Thanks for the reply. I agree also, but was being accused of being too picky by relatives (I was vacuuming the backs of tiles as well as the floor). They said any dust or dirt would be absorbed by the mortar and wouldn't affect adhesion. Thanks again.
Here's the deal. Fired clay tile surfaces are contaminated with what is known as "kiln release". It is a powdery silky product not unlike talcum powder that is used in the manufacturing process. The tiles in most cases are cleaned but only with compressed air. This leaves a powdery residue that can in fact repel any tile adhesive. I have seen this in re-dos many many many times over the years when repairing a tile installation failure due to improper timing of tile placement and allowing kiln release to remain on the tiles.
True the adhesive will absorb some dust but not all.
Vacuuming the tiles isn't enough and isn't the industry recommendation either. The use of a wet sponge to clean the tiles and the substrate is the recommendation. In addition, you should always clean any cementitious substrate with a wet sponge just ahead of spreading thinset mortar.
Then there is the matter of common-sense. What would common sense say to do?
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