Hot versus Cold Alum Mordant for Wool before Dyeing with Dyers Chamomile

I came in out of the rain, pulled off my Wellingtons and announced with some satisfaction
"Soon as we get some sunshine, the Dyers Chamomile plants will be covered in flowers. This time next week, I shall be transmuting base wool into gold."
My companion, Elinor Gotland, put down her book to wipe droplets of water off her specs.
"You've already transformed the kitchen floor into a mudbath, Beaut. Couldn't you put a towel over that dog before she shakes herself again? I've already had a bath this morning."
"You'll have to make do with showering for the next few days. I'm going to take over the bath to mordant all the undyed yarn I bought at Wonderwool."

Elinor rolled her eyes, but she wasn't surprised. It has become my habit to mordant in bulk before the dye plant flowering season begins. When mordanting large quantities, I add the alum to water in the bath in my bathroom. Himself does make me promise to rinse it out afterwards, though considering the distinctive aroma he develops while cycling in lycra, I wonder if alum's long history of use as a deodorant might mean his bathing in it could be a good thing. Anyway, I divide up the fibres I want to mordant into seven equal portions, then weigh out and dissolve sufficient alum to mordant four portions. For example, if I had 700g yarn, that would mean dissolving 40g alum to mordant 400g of it at 10% by weight. I leave the wool to soak in the bath for 24 hours. I understand that fibres will only pick up half the alum from any mordant bath, whatever the concentration, though I don't understand quite why that should be so. This means that once the hypothetical 400g of wool has been taken out, there would still be 20g of alum in the bathwater, enabling me to mordant another two 100g portions of wool by leaving them in there for the next 24 hours, then finally the seventh 100g portion has its day of cold alum mordanting in the remaining 10g alum. Although by this logic 5g of alum must remain in the bathwater after the last skein is mordanted when I pull out the plug on day four, I can feel gratified to have minimised the cost, wastage and pollution involved in the alum mordanting process.

Wool tops, roving and fleece are less likely to become felted during cold mordanting than hot simmering and the bath is ideal for big quantities, especially if I want a large piece of fabric to float freely and mordant evenly. The process does work, I've been using plants to dye cold mordanted wool and silk for the last five years and not noticed a problem.

Recently, however, I have begun to suspect that cold alum mordanting may not result in such strong final dye colours as when alum is used with heat. As a small trial before a major mordanting session on this year's haul of silk/wool blends and superfine Falklands yarns, it seemed timely to make a direct comparison using good old DROPS 100% wool aran yarn.

Two 50g balls were made into four 25g skeins and tied loosely at four points, soaked with a bit of detergent to scour them, then rinsed and soaked overnight. In order to be as precise as possible with tiny weights of alum crystals, I dissolved 20g in 200ml boiling water to make a solution containing 1g per 10ml, then used a syringe to draw up 50ml to be sure each bath got the same amount of alum.

Two skeins of yarn went into a pot of water with 5g alum and were heated up, simmered for an hour then removed and rinsed once the water had cooled to hand hot. The other two skeins went into a bowl of cold water with 5g alum and were soaked in there for 24 hours before rinsing.

Dried Dyers Chamomile flowers can be kept for years in a paper bag and will still release their dye when soaked and simmered. My impression is that the dye per weight of dried flowers is stronger if you use them within a year, though it is hard to be absolutely sure, when so many factors influence the amount of dye an individual flower contains - where the plant grew, whether it was picked in June or August, whether that particular month or year was hot or cool and damp. What is clear to me is that fresh flowers give much more dye than dried ones, because even though they weigh so much less after drying out, it still takes about double the weight of dried flowers to fibre to impart a strong colour. They are still worthwhile having, because picking a plant bare even when you aren't ready to dye does mean it will go on producing new flowers throughout the summer and drying them does provide supplies for times like the present, when no fresh flowers are blooming.

For this trial, I soaked 100g dried Dyers Chamomile then simmered the pot for an hour to extract the dye before adding one 25g skein of wool mordanted with hot alum and one 25g skein that had been mordanted cold. After simmering for an hour, as soon as they had cooled enough to handle, I pulled up lengths of wool from each skein to have a look at their dye colour.

"So, you won't be hogging the bathroom after all, then, Beaut?"

"There's no need to look so pleased about it, Elinor. The cold mordanted skein has still picked up a decent yellow."

"Not as good as the hot one though, is it?"

"The two skeins might even up after I've left them in the pot overnight. Or if I added some soda ash to alkalinise the lot and make the whole dye job brighter. Or maybe I shall just have to hot mordant everything if I want the best results. What a bloody palaver it all is." I turned away and traipsed back into the house. My companion followed.

"Come on, let's put the kettle on, have a cup of tea and a cigarette. Hot alum mordanting isn't the end of the world."

"I don't want to be forever turning on the gas and heating pots up, I wish I didn't have to bother with mordanting at all. I don't really like having to buy any chemicals. I've read about plants like mosses and heuchera that concentrate natural alum from the soil, but I never did grow any. I wish I had, it would be so much more authentic to grow my own mordants."

"More authentic? Authentic how?"

"More traditional, I'd like to use processes that predate industrialisation."

"Hmm, the Industrial Revolution began in the eighteenth century, Beaut. I think you'll find British dyers were mordanting with alum crystals long before that."

"Oh, surely not."

"It's a matter of record which you'd already know if you'd read the classics. Pliny the Elder wrote plenty about alum and its uses. It didn't come from plants in his day, he called alum 'a sort of brine, which exudes from the earth.' The Romans probably brought some with them when they invaded Britain over two thousand years ago. There wouldn't have been anything half so good locally for mordanting the wool from all our sheep. More than likely the British tribes had already been importing alum along the ancient trade routes before the Romans ever turned up. Iron Age Brits were already drinking wine and eating olives, so I expect they would have been pretty interested in trading for Mediterranean alum too."

"Why wouldn't they have made their own British alum?"

"Ah, well, it's all the rain, see, Beaut. Alum dissolves in the wet, so you wouldn't find crystals lying about on the ground anywhere in the UK. You have to get much more modern for evidence of local suppliers. It wasn't til the seventeenth century that someone cornered the market by extracting alum from shale in North Yorkshire and got a British monopoly going." 
We drank our tea and had a contemplative smoke. 
"Feeling better now, Beaut?"

"Well, yes, I suppose I am. If my ancient ancestors had to sort out how best to buy and use alum, then I can do it too." 

"That's the spirit. For a minute there, I thought you were going to insist on growing your own tea and tobacco."