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Cleaning Slate – Expert Advice on Cleaning Slate and Slate Floors – Part 1

Cleaning slate isn’t too difficult. It’s vital, however,to know what kind of dirt or contaminant you are trying to remove before you can make a decision as to which cleaning product is best suitable.

Slate comes in many different colours and finishes. Some of these are smooth whereas others have a textured surface. Undoubtedly, slate is a very popular material for flooring right now.

Slate generally has good resistance to mild acids so this gives you a much wider choice of cleaning materials than you would have with more acid-sensitive stones like limestone or marble.

So, how do we go about cleaning our slate floor?

First, you need to check whether the contaminant is mineral based? e.g. grout staining, cement, rust or general dirt. If it is, you would be advised to use a mild acid cleaner such as one based on phosphoric acid. Beware though as any acid cleaner may etch the grout also.

In most other situations where you have, for example, general dirt and grime, wax, oil or a general build up of old polishes etc. I would recommend using a strong alkaline cleaner for periodic intensive cleaning. For everyday cleaning I’d recommend using a mild, neutral cleaner.

Here’s an Expert’s 6 – Step Guide to Cleaning Slate:

1. Ensure your slate is protected with a high quality sealer: either an impregnating sealer which will give you a natural look with a mat finish and below surface stain protection. Or, if you prefer the look of wet slate, in so far as the darker colours, but still want a natural, mat, no-sheen finish, then look a good quality enhancing sealer such as Enhance ‘N’ Seal. Alternatively you can use a coating sealer for a pleasant low sheen gloss which really helps any ongoing cleaning.

2. You must eliminate all grit – this is the major cause of all wear to slate floors. Ideally, place a dust mat both inside and outside the room which will help remove grit from shoes and prevent it from spreading to the slate floor.

3. Sweep and vacuum the slate floor regularly. This will also help remove grit from the surface of the floor.

4. Mop up and soak up isolated spillages as they happen. You shouldn’t leave strong contaminants such as coffee or wine to dwell on a slate floor.

5. Wash the slate regularly intervals using a mild, neutral cleaner. This will clean the slate but it won’t damage the stone, grout or the sealer used.

6. Less frequently, carry out a deep clean. For this, we recommend using a high alkaline cleaner. More in Part 2.

Update 22/01/13 – Check out this case study on cleaning slate

 

 

 

Copyright Ian Taylor and The Tile and Stone Blog.co.uk, 2013. See copyright notice above.

33 Comments

  1. Hi Ian – We moved into a house with a slate floor and a slate bench in the shower. It is really cloudy looking. I bought StoneTech soap scum remover, stone tech mold and mildew remover and sealant. I have used the soap scum and mold and mildew remover a couple of times. It still looks cloudy. Did I wait too long? Do I have to do it more times? Do I need to or can I seal it while it looks cloudy? There is also white “stuff” that I can’t scrub away with an OXO nylon brush on the grout. Any advise is appreciated.

    Thanks, Mindy

  2. Hi Mindy,

    OK, well the cloudy marks could be efflorescence or damaged sealer. Some sealers will go cloudy upon curing if there is too much moisture present in the stone at the time.

    If the cloudy haze ‘disappears’ when wet, only to reappear when dry then it can suggest efflorescence. If however, it stays visible through the water, then it might indicate a sealer or come kind of coating/deposit other than efflorescence.

    If the latter, then try a small amount of old fashioned nail varnish remover (the clear type with acetone). Just rub a little NVR on a cotton pad onto the affected area, see if it improves it, removes the haze. If it does, then this indicates that you need to get a sealer stripper.

    If not then eff is more likely, and for this you would need an acid based cleaner, not though one based on HCL/muriatic. Instead look for a proprietary grout haze remover based on phosphoric or similar.

    The white on the grout is also likely to be efflorescence

    I would certainly try to clean all white marks before sealing. Don’t forget to thoroughly rinse and allow to dry before attempting to seal also.

    Hope this helps

    Ian

  3. I’ve recently bought a slate fire surround but it’s been painted in white gloss. What should I use to strip the paint off without ruining the slate?
    Cheers
    Rob

  4. Hi, the slate should be fairly resilient to chemicals like paint strippers so I would try one of those. You could also try a hot pain gun – You might find you can use heat and or a plastic scraper to remove the bulk and a paint stripper to remove the residue that is left after. I doubt any harm will come to the slate.

    What I cannot say however is why the slate was painted in the first place, you may uncover something that is not in great condition

    Hope this helps

    Ian

  5. Hi I have Chinese green slate tiles a number have developed central pale discolouration. Which we put down to a leaking central heating pipe. Well we have dug two up and cement dry😣. Could the heat from the pipe have created problem? Lyn

  6. Hi Lyn,

    Not heard this before, at least not heard about slate ‘changing’ it is normally pretty stable. One thought that I have is maybe the tiles were always like that but while wet (during installation and grouting etc) they looked more even, but upon drying out, they lost their moisture which was artificially darkening them – so not they are appearing as they really are? – It is worth a though, have any other of the slates change?

    Some Chinese material can lose its colour when washed – but this usually affects grey slate with autumn colours, where the reds and brows are formed due to h presence of r=free iron minerals and where the slate is quite porous and dusty and friable. Here the iron ‘rusts’ when exposed to the atmosphere and can be in the form of a surface powder/dust that is relatively easily wiped/washed away.

    I think this far less likely with a green slate, in fact the Chinese green slates I have seen are pretty stable but green also comes from iron so if you have a slate where the part of the stone that is coloured is not completely ‘fast’ in the stones matrix then there is in theory at least a possibility that water, chemical cleaning could remove them to some degree – unlikely I feel but possible.
    HOpe this helps Ian.

  7. Thanks Ian that is helpful. They have been down for 18 years and only started to discolour in last 6 months. No obvious change of cleaning so will just replace and do a grand strip and resell and see how it goeslyn

  8. Hi Ian

    I cleaned our textured slate floor with a steam mop. Unfortunately I left it to cool sitting on the tiles and there is now a large white mark on one of them.

    Any suggestions as to what I can do to get rid of it?

    Feeling stupid and would love your help!

  9. Hi Lorna,

    OK sounds like you have some kind of deposit – I can only guess but, normal use of a steam mop would spread minimal moisture over a large area, and as steam which would evaporate quickly. Where as keeping the mop in one place and allowing it to cool, means that a larger concentration of moisture will then have cooled, condensed, and then slowly evaporated. So much more moisture, if there was anything in the moisture (there almost always is, detergent residue, calcium from the water itself, dissolved dirt from the entire floor and whatever was on the mop etc) all this ‘stuff’ is carried by the water and the mop, all you have done is let it all collect in one spot and as the water has then slowly evaporated the ‘stuff’ has dropped out of solution and been deposited on the surface.

    There may be some mineral content or it could be largely grease base. IF mineral (calcium from hard water) you may need to try a MILD PHOSPHORIC acid based cleaner – you could try some white vinegar as a test.

    If it is manly grease based then a high alkaline degreaser should do it.

    Failing that you could try a cleaner with a mild abrasive in it

    Assuming of course scrubbing with water and or the steam mop does not remove it?

    Hope this helps

    Ian

  10. Hi Ian,
    Thanks for your reply. We have tried washing up detergent and alkaline floor cleaner with no success. I noticed in your reply to someone else that you mentioned ‘etching’ – as the marks disappear when they’re wet, I wondered if that’s the case with our slate? Would a fine emery paper followed by slate oil work in our case or should we try stronger cleaners first? Thanks very much for your advice.

  11. I do not think it deposit because it occurs under a bench where there is little cleaning and no wear and tear. It does however occur above a junction of underfloor central heating pipes, so I think the slate has become denatured due to constant heat over 20 years centralised by the fact that the main area is where everyone turns on their heels to go around a corner. We did dig the offending tiles up looking for what proved to be a non existent leak so can’t try any other solution. I will be replacing tiles with spares soon and will post if it happens again so that others can consider heat as a problem to slate tiles that discolour, loosing colour.
    Lyn

  12. Hi Sarah,

    It could be etching, although that is not typical with slate. Normally etching is associated with calcium based stones. However, there can be some slates with mineral content that is effected by acidic cleaners and so thqty can on occasion etch in much the same way.

    The other thing to consider might be that there was some kind of sealer/polish/oil/ coating on the slate that has been removed (from simply being kept wet, wioth steam or detergent or whatever)

    An etch wuld not only be white-ish in colour it would also be phyically diferent, duller, matt etc. If you can look at it under a mangifying glass, look for a difference in texture.

    It the surface has been etched then possibe rubbing with very fine wet & dry/emery paper with a little water may well be worth a go. Adding oil may help but it also may then look different tothe surrounding layers, but you might be able to blend this out over a few tiles.

    Hope this helps

    Ian

  13. Hi Ian,
    Thanks very much for your help. The slate oil has worked a treat and it looks so much better, as if it had never happened!

  14. Hi I have a natural slate patio and paths in my garden which looks stunning when wet. Is ther anything I can put on it to keep the wet look. Thanks

  15. Hi Karen,

    Yes you can use what is called an enhancing Sealer – there are a few around, be sure to test on a spare piece first as they are generally not reversible – so if you don’t like the look, they cannot be taken out again. Also you really have to use these sorts of product in good weather – they must be applied correctly and there must be next to no moisture around, before, during and after. I have a product called Enhance n Seal you will find in on Amazon – but there are others out there also.

    Hope this helps

    Ian

  16. Hello Ian,
    A very helpful site but I can’t find my specific problem. We have black natural slate floor tiles in the bathroom, which I think were sealed when laid, but they now have a few white marks and patches where bleach-based cleaners have sprayed or dripped onto them when cleaning the loo etc. Will it be possible to restore them? Any advice would be appreciated.
    Thanks.

  17. Hi, most likely scenario here is that the bleach based cleaners have taken their toll on the sealer or coating that was applied to the slate in the first place. It could be hard water deposits, if for example you are in a hard water area and over time the sealer has degraded and allowed better contact with the stone surface, so, again over time the deposits can build up.

    You could test stripping a section with a proprietary sealer stripper, then when rinsed and dried – see what it looks like, if it is nice and clean and a good surface, do the same to the whole installation and re seal after. This process will also clean the slate at the same time.

    If this is not the cause and it is hard water then try a mild, phosphoric acid based cleaner. This should help, rinse well afterwards and you may not have to re seal.

    IF you would like some product suggestions use my contact us page

    Hope this helps

    Ian

  18. Thanks Ian. We’ll try what you suggest and let you know how it goes.

  19. Hi, we have recently moved into a property with Lakeland slate flooring which has been covered with asphalt, is there any way that we can remove the asphalt from the slate without damaging it? Your advice would be invaluable.

  20. Quick answer is – not sure. If it is the thick, poured asphalt screed then you will simply have to try, it is normally quite brittle and may not be well adhered to the slate. IF the slate is very porous and textured then it might have a good bond. You will have to try mechanical removal initially – using a large scraper, hammer and chisel etc to see if you can get the bulk of it up. Whether or not you damage the slate surface in the process is going to be down to luck I think. IF you can get the main solid part of the asphalt up but are left with a residue, then you would need to look for a strong enough solvent to remove it. This may or may not be possible but it could be a little messy. I have seen some asphalt removers online – just googled it, these products will be a solvent and I guess all you can do is try.

    IF it is the thin, trowelled type – so like a thin levelling layer or water proofing membrane then again you may be able to remove some if it mechanically, but it may be far less brittle and more tacky/flexible. Again you may have a residue to deal with that would require some kind of strong solvent to remove it. This process may take several attempts also and even then you may not end up with a 100% clean and undamaged floor – suggest you try a small area to see what happens

    Hope this helps

    Ian

  21. Thank you Ian I will let you know how I get on.

  22. Hi Ian, can you remove bleach spotting from slate? We have a kitchen bar top of thin slate that came from the inside of a billiard table. I don’t think it had a lacquer seal applied originally: just slate oil or similar to give it a sheen..

  23. Hi,

    I am not completely sure what you mean by bleach spotting. However if there was some kind of slate oil or something, some kind of treatment applied to the slate, then the drops of bleach may well have removed or affected it – the bleach being a high alkaline solution will affect oil/grease type substances. So the bleach spots are really just ‘clean’ spots as opposed to them being dirt spots or deposits. Billiard table slate was often of good quality Welsh or other native UK and most of this type of slate would not normally be adversely affected by bleach.

    It may be that you have to just get hold of some slate oil and try to re oil the spots, may take several applications to get it looking nice and blended.

    Hope this helps
    Ian

  24. Hi Ian,

    Thank you for all your responses on this thread. I think i’ve a similar issue to a previous commenter, but just wanted to confirm. In cleaning some bricks with a strong acid based cleaner in a fireplace, unfortunately a bit dripped onto the slate hearth and etched an area before I could clean it up.

    It’s lighter in colour to the eye and rougher to the touch. Is my best bet some mechanical polishing with a some very fine wet/dry paper? The slate has no coating or finish on it. I’ve previously been recommended to use WD40 to clean and maintain its look.

  25. Hi David,

    OK, what you describe, the lack of colour and the rougher touch, is a great description of acid etching, it is less common on slate as much of it is acid resistant but there are some that will suffer. Basically the acid has removed some of the finer surface particles leaving a rougher texture behind which not only feels less smooth than the surrounding areas but also affects how light is scattered and reflected back, hence the less intense colour. So, logically if you can get the surface back to being as smooth again you will go some way to repairing the look and feel. So yes try some wet and dry work in a circular motion, with a little water spread out from the etch to an area about 20% larger to ‘blend’ it in, wipe and let it dry, repeat if it has done something but not quite got there. Depending on what the actual finish was you might have to resort to diamond hand finishing pads in finer grades, but these are easy to get online now and easy to use.

    Hope this helps

    Ian

  26. Hi,
    I have been removing high gloss black paint from natural rough slate on my stair landings and around my fireplace. I have been using a low voc paint stripper. I have uncovered some stones that have areas of light blue that don’t look natural. I am wonder if there had been some chemical reaction with the stripper but some stones came up perfect with no mottled spots. Possibly something on them before they were painted? The areas looked bleached. How do I restore it to the natural slate?

  27. Hi Debi,

    OK, I guess there could have been something on them before they were painted, but without knowing what I would not know what to suggest. However, some slate does have such random blemishes. If you google ‘mineral spots in slate’ you will lots of images – over here we often see dull, light-greenish spots in Welsh slate – it is just part of the stone, people don’t like it and think it is a fault, but it is natural, like knots in pine for example, completely random – some slate has them others do not. If you have this then it is part of the stone and there will not be anything you can do to remove it.

    Hope that helps

    Ian

  28. Hey Ian,

    Some apple juice spots showed up on our slate. I’m guessing the Apple juice was sitting on it for a while. The spots are look like faded, slightly white, circles. How do I get rid of them?

  29. Hi, hopefully, this is residue from the apple juice. If so then you could try a mild alkaline cleaner with water, or a microabrasive cream cleaner – there are plenty available in your part of the world.

    Worst case is that the acidic compounds in the apple juice have etched the slate a little (some slates will etch but not all are sensitive to acids) if this were the case then you may have to try rubbing the slate back with a very fine emery/ wet & dry paper and a little water, or use some diamond hand polishing pads – but this depends on the original surface of the slate.

    My bet though is that it is residue that should be possible to be removed

    Hope this helps

    Ian

  30. Hi
    We have a Black Brazilian slate floor in the kitchen which we have sealed with and enhancing seal to keep it as dark as possible (cant think of the name of the seal at the moment), I have been cleaning it with my steam mop – no products and the floor seems to have lost some of its shine…do you think this method has affected the seal, would you suggest resealing it?

  31. Hi Kirsty,

    OK well, it depends on what type of sealer you have. Two types of sealer can ‘enhance’ a slate in different ways. There are coating sealers, these typically leave a deliberate surface film and can be matt, low gloss or high gloss. They enhance or darken the stone (to varying degrees) by making them appear to have a surface sheen. The other and more typical type are impregnating sealers, these are designed to sit below the surface (not on the surface) and create the enhancing by making the stone appear to be saturated or permanently damp.

    IF you have the first type – i.e a coating sealer then they bound by design to wear off with use/traffic/cleaning – so over time they will wear and dull down (your steam-mop may help accelerate this but it will happen anyway) – with the gloss will also go any enhancement that was there – and so cleaning and reapplying more of the same should help.

    If, as I suspect you have the more typical impregnating enhancing sealer, then it is not so simple. Most sealers of this type are meant to sit beneath the surface and are not intended to give a sheen or any kind of glossy coating. If yours had one then it was most likely the product of the incorrect application of that sealer. That is to say, the sealer was not properly applied, with a residue being left on the surface (the shine) and over time this residue has been removed. Reapplying this type of sealer (correctly) will not add back any shine.

    A 3rd possibility is that the slate itself is not as hardwearing as some and has actually worn slightly dull. If this was the case then you would have to look at potentially getting the stone re-honed or refinished by a professional.

    Hope this helps

    Ian

  32. Hi Ian, great forum !

    I got a door entrance floor made of slate tiles, and after some winters, I can see a pathway (the color and texture of some slate tiles changed) probably caused by the salts the city puts on the sidewalks that sticks under the boots.

    I tried a thorough cleaning, and a sealer, but the result is the same… When I apply the sealer, with the wet look, the problem is gone. However, when completely dry, I still see the tiles where we frequently pass… the color is different.

    Any suggestion on how to make the floor more uniform in terms of colors ?!

    Thanks !

  33. Hi Pascal,

    Dealing with the salt deposits should be fairly straightforward: With a proprietary phosphoric acid based cleaner (not HCL or muriatic), diluted to low strength, pre-wet the steps, apply the dilute cleaner, scrub, rinse, rinse again and dry.

    A layer of salt deposit can affect the look of the slate, as it can encapsulate the surface and alter how light is reflected back to your eye. Wetting the stone can make the deposit translucent – hence the colour can appear to return, only to disappear again as the stone dries out.

    However, as you did a clean and re seal, you have most likely sorted this for now but for future information, that is how to deal with it.

    Regarding the loss of colour: As you note that the colour returns and looks fine when wet, it suggests that the slate is a) encapsulated by a water-soluble mineral as above) or b) worn, (ok we know this, but the point is it has worn to a different finish than the slate had originally. So, assuming that it is not simply a salt issue and that the slate is worn:

    Slate comes in a variety of surface finishes, rough-textured, riven/natural split, fine honed and on occasion, polished, for example. The degree of flatness/polish affects not only how smooth and even shiny the slate surface is, but also the depth of colour you see. The flatter/smoother the finish, the higher the degree of any sheen or even shine and the greater the depth of colour. As the surface wears, that makes the surface rougher, less smooth, resulting in a loss of both surface sheen and colour. Even riven slate may be rippled but the surface of the ripples may be smooth enough for good colour depth.

    Think of a lake surface, on a calm day a tree on the shore will be well reflected in the flat surface of the water, on a windy day, with the water surface broken up, light is scattered and you see no reflection – smooth stone surface = lake on calm day, good reflection of colour, rougher surface, light scattered, less colour depth. The reason it looks ‘ok’ when wet is that the water temporarily fills up the surface, leveling out all the unevenness and makes a temporary flat surface. As it dries out, the actual surface of the slate is revealed and so the effect fades.

    Different stones have different degrees of surface hardness, and slate can vary quite a bit, (slate from Wales and North America can be hard, some from Asia very soft etc). The softer the slate the harder to achieve a smooth surface, harder the surface, the easier it is to achieve a smooth/flat surface and the longer it can last. However slate is not as hard as say some granites, and on an external step, exposed to the elements, out-door footwear, carrying abrasive git etc – that surface is not going to stay smooth and colourful for long.

    What can be done? Well colour enhancing sealers are an option, these are sealers that give a wet look (not a topical or coating sealer which wears off really quickly – within one season in these circumstances) rather, an impregnating sealer that is designed to enhance the colour. These can be effective but they most often need raw/unsealed stone – as you have had a sealer on them, and very recently too, it might not be possible to use this type of product.

    The only other real thing you can do is re surface the stone. If you had a smooth, flat honed slate then you might be able to re create this using diamond sanding pads – these are available for use by hand as well as small 4/5 inch discs for use on hand polishing machines (available to hire). You may have to use several grades to ‘cut’ below the surface abrasion then start making it smoother and smoother. If, on the other hand you had a naturally riven stone then it is harder, as diamond pads would struggle in the texture. You could try flexible nylon polishing/scrubbing pads again you can get these for a machine. Try a medium course and go up to the fine polish (white).

    I you are able to restore the surface you could then use an impregnating sealer to sealer it. Depending on how much ‘sanding’ you do, the stone might now take an enhancing sealer but this is not guaranteed as enhancers typically need 100% removal of any previous sealer, any trace of an old sealer can prevent the enhancer from getting to where It needs to go in order to create the effect properly, so it can result in a very blotchy appearance, and it is not usually possible to reverse the application of an enhancer, so you are then stuck with how it looks).

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