Sure no problem.
More than likely I would tend to guess that the blade would've lost carbon on its surface, maybe a few thousands or possibly bit more, depending on how long it was in the fire and how hot it was. Unless the blade is in a reducing atmosphere (where more fuel is burning than oxygen) within the charcoal forge, which takes some knowledge on how to properly adjust the charcoal pile and where to put the blade within it, it's more likely to lose carbon when exposed due to an oxygen rich (oxidizing) atmosphere rather than absorb any from the coal. It's possible it picked up some carbon, but it's more likely it lost some on the surface. Also, it is were to gain any cabon, unless it was held at the right temp in a reducing atmosphere for long enough, it would be a very thin carburized surface which would end up getting ground off when finish grinding and sanding the knife
I'm not that knowledgeable with coal or charcoal forges and it's been a long time since I last used one, but generally speaking when it comes to those who are just beginning to learn to forge and use the forge properly, more often than not the steel tends to be over-heated and for a good amount of time. I'm not saying this is the case with your blade, but with the condition of the surface of your steel on the first knife with the "bubbled" effect, it sounds like the steel may have been over-heated, because I remember that happening to my knives a good bit when I first started making and heat treating them.
I think with the right forge design, the right size and type of charcoal, the right amount of air blown in, and proper tending of the pile, charcoal can be an excellent fuel to forge and heat treat high carbon steel with, but it surely requires a decent amount of knowledge and practice in how to get the most benefit from it, as with most things.
I would suggest next time you heat treat a blade, keep the blower down to a minimum and bring the steel up to temp slowly and evenly. Use a muffle (a steel pipe large enough to fit the blade which is capped off on the far end) buried in the charcoal to help more evenly distribute the heat while heating the blade, and slowly move the blade back and forth to avoid heating up one part of the blade more than the rest. Once you begin to see a bit of red in the blade begin testing along the edge with a magnet fairly often to make sure you're not getting too hot. Once the entire edge reaches a non-magnetic state, continue heating just a little longer where the steel gets just a little bit brighter, and then take it out to quench, moving the blade either up an down from tip to butt (for a vertical quench tank), or up and down from edge to spine while in the oil (for a horizontal quench tank), which helps to break up any vapor barrier that forms around the hot steel and hinders the oil's ability to cool the steel. You can also put a piece of charcoal in the end up the pipe to help burn up any excess oxygen within it.
You can take a scrap piece of high carbon steel and roughly shape the bevels to the thickness you would leave it prior to hardening, and use that as a practice blade to begin to familiarize yourself with the process and with your forge. Practice bringing the blade up to non-magnetic and heating it evenly, and then after you feel more confident in doing that, you can even practice holding it at that temp for a couple of minutes by pulling the blade in and out of the forge/muffle.
When bringing it up to heat, it also helps to pull it out of the muffle/forge to let the heat in the blade equalize for somewhere around 5 to 10 seconds, so that the thicker sections (spine, ricasso and tang) soak up the heat from the thinner sections (edge and tip) which will help to avoid uneven heating and accidental over-heating of the edge & tip. The most important part to have evenly heated is the edge, and since the edge and the tip will come up to heat the fastest, keep a close eye to not let it get too hot which can happen quickly in a forge. If the tip or edge starts getting too hot too fast, just take it out of the forge to let the spine soak up the heat, and keep doing this until it gets to the point that the edge/bevel area are heating up evenly to just past non-magnetic and then quench. Another thing you can do is heat up the tang first, so it draws less heat from the blade end and makes it easier to heat evenly. In the past and still even now and then, I've also used a small piece of kaowool to cover the tip while bring the blade up to heat to make make it less likely to over heat, then I brush it off when the temp is getting close to critical. Let me know if that all makes sense.
~Paul
My Youtube Channel
... (Just some older videos of some knives I've made in the past)
