If this term is reference for tiles quarried (cut) from the earth then see the sealing systems for slate.
If this term is reference for unglazed clay tiles then see the sealing system for Britti if high in porosity / or NCI unglazed if low in porosity.
* If the sealer has rechaed the point of degradation that it appears patchy, then it is recommended to strip and reseal rather than clean and reseal. Look for the stripper selection chart to determine which stripper to use before resealing.
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