I will admit that I didn't record the questions and answers, but I typed it up immediately after my book was signed. There might be some small changes in word choice, but I'm about 99% sure these are the words he used. He also said that First of the Sun is weird in that the apparent magic user (the person) is different from the vessel of the investiture (the Aviar).
I think what he's saying but not Saying is that you might need different metals to take different powers, just like in any other magic system. You would still need several spikes, if only because they each have a different bind point.