a singular noun after the word "mass" [closed]

Paper here is a mass noun (Grammar Handbook), which means that, rather than being a countable quantity of leaves, paper (and therefore the noun phrase charred paper) refers to the material of the mass. The material is being emphasized, rather than the form of the leaf or page.

Compare these examples, the first using another mass noun (iron) in a similar situation and the second using paper as a mass noun in another situation.

The ancient village of Newstead, where this mass of iron was found many years ago, lies at the eastern extremity of the valley of Melrose ... (Notice of a mass of meteoric iron..., 1862; iron denotes the material of the mass)

Make original stationery by stamping several sheets of paper with the stamp. (Creative Art and Activities, 2004; while there are multiple sheets involved, paper denotes the material and therefore remains singular)

Finally, it is also possible to have a mass of papers. That merely relies on the count definition of paper, and emphasizes the number of sheets of paper.

To do our work, we all have to read a mass of papers. ("Brevity," Winston Churchill, Memo, 9 August 1940. Quoted at University of York).