Welsh Slate is the only slate that will not frost, change colour or bleed rusty streaks. It is good for 200 years as many English Churchyards will testify. It is however difficult to obtain and expensive.
So what are unscrupulous house sign makers doing – using Brazilian slate instead and calling it Welsh.
Brazilian slate is OK for internal uses such as Hearths, floor tiles and worktops but its weakness on the riven plane makes it prone to delamination externally. Certain layers absorb water, it is slow to soak in or out, so if it either freezes or heats up rapidly, internal pressure builds up and the slate cracks.
How to check the origin of the slate.
Brazilian slate can be variable in colour from a medium to dark grey. On any one piece the shade may vary, nicknamed “(contoured) map of the world” . Welsh Slate is darker, nearly black when treated, and may have diagonal veins or bands of a very slightly different hue. Welsh slate above Welsh slate above
Brazilian Honed slate below , very attractive but prone to weathering.
