Hi Kisak,
I took a little time out to read that paper, though only the sections dealing with the clay and cooling rates.
The aim of covering a Japanese blade in clay is primarily to slow down the cooling rate in the parts of the sword relative to that of the edge. This is so that the edge can transform to the hard martensite desired and the rest of the blade into the more ductile ferrite + pearlite, with a transition zone in between.
Salt is put into quenching water to help break up the water vapour barier that otherwise would envelop the steel and slow down its cooling rate. From that paper, it would seem that a thin coating of clay over the edge region has a similar effect, whereas over the rest of the blade, where the clay is applied more thickly, the cooling is slowed down and hence the paradox referred to..
Cheers
Chris
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