When making quills, they needed to be degreased. People often saw swans preening their feathers by the water. They did not do so merely for elegance; more importantly, they spread a layer of oil over their feathers so they would not become wet when splashed, allowing them to float on the water.
Animal fat oxidized easily and then gave off an unpleasant smell. Even swans were no exception. At the same time, the iron in blood accelerated the spoilage of fat. Even refrigerated fish spoiled quickly, whereas butter and lard did not.
Pomona found a compound called plaister in Cassino's notes. It was somewhat like ointment, a thick oily substance, though harder than ointment, and it could be made into candles. The candles floating above the Hogwarts Great Hall had once been handmade. Anyone with nothing better to do than stare at them would notice that they varied in length and thickness.
The atmospheric charm synchronized the weather outside the Great Hall, making its ceiling resemble a glass roof. Yet rain and snow could still fall through it. If someone discovered that their soup had been chilled by rain while eating, they would surely lose their appetite.
Those candles served as protection. Rain and snow vanished on their own when they fell upon them. As for where the rain and snow fell, that was not something people could control, so candles in some places burned down faster and had to be replaced.
Pomona had originally planned to ask Albus to borrow a feather from his pet Phoenix, scrape off some oil, and mix it with the ashes of Fireseed Bushes burned to ash by dragon fire to make plaister.
"You misunderstand," Albus said with a smile. "We don't want the Fireseed Bushes to come back to life. We want them