Unglazed terracotta tile care
In the 19th century, unglazed terracotta floor tiles were laid directly on sand. With no damp-proof membrane beneath, moisture would percolate through the tiles to evaporate harmlessly in the air. To make them more resilient they would have been impregnated with boiled linseed oil so that dirt and spills were not absorbed into the tile.
Naturally, terracotta tiles become more porous over the years, allowing watermarks and stains to set in. You can test your tiles for porosity by placing water in the middle of a tile. If, after five minutes, the water has soaked into the floor the tiles will need to be impregnated with a suitable sealer. This will darken the tiles a little so, to get an idea of the finished colour, wet the tiles to assess the effect.
As long as your tiles have been laid on top of a damp-proof membrane, it’s safe to apply a floor sealer that will merely require sweeping or vacuuming to prevent dirt and grit becoming ingrained. An occasional wipe with a damp mop will freshen up the finish. Check every year whether your tiles are becoming porous again.