Reduction vs. Closed Phase: Two Causes

The standard

A diagnostic aeration test tells the two apart by their response to oxygen: reduction noticeably lifts with swirling or brief decanting, while a closed phase remains largely unaffected.

Criteria

• The aromatics change measurably after aeration, or they persist
• The cause is identified as chemical, or recognised as age-related
• The observation draws on several glasses, not the first impression
• The judgement stays provisional until the test is repeated

Distinction

Reduction ≠ wine fault: reduction is reversible and only becomes a fault if it persists. A closed phase ≠ reduction: it has no chemical cause and resolves through time, not through air. Without a diagnostic aeration test, any attribution remains a guess.