Highlight-Based RTI Equipment

The equipment required for highlight-based Reflectance Transformation Imaging (RTI) is essentially the same for all captures; a light source which can be moved (or a dome of multiple light sources) and a fixed camera perpendicular to the object being imaged. However the actual objects being studied do inform the exact choice of equipment. My current work is on Ancient Egyptian Predynastic palettes, as my next research project will be too, which typically are around 20-40cm (even the so called Narmer palette is only 64cm in its longest dimension). If you’re working with larger objects then some of my problem solving wouldn’t fit your needs.    

Figure 1 – H-RTI flashgun setup

Typical highlight-based RTI (H-RTI) setups use a string tied to the flash to ensure a constant distance between the head of the flash and the surface of the object. Rather than put string onto the flash which needs measuring for each set up (and can untie itself during capture and need more measuring), I’ve mounted a 1.50m (60 feetsies for my American colleagues) fabric tape measure with a plastic tip. It’s mounted to the flash on the underside by Velcro to a Honl SpeedStrap, but you could just use any Velcro mounting onto the flashgun. Whilst string gives you more versatility and distance, for the smaller objects I am currently studying this solution gives me a few less steps in setup – which an RTIsta will tell you takes much longer than the capture itself and any streamlining is welcomed. I found two minor issues with the tape measure set up when I first used it. Firstly the tape measure needs to be set back slightly from the front of the flash, or it interferes with the light spread, and secondly the black plastic tip is extremely hard to see in low light environments – thankfully something easily remedied with a piece of white plastic tube glued onto the tip.  I had toyed with the idea of having two laser dots to line up to get the correct distance, somewhat like a modern Dambusters, or even using an ultrasonic rangefinder but decided that simple was better. Although if/when I work on larger objects and need to be further away I might revisit the ultrasonic rangefinder idea instead of using metres of string.   My lighting setup is using an Olympus FL-35 flash and Phottix Atlas triggers, for no reason other than I already owned them. Any flash which allows the setting of light intensity setting and remote trigger, or continuous lighting, set up will work.  

Figure 2 – Engraved Thoth hieroglyph

The grip is a motorbike grip over a solid aluminium round bar, which was drilled and tapped to accept a ¼ inch (20 TPI) male to ¼ inch (20 TPI) male connector that then screws into the trigger unit. I don’t have access to a lathe, or it would have been nice to have turned the end of the bar down and threaded it to screw into the trigger (similar to the handle supplied in the CHI starter kit). The aluminium bar was chemically blackened and I’ve hand engraved a Thoth hieroglyph into the end of the bar (Figure 2).    

Figure 3 – Quick release camera mounting bracket, on adjustable copy stand column

My museum based research is on smaller objects and often in location where it’s not possible to set up a large stable tripod, and so a copy stand is much more suitable as it has a significantly smaller footprint. Traditional copy stands usually have their own baseboard and potentially their own legs, making them less ideal for transport to different research locations, and they also tend to be rather expensive. To mitigate both issues I’ve built myself a mobile copy stand which has a desk clamp rather than the traditional style stand (Figure 5).   This stand is built from a Dusrt M670 darkroom enlarger column and base with a compatible geared winder, mounted to a computer monitor stand desk clamp. The horizontal camera arm (Figure 3) was created with a 150mm long mild steel tube with a 25mm internal diameter, with an M6 wingnut style hand knob bolt to secure it to the winder and with a solid mild steel plug interference fitted into the end (with a 4mm grubscrew to ensure zero possible movement) which was drilled and tapped to ¼ inch UNC (20 TPI) and bolted to a Manfrotto quick release mount. Using the ¼ inch UNC (20 TPI) thread makes the unit compatible with standard photography equipment, such as tripod heads or camera brackets.  

Figure 4 – Desk clamp

The column was mounted to a desk clamp (Figure 4) using a 5mm thick mild steel interface bracket, which both clamp and column base were bolted to. This then allows the whole unit to be camped onto a desk, with the camera height being easily adjusted.

Figure 5 – Copystand with camera mounting bracket

Analysis into Predynastic Ancient Egyptian Malachite Processing and the Production of Microfibres

Figure 1 – Malachite processing using stone hammer against a sandstone block

The 19th century description of ‘grinding malachite’ to reduce it to powder for pigment is completely erroneous, and effective processing requires smashing or crushing – most likely with a hammer stone against an substantial anvil stone as shown in Figure 1 and also in video demonstration here: http://www.instagram.com/p/BksM2HQgT-s/

This percussive smashing and crushing produces small crystal shards which are extremely flyable. To contain these, the malachite would be wrapped in a material such as leather or linen. This material would most likely be scrap from clothing or other use, as it is destroyed during the crushing process. This destruction of the wrapping material imparts microfibres, with diameters ranging from 8μm to 23μm, into the resultant powdered malachite as can be seen in Figure 2.

Figure 2 – Powdered malachite viewed at 100x through a Dino-Lite AM4113T, with measurements taken in DinoXcope

The powdered malachite would be mixed with a base before use, such as a lipid base for use in ‘cosmetics’. The presence of the microfibres may help to reinforce the resulting pigment, thus helping to reduce shrinkage and cracking in a manner analogous to the reinforcement of  mudbricks with straw

This microfibre reinforcement may be a part of why green pigmentation appears to survive extremely well on the surface of palettes in the Predynastic era and also in the later Dynastic era (as can be seen in Figure 3).

Figure 3 – Decorrelation stretch vs ‘normal’ photograph of C. 1545 in the Museo Egizio

Steps to Create a Fish-Shaped Predynastic Ancient Egyptian Palette

This is an excerpt from my upcoming paper discussing the manufacture and use of Predynastic Ancient Egyptian palettes, highlighting the various steps which are required to create a fish-shaped (pisciform) palette. The replica was made from slate rather than siltstone/greywacke, as the material was easier for me to obtain. The steps are essentially the same but as slate is softer (Moh 3-4 vs Moh 6-7) it would take longer to create out of siltstone/greywacke and be potentially more abrasive to the tools.