If they are dulling down and going grey, then that is a sure fire sign that they could be a basalt, this would be indicitive of acid etching. It is also possible that a build up of alkaline detergent could also create this dull effect, but by leaving a dull patina on top rahter than actually dulling ths stone. This would be removable with a good clean with an alkaline cleaner (as long as you rinse well) and/or an abrasive such as Microscrub.

However, my gut feeling is for the former, based on what you have said. If this is the case, then an alkaline cleaner will not help (apart from cleaning it well). I am afrad the only way to fix acid etching is re grinding/polishing, and I have to say that with basalt, it is not always 100% possible, although it can help and improve the look.

An enhancing sealer can also help, but I am reluctant to advise this as any previous sealer may cause problems

The other thing to point out is that it is hard to just re surface 10 individual tiles, normally the whole floor would have to be done for a uniform finish.

a good alkaline cleaner for on going cleaning would be Xtreme Clean by All For Stone, if it is not on the site yet, you could call them

Hope this helps

Ian

Can you suggest an alkaline cleaner I could buy?

Many thanks

Your issue could just be a build up of dirt, and/ or detergenet resiidue. Often dull tiles come about due to lack of rinsing with detergents and over time it builds up a patina and dulls the surface, a good deep clean with a high alkaline cleaner would normally resolve this.

I would strongly recommend that you DO NOT use vinegar in any form. There is strong possibility that your ‘black granite’ is actually basalt or some other similar stone. In fact, many geologists will tell you that there is no such thing as a true black granite, what you have is an igneous stone, formed in the same way and with very similar characteristics but not technically granite. There is no problem with this, the stone industry is allowed to describe such material as granite as it meets many of the criteria for that stone type.

The main thing to be careful of is acidic etching, quite a lot of the so called black granites are susceptible to acid etching, in fact, this may be the reason that your stone has gone dull in the first place. If any of the black colour has turned greyish, that is another indication of etching. Unfortunately, there is liitle that can be dine to bring back the shine on this type of stone if this is what has happened, it may be that you need to seek the help of a professional stone restoration company (like Marblelife Ltd. for example) who may be able to re-grind and polish the stone back to its former glory – but, if this was possible then it it quite a costly process.

Try the alkaline deep clean first,

Hope this helps

Ian