Everything you need to know about Slate – Slateplate
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Everything you need to know about Slate

WHAT IS SLATE?

Slate is considered as the finest grained foliated metamorphic rock. Metamorphic rocks are formed from the change in form of existing rocks, a process called metamorphism. Slate arises from the repetitive layering or foliation of metamorphic rocks, particularly through the low-grade metamorphism of shale or mudstone. It is widely used in building roofs and floors, for it being fireproof and a good electrical insulator. It has also been popular in the use for billiard table tops, blackboards, tombstones, and commemorative tablets. It is widely used as a roofing material because of its low water absorption index of less than 0.4%, making it resistant to frost damage and breakage secondary to freezing.

WHERE IS SLATE FOUND?

Slate can be quarried in a slate quarry or tunneled in a slate mine. The slate industry revolves around extraction and processing of slate for most of its uses such as roofing or flooring.

Slate mines are abundant in several cities in the world, including Wales in the United Kingdom, which is considered a major industry in the region. Slate quarries are also found in Cornwall and in the Lake District. Majority of the natural slates used in Europe for roofing is from the slate industry in Spain. Other significant slate sources in Europe can be found in Portugal, Italy, Germany, and Brazil. Wales is also a major slate mining region. In the US, slates are found abundantly in Vermont, New York, Virginia, and Pennsylvania.

The slate industry in Spain is the source of about 90% of slates used in Europe, the region of Galicia being the principal area of production. Slate production companies are located in Valdeorras in Ourense, and some others in Quiroga, Origueira, and Mondoñedo. In this region of norther Spain, slates are believed to be formed during the Palaeozoic period hence are about 500 million years old.

Slate mining in North Wales began in 1782 with the Penrhyn Quarry near Bethesda in the Ogwen Valley. By 1882, Wales has produced about 92% of Britain’s slates. The Penrhyn Quarry is now considered as the largest slate producing quarry in the world. Other essential slate producing mines in Wales include the Dinorwic Quarry, and Cilgwyn Quarry. There is also a number of slate producing quarries in the Blaenau Ffestiniog area. The slate mines in the Llangollen area are known to produce a darker colored slate, making them almost black.

In North America, slate was first quarried in 1734 along the borders of Pennsylvania and Maryland, USA. The first commercial slate quarry was then opened in Peach Bottom Township, Pennsylvania in 1785. Presently, several regions in Vermont, New York, and Pennsylvania have active slate quarries.

Slate extraction in South America comes from the Minas Gerais in Brazil. Brazilian slates, compared to the ones found in Spain, have higher water absorption indexes. This property makes slate more prone to breakage when subjected to freezing, hence they are less used as a roofing material.

WHAT IS THE COMPOSITION OF SLATE?

Slate is derived from shale-type sedimentary rock of clay or volcanic ash that underwent low-grade regional metamorphism. It is mainly composed of quartz and muscovite or illite. Some minerals like biotite, chlorite, hematite, and pyrite are also usually present in slate. Apatite, graphite, kaolinite, magnetite, tourmaline, and feldspar are sometimes present as well, although less frequently. Some compound mineral can also be found in slate. This includes aluminum oxide, iron oxide, potassium oxide, magnesium oxide, sodium oxide, silicone dioxide, and titanium dioxide.

WHAT IS THE COLOR OF SLATE?

The color of slate is determined by its mineral composition. Slates usually range from shades of light to dark grey in color. However, in some areas of slate industry such as in the town of Granville in New York, colored slate can be obtained. It can occur in various shades of green, red, black, purple, and brown. If hematite is abundant, a slate is usually reddish in color. Chlorite produces green slate, while sericite produces bluish-grey slate. Carbonaceuous materials make slate appear darker grey or black, while limonite makes it yellowish-brown.

HOW DOES SLATE FORM?

Slate is formed through the regional metamorphosis of mudstone or shale under low-pressure conditions. When shale or mudstone is exposed to heavy pressure and heat from a tectonic plate activity, its clay mineral components metamorphose into mica minerals. Mica minerals such as biotite, chlorite, and muscovite, are the main components of slate. One unique characteristic of slate is that it is formed through the process of foliation, which refers to the repetitive lamination of metamorphic rocks caused by shearing forces or differential pressure. Layers of rocks are then formed perpendicular to the direction of the pressure of metamorphic compression. This gives slate its ability to cleave along flat planes. It is considered as the finest grained foliated metamorphic rock, having 0.01 mm or less of space occurring between each layer or lamina.

HOW HARD IS SLATE COMPARED TO OTHER ROCKS?

The hardness of minerals is measured through the Mohs scale. This 1 to 10 scale has been widely used in mineral identification since 1812. The Mohs scale is a comparative tool, where it measures the hardness of a material by determining which other material can scratch it. For example, if stone A can scratch stone B, while stone B cannot scratch stone A, then A is a harder material and ranks higher on the scale. If they can scratch each other, then they are given the same rank and are considered to have the same hardness. Slate is considered to be in between 2.5 to 4 on this scale in terms of hardness, which means that it is, on average, almost as hard as marble and limestone, but not as hard as granite or natural quartz.

USE OF THE WORD ‘SLATE’

In the past, geologists often use the word “slate” loosely to refer to shale. “Slate” in reference to shale can be deemed acceptable because of the fact that slate is a product of shale metamorphism. However, the use of the term to refer to shale has resulted to confusion, as slate itself is actually different from shale. There had been incidences where the term “slate” was consistently being used to refer to a mining or quarrying area, when in fact it is shale that is present and not slate. Therefore, geologists are now cautious with the use of the word “slate”, so as not to confuse it with shale.

The term “slate” is also widely used to refer to different materials made from slate rock. A roofing tile made of slate may be simply called “slate”. In 1800s, a small piece of slate affixed in a wooden frame was widely used by elementary school students for their writing activities or arithmetic problems. These are referred to as “writing slate”. People then used a small pencil made of slate, soapstone, or clay to write on the writing slate. It could be wiped clean with a cloth. The phrases “clean slate” and “blank slate”, which refers to the philosophic idea of starting anew without considering what has happened in the past, came from this practice.

SLATY CLEAVAGE

Because of the parallel alignment of the minerals composing slate, it is able to undergo the process of repeated layering or foliation. The parallel alignment of minerals gives the rock the ability to break evenly along the planes of foliation. This is called the slaty cleavage, which gives the rock the ability to split into very thin layers. There is approximately 0.1 mm or less of space between the foliations. Slate, therefore, can be produced in thin sheets that are mostly used in construction and manufacturing industries.

What is Slate Used For?

Slateplate products are not microwave or oven safe.  Slate can contain trace metals within the stone, and uneven heating in a microwave or oven may cause the slate to break

You can pre-heat your Slateplate in a warming oven at 200F or less.  You can pre-cool your Slateplate in the freezer or refrigerator.  Slate is slow to change temperature (and won’t look different when hot) so be careful in handling hot slate.

Below are some fun uses you can try for your slate!

Serving food


Slate is well known for its use as for serving food, but there are many other great uses for this unique natural stone, that makes it the perfect slate cheese board. The reason that slate is widely used for serving food is three-fold. First, and most notable, is that the dark color of the natural stone brightens your cheeses and other foods making your dishes or offerings more vibrant and attractive. This is why slate is used in many fine restaurants as a serving plate. Secondly, despite being so thin, slate is very strong and is also food safe which together make it the perfect natural plate. We seal our slate with mineral oil to maintain its deep luster and this also keeps anything from sticking to it making cleanup a breeze. Lastly, slate works great as a chalk board so it takes an ordinary serving dish and makes it different and fun.

Warmplate


Slate is very, very old- in fact, most slate is densely compacted volcanic ash! It is fireproof from creation and certain pieces can handle hot temperatures. Because of this, your smaller slate pieces may work great as a trivet (we recommend on sizes 8x8 and smaller). Be aware, although your slate can handle heat, it is equally an amazing conductor of that heat - so therefore the slate itself will become hot and stay hot for a long time so be careful if using it in this application! That said, this makes your Slate a great Warmplate to keep food warm on the table. Do not put your slate in the oven or microwave.

Coldplate


Just like its ability to handle intense heat, your Slateplate can handle intense cold as well. If placed in the refrigerator your slate will stay cool for quite awhile, if placed in the freezer your slate will stay cold for a much longer time. This allows you to effectively serve desserts and other cold items on your slate that you might not otherwise be able to, such as pies, cakes, and tortes. Just keep in mind if you are serving something that will melt, much like an ice cream cone, you'll want to eat it before it melts too much!

Food Service


There are a number of restaurants that use Slateplates for serving their customer's meals, desserts, and side dishes. They are especially nice for serving colorful dishes like sushi because they bring out the food's brilliant colors and can keep it chilled at the same time. Slateplates are food safe, so you can use them to serve just about anything. Pretty much the only thing you can't use them for is dishes with thin sauces since the Slateplates are almost completely flat.
 

Bath and Decor


Slateplates aren't just for food! They also make great decoration and utility pieces. Many of our customers like to use them for candles because if the wax spills over it just peels right off, plus it provides a fireproof base in case they were to fall over. Others like to put flower arrangements on them using the black as a stark contrast giving those natural colors a real 'wow' factor. Slateplates are completely waterproof and make great easy-to-clean soap dishes for use in or out of the shower!


We've been told of many uses for Slateplates other than what we listed here. One customer paints them and sells them at art shows! Another uses the larger ones as slate placemats she leaves on the dining table to make it look nicer when not in use.

Other Uses Of Slate

As mentioned, slate is widely used as a roofing material. It is an efficient roofing material because it can be cut into very thin sheets. It also absorbs minimal water (absorption index being 0.4%), and is almost resistant to breakage when exposed to freezing. It is also favored for its resistance to moisture and good insulating capability. Roofs made of slate can last for hundreds of years. However, slate is quite more expensive than other available roofing materials, and its installation cost more as well. Hence, the use of slate in the more recent times has been mainly restricted to high-end projects and prestige architecture.

Slate is also used for outdoor and indoor flooring, and cladding. Floorings of porches, basements, bathrooms, and kitchens, can be made of slate. They are very durable, elegant-looking, and require less maintenance. Some slates that are used for indoor flooring have a wide variety of finishes, patterns, shapes, and colors.

Landscaping also make use of slate rocks, taking advantage of its resistant property to weather and pollution. Pavements, swimming pools, patios, and even contemporary fountains make use of slate either as a primary material or decorative stone.

Slate can also be used as billiard table tops, commemorative tablets, and tombstones.