmering my scoured yarn for an hour. After soaking overnight, the yarn came out a nice deep pink and the dye bath was still so dark I could barely see the peeled bark floating around at the bottom of the pot. Retesting with indicator paper later that day, it seemed that the bark must still be fermenting despite having been simmered, as the pH had dropped again. Over the next five days, I kept adding a little more soda ash to keep the pH neutral and dyeing successive batches of scoured yarn by simmering for an hour and leaving them in the pot for an overnight soak. By the time the dye bath was giving only palest pink, the original 600g bark had dyed 1.4 kg of fibres - some of them shown in the photo above. The fluid still looked dark. Nonetheless, I feel clear the dye bath is exhausted - or possibly, vice versa.