Is it safe to cook on uncoated aluminum?

Yes, it is. It is done all the time. Make no mistake about it. Aluminum on the atomic level (or a bit more if a cook is overzealous with utensils in the vessel) gets into the food cooked in uncoated aluminum vessels. But the amount is so small that no toxic threat is posed and no change in taste is effected. (There is no documented evidence.) Aluminum is being linked to Alzheimer's disease (dialysis dementia). It is only fair to mention this. But the link is tenuous. Do your own research. The chemistry of aluminum won't change for the reading. Aluminum is poorly absorbed by the body and is quickly passed through the system. We have been using (uncoated) aluminum cookware for years with no demonstrated ill effects. Research is ongoing. Aluminum resists corrosion by the simple mechanism of "putting on" a thin coat of oxide when exposed to air. This "blocks" further corrosion. When we scour the pan, we remove this thin layer. Don't panic - it's only atoms thick. A good rub with a "scrubbie pad" won't hurt. The pan needs to be clean. Some things we cook in the pan will "pull off" some of the aluminum. Tomato sauces are famous for leaving a bright shine on an uncoated aluminum pot where they are simmered. Aluminum is frequently the choice of professional chefs, and the uncoated kind of aluminum, too.