Slate- Blaenau Ffestiniog

[John Twilley <jtwilley@xxxxxxxxxxx>]

Ann,

One of the main things to avoid in slate is the presence of iron pyrite.  The
conditions in which slate forms are also favorable to the formation of pyrite.
Most slates have individual strata that may contain pyrites and these need to be
excluded from material that you want to be durable.  Pyrites can be recognized
as brassy particles or nodules, when fresh, and rusty spots, often trailing red
stains, when weathered.  Not only will the oxidation (or rusting) of pyrites
lead to stains, the iron rust occupies more volume than the pyrite, resulting in
swelling and splitting as it develops.  This can lead to popouts or
delaminations of the slate.  The pyrite releases sulfuric acid when it oxidizes,
so that once the process starts, it tends to be self-perpetuating unless in kept
continuously in a very dry environment.  Some pyrite may be microscopic in size
and harder to recognize.  This often degrades rapidly and may lead to crumbly
areas instead of popouts.  The best way of avoiding all these problems is to
select material that has been quarried and left outdoors for a few years without
signs of developing these problems.

John Twilley
Conservation Scientist

Petropict@xxxxxxx wrote: