It's simply compacted dirt and grease from neglect and poor routine cleaning. Use an alkaline (degreaser) cleaner, scrub with a grout brush, rinse, and extract.

On Oct 22, 2015 8:45 PM, "Roger Konarski (via sccpartners list)" <nobody@simplelists.com> wrote:

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I looked @ a job. White grout lines have a black residue on top. I can scratch away the black residue with a key. The question I have, why the black build up? See attached pitures.

Thanks,

Roger
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