Slate is a natural stone becoming increasingly popular. So, what is it exactly? Well, in geological terms it is fine-grained, foliated, homogeneous, metamorphic rock. How is it derived? Well, in short from original shale type sedimentary rocks made up of volcanic ash or clay through low grade regional metamorphism. Most people know it as grey in colour from the many thousands of roofs it is used on across the world. However, slate can come in varieties of colour even that derived from the same source location. For example, slate coming from North Wales can come in numerous shades of grey from very rich, deep and dark to that a lot paler in colour. It can also come in shades of cyan or green.
So, what is slate used for? Well, commonly it is used in roofing as roofing slates or something commonly called shingles in North America. One of the advantages of slate as a natural stone is that it has 2 lines of breakability – grain and cleavage – and this makes it very easy to split into thin roofing slates.
Very fine slate can also be used as whetstone to sharpen and hone knives. Historically, and especially during the 18th and 19th centuries it was also used as a material for blackboards in schools and also as individual writing slates. Slate is also a good electrical insulator and is fireproof and so was used in early part of the 20th century as a component in large electrical motors. Slate also possesses thermal stability and is still used as the base in snooker and billiard tables.
Today, slate is becoming increasingly popular for both interior and exterior flooring. Generally, slate tiles are fitted and set on mortar and then the tiles are grouted along the edges. A variety of sealers should be used on the slate tiles to improve durability, appearance, enhance stain resistance, reduce efflorescence, and increase or lessen surface smoothness of the tiles. We will talk more about this soon.
Copyright Ian Taylor and The Tile and Stone Blog.co.uk, 2013. See copyright notice above.
Hi Ian we have recently had a black slate floor installed and are unhappy with some with some light grey areas that appear on quite a few of the tiles which spoil the look. The marks on the tiles vary in size and shape but many are circular, light grey in colour and duller in finish that the black slate around it. We were unhappy when the floor was finished so had asked the filer to strip the sealer off and wash the floor with light acid before reapplying the sealer (2 coats). Definitely not limescale marks. Any ideas can we get these areas polished out? Grateful for any advice. Graham
Hi Graham,
Without seeing pictures it is difficult to be sure, but it sounds like a common issue with some slate. Basically it is a naturally occuring spot, a ball of impurities in the stone that when cut into slices (tiles/slabs) creates a circular like splodge. It is basically a concetration of similar minerals that are not the same colour as the rest of the stone, often a bit softer so can be more dull in the slab (as they don’t polish as easily, the harder the material, the more glass like, the softer the material, the more like dried mud etc).
Slate is a metamorphic stone, meaning it used to be something else (a mixture of different minerals and often a lot of clay/silt/mud etc), then it was subjedcted to long periods of pressure and or heat and 99.99% was phyically changed but there may have been pockets of other materials or impurities the stone that were like little balls or clusters, and maybe they reacted to the heat and pressure at a different rate – a bit like having a few chocolate chips in cookie dough or cake mix – they are still in tact as choc chips in the baked cake.
EDIT: “I just spoke with a geologist friend and he says that the spots are not so much, pockets that did not metamorphose as I speculated above, but more that certain minerals within the mix for whatever the slate was made up of, during metamophosis, seem to be drawn togehter and coalesce to form these spots, they do not know exactly how or why, but the result is the same – natural spots”
If this is what they grey spots are, then I am afraid they are a naturally occuring (and therefore normal) part of the stone. If this is the case then you will not be able to remove or even disguise them. Your only argument may then be that a) the sales people did not adequatley warn or inform you about the possibility of recieving a number of these as being ‘normal and reasonable, or b) the samples you saw were all ‘perfect’ possibly selected to show those that are blemish free, and therefore one might argue, not representitive of the actual tiles you were about to buy.
If you wanted to challenge them on this, I would first check their displays and any quotes/paperwork or contracts you may have singed, if their displays do show some of these marks, and/or if you have signed some document or they can point to signs or posters in the showroom etc that point out somehting like :”all natural stones are naturally variable and some may have marks or blemishes etc…..” – or words to that effect, then youu may have a hard time persuing it.
Sorry I cannot be more helpful
Ian