This is part 3 of a series. Part 2 can be read here. These questions exist to prefigure the likely impacts, throughout the history of a world, of a specific magical power.
Imagine this world is now analogous to France during the European Renaissance (roughly 1400-1500). Importantly, it is not France, or earth. This merely serves to provide an idea of the general social and technical context for this question. A small group of people (five, for the purposes of this question) have the ability to magically induce redox reactions at-will.
This means they can do things like:
Melt through the walls of vaults. Speed the growth of plants, and make draft animals more robust. Explode trees. Light people on fire. Charge chemical batteries at-will. Easily create advanced alloys. Ignite hydrocarbons. Render land infertile.
Each person can use this ability to effect a total weight of 3 tons, per day. They can control the speed of the reaction, the scale of the reaction and remove the need for any activation energy. The effect (for the purposes of this question) must be induced within eyesight.
If they wanted to gain economic dominance over nearby nations, using the least amount of this power, how might they do this?
They are in complete agreement about this goal and the means of achieving it. Nobody will be founding new religions around these powers. There are no particularly important figures for them to assassinate, though warfare is an option, if it were magically cost-effective. The environment and tools are otherwise typical of the time and place. The power can be used on the magic-users themselves, inanimate objects, and non-human animals.