John Lightfoot on Matthew 6:16

[They disfigure their faces.] That is, they disguised their faces with ashes; as he heretofore upon another cause, 1 Kings 20:38: "In the public fasts every one took ashes, and put upon his head. They say of R. Joshua Ben Ananiah, that, all the days of his life, his face was black by reason of is fastings. Why is his name called Ashur? (1 Chron 4:5). Because his face was black by fastings."

Here let that of Seneca come in; "This is against nature, to hate easy cleanliness, and to affect nastiness."