Why are my Black Galaxy tiles being discoloured?

Hi all, I hope someone can help me. 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 greyed the granite after only 10 mins. The granite didn't seem to significantly grey any further with lemon juice applied for 30mins, 60mins and 12 hours (refer LemonTen30Sixty12hrs.jpg showing 10, 30, 60mins and 12hrs from left to right). 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:

 "Spoke 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 did a similar test on a sample of Black Galaxy from a different supplier and found there was no mark after 30mins for lemon juice or white vinegar. However, there was a slight mark left after I let some lemon juice and white vinegar total dry after 24 hrs over a very hot day (refer DryLemonVinegarOtherSup.jpg).

I have heard that wetting the back of the tile reveals the true colour. I submerged the tile in water for six hours face down in a paint tray and put another tile face up next to it (refer BackFrontColorDiff.jpg). There is a significant difference. However, the sample from the different supplier seamed to be even lighter when submerged and placed on top of the tile (refer OtherSupBackColorDiff.jpg).

There doesn't seem to been any substance on the side of the tiles to indicate any doctoring (refer SideColor.jpg).

I put hydrochloric acid on half of one tile and let it stand for 24 hrs to see how bad acid could discolour the tile as a worst case scenario. The standing hydrochloric acid turned yellow (refer Hydrochloric acid on tile.jpg). 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 (refer Stained rag.jpg). I put the rag on the clothes line to dry and when it did the yellow almost totally disappeared (refer Dried rag.jpg). 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 (refer Tested and untested tile.jpg). I did not dilute the acid which was the strongest type from the hardware store (i.e. probably 30%).

The acid may have etched the tile as its reflective ability seems slightly diminished (refer Reflection.jpg). The surface feels noticeably rougher than the untested section when running a rag across it with your finger.

I put hydrochloric acid on another sample of Black Galaxy from a different supplier and let it dry for 72 hours over very hot weather (refer DryHCLOtherSup.jpg). I wiped off the substance with a white cloth (refer Dry HCLWipedOff.jpg). This sample was significantly greyed.

Could there just be a colour enhancer or resin coming off the tiles? There does not appear to be any case for resining the tiles (other than to change the colour) as the tiles seam to be solid and free of imperfections.

 

I did the olive oil test over night on the HCL tested tile and HCL tested piece from another supplier. It darkened the greyed area almost instantaneously (refer Oil test after1min.jpg). I left it overnight (refer Oil test after 12hrs.jpg). I vigorously cleaned both pieces with a soft sponge and dish washing detergent and couldn't get the stain out (refer Oil stained tile.jpg and Oil stained Other Sup.jpg). I would imaging the MC will do the same thing as the HCL and make the stone more pervious. At the moment I do not have to seal the tiles, I like the colour, but will have to be careful with acids. If I strip the tiles I will have to seal them and do not like the colour.

It seams that whatever is darkening the tiles, be it colour enhancer or resin, is also sealing the tiles. I'm tending to want to put them down as they are and work out a way to best maintain them. To best do that I need to know what is providing the colour.

If I do the oil test on the back of the tile and it does not stain, does that mean it has been resined? Or is the resin just applied to the face and not right through the tile? If I knew that, I could try and source some resin and just re-apply it as needed to maintain the current look.

Or if it has been resined, does this mean the tile doesn't breath? If this is the case I can just put a topical acid resistant clear coat to maintain the finish.

How do I determine if the tile has just been colour enhanced on the face? If I knew this I could just source similar colour enhancer and re apply as needed to maintain the finish.

Will the MC react with the stone like the HCL or will it just strip the colour enhancer of and will I be able to see the stuff on a white rag after wiping the MC off?

 Does anyone know what pH the Methylene Chloride is as I don't want to etch the tiles if I use it to strip them?

Any answers to the questions above would be greatly appreciated so I can work out what to do.