Igros Moshe (OC I #113) Question: Can one wash with soap on Shabbos and Yom Tov?Answer It is clearly prohibited because of the prohibition of smoothing and erasing (memachek). Even if it is as soft as water which people are customarily lenient it is not clear to me that it is permitted since in the washing process it spreads out which indicates there is still a process of smoothing. It might be that the prohibition of smoothing doesn’t apply to a liquid. However in my home I do not permit the use of liquid soap and it is best for all to be strict.
Soap is produced from fatty acids which are oils, eg castor oil, olive oil etc. I don't know what kind of liquid soaps were available 60 years ago.
ReplyDeleteOils and fatty acids are more viscous than water, as is honey.
So what is the definition of spreading? Can you spread water? Honey?.