The plant which first induced John di Castro to search for alum was that evergreen, prickly shrub, the _Ilex aquifolium_, or holly, which in Italy is still considered as an indication that the regions where it grows abound with that salt. But though it is undoubtedly certain that the quality of the soil may be often discovered by the wild plants which it produces, it is also true that this shrub is frequently found where there is not the smallest trace of alum; and that it is not to be seen where the soil abounds with it, as has been already remarked by Boccone[528] and Tozzetti[529]. Among the earliest alum-works may be reckoned that which was erected at Volterra, in the district of Pisa, in 1458, by a Genoese named Antonius[530]. Others say that it was constructed by an architect of Sienna; but this opinion has perhaps arisen only from the work having been farmed by a citizen of Sienna, or built at his expense. On account of this alum-work an insurrection of the inhabitants of Volterra broke out in 1472; but it was at length quelled by the Florentines, who took and plundered the city[531]. Brutus, who wrote his History of Florence in the year 1572, says that this alum-work was carried on in his time: but this is certainly false; for Raphael di Volterra[532], who died in 1521 in his native city, expressly tells us that in his time alum was no longer boiled there; and this is confirmed by Baccius[533], who also lived in the sixteenth century. At present no remains of it are left; so that Tozzetti was not able to discover the place where the alum-stones were broken[534]. It appears from what has been said, that the art of boiling alum in Europe was first known in Italy, but not before the year 1548. That document therefore of the year 1284, quoted by Tozzetti, and in which alum-works, _alumifodinæ_, are mentioned, must, as he himself thinks, be undoubtedly false[535].