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What is Slate?


Stone Care Recommendations

What is Slate?

Slate is metamorphic rock, like marble. However, instead of forming from a pre-existing limestone (like marble), slate is formed from the low-grade metamorphism of the sedimentary rock shale. Slate, like shale ("mudstone"), is a very fined-grained rock of mostly microscopic clay minerals with some microscopic quartz and calcite. Slate can also contain some of the same minerals found in granite, which make some slates iridescent and/or hard. The alteration of shale by heat and pressure produces the pronounced partings (slaty cleavage) that give slate its characteristics. Like limestone and marble, the color comes from trace metals. The wild colors on most Chinese and Indian slates are the result of splitting the slate along natural layers, which exposes the metals to the atmosphere, and they oxidize (rust).

Most commonly asked questions about care and cleaning of natural stone.

         Question: Should the natural stone we have selected for our home or workspace be sealed?

         Answer: Yes, all natural stone should be sealed. This includes granite, marble, limestone, sandstone, slate, travertine, and onyx

         Question: What should stone be sealed with?

         Answer: There are basically two kinds of sealers. Impregnating sealers are absorbed, leaving an invisible barrier of silicone in the pores of the material making it difficult to stain. Stone Protect made by Stone Medic is easy to apply and is a high quality impregnator. Color enhancing sealer can be used on textured stones (i.e. slate, tumbled stone, sandstone, etc.). The enhancer will deepen the color of the material, making it look wet but with a matte finish. SIM Enhancer is easy to apply and maintain.

         Question: How is sealer applied?

         Answer: Application is simple, when using Impregnators, a white cotton cloth or lambs wool applicator should be moistened with the sealer and wiped evenly over the surface. Streaks or puddles are an indication that the product is too heavy and should be spread over a larger area. SIM Color Enhancer is best applied with a disposable paintbrush to allow proper coverage over the textured surface. For best results, two coats should be applied (three on limestone) and spaced ten minutes apart. After the final application, the entire area should be buffed with a clean dry cloth to remove any excess product from the surface.

         Question: What cleaning product should be used on natural stone?

         Answer: Stone Soap (manufactured by Stone Medic) is the best general purpose cleaner available. It is a liquid concentrate and can be used to damp mop a floor or spray and wipe a counter top or wall.

         Question: How often should sealer be reapplied?

         Answer: Stone Protect will provide about four years protection if stone soap is the only cleaning product used.

The Marble Institute of America offers a brochure that includes information on treating spills and stains on natural stone surfaces. For a copy, please send $1.00US along with a self addressed, stamped business size envelope (37 cents) to the Marble Institute of America, 28901 Clemens Road, Suite 100, Cleveland, Ohio 44145 USA.