Lichens of the species Ochrolechia and Umbilicaria give beautiful brilliant purple and red shades when treated in stale urine or a solution of water and ammonia and no mordant is required for these dyes. This purple colour seems to have been used relatively rarely by the Anglo-Saxons, probably because the lichens needed to produce it only occur in restricted regions of Britain, mainly in the North and West in hilly areas or rocky, coastal districts
Analysis of the dyes used in textiles from the early Anglo-Saxon period shows that purples from lichens were used in embroidery, narrow woven bands and accessories, such as bags and headdresses, rather than to dye larger fabrics. Bearing in mind the scarcity of purple-producing lichens in southern and eastern England, this is perhaps not surprising.
This photo gives some indication of the beautiful purples available from lichens but does not do justice to the brilliance of these colours, which sadly are not very lightfast.
I do not recommend using lichens for dyeing, except in very small test dyebaths, as lichens grow very slowly and may take a long time to regenerate. Lichens should never be harvested indiscriminately and some may be protected species and should never be gathered. It is very important to be sure you have correctly identified each lichen before even considering collecting any. However, even a small piece of lichen the size of a large coin can yield enough purple dye for most test purposes.
Purple-producing lichens are prepared by soaking them in stale urine or in a solution of 2 parts water to 1 part ammonia. Use a strong glass jar with a well-fitting lid and shake or stir the solution every day. It can take several weeks for the purple colour to develop. When the solution is a rich purple in colour, strain off the liquid into a dye pot and add the fibres to be dyed, plus more water if necessary. If you have used ammonia, make sure not to inhale any of the rather unpleasant fumes. (Stale urine can be equally unpleasant, of course!) Then heat the solution gently to simmering point and simmer gently for about 30 minutes. Take great care when heating the solution, as ammonia can catch fire very easily. Then allow the fibres to steep in the solution overnight. Then remove the fibres, squeeze the excess liquid back into the pot and re-use the solution until it is exhausted.
If the dyed fibres are steeped in an acidic modifier (for example in a solution of clear vinegar and water) they will become redder in tone. Using an alkaline modifier, such as wood-ash-water, will make the fibres more purple in tone.
The 4th skein from the left in the photo below shows some of the variations from acid and alkaline modifiers.