Final answer:
Steam acts as the mordant in the Schaeffer-Fulton endospore staining method, facilitating the penetration of the primary stain malachite green into the endospore.
Step-by-step explanation:
The substance used as a mordant in the endospore staining method described is steam. A mordant is a substance that helps fix dye on a material. In the context of endospore staining, particularly the Schaeffer-Fulton method, steam acts as a physical mordant that increases the permeability of the endospore coat, allowing the primary stain, malachite green, to enter and stain the endospore. The vegetative cells are then decolorized with water and counterstained with safranin, resulting in the endospores appearing green and the vegetative cells appearing pink.