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An Analysis of Case Hardening
Phaedra Culley
Geography Department
Mesa Community College – Red Mountain Campus
Mesa, AZ, 85207
and then
Geographical Sciences and Urban Planning School
Arizona State University
Tempe AZ 85287-5302
1. Introduction to RASI Element: Case Hardening
Case hardening occurs when an outer crust or shell forms on the very surface of
a rock surface. This protection comes from the addition of hardening agents such
as silica and iron to the outer few millimeters of a rock. Figure 1 shows and
example of case hardening.
Figure 1. This basalt boulder from has an ancient Hawaiian petroglyph carved
into a surface protected by case hardening. In this example, silica glaze has
moved into the weathering rind and indurated (hardened) the outer shell. The
sunglasses provide a sense of scale. This image was taken by R. Dorn and was
downloaded from Cerveny et al. (2007).
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2. Brief Literature Review of Case Hardening
Case hardening is formed by two processes: softening of the inner rock (or core)
and hardening of the outer layer. Due to its sedimentary nature, sandstone tends
to harden on the outside because of how the grains are held together. When a
rock varnish begins to form, much of it can dissolve, and the amalgam of
minerals and chemicals re-deposit into the pore spaces (or holes) in outer layer
of the sandstone. These agents then begin to harden the exterior of the rock
(Boxerman, 2012) (Figure 2). At this point, the rock begins to form the hardened
outer shell, and this cycle continues until the stone has a protective coating
(Dorn, 2004).
Figure 2. Case hardening is associated with a weathering form called tafoni, or
holes decayed out of rocks. Tafoni often starts out as alveoli, or smaller holes,
that are sometimes seen on petroglyph panels. This is not an image of a
petroglyph, but comes from the Lyons Sandstone at Red Rock Canyon,
Colorado. Source: Photograph by Brandon Vogt in Boxerman (2012).
Other types of rock such as granite tend to soften from the inside out (Dorn,
2004). The problem with case hardening is it leaves the area beneath it
vulnerable to decay; therefore, the rock erodes differentially, and the core stone
beneath decomposes at a faster pace than the outer crust. This often results in
loss of the surface material from the original rock. This is a particular problem for
petroglyphs, because at this point, they can scale off and become separated
from the rock itself, resulting in loss of the panel, or a portion of it.
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Case hardening can also refer to the deliberate hardening of steel by altering
metal and forming a thin layer of alloy (Fitzgerald, 2006). In addition to areas like
the carvings at the Petrified Forest, Arizona and other western states, case
hardening examples have been cited in the carvings at places such as Petra,
Jordan, and even some rocks on Mars. “Pot of Gold” and “Bread Box” are
examples where the process occurred along joints in volcanic rock and formed
similarly to stone previously described in this paper. Case hardening probably
exists on Mars:
“The case-hardened portions of these rocks have since been
etched into relief by the wind, with the interiors having been largely
removed by wind erosion. The major factor controlling this style of
weathering was liquid water.” (Farmer, 2005)
Case hardening creates a protective shell for whatever surface it forms on.
3. Visual Case Study of Case Hardening at Petrified Forest
National Park
The GIS of RASI forms at Petrified Forest National Park (Gutbrod, Cerveny,
Dorn, Allen, & Gibson, 2012) provides an opportunity to study the nature of case
hardening up close. By downloading and then zooming into different locations, I
hypothesize that case hardening is associated with rock art preservation (Figure
3). It appears that the surfaces of sandstone rock faces are undergoing erosion.
This destroys the rock art, and the last places where the rock art appears to be
preserved are case hardening.
Figure 3: Case hardening preserves rock art. The sandstone at Petrified Forest
National Park appears to undergo spalling regularly. Spalling is the loss of the
surface by detachment of pieces of different sizes. The case hardened areas,
however, appear to be the last locations to spall. Thus, case hardening preserves
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rock art more than the regular sandstone surface. Source of image: Gutbrod,
Cerveny, Dorn, Allen & Gibson (2012).
On the other hand, case hardening also appears to be associated with
detachment of rock art. The rock directly underneath the case hardened shell
appears to be the weak point. It has decayed so much that the petroglyph and its
case hardening shell can no longer remain attached to the rock (Figure 4).
Figure 4: This image shows the loss of rock art due to detachment of case
hardening This panel from Petrified Forest National Park has formed a hard outer
shell, known as case hardening. Due to the decay of the rock beneath (forming a
weathering rind), portions of the panel have fallen off, resulting in the loss of part
of the petroglyph. Source of image: Gutbrod, Cerveny, Dorn, Allen & Gibson
(2012).
4. Spatial Analysis of Case Hardening at Petrified Forest
National Park
For my spatial analysis, I selected a cluster of rock art panels on the easternmost
area that was analyzed at Petrified Forest National Park. The inset map in
Figure 5. I selected this area because I do not understand the distribution of
case hardened panels. Almost all of the panels in this area are scored as not
having case hardening, except for a few. Given the dramatic pictures on how
case hardening can impact sandstone rock surfaces (e.g. Figures 2-4), I
expected to see a lot more case hardening scored at Petrified Forest National
Park.
The general absence of case hardening is thus perplexing to me. The next step
in figuring out why this weathering form is so infrequent would be to visit this
national park in person. The next best thing is to go to http://images.google.com/
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and search for petroglyphs Petrified Forest National Park. Many of the searched
images do appear to have case hardening. Thus, the mystery depends for me as
to why so many of these panels are scored as case hardening not present.
Figure 5. Map of case hardening of an area at Petrified Forest National Park. I
first zoomed in to the cluster of panels to screenshot the data. Then, I zoomed
out to make the inset map. Both screen captures were turned into .jpg file, and I
combined them using the website http://pixlr.com/
Source of image: Gutbrod, Cerveny, Dorn, Allen & Gibson (2012).
5. Comparable Feature in My Local Built Environment
Now that I am aware of case hardening, I tend to see it in many different settings.
Case hardening can help old buildings and structures such as the carvings in the
cliff faces at Petra in Jordan, giving them the ability to resist wind or water
abrasion. In the case of wood, paint protects fencing from weathering agents,
and in metal, allows the surface to harden, while maintaining the integrity of the
inside portion. On many homes, we use a Stucco layer to protect the walls, and
stain or lacquer to protect furniture. I also think of case hardening when I see a
callus, because the thickened skin is hardened; of course, the processes
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producing these similar forms are very different. However, I also see case
hardening around my college campus at Arizona State University (Figure 6),
where different boulders used in landscaping show case hardening.
Figure 6. Case hardening seen on sandstone sculpture at the Arizona State
University Law Library. The raw material appears to be sandstone. The location
of the watch shows ripples from when the sandstone was sand flowing in a
stream. The ripples are hardened by what looks to be a light reddish agent, that
could be silica glaze. Source: Phaedra Culley.
6. Conclusion: Importance of Case Hardening to Rock Art
Conservation
Case hardening’s importance to rock art conservation rests in its ability to
preserve petroglyphs. The hardened outer shell of a rock art panel seems to
allow the rock art to last longer. This is a very good thing in terms of helping to
protect the art.
At the same time, the images of fragments of panels (e.g. Figures 3 and 4)
should be a warning sign that the remaining bit of art could break off at any time.
The weakness of the weathering rind under the case hardened shell is the ‘weak
link in the chain’. When site managers are made aware of panels that appear to
be ‘hanging by a thread’, every effort should be made to preserve the art in other
ways, such as extensive photography. The next time someone visits, the art
could be gone.
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7. Acknowledgments
I owe Niccole Cerveny a great debt. Her gift of caring involved spending a lot of
taking me and other students to Petrified Forest National Park, where my life was
changed by conducting research. I encourage all students reading this article to
get involved in an undergraduate research project, whether you are in a
community college or a four-year institution. I would also like to acknowledge the
producers of Battlestar Galatica for giving me something to listen to while
researching case hardening.
8. References Cited
Boxerman, J. (2012). Tafoni. Retrieved from http://tafoni.com/
Cerveny, N., Dorn, R. I., Gordon, S. J., & Whitley, D. S. (2007, October 24). Atlas
of petroglyph weathering forms used in the rock art stability index (RASI).
Retrieved from
http://alliance.la.asu.edu/rockart/stabilityindex/RASIAtlas.html
Dorn, R. I. (2004). Case Hardening. In A. Goudie, Encyclopedia of
Geomorphology (pp. 118-119). London: Taylor & Francis Group.
Farmer, J. D. (2005). Case-hardening of rocks on Mars: evidence for watermediated weathering processes. Retrieved from 2005 Salt Lake City
Annual Meeting (October 16–19, 2005):
http://gsa.confex.com/gsa/2005AM/finalprogram/abstract_96845.htm
Fitzgerald, C. (2006, March). Case Hardening in a Home Garage. Retrieved from
www.hemmings.com:
http://www.hemmings.com/hsx/stories/2006/03/01/hmn_feature20.html
Gutbrod, E., Cerveny, N., Dorn, R., Allen, C., & Gibson, S. (2012). Petrified
Forest National Park GIS of petroglyph weathering forms used in the rock
art stability index (RASI). Retrieved from
http://hosted.seservices.us/elyssa/test_v1.html
9. Suggested Glossary Entry
Case Hardening — Hardening the outer millimeter of an exposed rock surface
by infilling of pores (tiny holes) in the rock with strengthening agents such as
silica, iron, and manganese.