Yes as I see your explanation of the process better here than on FB, I see thay Alan is right. They are doing a direct reduction process. Not really sure why they use borax in this when all you really need is silica.
The method of adhering the cast to the iron is neat. Never heard of it as a process as Alan points out though until now.
It's similar to how I combine cast iron made in the hearth to actual oroshi steels. Piling them on and welding at lower end heats before the cast crumbles and after a few heats of this, it diffuses enough into the steel to behave with the rest of it. I fully believe that in the beginning, the welded steel to the cast actually allows it to forge. Much like the ductility imparted to steel when you forgeweld iron to it in sanmai.
When crossing into really high carbon ranges and cooling slowly, you create widmenstatten cementite. I create these structures in my 1.5-3% steels and cast irons in oroshigane. The only issue with these structures present in the steel is that it is seemingly difficult to break down and diffuse the cementite into adjacent welded steels or irons without high heat. Even then, I've had it still leach out carbon much slower than cementite in grain boundaries.
To me, this is why his mix of that material and something like wrought shows a distinct difference of light and dark. The excess cementite hasn't diffused into the lower carbon irons enough to break apart and this etches bright.
I took a micrograph of the cross section of one of my oroshogane blades that used material like this and behaved as I described and it really look like this. It may also explain why I was having weird spectro analysis on some of my oroshigane. Areas after folding a bar 4-5 times had concentrations of ~.5% C, and others much, much higher.
This steel in micro below was 2 different billets thst were independently folded 6 times and then alternated stacked on eachother in a 4 layer stack and folded 6 more times. Notice that carbon even after 6 folds did not diffuses readily into adjacent material.
Edit: Orrrrr it's something Phosphorous related. ???