Sumi-kiri, the elaborate cutting of charcoal by apprentices is said to take three years (san nen) to assimilate. I'm obviously a slow learner for it's been over five years and my charcoal is far from all right. But what's all the fuss?!
If you ever tried shoveling gravel, you'll have noticed it's a lot easier when all stones are the same size than when it's a melting pot of sand and gravel of various sizes. Same with charcoal in the forge, if the chunks aren't of a given size, the stock or object being heated won't move in and out smoothly, spreading coals all over the place every time it's moved and thus allowing the hearth to cool as well as wasting much time (and I don't mention the lack of elegance!).
Splitting along the grain first
The very size of the chunks also influences on the depth of the heating area while allowing for gases to pass through more or less easily. Big chunks will let air flow further than tiny bits, and this way we control the heating environment very precisely. To heat huge blocks of raw steel during tanren, the biggest chunks (2-4cm sides, depending on each smith) are used, but for quenching, when the area of the sword the be heated must be controlled precisely, much smaller bits are used (0.5-1.5cm sides). Granules and powder are also used, the former at the bottom of the forge and the later as an ingredient in many products. Not a bit of the charcoal is wasted.
In addition, the bark must be scraped off and kept separate from the fuel because it tends to spark and explode noisily, making it impossible to listen to the fire's breath, the «boiling» steel or simply stay focused enough to properly judge the flames color and the nature of the flying sparks, all vital indicator of what's happening below.
…then cutting crossgrain.
It is said that a good charcoal cutter can cut to exact dimensions after having been shown a sample once, and each chunk is found surprisingly square and sharp. I've seen such charcoal, and I've seen some messy one too. The charcoal shape shown in these pictures (mine) is pretty poor.
Pine is the favored wood for blacksmithing charcoal. For the same reasons potters prefer it for their kiln, it gives great heat in a short time and burns cleanly. Hardwood charcoal cannot be used because it burns too slowly, being dense, and forbids higher temperatures. Most forging charcoal in Japan now comes from Iwate prefecture, in the North, while other places specializes in other types of charcoals, each with their own application (tea ceremony, water and air purification, soap, moisture absorber, etc). Charcoal is like a sponge in many ways: it sucks moisture out of thin air, and oil out of your fingers. A full week of charcoal cutting usually cracks virgin fingers.
Charcoal sucks the oils from the unaccustomed skin.
Most modern blacksmiths switched to coke (produced by burning coal), but its higher phosphorus and sulfur content make it a bad choice for very high temperature steel treatments, such as tanren and welding. On the eco side, charcoal is perfectly harmless to the human body, even when inhaled as dust, whereas coal and coke dust slowly kill you.
Sumi-kiri san nen… I think they were trying to be optimistic. It takes a lifetime to properly become able to prepare the charcoal. And I'm slow to start with.