What is Slate?
Slate is a metamorphic rock formed over millions of years by the deposits of silts rich in minerals. These silts consist mainly of tiny grains of chloride, quartz and white mica.
The sedimentary deposits underwent a dramatic change due to pressure and heat which pushed the mineral grains of shale into sheet-like layers (clevage).
The colours of slate are mainly grey/black but can be combinations of all colours. Another cause of colouring is by intrusion where mineral oxides penetrated the slate clevages and deposited a thin layer of colour.
Indian slate is an excellent example of this type of colouring. Plant and insect life can also contribute to the colouring of slate.
Requisite of a Good Slate
Slate as a natural product formed by nature varies dramatically. As the purchase of slate for a floor is a lifetime investment, care and knowledge save money and disappointment.
Slate to be suitable for a flooring medium must possess slaty clevage which will allow the slate to be split to an even thickness and provide a reasonably flat surface. The clevage must be strong enough to prevent flaking. This straight clevage is not as important when considering landscape-type slate or strata material.
As previously mentioned slate was formed by minerals, but excess minerals can occur and be detrimental to the performance of the slate.
Calcium is contained in most slate but a surplus of free calcium will cause the slate to whiten rapidly when exposed to water and/or sunlight. An example is Italian slate which generally contains an excess of calcium and should be avoided unless the supplier can provide an effective sealer which will improve the abrasive resistance of the slate and prevent scuffing.
Pyrites can be responsible for some of the beautiful colour of slate but in some circumstances iron oxide will cause rust stains to appear when subjected to a wet application. Local Australian landscape-type slate can contain pyrites.
Slate has been used for centuries as a roofing material in many countries throughout the world and is recognised for its low moisture absorption. Porosity of slate is the amount of water a slate can absorb. Porosity of slate becomes important when considering slate as a flooring medium, i.e. as a material which will absorb water will also absorb grease, oils, fats etc. Fortunately slate has a very low porosity, however, in applications where grease and fat staining could occur it is easily overcome by the use of a slate sealer.