Okay, I was going by this information after a cursory search trying to understand why?
Dry fasting is also known as an Absolute Fast, Black Fast, and Hebrew Fast. This is fasting at its extreme, and usually only attempted for spiritual clarity and/or union.http://www.allaboutfasting.com/dry-fasting.html
I have always ended up with a clean tongue after water fasting, [even my 10 day fasts]. 30 days if done right should leave you with a clean tongue, but then again I only have my personal experiences to go by, not being a health professional.