Ruling 185

The sun purifies earth, buildings, and walls on five conditions:

1. The impure object is sufficiently wet, such that were something else to come into contact with it, the latter would become wet. Therefore, if the object is dry, it must be wetted by some means so that the sun can then dry it;
2. No intrinsic impurity remains on the impure object;
3. Nothing prevents the sun from shining on the impure object. Therefore, if the sun shines on the impure object from behind a curtain or cloud etc. and makes it dry, the object does not become pure. However, there is no problem if the cloud is so thin that it does not prevent the sun from shining on the object;
4. The sun must dry the impure object by itself. Therefore, if, for example, an impure object is dried by both the wind and the sun, it does not become pure. However, there is no problem if the drying of the object can be commonly attributed to the sun shining on it;
5. The sun must dry the building that is impure in one go. Therefore, if one time the sun shines on impure earth or a building and it dries its surface  and another time it dries its underside, then only its surface becomes pure and its underside remains impure.