According to modern potion classifications, preparations could be divided into seven categories:
First, decoction: the common method of placing ingredients into a cauldron in sequence and boiling them in water.
Second, infusion: pouring boiling water over herbs as if making tea, then waiting fifteen minutes.
Third, maceration: placing ingredients in fat, like making an oil salve, until the herbs lost their original color and the fat gave off a strong herbal scent.
Fourth, ointment: placing herbs into solid fat and treating the herbs, the ointment, or both.
Fifth, poultice: soaking herbs in boiling water, pouring out the water once the herbs were thoroughly saturated, then wrapping the herbs in gauze and applying them directly.
Sixth, tincture: the proper method was to steep herbs in alcohol for at least two weeks.
Seventh, wash: when Slughorn had served as Potions professor, washes had been fashionable. Whether it was Lockhart's shampoo, the Potter family's hair lubricant, or Slughorn's bath solution in the Prefects' Bathroom, all belonged to this category.
The headmaster at the time had followed Slughorn's advice and allowed him to provide bath gel for the Prefects' Bathroom through the method of an "Eternal Potion." An Eternal Potion did not mean that drinking it would make someone remain "eternal" in their current state, although its ingredients included vampire ashes. It referred to a state of inexhaustibility: once used up, it refilled itself.
At first, no one had thought much of it. They simply wanted to save money and trouble, considering how large the pool in the Prefects' Bathroom was. It was only after Severus began using bath gel in the resident teachers' bathroom that better suited each individual's traits that everyone realized replacing the bath solution in the prefects' bathroom was no longer possible.
According to Albus Dumbledore's understanding of the Fireseed