In the 15th century, Alum was very important, used in the refining industries and also used for cleaning. Europe could basically not survive without Alum. So, of course, if a thing is very useful, then of course it must be scarce or hard to get. The only reliable source for Alum in sufficient quality was a spot in Syria owned by the Geonese. The Geonese were experts, and they knew that the way to get the highest price was to put out only a little at a time.

Unfortunately, the Geonese were driven out by the Turks, who then took control of the mines. The Turkish Alum was expensive, and none of the Europeans were getting the money from it’s sale, so many people began looking for new sources. Finally Alum was discovered in the region of Tolfa, and was immediately under the possession of the Medici family;  very rich bank owners. The Alum from Tolfa was immediately tested, and turned out to be of equal, if not superior quality to the Turkish-owned Alum!

So it turns out that one of the reasons the pope (who was receiving a percentage of the Tolfa Alum sales) called for one of the crusades was to put down Turkish Alum competition.