Question: I walked through some muddy grass on Shabbos. Can I wipe the mud off my shoes before going back inside?
Answer: There are a few potential issues with scraping off mud from one’s shoes on Shabbos:
The Gemara (Shabbos 124b) relates that Rava’s shoes got muddy one Shabbos and he used a piece of earthenware to clean them off. The Gemara (ibid 141a) discusses whether one may use the back of a knife, and whether this applies equally to old and new shoes. The Gemara discusses whether one may scrape one’s shoes on the ground or wall as doing so may be fixing a small crack, an issue of boneh, building. According to the Mishna Berura (302:28) this wouldn’t normally be so problematic, especially when rubbing them against a wall (See Shevet Halevi 5:37).
The Shulchan Aruch (OC 302:7) writes that one mustn’t remove mud that has properly dried up as scraping the shoe would cause the soil to crumble which would be considered tochen, grinding.
A third concern that the poskim discuss is memachek, smoothing the shoe. Thus, the Mishna Berura (302:26) allows one to scrape the mud off using a blunt mud bar, though not with the back of a knife (See Kaf Hachaim OC 302:47; 59). Likewise, the Aruch Hashulchan (OC 302:17) writes that one may use something blunt to remove any mud.
R’ Shlomo Zalman Auerbach (Shemiras Shabbos Kehilchasa 15:39) writes that this issur applies equally to non-leather shoes.
R’ Benzion Abba Shaul (Ohr Letzion 2:24:5), however, writes that as leather is processed differently nowadays, there is no issue of memachek.In conclusion, one may scrape the mud off one’s shoes with something, providing that it isn’t too sharp.