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She ducked into a boy's bathroom and closed a cubical door behind her firmly. In her pocket, next to Archie's miniaturized trunk, was a vial of modified Polyjuice potion that Harry's fingers were itching to uncork.

It had taken her several days of straight research (in between breaks for Remus' merciless training exercises) to find an ingredient to add to the Polyjuice recipe that would make it permanent but also reversible. It turned out that the key wasn't in adding an ingredient to the actual recipe, but to the cauldron while the potion was being prepared. At first Harry didn't see the difference, because if you added something to the cauldron while you were making a potion, then surely you were adding it to the potion, but in the case of certain additives, you could add without adding, if that made sense.

Amber was the key. She had never used stones in potion making before, which was perhaps why it had taken her so long to come up with the solution, but once she started looking into it, she found that amber was a stone of permanence. The ancient Egyptian wizards used to place amber stones inside caskets, because the magical energies the stone gave off preserved the body and ensured that it would forever remain whole. Gems, like many things in the world, were excellent magical lodestones. They absorbed natural magical energies from the earth, but because each type of stone had different properties, each stone shaped the wild magic a different way once it was absorbed.

It was similar to the way a witch or wizard's magical core shaped magic in different ways depending on the wizard's personality, physical condition, disposition, etc. Each type of stone imbued the wild magic it absorbed over time with different tendencies as it shaped the magic—passively, of course. Stones were not like wizards, who could overcome their natural tendencies toward shaping magic with simple intent and willpower. The magic in an amber stone would be similar to the magic in all other amber stones, because the nature of the stones were similar, and that was the entire determining factor in how the magic from the stone was shaped.

When a stone was added to the cauldron while a potion was brewing in it, the magic interacting in the cauldron drew out the magic stored in the stone. It was much like what she'd done with the modified Weightless Draft, in that it was a matter of adding shaped magic instead of imbuing the potion with only raw magic, only the shaped magic came from the stone, not from her. She couldn't have accomplished the same effect with her own magic because unlike with the imbued Wingardium Leviosa spell, she didn't know how to shape it into the sort of magic that would cause permanence on the scale she needed. The stone, which was dipped in a protective potion to protect its physical form, didn't interact with the potion in any way except to release its magic into it. The type of magic released depended on the stone. Amber was not actually made of minerals from the ground, as other gemstones were, but was in fact tree sap which had fossilized over millions of years. Its magic, when included in a ritual, could lend anything from power to protection to the wizard invoking it, depending on the controlling runes involved. In a potion, however, it almost always increased the length of time the potion was effective—exponentially.

If her calculations were correct, the Polyjuice she had brewed on one pea-sized stone of amber should hold the ingredients which effected the reverse transformation in stasis for a little less than a year, thereby keeping the drinker in the transformed state for at least that long. Which was perfect, because she and Archie were going to re-do the blending spell every year and re-take the potion anyway. The only down side was that, because the potion could only be reversed by waiting for it to wear off, they would be unable to resume their original appearances immediately if the need arose. Not that she thought such a need was likely.

Probably the reason no one had done serious studies in the use of amber in Polyjuice was because so little increased the active time by so much that it was impractical for anyone who didn't want to remain transformed for months at a time. Really, who would want to be someone else for that long? The need was for a Polyjuice that worked for three, maybe four hours at a time, not three or four months.

Unless of course you were prepared to lie both explicitly and implicitly to everyone you knew for several years, in which case amber was the perfect solution to the Polyjuice problem.

http://www.fanfiction.net/s/8239413/3/