Moran started with 1/2" stock, three pieces. That's a lot of time at welding temp to get enough layers. I "cheated" when I went to Zowada's and we made my first billet from 1/4" stock. There couldn't be squat left for carbon in there. And, it's bothered me for many years I have no idea of the carbon left in there when I finished making a billet.
I had just hoped in my years-long absence someone had done a graduate study on the topic or someone of note would create samples and have a cousin or something in a metals lab...
If one searches the article cited above, in addition to the regular journal article there's a pic of a poster-board style display. It leaves me guessing it was presented at some scientific conference regarding Materials Science. A grad school project on a budget, he likely lacked funds to get post-production chemistries run and estimated the migration based on the amount and distribution of pearlite instead. Not the best way, but still telling as the carbon keeps creeping toward the edges as it's burnt away during forging.