After reading the advice given here, its obvious to me that nobody seems to use aqua fortis the way I do. I first make sure it is a good strong mixture, that will no longer eat up metal filings. I then slather a good heavy coat on the previously prepared stock. Now, I tend to like the curl very dark, so when I apply the heat, I don't stop until the stock looks like its been given a good coat of flat black spray paint. I neutralize with a baking soda bath, but not too heavy on the soda. I would rather do a couple of light washes, rather than one heavy one. The stock will whisker up, but, what the heck, It still pretty black. The whiskering process will remove a lot of the black, and convert most of the black spots to a dark brown. The light spots will usually either be a light reddish brown, honey gold, or a combination of the two. If i want additional color, I either use a spirit stain, or tint the finish.
I have been experimenting with a tannic acid treatment, before the aqua fortis treatment, as shown on this forum. So far the scraps of curly maple I've done have come out very well.
Hungry Horse