The raw materials for soulforging can be anything made out of plasm. This includes wraiths, Spectres and Plasmics. Presumably Stygian horses as well; however, nobody has ever dared to soulforge the Stygian steeds.
Given that soulforging is tantamount to a death sentence, there is a definite hierarchy of preference in what wraiths are used to manufacture raw materials. As a rule, wraiths prefer to use Drones, since they aren't sentient anyway; soulforging is rationalized by Stygia is a preventative measure, so Stygian wraiths generally prefer to soulforge those wraiths who were "heading for Oblivion anyway". Spectres are considered fair game for soulforging, as are plasmics.
Stygia also regularly soulforges criminals. Harrowings mean that anything that would generally be considered a death sentence would feed oblivion, so by soulforging offenders, the Hierarchy shores up its defenses.
Theoretically, a piece of a wraith can be soulforged. The tithe of the dead used this approach, and artificers are initiated by soulforging a part of themselves. However, this damage is apparently permanent and not a viable technique for producing mass materials.
Soulforging is used to manufacture "hard goods", such as wheels, beams, walls, bricks and boards. The product of this activity is substance generically called soulsteel. Soft goods, such as cushions, are manufactured using Moliate. Soulforging is a craft, not an Arcanos, but is traditionally controlled by the Artificer's Guild.
The raw materials are exposed to the soulfire until it begins to "melt" and lose its integrity. This Corpus is then placed on the anvil and hammered into shape by the forger; repeated exposure to the flame and anvil are applied as necessary. The process is incredibly painful for the subject, and most wraiths go screaming to the forges.
As a rule of thumb, a single wraith can be made into at most one good - such as a brick, a sword, or the like. Since wraith is not science fiction, conservation of mass does not apply - the same Corpus could be moliated into a small brick or a large suit of armor. Certain exceptions exist - Stygian chains are made out of multiple wraiths linked together, with each individual wraith providing several dozen feet of chain.
The question that nobody deals with when discussing soulforging is whether the target is still self-aware after the event. Soulforged artifacts tend to mutter and moan, but everyone hopes that this simply means that they can vocalize and nothing else. Nhudri and several artificers believe that the act of soulforging obliterates the consciousness, and the Steel Martyrs of the Silent Legion take this as an article of faith. Soulforging, however, is irreversible, so nobody truly knows.
Alternative Techniques Edit
As noted above, moliation is the major Stygian counterpart to soulforging. In addition, the Way of the Farmer is used to create White Jade, used for the same purpose in the Yellow Springs. In Swar, the Tvashtriya use their distinctive arcanoi to manufacture goods.