Why do you heat the wood after applying iron nitrate stain?
Heating triggers the chemical reaction that changes the stain from a dull gray-green color to its final orangish brown color with red highlights. A heat gun on a medium setting works well for this. Traditionally builders used a forge or a heated bar of iron instead. A hair dryer will not get hot enough to work, and the wood needs to be heated enough to see a dramatic color change, without scorching it.