To be honest, I don't do much with alum in general as it seems to make silk a bit stiff. After steaming red cabbage either in an eco print or by itself I do give a post steam rinse with a small amount of alum in the water. This turns the cabbage from red-purple to some shade of blue. The red-purple seems to be fairly fugitive while the blue lasts. I maybe use a teaspoon in a gallon of water per scarf. This is a guesstimate as I am usually working on more than one scarf at a time.
I did a scarf with Himalayan Blackberry on a rusted steel wool pre-mordant. Nasty plant, wonderful print-isn't that always the way! I like the stark, hand drawn look of the prints with the purply "bleed" from the end of the stems.
When I have put them in with red cabbage they appear to go pink after the alum rinse. I thought it would be interesting to see what they looked like by themselves with the after rinse. I heated up some water and dissolved the alum powder in it, then let it cool off. I immersed the scarf and kept an eye on it for changes. After a few hours I decided it didn't look like it was changing any more so I took it out and let it dry. I gave it a rinse with shampoo the next day. I didn't add any alkaline to the alum water as the scarf was already exposed to salt as part of the rust mordant done before the steaming. How much difference a cream of tartar addition would make I am not sure. I used a teaspoon of alum in a gallon and a half of water. I decided if I was writing a post about it I should make note of the details. As you can see in the next picture, the prints stayed dark and the background turned a limey green.
The conclusion I came to is that the leaves themselves are bleeding or "dyeing" the background, similarly to what the cabbage does, but in this case in it isn't readily apparent until you expose it to the alum. I liked this, but liked it without just as well so I decided to do another scarf with blackberry so I would have one of each. This also allowed me to take a picture side by side so the difference is really clear. Also proving that this can be done over and over with the same result.
A note about working with Himalayan Blackberry, it has miserable thorns. Gloves are advised and although I have never had it make an actual hole in the fabric I do trim the ends of the thorns before placing the leaves.
So, I guess the conclusion is that sometimes an after bath in a small amount of alum can bring out colors you didn't know were there. As to whether you like it or not is another matter altogether, so using scraps to experiment with might be a good call. I have done this to some things that I really didn't care for the outcome. The items in the print that changed color did so in a way that did not mesh with the rest of the printing. Knapweed, Arrowleaf Balsamroot and Artemisia Wormwood usually come out some shade of yellow when an alum pre-mordant is used, sage green with rust pre-mordant, so when mixed in with leaves that may have tannins etc it may be the influence of the other leaves that causes the after rinse to produce less than ideal results-kind of an indescribably ugly color somewhere between dirty yellow and a not good lime green.