Severus had done extensive research using not only the less-than-savory book shops on Knockturn alley, his parents' old library, but also the restricted section at Hogwarts. Sometimes he wondered why Dumbledore allowed him in there during the summer holidays. The man had to know what the information was used for. Again, it was another example of Dumbledore's behavior that Severus found maddening. He prided himself on being able to determine what motivates an individual, but Dumbledore was the single individual who still mystified him on a regular basis.
Unfortunatly, many of the sources contained contradictory information, and sussing out the true formula was proving exceedingly difficult. He'd spent the past few evenings and any other time he was not busy restocking Rook's Potions engaged in failed experiment after failed experiment, and 36 dead rats were all he had to show for his labor. Oh, the potion would kill, certainly, but the manner of reaching death was never as it had been described in the texts. It was swift instead of slow, or nearly painless instead of agonizing or was clean instead of bloody. He had discovered 36 formulas for a very efficient pesticide, but no Heartsbleed. The Dark Lord would not be pleased.