Bleaching (Without Bleach)

I write a series of stories that take place in the Dreamlands, an alternative universe that certain people can visit while asleep. One of its idiosyncrasies is that it contains no technology, device, or material that appeared in the Waking World after 1500 AD.

This pre-sixteenth century limitation means that, with some exceptions, everything has to be done in the Dreamlands using pre-industrial methods, technology, and materials. This in turn means I have to conduct research to make sure I get the details correct. (I am a devotee of Poul Anderson's "On Thud and Blunder", in which he castigates fantasy writer's for being too lax or even outright wrong about the environmental, technological, and social details of their quasi-medieval world.) Most of the time I'm able to learn what I need in short order, but some topics are so complicated, esoteric, or mundane that good complete information is hard to find.

Take bleaching for example. Just how do you bleach cloth in a world without chlorine? For a long while, the best I could find was the ancient use of the "bleachfields"; literally, open fields where wet cloth was laid out to allow the sun to bleach it white. That could work, but I figured there had to be other methods between that one and modern chlorine-based bleaching.

Well, recently I discovered just how that was done. It's called the Dutch method, because in the 18th century the Dutch had a virtual monopoly on bleaching cloth. Brown cloth, made from wool or flax, was sent to Holland in March and was returned bleached in October.

Okay, this is a post-1500 method, but one exception to the technological limitation is any method simple enough that it could have been invented before 1500 if some bright person had put all the components together.

Anyway, what the Dutch did was to first steep the cloth in soda lye, then soak it in hot potash lye for a week. Afterwards it was washed and resoaked in buttermilk for a week before being washed and laid out in a bleachfield for several months.

An alternative method, developed by the Scots, involved a lye soak for several days followed by washing and exposure to the sun for several weeks. The cloth was then soaked in sour buttermilk for several days, washed, and re-exposed to the sun for several weeks. The whole process was repeated five or six times, with the strength of the lye reduced each time, until the desired degree of whiteness was achieved.

Incidentally, the lye soaking step was called "bucking", the milk soaking step was called "souring", and the sun bleaching step was called "crofting".

The long periods spent bucking, souring, and crofting were the reason it took all summer to get brown cloth white. What the Scots eventually did was to substitute oil of vitriol (sulfuric acid) for sour buttermilk. That resulted in a reduction of the souring time to 12-24 hours. (My source is a bit confused on how long milk needs to sour the cloth, stating in two places several days to a week, but in a third 6-8 weeks. Nonetheless, this is still a substantial reduction in time.) That reduced the overall method from 8 months to four.

So, in the Dreamlands, cloth is bleached by a multi-step process involving lye bucking, vitriol or milk souring, and bleachfield crofting, with a standard time period of 3-4 months (8 in the case of milk).

But how can you make lye and industrial acid in a quasi-medieval world?

That's a subject for another post.