First Post so bear with me on this. lol
as i understad allomancy metals come in pairs. one metal and its alloy the metal pulls and the alloy pushes these then can be paired with another metal and its alloy to form a group of four. one pair is intenal and the other external. ie the physical group contains iron and tin and their alloys steel and pewter. if we take this onto the final group which presumably contains two metals and their two alloy's. we now know from mb2 that the one pair is alluminum and Duralumin the other pair is presumably the metal bead found at the well and its alloy which i am going to call Metal A and Alloy Z. If we assume that alluminum and Duralumin are the internal pair then 'Metal A' and 'Alloy Z' are the external pair. But wait i hear you cry Metal A grants allomancy surely that is an internal attribute. but i dont think so. i dont want to speculate too much but i think the reason they are called mistborn factores heavilly into how this last metal works. have to wait for mb3 to find out but i digress...
I think the 15th metal grants the gift of allomancy on whoever swallows it.The alloy of this metal will be an external power. So maybe it acts like a copper cloud but with the effect that no one within the cloud can perform allomancy when you burn the alloy. I dont think it would remove the ability to burn metals because thats what alluminium does and as i said earlier i believe that this metal will have an external effect.
will have to wait till mb3 comes out. Cant wait October is going to take forever too get here.