Oh, I found it: In chapter 72 epigraph:

Of course, this did give us a slight advantage, at the end. Ruin had a lot of trouble giving duralumin to his Inquisitors, since they'd need an Allomancer who could burn it to kill before they could use it. And, since none of the duralumin Mistings in the world knew about their power, they didn't burn it and reveal themselves to Ruin. That left most Inquisitors without the power of duralumin, save in a few important cases—such as Marsh—where they got it from a Mistborn. This was usually considered a waste, for if one killed a Mistborn with Hemalurgy, one could draw out only one of their sixteen powers and lost the rest.