QUESTION

Granite Tiles Staining - Hi, I purchased 43m2 of Black Galaxy tiles and did the oil test to work out if I should pre seal them. I left olive oil on for 24 hrs and it just wiped off with no stain so I was happy that I didn’t need to seal them. However, I also did the lemon juice test and found that it noticeably greyed the granite after 10 mins and 3 times as grey after 30 mins. I asked the supplier why this is happened because granite should not be affected at all by any acid other than hydrofluoric acid. His reply is as follows:

“He has spoken to the manufacturer and there is no coating of any kind whatsoever on your granite tiles, they are pure granite, they have not even been sealed, only polished. The tiles have come from the most highly regarded manufacturer and none of their products have any coatings on them.

They have done an investigation on your tile and have said:

Your particular granite tiles are not 100% non porous, they are the tiniest amount porous (0.03 to 1 porous) and will therefore need to seal your tiles.

The reason you got the mark is because the acid is eating away at the 'oxide' in the granite, it has nothing to do with having a coating of any sort on the tile.”

I put hydrochloric acid on half of one tile and let it stand for 24 hrs to see how bad it could possibly get. The standing hydrochloric acid turned yellow. I wiped it off (being careful not to get any acid on the other half of the tile) with a white rag and it turned it yellow. I put the rag on the clothes line to dry and when it did the yellow almost totally disappeared. I then hosed down the tile with water (again being careful not to get any acid on the other half of the tile). The tile was very grey where the acid was but also went significant grey where there was no acid presumably just from the acid fumes (I did not dilute the acid which was the strongest type from the hardware store i.e. stronger than 10%).

ANSWER

ANSWER - True geologically classified granite stone is normally very resistant to staining and chemicals.  There are some stones sold as granite that are not true granites and they could be more susceptible to staining and etching.  Some overburden stone (layers near the earth surface) will tend to be less stable and more absorbing, although they may be more colorful and interesting.

Most slabs of true granite or more likely commercially sold granites are being treated with a resin at their processing plants to help stabilize it. This resin tends to enhance the color.  when subjected to a solvent of one type or another it will smudge the surface giving it a dull look.  Unless you know what type of resin they used you may have to repolish the surface to make the smudge go away.

Your hydrochloric acid test isn't a fair test.  The lemon juice test is a reasonable and more realistic test to determine its resistance to acid.

So your stone is likely chemical sensitive due to its composition, or it has been treated with a resin.

It is always recommended to seal your granite tiles or slabs to help make it more easy to maintain.  There is no guarantee that sealing the stone will protect it from being etched from being subjected acidic substances. Good luck.