Yes, cow dung, amongst other things - that's what was used to bleach linen in medieval times, and I plan to do some experiments. On Saturday June 2, 2012, I submitted a challenge to the White Wolf Fian group - an Ealdormerian group within the SCA that seeks members who challenge themselves to undertake projects of their choosing at least at the intermediate level that stretches their abilities either by taking something they are already familiar with to a higher level or starting with something completely new, and completing it within one year.
During my early days researching about growing and processing flax I came across a reference to bleaching/whitening linen having been done in the middle ages, and earlier, by using cow dung - gross as it sounded, it piqued my interest. I tried whitening one small linen towel with just water and sunlight, with some degree of success,
but now I plan do to some experiments with cow dung, sheep dung, urine, wood ash, potash, bran, sour milk or buttermilk etc. along with sunlight, moonlight, frost and freshly fallen snow, that was used before the advent of modern chemicals.
I have some cow dung from a dairy farm, a shepherd I know will be happy to provide me with sheep dung, urine has to be stale - at least two weeks old - I have one yard of linen that I will cut up into 16 pieces 5" x 10". I will be doing at least 9 experiments for my White Wolf Fian Challenge but I may do some extras as well.