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How to seal travertine 
Test whether sealing is necessary. 
Polished travertine often doesn't need sealing, but you don't need to guess whether or not it's a good idea. It's easy to test whether sealing is required by dropping a few drops of water in a few inconspicuous areas.
Let the water stand five to ten minutes, then blot dry. If the travertine stays darker in color for more than a few minutes, sealing is probably a good idea to protect it from other liquids that could leave a permanent stain.