Exposed to today's tailpipes and smokestacks, stone is deteriorating faster than ever before. A solution, one that is in the building owner's control, is to strengthen the stone. THE POTIONS BEHIND THE MAGIC Materials used to strengthen stone are called stone consolidants. Brushed, rolled, or sprayed onto the stone, they strengthen it by restoring the bond between adjacent stone particles. A stone consolidant restores the bond between stone particles in one of three ways: replaces natural stone binders that are decaying; precipitates chemically resistant material that lines the stone's pore space; and partially fills pores with polymer. WHICH KIND SHOULD YOU USE? Determining what consolidant to use is a difficult task. No material has been shown to work equally well on all stone in all conditions. Several factors should be considered in selecting a stone consolidant: type of stone; causes of decay; amount of decay; present and future environment; amount of stone to be consolidated; and the importance of the building or monument.