Once again, I do not own Harry Potter or Severus Snape or Hogwarts or anything else in this story. Well except for anything you do not recognize as the work of JK Rowling. Which might include a few characters and the general lack of plot.

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Severus Snape was determined to spend the evening in quiet isolation. He had ordered his dinner from the kitchen. His classes were over for the day and he was not assigned corridor watch until nearly dawn.
He most especially was not going to think about James Potter's insolent-brat-of-a-famous-son.
Snape opened the book he was studying. He soon immersed himself in the intricacies of potions. It was incredible how a dozen different procedures could yield the same results. Conversely, it was fascinating how sometimes changing one ingredient in the slightest way could warp the results into something else entirely.
His meal had appeared unnoticed, and he was almost to the section of the book that he most wanted to review—drat Longbottom anyway—when there was a timid knock on the door.
Snape looked up, scowling. For a moment he considered pretending he was not there. It was his night off, after all. Oh, why hadn't he gone up to London after gillyweed or some such thing?
The knock sounded again, firmer this time.
"Yes?" Snape bit out. "Who is it?"
The door opened slowly and the head of a Hufflepuff first year appeared.
"Professor?" A girl asked quietly.
"Yes, come in." Snape said with a sigh. "What can I do for you Miss Pyrene?"
"I was just wondering if you could recommend a book on potion ingredients for me." She spoke hurriedly, as if fearing interruption. "I have been reading through Magical Drafts and Potions. Some of the formulas are just like the ones in the book my father uses, and some of them have one or two different components." She drew in a deep breath then continued. "I was just curious what specific characteristics of those particular ingredients allow them to substitute for each other in those spells and not in others." The girl stopped there and adjusted her glasses.
"I see." Snape drawled. "Well, there are a few titles I can recommend." He scrawled on a scrap of parchment. "These should be in the library." He paused before handing it to her. "They are a bit advanced for a first-year. Most students do not make the connection that similar ingredients can occasionally be substituted. It can be very dangerous."
The girl rubbed her chin. There was a small, irregular burn scar there. Snape would not have noticed it if she had not touched it.
"I have learned that lesson firsthand, sir." She said with the firm assurance of a much older person. "Magical laws are a bit different in the US. I have brewed many potions, most of them very simple. I have been apprenticed to my father practically since I could walk."
"Pyrene?" Snape said thoughtfully. "Not Flavius Pyrene?"
"Yes, sir. He said he knew you." She colored and pulled a small parcel out of her robes. "Actually he asked me to give this to you. It arrived this morning in a larger parcel."
Snape took it and opened the letter first.

Severus,
I do hope that you remember me fondly, in spite of the argument we had when last we spoke. I have been living quietly in America with my wife, Rowena, who you know, and our daughter, Gwylan, who you have met by now. We have tried to raise her in both magical and muggle worlds. (which I know you do not approve of)
I have sent you something that I think you might enjoy. Take care that you do not give my daughter any favoritism or hardships for my sake. I have sent her away because I do not think I can teach her further without prejudice.
Once your friend,
Flavius Pyrene

Snape folded the paper and replaced it in its envelope. Then he opened the little package. It was a small, leather bound book entitled Magical Potions of the Ancient Mayans by Flavius Pyrene. Snape tossed the wrappings into his fire where they turned the flames blue.
"Thank you, Miss Pyrene, you may go." He scrawled one more title onto the parchment before handing it to her. The girl left quickly.
Snape smiled when she had gone. An actual, genuine smile.
Flavius.
They might have been friends, almost. Signs pointed more towards them having been mortal enemies. They had been forced to sit next to each other in potions for several years. Their professor had been so fed up with the antics of Potter and his gang that he assigned permanent seats that separated all housemates from each other. He and Flavius had been lab partners ever since. They were forced to cooperate on all manner of assignments inside and outside of class. Throughout school Snape would have said they loathed each other.
It was Flavius who had stirred his love for potions. Flavius could tell you the exact properties of any spell component. It was tough to keep up with him sometimes.
Flavius had sent his daughter to him to teach. It felt unreal. Snape felt deeply honored.
Snape thought back to the last time he had seen Flavius. It was a few years after they had graduated. They had argued terribly. About Lord Voldemort and about Rowena, Flavius' muggle fiancée.
"Severus, is that you?" Snape's fire suddenly asked.
"Yes, Flavius." Snape answered wearily. "I'm here."
"Hah! I thought you would throw the wrapper into the fire. Habits, old boy, habits." Flavius's head chuckled merrily from among the flames. "So, can you forgive me, Severus?"
"For what?" Snape drawled wryly "For being right or for marrying Rowena?"
"Both, either, for not being more of a friend." The head in the flames shook sadly. "I followed your trial from our hiding place in America. I suppose I should have gotten in touch then, but…"
"We both said unforgivable things that night." Snape broke in. "It is better not to dwell on them. How is Rowena?"
"She is doing very well. We make a good match, she and I. How about our little Gwylan? Not a bad kid, eh? I have taught her a lot of potion theory, and only a little of the practical. She seems to have the knack for it."
"We'll see about that." Snape growled good naturedly. "Sometimes too much theory can be a bad thing. I will keep an eye on her."
"Thank you, Severus. I have regretted our argument over the years."
"I was supposed to kill you that night and Rowena too." Snape said suddenly. "I could not do it."
Flavius drew in a deep breath. "Perhaps we could be friends again."
"Were we ever friends, Flavius?" Snape asked almost wistfully.
"Perhaps, Severus, perhaps. I have certainly missed our discussions." Flavius turned as if listening to someone else. "I have to go. I have linked our fires if you would like to chat again." Flavius grinned. "Bye."
The face in the fire disappeared with a pop.
Snape sat back in his chair, his research forgotten. Had he and Flavius been friends? It was an interesting question. Flavius seemed to think they had been. They had studied together, but throughout school they had maintained a façade of hatred and rivalry.
Snape thought that perhaps Flavius understood him. At least as well as anyone ever had. Well, except for Dumbledore.
Snape touched the book that Flavius had sent him. He flipped through it chuckling. They had discussed this sort of thing while they were in school. The possibilities of making potions using plants that grew only in the Americas.