This thread was dyed with bark of alder. Bark of trees contain tannins, what makes them excellent dystuffs, providing effective and lightfasting colours. It was used for dyeing from centuries. In a case of alder, you can use both bark and 'cones'(which are in fact flowers!).
The yarn was mordanted with iron, just by adding scrap iron to the pot. This method of mordanting the colour was also practised by Medieval Age dyers. They could also use water from the quenching bucket (received from the local blacksmith!).