Even if we did have cookbooks from the era, they didn't have thermometers. We'd have things like Cai Xiang's Record of Tea.

If you see tiny “shrimp eyes” in your water, he wrote, the water is roughly 155°F. If you see “crab eyes”—a slightly bigger bubble—then you’re reaching 175°F. Larger “fish eyes”: 185°F. String of pearls: 195°F to 205°F. And finally, when your hot water has reached a rolling boil, or a “raging torrent,” you know that your water is 212°F."