When breathed upon or otherwise rendered moist

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Two hundred year old roof, Lake District UK. Lead dowel used to fix slates to battens. It is more usual to find timber dowels or iron nails.

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Westmorland slates reclaimed from the roof.

Slates are tested for quality by their thickness, clean smooth cleavage, toughness to allow of holing for nails, and resistance to water. A good slate partially immersed in water should not absorb water to any appreciable extent above the water line…It should also give a clear ring when struck with the knuckles and when breathed upon or otherwise rendered moist, should not emit a clayey odour.

From Architectural Building Construction Vol 2 by Jaggard & Drury, Cambridge University Press 1945

Uncovering a roof photoset