Sandblasting Cave Graffiti:
Sandblasting is one method of removing the visual impact of graffiti. There is other benefits to consider as well. Sociological theories such as Broken Windows suggest that visible vandalism creates an environment where more vandalism is likely to occur. The theory is that some people who would vandalize are more likely to target sites that are already degraded. There is supportive research of this theory. None of that research is cave specific. Anecdotally we are seeing in Oregon that cleaned caves with graffiti removed generally need a minor touch up every 2-3 years. Gated caves are secure and remain clean. Heavily vandalized cave sites here open to the public seem to have a much lower reoccurrence post clean-up.
We have developed a sand blasting system to remove cave graffiti. The system is powered by two gas air compressors in an enclosed cargo trailer that is parked on the surface near the cave entrance. The system uses two 6.5hp Honda motors, similar to a lawn mower. The exhaust is up high in the trailer producing no sparks or fire hazard. We also keep a fire extinguisher on site just in case and keep a person with the engines to monitor the system constantly. Air hose is laid in the cave to the removal site. The blasting media is hauled to the removal site. We have over a mile of air hose to reach very deep inside caves. Compressed air comes into a portable metal tank that holds the media. The compressed air forces tiny media particles through a nozzle to remove the paint from rough cave walls and rock. The process is so delicate we can remove paint from calcite and gypsum crystals with very little impact. We have even sandblasted paint off of moss with very little impact on the living moss. We typically start at the end of the cave and work our way towards the entrance. We lay out tarps to collect and reuse the media. There is always a little media left behind post cleaning. The media we typically use for lava tubes is crushed green quartz which is natural and chemically inert. It is chemically very similar to the pumice sand soil already present in caves. It does not have any effect on the cave or life inside.
To prevent contamination (primarily to guard against the spread of White Nose Syndrome) between caves we run decontamination to USFWS standards. All gear used at the cave will undergo full decontamination prior to use. It will be decontaminated again after use. We have two 30 gallon hot water decontamination pots with propane burners, isopropyl spray bottles, Clorox wipes, bleach vats and a pressure washer.
Biology:
Sandblasting should not be done directly on animal life. Compressed air and the impact of sand could cause harm. Generally invertebrates should be relocated at least 5 feet away from operations. Bats should have a 100 foot buffer. Often simply blasting several hundred feet from bats will result in them moving further away from the operation. Salamanders and frogs should be at least 10 feet from operations or relocated away from the area being worked.
Sandblasting Methods:
Sandblasting is a simple process where sand, small crushed rock or other blast media are propelled at high speed to remove paint. The sand particles are very small and have very minor mass. Blast media can be used on gypsum, calcite, rock, lava, metal, wood and other surfaces with very little impact to the surface. Microscopic and loose larger parts of the surface may be removed as well. The process can be dialed in to lower or higher pressure to work on specific surfaces. The skill of the user is a variable to this process. We have sandblasted paint off of moss at cave entrances with the vast majority of moss left in place. I have sandblasted paper thin gypsum crusts with very minimal impact. With skill and time this process can be remarkably gentle. Higher air pressures results in more impact to fragile surfaces like gypsum crusts. Surfaces like basalt lava tube walls or bare limestone can be cleaned at high pressure with very little impact.
Blast Media:
Blast medias very widely. Sourcing an appropriate media is an important factor. An ideal media would be chemically similar to the local cave soil. Most blast media I use is high SiO2 content rock mined and processed into sand. The dominant types we prefer are garnet, silica sand, and crushed igneous rock. SiO2 based rocks and sand are generally non-reactive. Garnet is the most expensive. The advantage of garnet is that many products are EPA certified as safe to use in watersheds. Garnet tends to be bright red and has a visual impact in areas it is left behind at. In many environments the garnet washes or mixes into the native soils and disappears quickly. Silica sand is also inert and similar to many cave soils like clay or water borne sediments. Silica sand tends to clump and clog blast equipment making reuse of the sand difficult and frustrating. Crushed igneous rock is very good at removing paint. It is non-reactive. There are generally minor impurities in the rock with a variety of colors like green or black. It is very similar to the natural cave soils in lave tubes. Overall crushed igneous rock is an excellent choice operationally with very low impact to the cave environment. Recycled glass bead media is widely available and operationally works well. Media like soda, metal slag or walnuts are reactive. These media types would have unknown effects on cave environments and likely should be avoided. The Safety Data Sheet (SDS) sheet (sample) of any media should be checked prior to operations to understand the contents of the blast media. Tarps should be used to collect used blast media for reuse and removal from the cave. Some media will always be lost and left behind.
Safety factors:
Blasting creates dust. Most good blast media for cave use is high in SiO2. SiO2 is associated with dust pneumonia, black-lung and other respiratory issues. Blast media should also not impact the eyes. A full face respirator with a P100 filter should be used by anyone close to the blast operation. A 3/4 respirator with a P100 filter should be used by anyone in the cave during blasting. Respirators should be fit tested at the beginning of operations and periodically during work. All participants should demonstrate fit testing their respirator prior to entering the cave.
Gloves and long sleeve clothing should be used to protect the skin from impact by the media. Compressed air can be harmful in it hits the skin directly. Gloves and eye protection are advised to prevent this risk.
Gas air compressors put off exhaust fumes. Gas engines should be far enough from the cave as to minimize the chance of exhaust entering the cave.
A volunteer should be present with the compressors at all times to monitor operation, fuel level and fire safety. A fire extinguisher should be present outside the trailer to deal with any issues.
Operations:
Sandblasting equipment varies widely. All have the same concept. Compressed air drives sand at high speeds into the paint removing it a microscopic chip at a time. Maintaining a reasonable air pressure for the specific equipment used and type of natural formation being cleaned is critical. Having a mechanically inclined team lead is critical to the operation. Sandblasting equipment needs regularly fixed and rebuild, often in the field. Having the correct tools and spare parts easy to access is a vital resource.
Cave Related Challenges:
Sand is heavy. Usually people must haul 50lbs bags of sand and metal blast tanks deep into caves. Surfaces are uneven and sometimes slick. Air hose is a tripping hazard. Having a strong crew able to move under load in challenging cave environments is critical. Mountaineers and expedition cavers are often well suited for the roles.