Our client complained that installed floor tile emitted a wet clay odor when wet. We undertook an analytical investigation to determine why this might be the case. We applied optical microscopy, XPS surface analysis, and XRF sub-surface analysis to determine why this was happening. The ceramic floor tile had a total thickness of 9 mm, of which about 0.5 mm was a hard, white layer over the baked clay substrate. A surface glaze was applied to the top of the hard, white layer.

Distilled water was put on the floor tile surface and it beaded up into puddles. The water was allowed to dry in laboratory air at room temperature with a low speed fan blowing air over the surface. When the tile surface was dry, there were water spots on the surface.

Since the water was distilled water and the laboratory air was clean, the deposits left on the glaze surface must have been due to the water leaching ions from the underlying tile materials. One does not expect the glaze to be subject to leaching, so defects in the glaze are to be suspected.

The many round defects in the glaze observed in the above photomicrographs suggest that bubbles formed in the glaze and may be providing pathways to underlying material in the hard, white 0.5 mm thick layer under the glaze. The water then leaches ions out of the material in this white underlayer and they are deposited on the top surface when the water evaporates, leaving the water spots.

XPS surface analysis was performed outside a water spot, inside a water spot, and at the heavy deposit at the water spot perimeter. About 90% of the XPS signal comes from the outer 20 nm of the surface for the photoelectron take-off angle used for this data.

Both organic and carbonate carbon concentrations increased in the water spot deposits relative to the clean glaze surface where no distilled water sat and evaporated over time. The sodium concentration also increased greatly.

XRF analysis was utilized to determine the composition of the hard, white layer under the glaze to see if it was a source of sodium. XRF looks more than 100 times deeper into a material than does XPS. The white underlayer proved to have an appreciable concentration of sodium.

High energy resolution analysis found that the surface deposits were organic materials and sodium carbonates.

In conclusion, distilled water on the top surface of the ceramic floor tile leaves water spot deposits on the surface consisting primarily of organic material and sodium carbonates. The surface glaze should have been formulated such that water would not dissolve its component materials in such a manner. The microscopic bubbles observed suggest that these caused pathways to the underlying hard, white layer of material. That material had a significant concentration of sodium in it. Since that layer was supposed to be protected from water exposure by the surface glaze, it is reasonable that the white layer was not formulated to prevent the leaching of organic materials and sodium ions. These water spots on the surface are certainly undesirable for the appearance. They also indicate a likely mechanism for the reported wet clay odors.

The complete analysis report may be examined.