Approach:
| Object | Identify pictograph colours as references to find pigment particles in the sublevel |
| Input | Pictograph image and the fallen pigmented chip |
| Process | Identify reference colours from the motif and its background |
| Sublevel Scan | Isolate relevant colour for detailed examination and analysis |
Initial Findings:
Red, clay and beige are easily seen, with the motif red and clay, and background beige. The rock surface colour is obscure, but possibly blue-grey, with magenta and purple identified as background components.
When combining clay and red to scan the motif, red seems faded, indicating clay was applied after red. With magenta added, the ‘red’ part of the motif is partly recovered. The fallen chip, both front and back, is clearly identified by scanning with red, clay, magenta and purple. Sublevel 53 was scanned with clay, magenta, purple and red colours to isolate colours that may identify pigments.
Colours : Motif is a red and clay composite with background of Beige, Magenta and Purple.
Colours : Scan recovered chip with pictograph colours
Colours : Scan sublevel with Clay, Magenta, Purple and Red
Observations and Additional Analysis:
With the component colours identified, both pictograph and sublevel images were combined for a ‘validation’ scan. The combined scan locates similar colours even though the photos have different details due to different focal lengths.
A red scan from the front of the chip yields only partial coverage. With the ‘red(R)’ boundary set to maximum, ‘additional’ red was found. The original ‘RED_G5’ limit was (R)80-200 when scanning with (R)201-250. There are twice as many ‘red pixels’, most likely due to the closeness of the object to the camera.
The red pixel density at the back of the chip looks like ‘Swiss cheese’. By adding grey and combining multiple colours, the scanned image produces a more complete outline.
With added grey now identified, the pictograph was re-scanned. Grey seems to be the base for the rock surface, since combined clay and grey contrasts with the creamy part of the lightning. As added beige and grey does not ‘fade’ the motif, beige can be considered to be the first colour applied before the motif were constructed with red then clay.
Colours : Scan composite of pictograph and sublevel with pictograph’s colours
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Combine the pictograph and sublevel 53:
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Colours : Scan recovered chip with additional pictograph colours
Colours : Scan pictograph with additional pictograph colours
Work Notes:
Chronological Colour Layers
- Grey (bluish) as the base for rock surface
- Beige (either natural covering like dust, or man-made as background wash)
- Magenta and Purple (derived from compounded Red/Clay/Blue?)
- Red (main motif colour)
- Clay (main motif colour)
Different Distances Creating Colour Variation
It is estimated the chip photo alone comprises about 2,000 pixels, the chip on the pictograph about 8,000 pixels, with the chip actually about 800,000 pixels. With more pixels the chip shows the details like ‘crystalized quart (white)’; and the back side shows very spotty red and can be ‘completed’ with beige/clay/magenta/purple. The sublevel shows a more ‘blended’ colour (magenta or purple), suggesting a similar distance should be used digital photos for better colour inspection of motif, rock fragment and buried sublevel photo.
Recoverable Artifact Sizes from Sublevel
When recovering artifacts from the sublevel, fragment separation by size can be quite productive. Besides recovery for labwork, the digital photo can benefit from better detail, with the following ‘scales’ considered during field work :
- Dust and Sand (2mm and below) – glue sheet pickup
- Granule (2mm to 4mm) – glue sheet pickup
- Pebble (4mm to 64mm) pick up before scraping
- Cobble (64mm and above) pick up before scraping
Environmental Photograph
In order to identify rock surface colour, the area beyond the motif must be studied. The beige colour detected from the Lightning Bolt can be a background coating created by a wash, or it may be wind-delivered dust. If a ‘boundary’ can be found, it can be investigated for pigment material and binder. If all surfaces are coated with the same ‘colour’, then the ‘dust’ can be readily identified as well.
Petroglyph Dating
The approach used in this study is based heavily on photo motif colour and contrast. For petroglyph dating, pigment will likely be absent. But if the lighter motif and darker surface colours can be identified, the sublevel can be scanned for these colours on bifacially-coloured fallen pocked particles resulting from hammerstone blows. It just demands better colour/detail references.