Manufacturing Greenstone Slate®
Greenstone Slate is produced from our 58 quarries of very high quality slate. It is cut, split and otherwise manufactured into roofing slates and warehoused in our facilites in Poultney, Vermont.
Step 1 – The Quarry
The first step in mining slate is blasting the stone. After drilling blast holes in the mountain, dynamite is packed into the holes along with a tamping compound made of water and tiny grains of slate. The heavy mud-like compound helps the dynamite to blast the slate away from the mountain. Raw slate is so strong that if not for the tamp, the explosion would simply shoot back out of the blast holes. Once the slate has been blasted it must be carted out of the quarry by huge dump trucks and back hoes. The piece of slate being removed weights between 3 and 4 tons. Using jackhammers, this piece will be broken into smaller more manageable stones for the first phase of cutting.
Step 2 – Cutting the Slate
After the stone is broken into smaller pieces, they are brought into a warehouse where radial saws cut the slate into thick blocks approximately the size of the final slates. The blades used are diamond-tipped and cost about $1000 each. Greenstone uses between sixty and one hundred of these blades every year. At each station there are two saws, one to cut the length and another for the width. A third craftsman cuts off the excess layers from the top and bottom of the blocks so that all sides of the slate are smooth before moving on to the splitting phase of the slate process.
Step 3 – Splitting the Slate
Splitting is by far the most specialized skill involved in the entire process of mining and shaping slate. Each piece is hand split from the blocks created in the "cutting" stage. The first step is to split the block into pieces that are twice the thickness of the final slates. These pieces are then split in half. Due to the delicate nature of this stage a huge amount of slate becomes damaged due to an unexpected split or crack. In all, 85% of slate becomes unusable or damaged during the manufacturing process.
Step 4 – Trimming the Slate
Trimming the slate is the final step in shaping the stone. Every slate is individually trimmed to your specifications. If desired, this step also removes the cut edge and gives each piece the common look of slate tiling that we are accustomed to seeing on most roofs today.
Step 5 – Drilling the Slate
If you are using slate for tiling a roof, the slate must be drilled for nail holes. Nails cannot be driven directly through the slate as the stone would break and split. Once attached to a roof, these holes will be sealed with glycerin to prevent water damage. Slate is drilled in small groups or one by one, to ensure that nail holes are placed at the ideal point for strength and durability.
Step 6 – Shipping
The final step of the slate process is packing and shipping. Each and every palette must be packed by hand to insure the least amount of breakage during shipping. Even the greatest care cannot keep some slates from being damaged, and when estimating, your architect or contractor will add about 5% to account for this. Once each palette is packed tight enough that the slates won’t move and shatter during transport, a flatbed is loaded and the order is delivered.
