When can you pluralize uncountable nouns?

Many nouns which are normally uncountable are potentially countable in certain contexts. It’s a matter not so much of grammar as of the nature of the object to which the noun refers. There can clearly be different kinds of foil, and that makes it possible to speak of foils. Matter is a concept that that lends itself less well to pluralisation, but the Corpus of Contemporary American English has this record:

A 1992 study estimated that CSOs release between four and fourteen billion pounds of solids and organic matters on a yearly basis nationwide.

Organic and inorganic matters might in some contexts be an unwise formulation, because it could be taken to mean ‘organic and inorganic topics’.

David Crystal comments on the pluralisation of uncountable nouns in the latest post on his blog.

If the company is producing aluminium foils of different dimensions, their produce is not single hence it is right to say FOILS, as one foil is different from the other.

The word "matter" can be referred to in several contexts such as "Substance", "Tpoic" etc. In the context of "substance", it is the common material using which everything in this world is created. There is no "This matter" and "That matter". hence it is both singular and plural. In this context, it is wrong to say "matters". In the context of "Topic", one topic is different from another. hence it is right to say TOPICS.