Photographing Casa Grande’s Great House With Moving Light

The Great House at Casa Grande Ruins National Monument has walls that hide things in plain sight: three plasterglyphs about ten feet up, too high and too dim to study closely, and 19th-century graffiti that is hard to see. A project funded by a 2018 Western National Parks Association grant cleaned some interior walls and then photographed them with reflectance transformation imaging (RTI), in which the light can be moved after the picture is taken. The article describing it, from the NPS journal Intermountain Park Science, is by Katherine Shaum, then an archeological technician at the monument, and Neil Dixon, a photographer who owns The Front Standard Photography.

The Great House, its roof shelter and the lower walls of Compound A, from the northeast. NPS photo.

The Great House

The Hohokam archaeological culture existed in southern Arizona from roughly A.D. 450 to A.D. 1450. The Great House, a monumental 11-room structure of three to four stories built of puddled mud, was likely built around A.D. 1325 and was no longer used on a permanent basis by A.D. 1450. Archeological research and historic accounts suggest there may once have been 12 such Great Houses in the Phoenix Basin; this is the only one still standing and open to visitors.

The interior was closed to visitors in the 1970s over concerns about structural integrity and unmonitored visits, and visitors can see in only through two doorways, with no view into Stack C, the central room where the plaster is best preserved. The building is about 90 percent original, though stabilization work took place over the years, notably in the 1890s. A roof shelter built in its current form in 1932 protects the walls, but it also created a shaded habitat for birds, whose acidic droppings damage the plaster. The interior has pink plasters with a calcium-phosphate coat and a polished gypsum finish, plus three known plasterglyphs, wall art drawn into wet plaster or pecked or scratched into dry. Whether they date to the Hohokam occupation, to migrants who came after A.D. 1450 or to the period after Father Kino visited in A.D. 1694 is debated. Mural art is extremely rare in the Hohokam world: only a couple of instances have ever been documented, and all of them were at this monument.

Cleaning first

Rock pigeons, European starlings and great horned owls had left droppings on some walls. Wet, they can dissolve plaster, etch it with acid, dissolve existing salts and generate new salts that stain; they also hide decoration and clues to how the building was made. A conservation team working from scaffolding dry-cleaned the glyphs and the walls around them with bamboo skewers and porcupine quills, tools softer than the plaster, and used damp cotton swabs on heavy deposits.

How RTI works

In RTI the photographer takes many digital photographs from one fixed camera position while light comes from different, known directions. Here a hemisphere with fixed light sources was used, and about 48 shots of each panel were combined, with the help of reflective targets, into one file holding light-position and color data for every pixel. Hewlett Packard Labs developed the method in 2001. In the RTI Viewer software a user can then move an overhead or raking light across the image and adjust color and specularity, the amount of reflection. That brings out low-relief etching and carving, such as plasterglyphs, petroglyphs, dendroglyphs, murals, historic graffiti, headstones and some artifacts, and can help show which of two overlapping inscriptions came first. The team used a 36-megapixel camera and an electronic flash.

A typical digital photograph of a wall surface with historic graffiti. The panel may hold the oldest historic inscription in the Great House, “P. Weaver,” possibly Pauline Weaver. NPS photo by Neil Dixon.

The same wall in the RTI Viewer with raking light; many inscriptions, including “P. Weaver,” are easier to see. NPS photo by Neil Dixon.

The same wall in “specular enhancement” mode. NPS photo by Neil Dixon.

The team photographed all the newly cleaned walls, including those with plasterglyphs, and several panels of graffiti that visitors left in the second half of the 19th century. Most of it dates from the 1870s to the 1890s, after the railroad and a stagecoach route reached the area but before the monument was protected and had an on-site steward. “P. Weaver” may stand for Pauline Weaver, a mountain man and fur trapper who worked along the nearby Gila River; it is difficult to see under normal conditions. The team also photographed artifacts with low-relief designs, such as decorated shell ring fragments and prehistoric ceramic comals, flat griddles typically used for cooking in Mexico and Central and South America.

A traditional photograph of an impressed ceramic comal fragment from the monument. NPS photo by Neil Dixon.

The same artifact in specular enhancement mode, which gives more information about the overlapping impressed layers. NPS photo by Neil Dixon.

What it gave the park

RTI found no previously unknown plasterglyphs in the project area, but the photographs renewed interest in the glyphs and other interior surfaces. They appeared in historic research on Frank Lloyd Wright and the rock art of southern Arizona that inspired him, and aided research on prehistoric migrations and trade connections and the tribal consultation for the monument’s Ethnographic Overview and Assessment. They could also feed new interpretive materials such as a photography exhibit or walking-tour visuals. The images also work as a digital record that helps offset erosion from animals and weather, and researchers can study them without visiting a fragile site.

The authors caution that RTI needs advanced photographic equipment and knowledge, and recommend hiring an experienced photographer where possible.

Sources

• National Park Service, Intermountain Park Science, Katherine Shaum and Neil Dixon, A New Way of Documenting and Imaging Cultural Resources; rewritten in hubnx’s own words. One of the two authors is a private photographer; the article is paraphrased throughout.