Cleaning Historic Stone
Before cleaning any stone, carefully check its condition. If the surface readily falls away, or you notice other conditions that indicate the stone is brittle or vulnerable, do not clean it. Cleaning may irreparably damage the surface. Historic masonry should only be cleaned if the marker is stable on its base with no signs of flaking or sugaring.
- Use a non-ionic soap. One of the most readily available soaps is Orvus®, commonly used in association with horse and sheep husbandry. Orvus is also recommended for washing quilt, and can be found in feed stores. Mix a solution of one heaping tablespoon of Orvus® (it comes in either liquid or paste form) to one gallon of clean water.
- Pre-wet the stone thoroughly with clean water and keep the stone wet during the entire washing process.
- Thoroughly wash the wet stone using natural bristled, wooden-handled brushes of various sizes. Start at the bottom and work up. The use of plastic handles is not recommended, as color from the handles may leave material on the stone that will be very difficult to remove.
- Be thorough. Wash all surfaces and rinse thoroughly with lots of clean water.
- When cleaning marble or limestone, one tablespoon of household ammonia can be added to the above mixture to help remove some greases and oils. Do not use ammonia on or near any bronze or other metal elements.
- Lichens and algae can be removed by first thoroughly soaking the stone and then using a wooden scraper to gently remove the biological growth. This process may need to be repeated several times. Do not use force to remove deeply embedded lichens. If the growth cannot be removed easily with scraping, consult a conservator. Some lichen can have strong roots that may damage the stone if removed forcibly.
- Not all stains can be removed. Do not expect the stones to appear new after cleaning.
- Do not clean marble, limestone, or sandstone more often than once every 18 months. Every cleaning removes some of the face of the stone. However, occasionally rinsing with clean water to remove bird droppings and other accretions is acceptable.
- Keep a simple treatment record of the cleaning, including date of cleaning, materials used, and any change in condition since last cleaning (such as missing parts, graffiti, and other damage). These records should be kept at a central location where the condition of the stones can be monitored over time.
(Source: Developed for Texas Historical Commission from data supplied by John R. Dennis, Dallas Museum of Art Conservation Lab15.)