White markings or blemishes in slate are common and are hundreds of millions of years old.  Rather than being a defect, they are a natural characteristic of the stone and are normally only visible after the slate has been split.  However, because they contrast so much with slate’s dark colouring, they’re not always appealing – but, with the correct product, they can be removed without damaging the slate.

What is slate

Slate is compacted mud and silt, from ancient oceans and river beds, that’s been subjected to pressure in a hot environment, deep underground. Eventually, it is forced to the surface by the collision of continents. Dead creatures, older limestone rocks and marble pebbles that have settled on river or sea beds are crushed between the layers of silt and over millions of years harden to form a calciferous deposit and the tell-tale white marking.  

To safely remove the markings, we recommend application of ECOPROTEC Cement, Grout & Salt Residue Remover after installation and before sealing and grouting. A rapid-cleaning agent, the Remover is non-corrosive and has an acid-replacement formulation, so it’s safe to use on all types of unpolished natural stone.  Once in contact with the calcium carbonate deposits, they start to dissolve.  The surface is then agitated, residue extracted/removed and the slate is rinsed with clean water.   Here are the step-by-step instructions which include more detail:

How to apply to remove these marks ECOPROTEC Cement, Grout & Salt Residue Remover:

  1. Shake container well before use. Gloves should be worn and instructions carefully followed.  Please read the Technical Data Sheet on the product page.
  2. Apply undiluted solution to the surface of the stone and agitate immediately with an emulsifying pad. 
  3. Rinse thoroughly with clean, fresh water. 
  4. Extract the released calcium deposits with a sponge, rinse with water and leave to dry.
  5. Repeat the process until the lighter marks have been removed.

N.B. Always dilute by adding product to water, rather than water to product – and always rinse surfaces thoroughly with cool, clean water. When treating vertical surfaces, start at the top and work down.

Once the surface is dry, seal the slate with ECOPROTEC Colour Enhancing Sealer – a water-based impregnator that will enhance the stone’s natural colour tone, protect it from staining and repel water, oil and grease.  

For ongoing maintenance, we recommend ECOPROTEC General Cleaner.  This cleaner has a gentle, pH-formulation and gives a thorough, fragrant clean with no rinsing required.

ECOPROTEC Iron Stain Away is another useful treatment.  Formulated for unpolished natural stone, it removes brown stains caused by naturally occurring iron deposits. Our recent rust-focused blog provides some more information on rust stains and iron break-out.

Important note:

Efflorescent and rising salts can also cause white marks. This is a slightly different issue which can also be treated with ECOPROTEC Cement, Grout & Salt Residue Remover, although the application method is different and the treatment and the remover is generally diluted before application.  Before treating the slate, it is important to establish whether the markings are calciferous deposits or salt-based.  For more information on treating efflorescence, please see our blog – Removal of white blooms and chalky residue.

For more advice and information about other ECOPROTEC cleaning and sealing solutions, please visit www.ecoprotec.o.uk or contact the ECOPROTEC technical team on 01823 666213 and email technical@ecoprotec.co.uk  

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