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Slate: The Textural Countertop Material

by Daniel Morrell

Posted on January 01, 1970 12:00:00 AM

Slate is no longer in the domain of chalkboards, pool tables, and roofs; it has entered the North American market for cladding in a big way. Along with the variety of colors and textures, the slate is a stone consisting of rich tones, natural ridging, and earthy looks making it a versatile stone used in many indoor applications. However, slate brings some technical and maintenance problems along with it. These minor problems can be the bane of customers but should not be the end of slate in home construction.

There is a two-fold problem with the material. The first issue comes down to what slate is and what is sold as a slate. The second rests upon the general janitorial perception that all stone was created equal. Understanding these two issues will greatly benefit your decisions on whether to buy the stone, where to put the stone, and how to take care of the stone.

First things first, it’s probably best to explain what exactly slate is as compared to what it’s perceived to be. The MIA Dimension Stone Design Manual (DSDM) Version VII defines slate as…a fine-grained, metamorphic rock exhibiting “slaty” cleavage, which allows it to be split into thin sheets. It is a low-grade metamorphic rock formed from shale, which is a thin-bedded, fine-grained, clastic sedimentary rock compacted from mud of clay-sized silicate clay mineral.

This is indeed a detailed definition for geologists, but for laymen who don’t need scientific jargon, slate is simply defined as compacted mud. Essentially slate is formed by clay and shale minerals settling underwater which is compacted by the sheer weight of the body of water. Where and how slate is formed plays a big part in quality of material.

Many areas of the world were covered in water as earth’s terrain changed over millennia’s. As a result, slate of different densities and textures can be found all over the place compacted near the earth’s crust. Cleaning and maintenance of your slate is dependent upon the stone’s geographic origin and local minerals that compose its makeup.

Why exactly is this? Well, since no two stones are created equal, your slate floor, countertop, vanity top, island, or backsplash may or may not be absorbent according to the exact mineral composition. You can have a tile and stone slate floor made up of four general colors and only one will be absorbent, or two of the four, or three out of the four, or all of them. This is only because no stones have equal minerals, nor is the amount of compaction going to be the same in its formation.

Think of it like making snowballs. The harder it is squeezed, the denser the snowball becomes. If you use wet snow, it will create a denser, heavier ice ball the more you compact it. The consistency is dependent on the type of snow (fluffy, compact, slushy, etc.) and the amount of compaction. Understanding that slate differs in make-up from one location to the other (even from within the same quarry depending on formation) we understand that installation and maintenance needs to take into consideration the properties of the slate at hand.

Slate is siliceous stone, making it resistant to acids found in lemons, alcohol, and cleaning products. These acids won’t etch or mar the surface making it ideal for kitchens. However, like soapstone, the slate is prone to scratching, especially around the edges and becomes weathered looking over time. These scratches can usually be visually erased with mineral oil or MB-6 Color Enhancer. In some cases, sandpaper will do the trick. On the other hand, some people like the look of the stone after naturally taking damage which gives it charm.

The real problem lies in what is being sold as slate, or commercial slate. The properties of some stones do not fall into the same category, yet is labeled as slate. Several quartzites are sold as slates (and even sandstone and granite). When people see a cleft on a surface, they assume its slate. Differences in mineral composition mean stones with varying needs.

Although I initially stated that slate is acid resistant, the real situation is a little more complicating. Since slate comes in a multitude of states and compositions, it is impossible to have a general rule that classifies all commercial slate as acid resistant. Slate usually is resistance to mild acids giving it a much wider choice of cleaning product than acid sensitive stones such as marble and limestone. It is an absolute necessity for clients to find out the maximum tolerance of their stones before actual cleaning to avoid problems. The best solution is to test every stone that you plan on using for your floor or countertops before even installing. This is especially true if you are using several types and colors to complete your project as one might be incredibly resistant while others are not.

Slate is beautiful in its natural state. However, it is common practice to use manmade chemicals on it, which, according to a person’s taste, adds or takes away from the stones beauty. The need for sealing, coating and/or color enhancing has become a popular choice for many.

Problems can arise with future maintenance when treating the stone with these agents. For example, after 20 years or so of installation, I tend to see something other than the stone itself. 9 out of 10 times its been coated several times with either a commercial acrylic sealer or the worst acrylic products known to man, sold from the shelves of your local grocery store. Acrylics are nasty to strip and will leave a white residue (especially in the clefts) that will most likely never come off without a method that will damage the finish.

Also on the softer slate finishes, you get a situation where coating will soak into some of the more-porous areas and never come out. Not to mention you also have the grout to contend with as it holds old waxes and looks unsightly. For these reasons, you should try to avoid coatings on a new installation of slate unless you will maintain the surface with these products on a regular basis.

Impregnating sealers and color enhancers work very well if applied properly. Remember, though, if applied as a coating, your problems will be even worse than the ones with acrylics-sealers and enhancers which are not made to be stripped and re-coated. Color enhancers offer the option of bringing back the fading from natural wear on the stone. As the traffic areas dull, a good deep cleaning and application will bring it back.

There has been a marked increase in the use of slate for kitchen countertops over the past decade and a half. Due to its subtle appearance, architects and designers are especially fond of the honed finish for slate countertops. Good hard slates are similar to granite and serve well as countertops, adding an earthly colors that exude warmth and gives a real classic old world feel to your kitchen.