Couple of things fwiw:
(1) AF will very often turn a stock green prior to blushing with heat. Usually red maple will REALLY go green, sugar maple will also but usually not to the same extent. I am not a chemist however I believe this is due to the residual acidity in the stain reacting with minerals in the wood. Red maple is softer and more porous, hence there is more stain penetration and subsequently more mineral/acid interaction. I think it really just comes down to the chemical and structural composition of the particular stock blank. Some may be seen to turn kind of a pukey yellow-green, some dull gray, some a weird orange etc. In the end, however, it all heats out.
(2) The need to be aware of what steel you are using in your AF is indeed a very real concern. Some may state that I am full of $#@*. Ok. Please consider: in a perfect world, your AF mixture would contain the *perfect* balance of residual acidity to dissolved iron, so that when you heat the stock and create ferric oxide, all of the acidity is 'spent' and - assuming you neutralize the stock with something to kill any residue - there will be NO FURTHER REACTION. This is a key point. We do not live in a perfect world. Some of us do not neutralize the stock. Usually there is always some residual acidity remaining and the stock is of an acidic pH. Now - if you use pure iron, or old wrought iron, or low carbon nails or whatever - no, it probably will not make any difference. However, there are a thousands of steel alloys in existence and some of them have weird things added to them for specific manufacturing or service purposes. Let's say you happened to use a steel alloy that was loaded up with chrome: what happens to chrome when it oxidizes? One thing that can happen with some of the chrome oxides is that they will turn a lovely shade of green. I'm sure all of us have at one point or another seen some old stocks finished in the 1970s that were stained with the then-popular Chromium Trioxide. Many of them now will match your lawn. A number of the chrome oxides will turn various shades of brown when heated, but over time will revert back to green - is this something that ANYONE would want? Chrome is not the only additive to be found in steel alloys which will oxidize green, and if you are working with mystery steel, it may come out nicely or maybe 10 years down the road someone may accidentally run over their rifle with the lawnmower. Which would YOU prefer? For this reason, I have always preferred - and will continue to prefer - to at the very least try to work with relatively basic steel alloys of known composition, or composition which can be reasonably guessed-at. It has nothing to do with a cork-sniffer approach or a purist attitude, I simply want to ensure the complete absence of dedicated St. Patrick's Day gunstocks. It only takes one to really ruin your day!
