The Curious Case of the Gridlines on Tiles.

We encountered a very curious case recently which mystified us for days but we are pleased that it has now been solved after some careful detective work.

A commercial space had been tiled with large white porcelain tiles and it was reported that the tiles had discoloured and that white gridlines had appeared.

Now as I wrote last week, tile just don't darken or fade. Almost the only explanation for the darkening could be moisture seeping into the tiles. However this seemed very unlikely as porcelain has a very low water absorption (<0,5%).

The appearance of the white gridlines was even more mysterious. At first we could only conjecture that somehow moisture had entered the tiles from below and some form of efflorescence was taking place, transporting dissolved salts through the tile body from below replicating the grid pattern on the underside of the tile. As it turns out however, we were entirely wrong!

When we examined the tiles on site we discovered firstly that the tiles were simply dirty and that explained the dark patches. The floor had been cleaned but not rinsed. Over time soap residue had built up and attracted dirt which appeared as darker patches on the tile. This in turn led to the faint white gridlines to become visible. Previously the gridlines were invisible against the white surface of the tile.

But where did the gridlines come from and why did only some tiles show this pattern? We found some boxes of spare tiles around the back.


I mentioned that the tiles were larger than normal. As a result they were packed three to a carton rather than four. As you can see two tiles were packed face to face but the third tile's face was packed against the grid-patterned underside of another tile.

I'm sure you will have noticed a fine white powder on the underside of tiles before. This is a release agent applied to unfired tiles to ensure they don't stick to the rollers in the kiln. This powder had been transferred to the face of every third tile during transportation!

So some simple solutions. Firstly, clean the tiles with heavy duty cleaning agent to remove the white deposits and properly clean and rinse in future. Secondly alert the factory of the problem and pack the tiles with foam dividers or at least with the faces turned away from the base.

Although we were wrong about efflorescence, I'll write in more detail about that problem next week.