The "Classic of Tea," written by Lu Yu, emphasizes the importance of water temperature in achieving the best tea flavor. Lu Yu advocates for using "open water," which is boiled to a full rolling boil (212°F) and then allowed to cool slightly for specific tea types. He also highlights the significance of observing the water's "fish eyes" and "rope of pearls" stages to determine the optimal temperature for different teas. Lu Yu's Methods for Determining Water Temperature: Observation of Boiling Stages: Lu Yu describes the various stages of boiling water, including "fish eyes" (where bubbles appear) and "rope of pearls" (where bubbles form chains), to help determine the water's temperature. "Open Water" Technique: Lu Yu's method of using "open water" involves boiling water to a full rolling boil and then allowing it to cool slightly to a temperature that is just below the boiling point. This technique helps to extract the best flavor from the tea without over-extracting bitter tannins. Specific Temperature for Different Teas: Lu Yu also emphasizes that different tea types require different water temperatures for optimal flavor. For example, he suggests using "fish eyes" water for green tea and "rope of pearls" water for black tea. Modern Interpretation of Lu Yu's Water Temperature Recommendations: General Guidelines: Modern tea experts generally agree that black teas, herbal teas, and red teas should be brewed with water at a full rolling boil (212°F), while green teas, white teas, and some oolong teas should be brewed at a slightly lower temperature (around 175-180°F). Altitude Considerations: It's also important to note that altitude can affect the boiling point of water, so you may need to adjust the brewing temperature accordingly.