The origins of slate go back 450 to 600 million years when very fine clay silts were deposited at the bottom of riverbeds, lakes and seas. Over millions of years these sediments were compressed and heated, altering the arrangement and alignment of the minerals. The resulting stone has a unique parallel alignment of the minerals that allows the stone to be separated into layers (split) - a distinctive characteristic of slate.

Inhabitants of northern Wales are believed to have been among the first to take advantage of slate’s natural cleavage for the production of roofing slate around 1300. Over several centuries the Welsh developed a proud tradition of slate production, one that continues to this day at the famous Penrhyn Quarry operated by Welsh//Slate. In the mid-1800s Welsh immigrants brought their slate working experience to the northeastern US, starting what was to become a major industry in the area.

The first slate quarry to operate in Vermont began in 1839, however it wasn’t until the mid 1800’s that slate quarries became commercially viable. Welsh immigrants experienced in making roofing were a valuable work force for the Vermont quarries and catapulted this fledgling industry to the national and international stage. Slate was a common roofing material in the late 1800’s, and the early 1900’s marked the high point for slate production in Vermont and other slate quarries in the United States. Today natural roofing slate competes against many other man-made products but it continues to be the standard for most colleges, universities, churches, institutional buildings and custom homes.

If you are interested in learning more about the history of slate, the Slate Valley Museum in Granville, NY (www.slatevalleymuseum.org), the National Slate Museum in Wales (http://www.museumwales.ac.uk/en/slate/) and The Slate Roof Bible by Joseph Jenkins are all great resources.

In Vermont and New York there are many different colors of slate - greens, greys, black, purples and red. There are also slates that are mottled with two or more colors - green and purple, grey and black for example. Neighboring slate quarries can yield stone of completely different colors - a grey/black quarry is next to a weathering green quarry - and even within a single quarry different colors can be extracted. There are no certainties with regard to color, quantity, quality or direction of the vein when quarrying slate.

The slate beds in Vermont and New York generally lie at a 45-degree angle to the ground so large surface areas have to be stripped away to expose the slate deep below.  In other parts of the country the slate beds are more vertical and smaller surface openings are needed to quarry the stone. Drilling and blasting are the general rule for removing large blocks of stone for processing at the mill but some quarries can utilize large wire saws to cut out the blocks and minimize waste from blasting.  In most quarries the finished slates represent less than 10% of the material handled!

Slate From Other Parts of the World

Being located in the heart of the Vermont/New York slate valley, we are proud to offer a full range of locally produced slate. Time tested and made in the USA, Vermont slate is some of the finest slate produced anywhere in the world. In addition, we have chosen a select group of other slates to compliment our Vermont produced slates. These include the Penrhyn Unfading Purple from Wales, North Country Unfading Black from the Glendyne quarry in Quebec, and both the Trinity Unfading Green and Trinity Unfading Purple from Newfoundland. These three sources produce exceptionally good quality slate and are respected around the world. Learn more about them in our section on Roofing Slate Colors.

Slate, and slate-like stone, is produced in other parts of the world as well. There is a tremendous volume of black slate quarried in Spain. China has been exporting slate for a number of years. More recently we have seen Brazilian slate enter the US market, and even some material from India. To date we have chosen not to import or promote roofing slate from these regions for a variety of quality related reasons, including:

• Prevalence of oxidizable iron pyrite inclusions that can rust and discolor a roof or create holes in a slate
• High carbonate content which can cause the slates to turn white
• Exceptionally brittle or glassy stone that is difficult to cut
• High amounts of broken slates
• Inconsistent product
• High water absorption leading to delamination
• Debate about whether some material being marketed as slate is slate. The Galacian & Spanish Slate Association has published a very interesting report on some of the slate being produced in Brazil and how it differs from other sources. http://www.spanishslateuk.com/slate_from_brazil.asp