John Lightfoot on Matthew 23:27

[Ye are like whited sepulchres.] Sepulchres are distinguished by the masters of the Jews into a deep sepulchre, which cannot be known to be a sepulchre; graves that appear not [Luke 11:44]; and a painted sepulchre, such as were all those that were known, and to be seen. Our Saviour compares the Scribes and Pharisees to both; to those, in the place of Luke last mentioned; to these, in the place before us, each upon a different reason.

Concerning the whiting of sepulchres, there are these traditions: "In the fifteenth day of the month Adar they mend the ways, and the streets, and the common sewers, and perform those things that concern the public, and they paint (or mark) the sepulchres." The manner is described in Maasar Sheni; They paint the sepulchres with chalk, tempered and infused in water. The Jerusalem Gemarists give the reason of it in abundance of places: "Do they not mark the sepulchres (say they) before the month Adar? Yes, but it is supposed that the colours are wiped off. For what cause do they paint them so? That this matter may be like the case of the leper. The leprous man crieth out, 'Unclean, unclean'; and here, in like manner, uncleanness cries out to you and saith, 'Come not near.'" R. Illa, in the name of R. Samuel Bar Nachman, allegeth that of Ezekiel; "If one passing through the land seeth a man's bone, he shall set up a burial sign by it."

The Glossers deliver both the reason and the manner of it thus: "From the fifteenth day of the month Adar they began their search; and wheresoever they found a sepulchre whose whiting was washed off with the rain, they renewed it, that the unclean place might be discerned, and the priests who were to eat the Trumah might avoid it." Gloss on Shekalim, and again on Maasar Sheni: "They marked the sepulchres with chalk in the likeness of bones; and mixing it with water, they washed the sepulchre all about with it, that thereby all might know that the place was unclean, and therefore to be avoided." Concerning this matter also, the Gloss speaks; "They made marks like bones on the sepulchres with white chalk," &c. See the place.