Red Onion Skin Dye On White, Grey and Alum Mordanted Yarn

"More onion dyes, Beaut?" My companion heaved a sigh. "Whatever could you want more ginger wool for? Thinking of knitting yourself a uniform and joining the Brownie Guides?"
"This week, I want to try dyeing white and grey wool with red onion skins. I expect red onions won't give me ginger so much as plain brown wool - which might do for a Brownie cardigan, except I'm just that little bit too old and I think my niece is in the Rainbow Guides."
"Never seen any brown on a Rainbow, Beaut. Brown is just a muddy old mixture and brown overdyed on grey wool will be as dull as ditchwater. Why don't you mordant this yarn with alum? They say alum makes red onion dye turn green."
One of the great things about onion skin dyeing is that you don't need to mordant your fibres beforehand to get strong and lightfast colour. Dye things well in the first place and I've found onion skin dyes only fade much if they have to be washed. Still, I had to agree this experiment would be more interesting if I divided my three balls of yarn into two skeins and mordanted one of each with alum, before dyeing all the skeins in the same dye pot.

To extract the dye, 80g red onion skins were boiled in water for an hour or so and left in the pot overnight. The six skeins, weighing 150g in total, were simmered in the dye bath for a couple of hours. Big disappointment when I fished them out next day.
"Overdyeing grey yarn with red onion skins may have given me dull and predictably darker browns, but I might as well not have bothered with the alum mordant. There's barely a hint of green to be seen."
"You probably did the mordanting wrong. Wouldn't be the first time, would it, Beaut?"
"I did my best. Three of those skeins had an hour simmering in a 10% alum solution and then 24 hours to soak afterwards."
"You wouldn't have reused that alum solution that's been sitting on the patio for weeks?"
"So what if I did?"
My companion wandered off sighing and tutting under her breath. I don't usually store mordant solutions for more than a few days, but I've heard people say they keep theirs indefinitely. Though perhaps not outdoors in the snow.

I did actually have some of the same white DROPS Alaska yarn that I accidentally mordanted with 15% alum, ages ago. A 50g skein went into the red onion skin afterbath for a simmer. Result next day was quite definitely green, so poor mordanting must have been the problem previously. Remind me not to bother keeping used alum solution hanging about.

Looking at my onion skin dyes of 2019, after all that experimenting, my companion pointed out that both the brown and the red onion skin colours look nicer on white than grey wool. 
"Still, the alum mordanted skein from the red onion bath makes me think green and ginger brown colours could go well together. Instead of dyeing the last few balls of this wool with bark like I planned, I might wait for the trees to come into leaf. Greenish yellow leaf dyes should come out well if I'm overdyeing grey yarns."
"Yes, Beaut. So long as you mordant them properly."