Documenting textile impressions or pseudomorphs on archaeological objects is very challenging. In my own experience, I’ve found trying to photograph textile pseudomorphs, especially when they are poorly preserved, very difficult and involves taking multiple shots with varying light angles, which still often results in poor quality images. This is why Emily Frank‘s paper was of particular interest to me because it provided an alternative to digital photography that would be feasible and more effective in documenting textile impressions: Reflectance Transformation Imaging (RTI).
RTI is a computational documentation method that allows for multiple images of an object to be merged into one and viewed interactively to allow the direction of light to be changed so that surface features are enhanced. The process involves changing the direction of the light when each photo is taken. Using open source software, a single image is rendered using various algorithms that allows the viewer to move a dial and change the direction/angle of light the image can be viewed at. Additional components in the software allow for the images to be viewed using different filters or light effects that make visualization of surface features easier. RTI is gaining in popularity as a documentation tool in conservation due to its low cost and feasibility and several papers presented at this year’s conference touched on the use of this technique (including this paper I also blogged about).
There are two general light sources used for RTI. One uses a dome outfitted with many LED lights that will turn off and on as photographs are taken. An RTI light dome is pictured on Cultural Heritage Imaging’s website that was used at the Worcester Art Museum (CHI is a non-profit organization that provides training and tools for this technique). However, most conservators use a lower tech method where a light source (a camera flash or lamp for example) is held at a fixed distance from the artifact and manually moved around at different angles when each photo is taken. You can see an example of this method used in the field in this blog post from UCLA/Getty Conservation Program student Heather White.
In her paper, Emily focused on documenting textile or basketry impressions on ceramics and more ephemeral impressions, such as those left in the soil by deteriorated textiles or baskets, using RTI. By using the various tools offered by the RTI software (changing light angle, using diffuse light or changing it so that concave surfaces of impressions look convex), she was able to see fine features not clearly visible with standard digital photography, such as the angle of fibers, striations on the surface of plant material or the weave structure. For impressions of textiles left in soil (these were mock-ups she made in potting soil) she noted that digital photography was not very effective in recording these because there was no contrast and the impressions were so fragile that they could not be lifted or moved for better examination or imaging. However using RTI she was able to clearly see that the textiles were crocheted.
In describing her set up and work flow, Emily took photos of the impressions indoors, as well as outdoors (for the soil impressions). She was able to take good images outdoors, but it was better to do RTI at dusk with lower light. She took a minimum of 12 shots per impression at 3 different angles. For her light source she used a flash. In all, she said it took her about 10 minutes to shoot each impression.
When compared to digital photography, RTI is a useful and feasible technique for the documentation of impressions, and worked well for most of the impressions Emily tried to record. It seems that RTI worked well as the stand alone documentation method for impressions in about 40% of the images she took, but is more effective as an examination and documentation tool in combination with standard digital photography. RTI is on its way to becoming a more standardized documentation method in conservation. It appears to be effective for recording low contrast, low relief surfaces, such as textile impressions, and may be the best method to record ephemeral or extremely fragile surfaces that are not possible to preserve. I’m excited about the potential of RTI for impressions and look forward to trying it out the next time I have to record textile impressions or organic pseudomorphs on an archaeological object.