It is related to the SI measurement of microns as 1 micron = 4 gauge.
What does Mil / Gauge mean when describing a Sleeve Mil/Gauge is the measurement of thickness of plastic Outers & Inner Sleeves. While bleaching removes the lignin, the paper making process produces other detrimental byproducts such as hydrochloric acid or sulfuric acid. Paper is derived from the cell wall of plants lignin, one of the chemicals contained in the cell walls, is unstable and light sensitive and decomposes into acidic products. Cheap, non-acid-free paper sleeves degrade with time.
I am currently testing the 30 day (Pro) trial version, but so far the biggest change I've noticed is the new and confusing user interface.It is nevertheless a safe bet that 99%+ % of Audiophiles & Record Collectors would agree that LPs shouldn't be stored in plain paper sleeves.
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