Ever since I first read about Saxon blue, produced by reaction indigo with concentrated sulfuric acid, I’ve really wanted to try it.
The lawyer Johann Christian Barth is credited with inventing the Saxon blue reaction in 1743. He treated natural indigo with sulfuric acid, then known as “oil of vitriol”. According to de Keijzer, the dye was in use in England by 1748, and Jenny Balfour-Paul writes in her book “Indigo” that the dye “can be seen in some oriental carpets, most characteristically those made in Turkey during the second half of the nineteenth century, and also in late eighteenth century Kashmir shawls”. The dye was relatively popular, even though its light- and wash-fastness is not as good as that of indigo itself.
Balfour-Paul calls the color “bright turquoisy blue” while de Kaizer mentions “bluish-green” shades.
The story about this caught my interest because it seems to be a midway point between truly natural dyes, and the synthetic dyes that came after Perkin’s discovery of mauveine in 1856. If made from natural indigo, Saxon blue is not really a synthetic dye. But it’s not fully natural, either, and the process that it was used in clearly seems to fit better into what we think of as an industrial process.
The problem for trying this at home is that you need to use concentrated sulfuric acid in order to produce Saxon blue. This is not something you can just go out and buy, and there’s a good reason for that. It’s a quite dangerous acid that reacts with carbohydrates like bread in a way that makes it look like the bread is on fire.
But now, the perfect opportunity came up, the exam project for teaching chemistry that I’m working on right now. So here’s my little experiment with Saxon blue. I tried this in a chemistry lab, inside a fume hood, wearing lab coat and safety goggles. DO NOT TRY THIS AT HOME!!
I mixed 0.5 g of indigo powder with 5 mL of concentrated sulfuric acid, and then heated it over a simmering water bath for about 10 minutes (left photo below).
Then I diluted the indigo into water, and put in alum mordanted wool. I heated the wool in the dye bath for about 40 minutes (right photo below). Even after diluting, the solution was very acidic (pH 1).
This is how the skein of wool turned out after rinsing out the excess color (there was a lot). A very clear blue, that’s actually very similar to the shade of blue you would get with indigo used as a vat dye.
But the chemistry behind this blue is different from the usual indigo chemistry. The reaction between indigo and sulfuric acid produces a compound called indigo carmine (this is what is called Saxon blue). Indigo carmine is an acid dye, not a vat dye. That means that it will bond to aluminum that was attached to the wool during alum mordanting.
Notice the cotton thread tied around my Saxon blue wool skein below. It’s only slightly blue-tinged. Alum does not react well with cotton, so there were only very few sites on that thread where indigo carmine could bond.
Now compare with the blue on the pile of cotton in the back. It just happened that I used the very same cotton thread for tying around clothes that I shibori dyed with indigo using the usual method. Notice how parts of the thread in the back are quite dark blue. They were exposed to the indigo vat, and the color took well, because indigo can deposit directly on cotton (there are also white parts, but they were just not exposed).
It was fun to try dyeing with Saxon blue (indigo carmine), but I don’t really see myself repeating the experiment for the purpose of actually dyeing wool. The fact that the light-fastness is low and the process uses concentrated sulfuric acid means that the comparison with indigo itself does not fall out in Saxon blue’s favor.
But if you are wondering what Saxon blue is up to these days, check your candy wrapper. It shouldn’t be difficult at all to find yourself some candies containing FD&C Blue #2 in the US, and E132 in the EU. That’s indigo carmine, or Saxon blue. The stuff in food does not come from natural indigo, it’s synthetic.
If you have appetite for some more dyes, you can also look for natural red 4 (US) or E120 (EU). It may also be written as carmine. Around here, it’s known as cochineal. In this case, the coloring in food does actually come from the natural source. Some people find this disgusting, but having ground the lice so many times for dyeing, I actually find it quite unoffensive.