When port is first bottled its fermentation is not complete; in time it precipitates argol on the sides of the bottle, where it forms a crust. Crusted port, therefore, is port which has completed its fermentation.
The “crust” is composed of argol, tartrate of lime, and colouring matter, thus making the wine more ethereal in quality and lighter in colour.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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