Was it possible to reach a purity percentage of 100 per cent? Practically, it was still impossible. There was always a small unmeasurable percentage of impurity at a level where filtration could not be carried out further. That was the conclusion that alchemists had come to after years of attempting to create a pure potion.
The purity of a potion was tested using a switch stone. The purer a concoction, the closer it came to a perfect theoretical ratio of magic to solvent. A switch stone dropped into a vial of potion would react to the presence of magic in it and change from a white to a blue colour. Through a process of trial and error, and tedious calculations, alchemists had come up with a switch scale that could identify the level of purity to a very precise degree.
