Slate: What is it?

Slate is quite often associated with the 1970’s and 80’s as far as western markets are concerned, however it seems to be making a bit of a come back and in many Asian countries has never gone out of fashion. So I thought it appropriate to have a closer look at slate and how we should protect and maintain it.

Slate is a fine-grained metamorphic stone derived from sedimentary rock composed of mainly clay. It was formed in simple terms when volcanic lava flowed over the ground super heating the clay. This then mixed with the volcanic ash, cooled, forming slate.   

Slate is composed mainly of quartz and muscovite or illite. Muscovite and illite are both phyllosilicates (layered silicates). Muscovite is the most common mica found in granites and schist high in silicon, aluminium and potassium. Illite has an even higher silicon content along with iron and magnesium. Other main components are biotite, chlorite, hematite and pyrite. With the main components being silicates slate has characteristics that make it a very chemically resistant (almost inert) and thermally stable stone. In the early 20th century slate was used to construct electrical switchboards due to it being fireproof and a good electrical insulator. This same chemical resistance and thermal stability is also the reason why slate has been used for roofs as well as laboratory bench tops. Slate can however have some soluble content. This is mainly in the form of soluble salts (efflorescence) and some of the iron compounds (such as the mineral pyrite) that give some of the slate its brown and red hues. However these are relatively rare in slate in a soluble form. So does Slate need any protection if it is so chemically and thermally stable? The answer is yes. This is because slate has a relatively high range of water absorption varying between a low of 0.5% and a high of 12-15%. If something can absorb water then it can absorb liquid contaminants and can benefit from sealer protection. Slate is a stone that can benefit from all three classes of sealer; penetrating, enhancing and coating. In most situations, both interior and exterior, a penetrating sealer will be the most commonly used sealer. It will reduce the slates rate of water absorption and in doing so increase its stain resistance, but will leave the slate looking natural. However slate was traditionally sealed with oil based sealers that enhanced the colour variations within the stone. This was seen as a benefit and hence is a pre-requisite by many clients when they select a sealer today. The problem with the oil based sealers was they offered poor stain resistance as well as poor vapour transmission sometimes trapping water and soluble salts turning the slate white. Today we have the new generation of penetrating enhancers that have none of these side effects. They give the same stain protection as normal penetrating sealers but enhance the natural colours in the same way the old oil base sealers did.Due to the foliated nature of slate some slate can break under load making flaking damage relatively easy. Slate does have two lines of breakability, cleavage and grain and this can make some slate in some situations more prone to surface damage than other types of stone. This is where a coating or topical sealer can assist. When a coating sealer is applied it tends to bind the surface together giving added protection from scratching, loading and flaking. It must be noted that in most cases coating sealers should be applied in interiors only.

How should slate be maintained? The answer to this lies in its chemical composition and relative stability. The first problem that commonly arises for a slate installation is grout residue. The textured nature of most slate surfaces makes it easy to get grout caught in the small micro environment and so the first recommendation is to pre-seal the stone prior to grouting. However if grout reside is present then it can be removed  by either the new Nanotechnology based cleaners such as Aqua Mix NanoScrub (a chemically inert and safe cleaner) or the more traditional phosphoric acid based grout cleaners. Perhaps the most important point here is that Slate in general is resistant to even acid based cleaners something that is not always widely recognised. However it is always safer to recommend non-acidic solutions where possible because these will not damage the surrounding grout joints. This also holds for the removal of any soluble salts from slate. Traditional acids can be used, however the newer non-acidic formulations such as Aqua Mix Eff-Ex are safer as they do not damage grout joints and other surrounding materials that may be damaged by acid.   

Slates chemical resistance makes it inert to alkaline chemicals that are ideal for removing oil based contaminants. However acid and alkaline chemicals should not be used for daily routine cleaning. This is best done with ph neutral cleaners that will not only be safe on the slate but also safe on just about any surface found in a domestic or commercial installation. In summary Slate is a very durable stone. It has great chemical and thermal stability as well as good relative hardness and is the reason why it has been used in so many different types of applications for thousands of years. The instances where you get efflorescence or some iron leaching due to soluble components are very rare. Due to its water absorption it has the ability to stain however a good sealer and proper maintenance schedule will result in a stone floor that can perform for many years to come.

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