One of the 39 forbidden Melachot on Shabbat from the Torah is the task of ‘Lash’. This can be described as kneading. The classic case is adding water to flour, and mixing it together in order to create dough. Essentially, it is the forming a dough-like substance from a solid and liquid. So this restriction prohibits the adding of water to dirt, as such a mixture would coagulate and solidify.
The question was asked about a Mohel who performs Metzitza at a Brit Milah on Shabbat. As we learned in a previous Daily Halacha entitled "The Issue of Metzitza At A Brit Milah", it is our custom for the Mohel to use his mouth and draw out some of the blood from the incision. After drawing out this blood, the Mohel would usually spit that blood onto earth. The Mohel usually brings some dirt with him, and he would spit the blood into that dirt. Does this action constitute Lash? Would this be forbidden at a Milah that takes place on Shabbat? This question carries even more weight, because many Mohalim mix some wine in their mouth with the blood, meaning they are spitting blood with wine onto the dirt. So in essence, the Mohel is spitting a liquid onto dirt, which as we know, will solidify. Is this permissible on Shabbat or not?
Rav Chaim Ben Attar (1696-1743, known as the Ohr HaChaim) in his sefer Rishon L’Tzion, writes that spitting blood onto dirt on Shabbat is forbidden. All the more so, it would be forbidden to spit blood mixed with wine onto dirt on Shabbat. The Rabbi explains that this is in deed a violation of the restriction of ‘Lash.’ Spitting blood onto dirt is actually the adding of a liquid to a solid, which will result in Lash. Rav Chaim Ben Attar also brings to our attention that it is the desire and intent of the Mohel to mix the blood (liquid) and the dirt (solid.) According to the Kabalah there is a belief that this blood when mixed with dirt is a Tikun (spiritual remedy) against the Nachash. The Zohar also has strong ideas about the benefits of mixing blood with dirt. So according to Rav Chaim Ben Attar, it is clearly forbidden for the Mohel to spit the blood onto the dirt, as this would be Lash with clear intent. That is the opinion of Rav Chida as well. So Lechatchila, the Mohel should refrain from spitting onto dirt on Shabbat at the Milah.
Chacham Ovadia Yoseph however, in his sefer ‘Liveat Chen’, siman 65, is lenient on this and does rule that it would be permissible for the Mohel to spit blood onto dirt on Shabbat.
Therefore, we come out and say that those who are lenient in this case certainly have what to rely on based on Chacham Ovadia Yoseph. But, it would be best to be strict on this like Rav Chaim Ben Attar and Rav Chida.
See Menuhat Ahava, Helek 2, page 288.
