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After cleaning a limestone building we noticed that there appears to be a brownish to yellow hue to the stone that wasn’t there prior to cleaning. Why is this occurring?

This is a common problem with limestones and some marble. Most of the time we find that there is a high content of iron in the water you used to clean the stone with. I would recommend using a chelating agent in the wash water, EDTA(ethylene diamine tetra-acetic acid) or equivalent are good choices. These chelating agent keep the iron suspended in solution so that they don’t settle on the stone and cause staining.

To remove the yellow hue from the building try re-washing with water and a chelating agent. If this doesn’t work a re-wash with a commercial rust remover may be necessary.

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