The following is an excerpt from the New Yorker:

The Newkirk Avenue building was nondescript, a hunk of brown brick. “Wow, it’s bleak,” Grant said, standing in the courtyard, which was covered in patchy grass. “The contrast between here and Point Dume of Malibu couldn’t be more extreme.” Grant, who has kept a diary for more than fifty years, has published two gossipy volumes littered with boldface names. “Who, and how, and where people come from so informs everything about them,” he said. “So now I’m in actual Streisand Land.”

-- From Richard E. Grant Hearts Barbra Streisand

My question is why is it 'a hunk of brick' instead of 'a hunk of bricks?'

Brick is countable.