Black Spots on Floor Tiles

Problem: A client responsible for maintaining the facilities in a public school district called with concerns of a possible mold problem.  Areas with high foot traffic, especially those where students tracked water in, were showing dark black spots in the floor tiles.  Aggressive cleaning and buffing of the floor would remove the problem for a while, but after several few weeks, the problem resurfaced.  The facilities management staff was convinced it was mold related, but sending samples, swabs, and air grabs to a mold lab for culturing showed no sign of fungal structures on the tiles.

Approach: An affected floor tile was submitted to determine if the previous mold testing had missed a source on the tile backing or mastic.  Additionally, a new tile from the same manufacturing lot was submitted for comparison.  The process of preparing and examining the sample and reference tile was documented.  Areas with darkened surface features were imaged and then cut out and examined.


Analysis and Results: While the dark spots looked very discrete when examined by eye, under top light polarized microscopy they appeared more diffuse at the outer edges.  The darkest areas surrounded what appeared to be particles embedded in the surface.

Scanning electron microscopy with energy dispersive xray spectroscopy (SEM-EDS) was used to identify the particles.  The SEM showed an elevated concentration of iron and iron oxide in the impacted areas.  The backscatter electron (BSE) image which correlates brightness in the image with atomic density, highlighted the iron particles that were embedded in the tile and the EDS spectrum confirms the PLM Image chemical composition of these higher density particles.

Also, the color EDS map highlights the iron particles in the mineral filled PVC floor.  These iron particles were concentrated in areas were the tile showed signs of impact which indicated some metallic object impacted the tile leaving behind small particles and over time the materials oxidized which created darker spots in the tile.

Conclusion: The client was able to determine that there was no sign of any mold or fungal growth in the tile or mastic.  Knowing that the iron was largely responsible for the dark spots, the client was able to apply a thicker wax coating in order to avoid particles from being embedded in the tile itself.


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