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In Wraith: The Oblivion, plasm is the generic "stuff" out of which most wraithly material is manufactured. Plasm is not a substance in the physical sense - lacking atoms, molecules or a dipole moment, but it is close enough for soulforging work.

Since Wraith is not science fiction, the actual behavior and composition of plasm (even, to a certain extent, what parts of the underworld are plasm) is open to debate. However, at the minimum, there is both liquid and solid plasm - the liquid is used to temper items made with soulforging, and the solid is everything else. In general, the only source of plasm is the bodies of wraiths; any artifact in the wraithly world has been made of plasm, and consequently is maede out of a wraith.