had a nightmare on job laid recently..50mts porc on ufh ..bifold doors on one end 3 rooms knocked into 1...massively out of level everywhere..started laying these 600x600 s with the vacum lifter which i always use on these with 3mm joints..noticed with light flooding in through the doors the lifter left a black ring in the tile .carried on laying thinking no prob will clean off during grouting etc ...wrong ..no way would the mark come off ..the tiles were made in poland and obviously un glazed but highly polished ..virtually zero info on the boxes...spent 2 days at my cost cleaning with every type of cleaner known to man..poultice ..alkaline...thinners ..etc ..has anybody else had this issue