Tears of the Salamander — Peter Dickinson

Alfredo, a choir boy in 18th-century Italy, loses his family in a fire, and his mysterious Uncle Giorgio spirits him away to their ancestral home below a volcano. There he learns that Uncle Giorgio is the Master of the Mountain; he can control the volcano. He is also an alchemist, able to make gold from the tears of the fiery salamander he captured from the heart of the mountain. Alfredo is his heir, the next Master; and as Alfredo learns the history of his family and its power, he begins to suspect that his uncle is actually a fearsome sorcerer.