When a strong flow of water is applied (pressure, compression) you must bear in mind several factors:
The water must not be allowed to concentrate for a long time on a single spot because it may damage the material.
Water containing abrasive particles should not be sprayed on the surface of the tiles.
If you use a rotating cleaning machine then the rotating speed must not exceed 450 revolutions/minute and the brushes must be flexible. It is not recommended to use cleaning pads. The machine should not be left to clean on the same spot for a long time.

It is better to fill the entire space between the tile and the wall/floor with glue. This avoids the breaking of the edges and corners when pressure is applied on them.

The different wear resistance of the faience and terracotta predetermine their application. Due to its lower water-absorption percentage, the terracotta can be applied on the lowest part of the wall (type of a socket).

The tiles are glued consecutively. Work is done simultaneously on a small area (preferably 1 m2).

The surface must be clean, smooth, free of dust or grease.

Now there are glues that enable such application. But before you start, remember that there is certain risk when applying new tiles on top of old tiles. If the old tiles have been glued on a limestone-cement plaster base, this could be a problem. If the plaster is old or if the old tiles have not been applied properly after some time a slight “bulging” of the tiles would appear because the old plaster cannot stand the weight of the old and the new tiles. Nevertheless, if you still decide to apply tiles on top of other tiles, remember that it is best to glue them up to the ceiling.

It is compulsory to leave spaces between the tiles (joints). They must be minimum 3 mm.