Jim: thanks for that demo on using tannic acid in conjunction with Ferric Nitrate.
I have done a couple of stocks with these chemicals, and I find the process tedious but the results are worth the effort. My experience with tannic acid followed by Ferric Nitrate, is a chemical reaction that turns the stock as dark as charcoal. And the stain is very deep in the wood, requiring nothing short of sandpaper to get down to the warm maple tones, and with carving in the wood, that requires a careful touch.
But if one is looking for maximum amplification of the curl, I know of no better way to achieve it. Here are two rifles I used this process on...I invite critique from anyone who cares to discuss it.
Sorry: the web server won't let me post pictures at this time...will follow up a little later hoping for a more favourable result.