Efflorescence is a crystalline, salt deposit that appears as an unsightly white plaque with a powdery appearance on porous stone and masonry surfaces. It is the most common salt related issue.
Water, full of dissolved salts (minerals) is absorbed into the stone from all six sides, then travels through the pores of the stone to the top surface where it evaporates, leaving the salts behind. Fresh efflorescence is often easily removed with a dry brush, but over weeks these salts react with the air and harden, until only acid washing or grinding will remove it.
There are a number of potential causes, including:
- Moist conditions
- Rain or dew
- Low temperatures