From a geologic point of view, slate is usually more compressed as well as finer grained. Surface tension is a factor of actual microscopic surface, so sandstone being more bumpy has more surface.

Larry2

---- [log in to unmask] wrote: 
> John,
>  
> When I split a piece of slate, and split a piece of bluestone (I have a  
> place three miles from the Helderberg quarries - the stuff is all over) I can  
> pour water on the slate and it will be absorbed at a much lesser rate than on  
> the bluestone. When we are judging wether a slate roof requires replacement, the 
>  water repellency (surface tension) of the existing is a factor. Is it a  
> metamorphic characteristic or binder/aggregate property that causes the  
> difference? 
>  
> Tw
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