‘And Yahweh said to Moses, “If her father had but spit in her face, should she not be ashamed seven days? Let her be shut up without the camp seven days, and after that she shall be brought in again.” '
Yahweh's reply was stern. It was important that Miriam learned her lesson. She must face up to her shame. A spit in the face was an insult, and depicted someone who had not done their duty (Deuteronomy 25:9), and when coming from someone who was unclean, it rendered unclean (Leviticus 15:8). This being like one spit in the face compares with Moses' meekness (Numbers 12:3 - see analysis in that verse). He was worthy, she was not. Furthermore to be spat on in the face by her father would be even more humiliating and devastating. It would mean that she had done something very dishonourable and was being virtually disowned. It may well be that to so be spat on by a father rendered a woman unclean for seven days, although we are nowhere told so. But whatever the situation was about that, Miriam was to go through a seven day cleansing outside the camp. It would in fact strictly be necessary because of her skin disease, even though it was presumably cured immediately, for a skin diseased person could not be clean until seven days after they were found to be free from their disease. But she had to recognise that it was because of a sin that deserved the utmost contempt.