What happens to a tombstone to fade the lettering?

Tombstone lettering fades due to exposure to various environmental factors, primarily weathering and erosion. Rain, snow, and humidity can cause the stone to deteriorate over time, while UV radiation from sunlight can break down the pigments used in the lettering. Additionally, biological growth like lichen or moss can further contribute to the fading by physically wearing down the stone's surface. As these elements act on the tombstone, the contrast between the lettering and the stone diminishes, making the inscriptions less legible.