Back in the late 1960s, there was a muzzle-loading group in the area where I live. One gentleman had personally made more than 50 rifles. He gave me some help & advise on mine though I did the work myself. Nitric acid was often used on the stock finish. Before I did mine he warned me occasionally one would turn Green, & he knew of no way to predict or prevent it. I went on with the acid. Brushed on the acid, then dried it with a propane torch After the acid treatment I coated the stock with Potassium Permanganate until it turned Black. This was rubbed then with steel wool dipped in linseed oil until the desired tone was reached. It turned out really good & has never gone Green. This was on Curly Maple.
Another method I discovered later is to coat the maple with a strong Tea solution then set the part in a sealed container, Cardboard Box will work, along with a dish of Stronger Ammonia. The Ammonia reacts with the tannic acid in the tea & turns it a most beautiful color. I have not done a stock this way but have done some pistol grips & several other items.
This process is said to have originated with Oak "Mission Furniture" except the oak had its own tannic acid & did not need the tea treatment.
