USA : how green is our Slate Valley ?


A problem common to quarriers, contractors, and designers alike, is introducing a slate color to our clients. We dust off our samples, wet them down, photograph them, then wax lyrical about their beauty and longevity, or, better yet, we bring a handful of slate for our customers to look at. In the glare of an indoor light or in bright sunlight, these can be studied to excess, until there is not a shadow of doubt regarding the texture, shading, or minute flecks of an intrusion — at least not until the first delivery.   

This roof is a mix of Weathering and Unfading Green slate.

Unfading Green is a good case in point. The ‘gray-green’ moniker is sometimes remarkably accurate. Other times ‘aquamarine’ might better describe the color. On the roof, however, all bets are off. The light creates its own version of green, and the client, who carefully chose a sample, may well gasp at a roof that doesn’t remotely meet their expectation. 

They were sold a beautiful slate, held it in their hands, imagined it on their roof, and now feel at a loss. Worse, in a few weeks the ‘green’ of one slate differs from the ‘green’ of its neighbor. What looked consistent in the pallet is now a patchwork of colors. Not the peach, tan and yellow of an oxidizing Weathering Green slate, but still a range of greens. 

To shed light on the subject I turned to our very own NSA guru, famed Belgian geologist and keynote speaker at the Philadelphia convention, Victor Cardenes. My question: Is there a test to determine that a green slate is in fact unfading?

Victor Cardenes responded with unusual speed — he is a geologist after all.


“No?” I asked.


“When you discuss this,” he eventually continued, “you should adopt a poetic perspective. You should speak poetically about the subject.”

“Poetically?” I felt the longest pause, a truly geological pause, and struggled to come up with the next question. Finally, it came to me, “What exactly is green slate?”

The prevalence of chlorite, a natural mineral, colors slate green. The elliptical flaky crystals help form an excellent cleavage, and the reason green slate can be split so perfectly. But slate is the kitchen sink of minerals and ubiquitous to everything is iron oxide (there are, in fact, thirteen kinds of iron oxide). 

To understand green slate we have to know at least two oxides: hematite and limonite. Hematite, a red iron oxide, occurs in Weathering Green slate and will oxidize in the atmosphere to form buff, tan and peach shades, a beautiful color palette that does no harm to the slate since it is a limited surface reaction. Limonite, a lemon yellow iron oxide, produces various shades from green to yellow in Weathering Green slate. Unfading Green slate shares limonite, in common with Weathering Green slate, but very little hematite.

That’s the theoretical chemistry, but even more obvious is that from one end of a quarry to the other, slate will show a variety of mineral content and shades of color, if not change color completely. And, from one quarry to another, slate has even more reason to be different.

Why on earth would we expect the same colors from the same slate? Slate color is a sliding scale in multiple dimensions. The exception is the rule. How many times have we heard that slate is a natural product? 

“Unfading” is the misnomer. It’s going to change. Everything changes. It can be said that Unfading Green will stay in the green to yellow range; there will be little evidence of hematite; there will be varying amounts of limonite. But is it definable? Not really. Every rock prized from the ground is going to react with its new surroundings. In fact, that is its color definition. The reaction. The reaction between the natural minerals in slate and the existing atmosphere. Put another way, a 500 million-year-old rock, prized from the depths of the slate valley, kisses the crystalline Northern sky, and born is an unfading slate of unique distinction. It gets a little poetic … like Victor said.

Unfading slate is going to arrive on the job-site in varying shades of green, react to the atmosphere at different rates, and begin its life a mosaic of greens. But it’s like doing your laundry in water with a high iron content. Eventually they all turn red(ish). Likewise, an Unfading Green roof, specific to its environment, settles to a consistent green(ish) cast. Take a look at the photos. These roofs are only a half-mile apart !

Two different versions of Unfading Green slate


Note: Victor Cardenes has moved to a remote island off the Norwiegan coast and has dedicated his life to writing slate haikus.    

Source : Tim Underhill, NSA Executive Director – September 2016




Slate Definition | Roofing slate maintenance | SLATE natural benefits