Those blades show classic signs of oxidation in forging. When I was teaching it would invariably show up on blades when students had the air blast too extreme or took too long to forge the knife. Both conditions expose the steel to oxygen long enough for large bits of steel to be turned into iron oxide and leave the blade, and also gets pounded back into the surface making pits which then oxidize deeper on subsequent heats.
There are two things that can happen to hot blade steel in the atmosphere, you can have the air take carbon - decarburization, or you can have the air take iron- scaling. Many confuse scaling with decarb when it is oxidized iron. Heavy scaling will give you things like this, while light scaling can actually help a little to fight decarb by removing iron from the decarburizing surface and maintaining a better ratio of the two. Carpenter steel did studies that involved heating steels in very oxidizing atmospheres which resulted in steels that had a higher concentration of carbon on the surfaces. This seemed insane, as everybody knows you must use a carburizing flame to carburize steel, until you stopped to think that there was no carbon added but instead iron was removed until the iron/carbon ratio was skewed in favor of carbon.
Why is a "reducing" atmosphere called that? Because it works very well in smelting to reduce iron oxide to pure iron. Atmosphere rich in carbon monoxide with an empty space hungry for another oxygen atom will strip it from iron oxide to form carbon dioxide and take it out the top of the furnace leaving purer iron behind. It is this funny balancing act that makes carburizing so tricky in anything other than an entirely closed atmosphere, and why the vast majority of claims of adding carbon to steel while forging is absolute bunk put forward by people that obviously have a very limited understanding of the complex chemistry involved. It’s complexity became very clear to me when I started messing with bloomery smelting and saw the chemical processes at work.
For many years now, all the scholars writing all the books stated that in ancient times only soft iron was made in the direct bloomery process, which then had to be cut up and carefully made into steel by carburizing in the forging process…
wrong. The reality is quite the opposite. Steel and even cast iron is quite easy to make in a bloomery hearth and the forging process is much more often a matter of reducing the carbon content and evenly distributing it
…. I have wandered quite far from pock marks in blades, haven’t I?
