My method depends on fallen microscopic pigment dust or droplets seen on the magnified surface of a freshly scraped 20×50 cm soil ‘peel’ vertically below the rock art. To date the art the 5mm-thick peel scraped to attain this surface must have AMS-datable natural fragments of twig, leaf or conifer needle or cultural fragments of bone, shell or wood. Fallen petroglyph chips also work. Before field use, we chalked artificial pictographs using red and yellow ochre from buried archaeological sites, such that the same ochre fell invisibly as particles to various sand types below. Thin, progressively more complex layers of white metallurgical, cement, playground, dune and ferruginous sands were sequentially added after each ochre application, and then scraped away as peels. A 5-10 MG digital photo and electronic scan of the magnified digital image of each excavated surface revealed a few fallen particles with similar RGB colour values as the ochre-covered standards placed on the same surface. Considering it is impossible to scrape to the identical surface where we let our particles fall, the few seen are normal.
We used ochre motifs on plywood leaning over a sandbox. After each peel was removed, we had a choice of storing the photo and moving on, or directly examining magnified digital images one peel surface at a time. This was done using our customized fast scanning program or electronically removing non-pigment colour with Photoshops’s Eyedropper, Paint.Net or GIMP, leaving only particles, which could be counted and plotted. If similarly-coloured soil iron oxides and hydroxides interfere in field tests, one must prove the particles are fallen ochre. In our past dozen field tests, we encountered no soil iron interference, thereby allowing us to use an ochre-coated neutral-gray paper strip directly on the peel as an RGB standard. In both lab and field tests we scraped peels with a vertically held rectangular masonry trowel, pushing the debris to one side. In field tests where AMS samples were needed, we sieved this debris through a kitchen strainer to collect all carbonaceous material. Debris was deposited further aside for final backfilling.
In our field trials we did not evaluate or alter our method due to time limits, only transferring camera memory card peel images to a laptop after work. This prohibited particle collection for later analysis and required the sieving of all peels for AMS sampling. A live feed camera-laptop technique requires work stoppage to collect particles, but permits optimizing camera settings and peel thickness, plus individual testing of the 50-100 peels often found in the field. Sampling involves maneuvering a lab spatula near a particle through several magnified digital images, taking a small soil sample with the particle, and a final image to ensure its capture. All camera controls (shutter, focus, zoom, ISO, etc.) are on the laptop.
Whatever method used to date wall art indirectly from buried particles, we must link them to the art. Particle colour on high resolution soil images must closely resemble that in the art, but physical or chemical proof is needed in uncertain cases, either in situ or via sampling. Samples are identified later with SEM or X-ray diffraction. In our initial fieldwork, scraped soil was first filtered, then pushed aside for final backfilling, along with its particles. We were unable to prove they were pigment particles other than through identical colour values, co-occurring ochre nodules or a nearby excavation having the same levels with particles. SEM and X-ray diffraction confirmations sound elegant but are not always better because they are slow, complex, time-consuming and expensive (add AMS dating costs) for most rock art enthusiasts. Their use on a regular basis favours a qualitative field test capable of distinguishing between the various iron oxides and hydroxides. Some field method to link the particles with the art would be far better.
Technique with Sample Field Identification
- Clear vegetation and disturbed sediment in area vertically below rock art.
- Drop pebble from centre of art to show path of fallen pigment/rock particles.
- Outline 20×50 cm rectangle in dirt from rock face, using pebble as centre.
- Attach DSLR (>6 megapixel with ample storage chip) with zoom to tripod.
- With two legs on dirt at rock wall, raise and level tripod to allow work space
- Adjust zoom lens so rectangle fills eyepiece (usually about 60-120 mm).
- Place tiny numbered paper in centre of rectangle; new number for each level.
- Focus on number and photograph. If insufficient sunlight or tripod leg shadow in picture, shade rectangle with umbrella or tarp to force flash.
- Evenly apply water-based glue to standard copy paper with >5cm brush, leaving several cm uncoated at each end for handling and later remarks.
- Apply wet side down over rectangle and pat evenly to attach surface dirt.
- Invert glue paper and rotate consistently vertically or horizontally for later comparison with level surface photo (coordinates will be reversed), add tag and re-photograph.
- Remove glue paper and sun dry before next paper is placed in order on top.
- With vertically-held mason’s trowel, scrape 5mm (1/4”) of soil into dustpan
- Erect second tripod and camera to view 1×1.5 m piece of cardboard.
- Sift dustpan soil evenly over cardboard, add paper tag and re-photograph as needed while sifting to see as many particles as possible.
- Collect cardboard contents in bucket or on ground for later backfilling.
- Wrap sifted wood, shell, charcoal, leaf or needle in Al foil with same tag.
- Return to 6 for next level, adding new level tag.
- Repeat to sterile soil or bedrock. Collect soil around buried rocks until they must be removed. Their voids will appear in photos and glue sheets.
- Fill in rectangle with backdirt, cover with leaf litter; remove footprints.
- Compress glue papers to avoid particle loss. Backup jpgs daily to laptop.