“Protein fibres prefer acid conditions; plant fibres prefer alkaline conditions” is a truism that I’ve repeated myself on many occasions. It’s a commonplace when discussing indigo dyeing, as the vats are generally alkaline: indigo is more soluble at high pH. Warmth only makes things worse: hot alkali is said to cause damage faster than cold. Liles and others mention the risk posed by the vat fluid “Indigo vat fluid may be allowed to dry on cellulosic fibres but should never be permitted to do so on protein fibres, since the alkali becomes more concentrated as drying proceeds” ( JN Liles, 1990. The Art and Craft of Natural Dyeing. University of Tennessee Press).

In accordance with this I don’t allow the vat fluid to dry on my silk and wool; indeed, I sometimes put dyed goods into a citric acid rinse as soon as they’re cool, then plunge them into cold, thoroughly oxygenated tap water. The theory behind the cold rinse is that not only does this rinse out alkali not neutralised by the citric acid and oxidise the indigo compounds, it washes out loose indigo particles while they’re still truly loose, not dried onto the yarn or fabric to wear off later onto skin or other garments.

But I’ve been wondering exactly what damage alkali does, and how quickly the damage occurs. Should I be quite so paranoid when working with wool and silk, counting the minutes that the dyestuff is in the vat? I decided to investigate.

Bombyx aka Mulberry Silk seems more fragile than wool and therefore more likely to reveal damage. I took samples of commercial silk fabric, yarn, and Bombyx spinning fibre, tied them into bundles and submerged them in a 1-2-3 Fructose vat. I tested the pH of the vat at the start of the session and every time I removed a sample: it was over pH11 the entire time. The vat was maintained at 50–55°C for the duration of the test save for a brief overheating to something like 65°C  for about 30 minutes between 2 and 3 hours into the test (I forgot to turn the gas off! Oh, for a water bath). The samples were removed from the vat, cooled slightly, then briefly rinsed in a citric acid solution (pH4) before further rinsing in cold tap water and air drying.

And this is the result of the test. From left to right:

Undyed silk; 30 mins; 1 hour; 2 hours; 3 hours; 4 hours 15 mins; 5 hours 15mins; 7 hours; 9 hours;  10 hours 45 mins (I had to go to bed, the next day was a gym day).

I think there’s a visible difference: after nearly 11 hours at 50°C or higher at >pH11, the silk on the end is less lustrous. As the photo suggests, it’s lost some of its drape, its flexibility. The difference is more obvious in the hand: that silk is ‘cottony’, it’s not as smooth as silk. So, that’s likely to be one aspect of pH damage.

To me, the most interesting thing is the time it took for the damage to be noticeable. (Note: the temperature peak 2–3 hours in is likely to have had some effect, but I don’t think it’s terribly significant – there’s no perceptible jump in the degradation at that point.) I can’t detect a difference between the undyed silk and that which was in the vat for an hour. I’m not even certain there’s a significant difference between the 1 hour and 3 hour samples; beyond that, the fibre does seem to be more cottony. The depth of the blue colour doesn’t change from 30 minutes, so there’s no benefit to leaving it in longer. It seems likely to me that the damage is cumulative, so caution is indicated when dipping repeatedly for darker colour, but still there’s far more leeway than I feared. I’m no longer going to count minutes and seconds when silk or wool is in a chemical vat for 15 or 20 minutes.

The colour change intrigues me. Again, the blue had a greenish tinge at 1 hour, so it’s not attributable solely to the temperature change. Is the alkali causing the silk to yellow? I need to try this again with washing soda and plain white silk.