To clean the stone you need to determine a few factors first and then proceed with the cleaning.
Following is the list that will help you clean and maintain you sandstone sculptures, seats or tiles.
1. Is the stone cleanable? Do not do any brushing or scraping as part of the cleaning process if the stone shows signs of chipping, scaling, flaking or other forms of deterioration. Apply a high-quality biological cleaning solution only or else you will do more harm than good.
2. Determine next what kind of stone is it? What technique should be used to clean a stone depends on the kind of stone it is.
3. The first rule is always to be as gentle as possible regardless of what kind of stone you are cleaning. To accomplish the cleaning objectives, use the least aggressive approach, safely without harming the stone. Long-term biological growth such as lichens and mould are highly variable in how fast they can be removed as they may have been attached to the stone for decades.
4. Completely saturate the stone to be cleaned with water and always begin with clean water, a soft scrub brush, and plastic scrapers.
5. There are many different types of brushes, which work well for cleaning sandstones. Always have a wide range of brushes on hand with multiple sizes and different stiffness of the bristles. Always begin the cleaning process with the softest brush and then progress to stiffer bristles only if needed. Do not use wire or metal brushes of any kind, as they may scratch damage or stain the stone.
6. To avoid streaking or erosion to the surface of the stone, always scrub in a random orbit motion. If the dirty water is allowed to evaporate before being rinsed from the stone, it may result in streaking and staining.
7. A non-ionic detergent is safe to use on nearly all types of stones as it has a neutral PH, which does not affect or harm historic stones. Do not expect miracles to occur while cleaning with a non-ionic detergent, you have to be patient as results are often less than spectacular.
8. Poulticing the stone is another highly effective cleaning method and it employs the concept a capillary action to wick away staining safely. A poultice is placed on a pre-moistened stone. A poultice is itself a clay type substance. To avoid evaporation the stone is then covered. The stone is rinsed clean of the poultice once it is uncovered.
These are all very safe techniques to clean sandstones and when properly employed they may be performed on very delicate surfaces as well, be it in museums or in your garden Sandstone Seats and sculptures.