Complex sentence: As the vessel is fired, any water left in the clay will tend to quickly turn to steam and explode the vessel wall in a spawl; it takes care and time to dry these vessels before they can be fired safely.
Split into simpler sentences: As the vessel is fired, any water left in the clay will tend to quickly turn to steam and explode the vessel wall in a spawl. It takes care and time to dry these vessels before they can be fired safely.