I'm in the middle of building a fantasy world with one core component – a reasonably rare magical metal that gets lighter the purer it is, without losing any of its mass. Alloys of this and steel are a magical equivalent of Damascus Steel – sharp, tough and resilient, with the added advantage of weighing less, but having a slightly greater mass than a steel weapon of the same size. How would this affect the weapons and armour of the setting?
So far, I've come to the conclusion that magical armour would be in high demand as lighter plate mail and chain mail equals somewhat better manoeuvrability. Where it comes to weapons, obviously as they are negating the weight weapons would be wielded differently.
I'd come to the same conclusions as many of those writing in the attached question, that oversized weaponry isn't much use, but that lighter weapons for the same mass would certainly give an advantage to things like hammers and axes in warfare. It should be noted that too pure a concentration and things start literally floating, so a purely magical sword would actually not be very useful at all.
My question is, am I right in my assumptions that weapons the same size with the same mass that were "lighter" would be more effective in battle? Specifically thinking of swords, axes, and arrowheads here.
Currently my idea is that the metal in question would impart a magical "upward" force equal to its concentration. 100% "unobtanium" would equal 100% upward thrust, akin to if gravity were acting on the item the exact opposite direction, with 50% equalling gravitational influence (as if you were using it in a weightless environment) and 25% making the item 50% lighter. The mass increase would be akin to that of a steel quantity 5% larger.