Title: That Subtle Science and Exact Art

Author: Jedi Buttercup

Rating: K+ / PG

Disclaimer: The words are mine; the worlds are not

Summary: "Oh. Uh, wow." Dresden reached up to smooth a hand over his mouth. "I don't know how to tell you this, but that's not how potions work in this world, at all." 1800 words.

Spoilers: Post-series for Harry Potter; vaguely post-"Proven Guilty" for the Dresden Files novels

Notes: For xiorlanth, for the wishlist prompt: "Dresden Files crossover. On a wildly different tangent, Professor Snape coming across the Dresden Files school of potion making would be interesting. :)" Originally posted elsewhere November 29, 2015.

"Hells Bells," Severus heard his reluctant host exclaim in the room above the lab where he stood.

"Hells Bells, indeed," he repeated to himself sourly, staring down at the mess on the table before him.

In all the days since he'd been found alive in the aftermath of the Battle of Hogwarts, clinging to life thanks to a prescient dose of anti-venin, only to find himself thrown unceremoniously through the Veil of Death without so much as an interrogation, the discovery of the lab in the subbasement of his new keeper's apartment had been the first ray of hope he'd found. That something in the strange world he'd found himself in instead of the Void was familiar; that he might have a place there, even with his magic refusing to respond properly to his wand and a restrictive new set of rules to learn.

(There were no soft Dumbledores in this strange world, apparently, where wizards carved their own foci and channeled the forces of the universe with words of their own choosing; the moment a wizard was declared guilty of breaking any one of their Unforgivables- which would instantly have doomed every Death Eater to cast Avada Kedavra or the Imperius, but strangely not the Cruciatus- his life was forfeit.)

But what Severus had found down the stairs concealed under the living room rug of the wizard who'd witnessed his unceremonious arrival had been so clearly the workspace of a skilled potioneer, he'd felt immediately at home there. Despite the unfamiliar layout, tools, and even ingredients, the careful labeling, the stains on the worktable, and the evidence of partial use in every container on the many shelves were signs plain enough for even a resident of another reality to read.

Unfortunately, however, whatever force governed the creation of potions in this place was as fickle as that flowing through a wizard's body. He'd been able to locate enough familiar items among the boxes, jars, and baggies to brew a simple boil cure as a test before trying anything more complex. The results had been... unsightly enough that he hadn't even realized Dresden had returned home until he'd heard the man curse.

"Snape? Is that you down there?" Dresden called down the stairs, an annoyed note in his voice.

"Naturally. Or do you commonly return home to find strange visitors in your lab?" he replied, bitingly. The damage Nagini had done to his throat had not been as severe as he had feared, fortunately; only a little hoarseness, and the pulling of the stitches that had closed the wounds, persisted in reminder. The venom had been the greater threat; this was the first day he'd been able to spend more than an hour or so on his feet since his arrival.

"Hey! No need to be sarcastic- especially since I didn't even show you I had a lab. Do you always trespass in your host's private rooms uninvited for kicks and giggles?"

"If you wished me not to know the room was here, you should have chosen a less obvious means of concealment," Severus drawled. "Or directed your familiars to guard it."

He didn't see the point of climbing up the stairs again, only to continue the farce of a conversation from different floors when the wizard inevitably jumped down to see what he'd touched; he remained where he was, still scowling at the reeking, gloppy contents of the cauldron. He'd made no substitutions, and the materials of both the cauldron and the stirring rods had seemed consistent with what he would have used back home. That meant the ingredients themselves must have interacted in unexpected ways. Would he have to relearn everything?

"Mouse, you traitor. No, I don't care how much he needs my help; you think any other Warden would have put up with this? Eb would have fried my ass," he heard Dresden mutter to the enormous gray beast that passed for a dog. Then he heard the sounds of something being dropped into the tin by the door, followed by heavy workboots stomping across the floor.

Nearly seven foot of lanky wizard appeared feet-first down the stairs, next. Severus scowled at Dresden as he descended, making no apology for the state of the worktable.

The wizard was still dressed in the muggle clothing he'd worn when he'd left the apartment two hours before; only the long, leather coat bore any evidence of magic. Another way in which Dresden's reality heavily differed from his own; here, there never seemed to have been any separation of the magical world from the mundane. Robes were a ceremonial or comfort item only, not a fashion choice. One of the first things Dresden had done after Severus' arrival had been to outfit him in jeans. Unlike many wizards back in the world he'd come from, Severus was at least familiar with them, but they were not his preferred choice for attire.

Dresden's face, when it appeared, was set in a scowl; he had a strong-featured face and a stubborn chin, and had his hair been any more unruly Severus would have immediately identified him as a Potter. Especially given his unfortunate first name. But he had neither the scar nor the glasses nor the green eyes of Lily's child, enough differences to weigh against the possibility that Severus had become entrapped in some sort of cursed illusion.

He took in the evidence of what Severus had been up to at a glance as his feet reached the floor, his jaw briefly tightening... but then his eyebrows drew together, and the visible anger transmuted into a more opaque emotion as his gaze lingered on the cauldron suspended over the Bunsen burner and the mortar and pestle beside it.

"Oh," he said. "You were working on a potion? I guess the ingredients must not be quite what you were used to at home, huh. What step were you missing?"

"None," Severus sniffed. "I chose a recipe I could concoct in my sleep, and one whose ingredients were all on your shelves; one I taught to eleven year olds for more than a decade. A boil cure potion."

"There's a specific potion for that?" Dresden's eyebrows went up; then he peered more closely into the cauldron. "Wait, there's a specific potion recipe for that, one that everyone follows? Exactly?"

"Of course. Dried nettles, crushed snake fangs, horned slugs, and porcupine quills," Severus growled. He saw no need to be more specific.

Dresden's eyebrows lifted further; he looked taken aback. "But that's only half a recipe. What base did you use? What sound did you put in?"

"Sound?" Severus shook his head in incomprehension. That did offer some explanation for the container labeled 'cock's crows'; but how did one capture a sound in a bottle? And what did that have to do with potion-making? "What do you mean, a half recipe? I've made gallons of boil cure with those precise instructions; it should have produced a cauldron of blue potion, of smooth, even consistency. Instead of," he curled his lip for emphasis, "this."

"Oh. Uh, wow." Dresden reached up to smooth a hand over his mouth. "I don't know how to tell you this, but that's not how potions work in this world, at all. I'm starting to get the impression your guys' magic system works a lot like science- inexplicable, but it follows rules. Magic here? Not so much. It's an art. Every potion has exactly eight ingredients; but each one is individualized to the brewer. It's more about what the parts mean to you than some universal list of values."

Severus stared, extremely nonplused. "If each one is individualized, then why eight ingredients?"

Dresden snorted, gaze finally settling to meet Severus' eyes. Severus braced himself, but there was no repetition of the extremely unsettling lance of Legilimency that had struck him upon their first meeting; apparently, that had been a one-time measure to ensure that he was what he claimed to be. "I'm not the right person to ask why; it just is. But I can tell you what they are. First, the base: a liquid to hold the others and provide a medium for mixing. It can be anything drinkable, from water to soda to coffee to alcohol. You get the idea. Then five parts symbolically linked to the senses, plus something for the mind and something for the spirit. Brew it up, infuse it with your magic, and voila." He shrugged.

"For the mind? And the spirit? Even given that meaning influences the choice differently for each wizard- how is one meant to choose such ingredients?" Severus prodded at the cauldron, still scowling. He would have to completely rework everything; no doubt nettles and porcupine quills alike would be categorized as provocative agents here, not soothing ones, no matter what sensory category one filed them into. And how would one even begin to approach something like Veritaserum?

"You just do?" Dresden wrinkled his nose. "Okay, how about an example: I made a sense-masking potion once to conceal myself from a werewolf. It started with a base of water. I used plastic wrap for sight; plain cotton for touch; deodorant for smell; a rustle of wind for hearing; a leaf of plain lettuce for taste; a blank piece of paper for the mind; and some elevator music for the spirit. The result was... well, boring is an adequate descriptor all the way around. But that was pretty much what I was going for. If you wanted to make something to cure boils..." He frowned in thought, then turned to one of the shelves and rummaged around, producing a somewhat stained notebook.

It took Severus a moment to realize what Dresden was doing, distracted as he was by the mention of werewolves: producing a stub of a pencil from somewhere and turning to a blank page in the notebook. Then he blinked. "You... intend to assist me?" he blurted incredulously.

Dresden looked up again, equally incredulous, dropping the notebook on the table. Then he shrugged off his coat and exchanged it for a bathrobe hanging on a hook on one wall. "Why not? You got something better to do today?"

"You know I haven't. How would it benefit you to adapt my magic to your world?"

"Besides maybe getting some new ideas for potions, and maybe getting you out from underfoot at some point? I kind of happen to enjoy experimenting with magic. Plus, I'm a little sick of roommates who don't pull their fair share of the rent, if you must know," Dresden added loftily.

Severus gave an aggrieved sigh, then nodded. That was a motivation he could understand... though he would remain on his guard.

"Very well, then," he condescended to agree, and began clearing away the remnants of his failure.