When do you use the plural form for nouns that are generally considered uncountable?

The plural form is used when the speaker wishes to emphasize a reference to different types of the mass noun in question.

At least in Am. E., "combustible" most often plural when it is used as a noun ("Put the combustibles in that closet") unless you are really singling out one category ("Is gasoline a combustible?", and even that sounds a little odd).

Since it is used more and is a more general term, "material" sometimes can refer to more than one type, where it is just referring to general stuff ("We'll need more material to build a house") but could just as easily be plural if you want to emphasize that it's not just one type ("We'll need more materials to build a house"). Liquid is similar, "Liquids go under the sink" implies a grouping of different liquids, but if it is all of one type, or the type is not important, then singular: "The host of You Can't Do That On Television was covered in liquid".

There is a very common construction in English that pluralizes mass nouns.
Of course, mass nouns don't take plurals normally, but that's the point -- if an unused
construction is floating around, it's very likely to be appropriated for special purposes.
There is no grammatical difference; mass nouns form plurals just like count nouns.

So countification of a mass noun X normally takes one of these semantic shapes:

Vast extenses of X: the seas, the deserts, the slops, the shits
Several different kinds of X: seven liquids, three combustibles, eighteen materials
(This is the second kind.)

There is also massification of a count noun Y, relating to some property of Y: