The comments on my posting "Commercial categories" struck me as useful, and also fascinating. They illustrated my observation that though technical, semi-technical, and everyday uses of expressions can be distinguished, these uses aren't fixed in stone, but can vary from person to person and time to time; and that both what's included in a category and also the label that's used for that category can vary in the same way.
Now comes a technical usage that was new to me, in a NYT Science Times article on Tuesday, "Anything But Clear", by Kenneth Chang, on glass. Two small points (none of them new): a technical usage that's an extension of everyday usage; and a mass-to-count conversion, taking a noun denoting a substance X to a noun denoting a type of X.
Here's the relevant bit (p. D4), with crucial expressions bold-faced:
For scientists, glass is not just the glass of windows and jars, made of silica, sodium carbonate and calcium oxide. Rather, a glass is any solid in which the molecules are jumbled randomly. Many plastics like polycarbonate are glasses, as are many ceramics.
Understanding glass would not just solve a longstanding fundamental (and arguably Nobel-worthy) problem and perhaps lead to better glasses …
Start with the first and fourth bold-faced occurrences ("glass is not" and "understanding glass"). Here glass is a mass noun denoting a substance — but not just the substance that the ordinary-language word glass refers to, but a wider class of substances that are structurally similar to ordinary glass. This is an extension of everyday glass for service as a technical term. Compare the use of metal as a technical term in chemistry, where it takes in the metals of ordinary usage, plus some other substances, like potassium, calcium, and thallium.
(Sometimes ordinary-language terms are adapted for technical usage by contraction rather than expansion: the ordinary-language term bug is used in entomology for a specific family of insects, for instance. Often the relationship involves complex overlap rather than simple expansion or contraction, as with the ordinary-language and botanical uses of nut and berry.)
Then we have the mass-to-count (M-to-C) conversion to denote 'type of [substance] X' rather than X. The converted M noun is now able to occur with the indefinite article a ("a glass is", above) and the numeral one and in the plural ("are glasses" and "better glasses", above), and so on. (For more than you probably want to know about C/M and related grammatical concepts, see my posting "Plural, mass, collective" here.) For substance-denoting M nouns, this conversion is so generally available to speakers of English that dictionaries rarely list the type-denoting C uses as subentries, even for M nouns that are very frequently used this way ("We have more than fifty beers in stock and more than two hundred wines"), not to mention those that are in C use in more restricted contexts (as with C sand 'kind of sand'). In fact, the examples dictionaries give often include both M and C uses, as in NOAD2's examples for metal:
vessels made of ceramics or metal [M]
being a metal [C], aluminum readily conducts heat
(Yes, there are other M-to-C conversions, with different semantics, as in the "serving" conversion of "Bring us three beers" 'Bring us three servings of beer'.)