I was thinking about trying alum, it seems to work well when dying wool. I've washed my hair three times and after the initial slight fading, the color is staying the same. My natural roots faded the most but is still very coppery and natural.
I did find out that high PH will turn it pink if left on for a few minutes but its slowly turning back to copper. I did a test strand with a liquid with PH of 10, left it on for about 10 minutes, it went fairly bright pink. Also did a test strand with CP shampoo bar and left it on for 2 minutes, no noticeable color change. I've been using store bought shampoo but I'm going back to CP bars now that I feel its safe.
I bet you could do a gloss like they do with henna to brighten your natural auburn. You mix the madder with conditioner instead of water. I was thinking about doing that, maybe it would make it easier to apply.