Changing the pH of a Japanese Indigo Plant Dye Vat

I think it could be better to harvest Japanese Indigo leaves just as the plant starts to flower, rather than cutting in July, hoping for a second crop of leaves to grow back.  Each pot full of leaves seems to have given a stronger dye bath, harvested at this time of year.  When you consider what a performance it is to process each vat, a higher indigo yield is worth waiting for.

This week, I cut plants grown outdoors for my fifth vat of the season.  Still following the method on Wild Colours website, only leaving the leaves to steep for 48 hours rather than 24, in case that is part of the reason I've been getting so much colour from them. There are so many botanical variables - sunshine, rain, earth, manure, season, not to mention so much about the dye process that I don't know for sure.
  

Vat Five was to be an experiment in changing the pH, working on the theory that a more alkaline environment might have been the cause of a purple shaded blue that appeared in Vat Three. I did try soaking a bit of indigo dyed yarn, one end in acid and the other end in alkali, without causing an appreciable change in the shade of blue, but given how strongly pH affects other plant dyes, I still thought the suggestion made on Ravelry was 

plausible.  So, now to change pH before the indigo is fixed.  After steeping, the leaves were squeezed out and drained into a bucket, enough dissolved soda ash was added to bring the pH up to the recommended 9, then the whole lot was aerated by whisking for ten minutes.  At this point, the bath was poured jugful after jugful into three separate pots.  The first had a splash of vinegar to get the pH down to around 8, the second was left at 9 and the third had extra

soda ash to get the pH up to 10.  I gave them all another few minutes whisking, heated them up to 50 degrees Centigrade then added the Spectralite and went to find the silk I had prepared for dyeing. Returning, I found my way barred.
"Have you gone completely tonto? You're never putting good habotai silk into vats you've buggered up on purpose?"
"Calm yourself, Elinor, you are witnessing the creation of a triptych.  While you've been away,

I've invented Double Daisy Tie Dye, which is practically art. Changing the pH in these vats should give me green and purple variations on an indigo theme.  I've already sewed slots in the silk so I can hang it on the wall after the front room has been repainted. My work will be titled 'Ultra Violet 8, 9, 10.'  Pretty cryptic, eh?"
I brushed aside my companion, swept the lids off the pots and let 

the handsewn silk banners float into their vats.  After ten minutes, they were hung out for the first airing.
"That's an understated purple, alright, Beaut. Minimalist, is it?"
"Probably not enough alkali, that's all.  I'll just put some more soda ash into vat 3 for the second dip."
I carried on, grimly dipping and airing that silk, then some yarn and finally, some washed sheep fleece.  In the end, I had to acknowledge that whatever the fibre, the pH 9 vat gave the strongest, clearest blue, the pH 8 was a little weaker in colour and the high pH just caused a somewhat muddier, possibly greenish dye. No purple tones anywhere at all.

"Reinvented the wheel, have you, Beaut?"
"OK Elinor, you can stop bumping your gums.  You were right.  In future, I'll stick to pH 9 like it says in the book."
"Any artist would tell you, blue is a cold colour and we want to be cosy in the front room this winter, so it's just as well you don't hang this silk up.  I suppose I can manage to find something to do with it.."