I would turn to the tile setter that installed your slate. Did he seal or enhance the slate? Commercial tile shops will have products that can help you but I would let a professional do the orginal cleaning. A home remedy that you may want to try is OXY-CLEAN and a hot water. It works for tile if sealed correctly but not sure how you will fair on slate. Good luck wish I could be of more help. If you wipe down your stale walls every time you use the shower that will stop some of the build up.

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Monica,

The likely culprit is efflorescence. Yes that's spelled correctly — google it and you'll find that it is a formidable foe. Efflorescence is caused by moisture carrying salts and minerals to the surface where the moisture then evaporates, leaving a whitish film, or in extreme cases, thick fuzzy deposits like hoarfrost. A slate mantel would probably not have any issues, but in a bathroom, the frequent moisture in the air permeates the porous slate and draws out the minerals.

When you try to wash it off, chances are you'll just re-dissolve the minerals back into the slate, where they will come out again later. It's a frustrating problem, and one that has literally changed the course of my career as an artist by making me avoid using cementitious products which are notorious for efflorescence. If it weren't for efflorescence, I'd probably have a nice line of bas relief cement tiles...but fear of customers calling a year after the install and saying, "my installation has a white deposit on it, what should I do?" keeps me from it!

A full treatise on the subject would take me all day, and others have already addressed the topic, so just google efflorescence and you'll find articles such as this excellent one which explains the causes in detail and offers various possible fixes:

http://www.johnbridge.com/how-to/how-to-remove-efflorescence/

Note especially the suggestions for dry methods of cleaning which do not re-dissolve the deposits.

Do your research, and testing, before applying any chemicals — acids may etch your slate or attack the mortar.

If it's any consolation, it will likely diminish over time as the minerals are carried to the surface and depleted...unless your slate is attached to a concrete wall, in which case the efflorescence will have an inexhaustible supply of minerals to bring out and show you!

Good luck!

Steve Eichenberger

Thanks you Steve! Great information!

Hijacking the post! Steve! How about a line of exterior tiles in concrete? I see you going all Batchelder with it? If you specify them for exterior and someone uses them inappropriately, you are not to blame...and I have another product to recommend in my files! And larger format too, 12X12, 12X24, etc.

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Hey Dave — I want more lives! Yes developing a new line of concrete tiles would be fun, but I'm already going in too many directions at once, don't tempt me ;-)

Several years ago I explored making bas relief glass tiles. I made several dozen prototypes that were gorgeous, but the process involved using silica flour which I finally decided I just didn't want to be around, so I didn't pursue it further.

I just went and dug up one of my cast glass tile prototypes in a flat file and took a snap to show you: