From the
Wikipedia entry:
"In the past, Horse-chestnut seeds were used in
France and
Switzerland for whitening hemp, flax, silk and wool. They contain a soapy juice, fit for washing of linens and stuffs, for milling of caps and stockings, etc., and for fulling of cloth. For this, 20 horse-chestnut seeds were sufficient for six litres of water. They were peeled, then rasped or dried, and ground in a malt or other mill. The water must be soft, either rain or river water, for hard well water will not do. The nuts are then steeped in cold water, which soon becomes frothy, as with soap, and then turns white as milk. It must be stirred well at first, and then, after standing to settle, strained or poured off clear. Linen washed in this liquid, and afterwards rinsed in clear running water, takes on an agreeable light sky-blue colour. It takes spots out of both linen and woollen, and never damages or injures the cloth."