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Thesis
Read through modern tea science, “第一煮水沸而弃其沫之上有水膜如黑云母饮之则其” is not a lab result but a set of variables: cultivar, soil, light and plucking standard.
Text
“When the water first boils, skim off the film that rises on the surface, like black mica; drinking it spoils the flavor.”
Context
水膜：头沸水面的浮膜。黑云母：黑色云母片。味不正：味道不正。
Evidence
- Collation: 各本一致。吴觉农注：此"水膜"为水垢与杂质浮沫，撇之使茶汤洁净。
- Existing readings: 吴觉农指出"弃其水膜"是唐代"洁水"的关键一步：水沸后矿物质与杂质浮于表层，撇去则味正——现代煮水同样讲究"刮沫"，此传统千年未变。
Analysis
Modern tea science would split “第一煮水沸而弃其沫之上有水膜如黑云母饮之则其” into cultivar, soil, light and plucking variables. 吴觉农指出"弃其水膜"是唐代"洁水"的关键一步：水沸后矿物质与杂质浮于表层，撇去则味正——现代煮水同样讲究"刮沫"，此传统千年未变。 They remain testable, but only with their conditions attached.
Conclusion
The section does not give a universal rule. It demonstrates a method: read the text, the annotation and the Tang context together before making a modern claim.
Source: The Classic of Tea, Chapter IV · The Vessels, section 8; the original is in the public domain.
Experience should be explained, not mystified.