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The bleaching of linen came up on the LH forum a while back, and someone offered a suggestion made by a historian at a conference. To wit, that putting linens onto grass or bushes helped bleach them by the oxygen given off by the plants. Or the ozone. Or something. It was all a bit garbled, and obviously oxygen alone doesn’t do much, since there’s plenty of it in the air. Remember, linen is brown in its natural just woven state, and nowadays bleach and other oxidising chemicals are used to make it brilliant white. Hence places that have the name ‘fields’ which might be from being called “Bleachfields” centuries ago.

Anyway, if you are trying to give a good portrayal of social differences in the past, you need to consider how the rich folk can afford good white well woven linen, and the poor can only afford rough, coarse unbleached linen. So we mostly think, but as usual, life is a bit more complicated than that.
How was linen bleached? Was it an expensive process? And what does authentic, well worn linen look like when peasants wear it?

I started some experiments back in April/ May of 2013 with the aim of whitening two kinds of linen.

A sample of each was kept for comparison. Soaked a sample of each in (1 drop bleach 400ml water) bleach solution twice for 24 hours each time. Two more pairs of samples were used, with one of each untreated, the other treated 5 times with potassium carbonate solution from ashes, but all 4 samples were stuck in wooden frames and left in the sun, often exposed to rain, wind and dew as well as sunshine. They received two weeks of exposure, albeit moderated by some wet or cloudy days as well as sometimes being blown over.
So here they are:
linen samples bleached

First, the bleached ones, you can see the difference clearly between the white bleached ones and unbleached samples.
linen samples sun and lye sun
Secondly, here we have the lye samples on the right, on the left the sunlight alone, with the unbleached pieces in the middle. These photographs were taken in strong sunlight in June, so the colours are a little washed out, nevertheless the difference is clear. However the top type of linen simply did not bleach well at all, because the weft or whatever it is is not exactly linen or has been treated or something; it was a poor choice of sample but I couldn’t tell at the time of starting. The plain woven simple linen has however bleached nicely.

So clearly bleach does a good job, and lye with sunlight does a good job. Sunlight alone is much slower, which would naturally be more expensive, although I don’t know if bleach and lye have any noticeable effect on the wear resistance of the cloth. The precise length of time will vary depending on the weather and latitude, I was lucky in that May was quite sunny.

Now, this leads inexorably to an interesting point – lye solution was used to clean linens in the medieval and Tudor times, and it is obvious that a shirt, when exposed to lye solution every few days and dried in the sun, will, over a month or two, bleach down to a fairly white colour. The obvious conclusion is that in real life it is less the colour of the linen that matters, and more the quality of the weave, since all but the meanest peasant will have a white shirt made so simply by being used and washed and dried.

This experiment has not involved live plants, I specifically placed the samples on my garage roof or a flat stone surface. At this stage, without further evidence forthcoming (And I can’t find any), the idea that plants are necessary for the bleaching is wrong. Why use plants when you have lye and sunlight? Or rather, what difference could they possibly make?

The reason why such ideas get traction is tha that like so many other things, information about bleaching of linen is a gigantic game of Chinese whispers. The internet has turned up alleged recipes involving everything from cow dung to moonlight, and of course nobody can be bothered to give their sources. The more sensible suggestions include the use of lye or urine, and the lye certainly works, but hordes of people out there basically rely on what they read in some old book. Here’s a hint – not everything you read in a book is true! And even if they give a source you have to check the source, there’s lots of books claiming that Atlantis was real which have lots of sources, but when you look closely you realise the sources are wrong or don’t say what they thing.

Now, onto the real sources and useful information:
This article in the World Journal of Urology says that even the Romans collected urine for use in bleaching sheets:

Improvements in bleaching in the 18th century, JSTOR article, with reference to F. Home “Experiments on Bleaching”, Edinburgh 1756
But this article
Makes it clear that one of the main solutions used was simply a plain old lye solution from plant or tree ashes.

Relevant LH thread:

Those wanting further information should find a copy of Dunbar’s Smegmatologia, from 1736 but the technology and techniques are old enough to be relevant to the medieval period.

Finally, one of the errors:
suggests the medieval lye was human urine, animal excrement, lime and wood ash, which would be a horrendous mixture. Their source is “Crowfoot et al 2001, pages 19-20 and 81”, which is the MoL book on textiles, except the author is a liar/ incompetent, because when I turn to my paperback edition, I find that the pages referred to, when corrected for the hardback- paperback transition, mention bleaching but make no statement about what lye is made from. Page 18, the section on linen,
“The natural colour of linen is brown and, therefore, it was often bleached to whiten it either before or after weaving. This was a slow process taking several months to complete and added considerably to the cost of the cloth.”

Seriously, that’s it. Nothing about the lye at all. This is how lies and wrongness get spread about, and it is extremely irritating. Anyway, my own experiment clearly shows that linen can be bleached with sunlight and lye in a month or less.