After a few experiments I've found that, at least to my eye, walnut ink crystals produce a shade of brown that has a green/blue cast while the brown produced by instant tea (Lipton Unsweetened Decaffeinated instant tea, in particular)has an orange/yellow tint. It's a subtle difference, but it helps explain my dissatisfaction with the walnut ink crystals. To me, brown with a greenish note denotes corruption and dirt, while brown with an orangish note conveys age and dissolution. That's an incredibly subjective judgement, of course, but one that's been subconsciously influencing my appraisal of documents produced with the walnut ink crystals from the beginning. In real life I have a severe aversion to dirt and grime. Having the staining process produce that effect, when I actually wanted an impression of age instead of dirt, helps explain my frustration.
On a related note, I'm going to try a few experiments using Diet Pepsi and Diet Mountain Dew as the liquid for staining baths. The first time the idea of using soda as the base was proposed to me I thought it was a joke, but it appears there may be some solid chemistry behind it.
Update: Tiffany over at Curious Goods has some more thoughts on the color shift:
"I don’t get the green tinge when dying with my inks – I switched to making my own ink a couple of years ago and it is a rich golden brown color. I did notice a green tinge on the walnut ink made by Making Memories, but I didn’t notice it on the ink made by 7 Gypsies. Perhaps it’s a brand related issue?"
Surprise, surprise...I just happen to be using walnut ink crystals from Making Memories, so it looks like it is brand related. After seeing examples of paper aged with walnut ink that did have a warm brownish-yellow tone I was beginning to think I was somehow screwing up the process.
Go read her entire post, because it has some great insight into the aesthetics of the aging process.
Doc also raised an interesting point in the comments that the artificial sweeteners in diet soda might promote mold growth. That doesn't trouble me too much, since I've always assumed that artificially aged paper has a shortened lifespan anyway. Nothing catastrophic, mind you, but I'd expect them to start crumbling after 20-30 years anyway. I have acid aged scrolls from 12 years ago that aren't showing any visible degradation. On the other hand, both the ones I have in my files and the ones I have displayed on the wall are shielded from UV light. I suspect the inevitable process of lignin breakdown would be significantly accelerated with even mild exposure.