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Weld, a tall woody stemmed plant, was used in medieval times to produce a yellow colour, or after a second dyeing with woad, green. I’ll get round to woad later, but in the meantime, here’s what I did.
I followed the simple instructions given in The medieval Dye pot” by Dee Duke, no. 3 of Woolgathering for dyers and Spinsters. Frankly, I didn’t bother weighing anything because I couldn’t be bothered and figured it would be fun to be a bit random about it. What I did do was alum mordant the wool and let it mature for 3 days beforehand.

There were two attempts, first, I boiled some weld with a caustic solution made from lime, for around 45 minutes. The instructions say to let it cool and strain off the dye, and it should be cold before you put the wool in and start heating.
Here’s the solution at that sort of time:
weld bits in yellow water

Then I sieved off the bits of plant, leaving a yellow-greeny liquid, which was transferred to the slow cooker. There, it was heated slowly for a couple of hours, and spent another hour at around 70C. The result was this:
Firsty dyeing with weld

Not actually that strong a yellow, the small bit of wool on the right hand side is a sample of wool I bought to make a doublet of a few years ago and shows you can get wool dyed the right kind of colour except by modern dyes. Possibly the caustic solution was a bit strong, or I overheated the weld. I remembered that I had some CaCO3 stomach pills, so the second dyeing, with this once dyed piece of cloth and a new piece with no dye at all on it, added them to the water, heating it slowly over a number of hours to extract the dye solution. Unlike earlier it didn’t boil, but whether this made much of a difference to the colour I am not sure. It certainly ended up more yellow than yellow/ green. The results on the wool dyed earlier, a new bit and asmall piece of linen (all boiled in alum solution for nearly an hour, 4 hrs before dyeing) was this:
second photo of overdyeing with weld
Twice dyed with weld

A deeper, better yellow with a hint of greeniness on the already dyed wool, and quite a nice yellow on the newly dyed wool. Using more weld than is recommended can give some useful results.
So far natural dyes have proved to be easy enough to work with.

Now the interesting bit of analytical chemistry is in working out what dyes are present in old archaeological samples of cloth. The best method nowadays apparently uses HPLC, that is, high performance liquid chromatography. The Perth High street finds were analysed in 1979, and techniques are far more advanced now than they were then, 35 years ago, and surely the textiles could benefit from a re-analysis using modern methods. The same of course goes for finds from London and elsewhere.