A newly installed black stone patio is an impressive sight – but its wow-factor often doesn’t last long. Black or very dark grey stone quickly fades, leaving you with a wishy-washy grey aesthetic. So, how can you restore the colour and protect slabs from future discolouration, or ensure that a newly installed patio retains its good looks?
Why does black stone fade?
A dark stone’s appearance can be greatly affected by UV light and normal rainfall because it causes a chemical reaction. Limestone and many other dark stones are calcium-based, alkaline in nature and sensitive to acid. They contain a soluble mineral called pyrite; when this is combined with water and oxygen, it forms sulphuric acid, which dissolves the calcium and changes the colour of the stone. Meanwhile, as rain falls, it absorbs carbon dioxide from the air, which dissolves to form carbonic acid. This makes it slightly acidic – around 5.6 on the pH scale, rather than a neutral value of pH 7.
Rainwater, humidity and UV-rays all contribute to discoloration, as does erosion caused by the wind and general wear and tear. Acidic maintenance chemicals can also cause additional damage, as can jet washing.
How to protect new slabs or restore an existing patio
Good quality water-resistant sealers go a long way towards reducing discolouration – and many can enhance colour too – but rainfall also damages the protection that they create. Topical sealers can be worn away and rainwater that seeps into the stone’s porosity will eventually compromise the effectiveness of an impregnator. Quality formulations that seal and restore, like LTP’s Blackstone Sealer & Restorer, provide a long-term solution by protecting the surface and creating a permanent colour enhancement. This particular product is a water-based, impregnating sealer which contains a permanent colour intensifying pigment. It can be used on limestone, marble, slate and concrete pavers. To maintain a very dark, pristine finish, annual treatment is recommended, although a treated stone will maintain a far darker aesthetic than an untreated stone for around 4-5 years.
Choosing a water-based treatment provides safety and environmental benefits. It can also be applied in damp conditions, so application is easy can be carried out at any time of the year, provided no rain is forecast for 24 hours either side of application.
Why good preparation is key
If you’re looking to restore a faded limestone, it’s important to ensure that tiles are properly cleaned and prepared before any restoration is carried out. When a restorative treatment like Blackstone Sealer & Restorer is applied to black stone, it mixes with any loose particles of limestone. These particles are caused by etching which occurs when the stone comes into contact with the acidic rain.
The etching creates a sediment which is carried into newly exposed capillaries and the sealer, in turn, draws the sediment out of open pores. When this residue mixes with the sealer, it creates a smeared and patchy appearance. A thorough clean will remove the sediment and condition the stone. If you rub your hand across the surface before cleaning, you’ll see a dusty residue; that’s the limestone sediment. Cleaning the surface opens the pores and prepares the stone for application of the sealer.
Less is more
Don’t be tempted to over-apply your restorative treatment in order to achieve a darker finish or speed the process up. It’s far better to apply several thin layers than one thick layer. If you apply too much product in one go it won’t be absorbed properly and the finish will be patchy. Any residue treatment should be removed with an intensive cleaner, like our Power Stripper; we recommend a diluted 1:10 solution. This is an important part of the process because the treatment only works in the pores of the stone, where the formulation cross links to make the sealer colourfast. This final clean won’t return the stone to its original colour, it will just remove any excess treatment that hasn’t been absorbed and is effectively just sitting on the surface.
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