Photographic mapping of stone deterioration

Yiorgos Marakis <yomaros [at] yahoo__com> writes

>I am looking for information on the use of Photography for mapping
>the deterioration of stone.

In the 1980's I did some investigation of deterioration rates of
petroglyphs on sandstone in Canada, in an attempt to answer a
request as to whether latex moulding of the petroglyphs would result
in accelerated deterioration (answer, as suspected: yes). Some
proponents of this moulding technique (which did have useful working
properties,were it not for the long-term implications) suggested
that the long-term didn't matter because the cliff faces were
deteriorating so quickly that it didn't matter anyway, and it would
be better to save what we could, while we could (a not
infrequently-heard comment).

In an effort to determine whether this was true, I examined
photographs taken of the site over a period of many years by a local
photography club. I started with the naive thought that enlargement
of dated photos of selected panels to comparable sizes might provide
some answers, but was foiled by the different lighting conditions:
the differences between the images were greater than the differences
between the dates taken.

I also examined tracings made by students of a dedicated high school
teacher over many years, only to find that these were inevitably
somewhat subjective. Without a clear understanding of the subject
matter, it can be difficult to know which scratches are artifacts,
and which are "naturefacts".

I had worked on archaeological sites in the middle east where
photogrammetry was used to record details of monuments, and
consulted Robin Letellier, then with the photogrammetry section of
Parks Canada. These discussions resulted in a pilot program of
normal photogrammetric recording of some of the principal zones (for
an objective record of both kinds of "-facts"), along with
micro-photogrammetry of some sample petroglyphs. Robin explained
that the methodology available at that time allowed the depth of
grooves on a Roman signet ring to be recorded and measured, and we
hoped that we could do the same for the grooves of the petroglyphs,
thus providing a baseline for future monitoring.

The pilot project appeared to be very successful in this, but
unfortunately did not receive funding for complete documentation of
the site and recording of the surfaces of key areas.

I am sure that technological advances since then will offer other
interesting approaches to measuring the depths of selected features
and documenting rates of stone deterioration. However,
photogrammetry does work, is fairly widely available worldwide
(usually from government sources that might be interested in
helping), is easy to use in remote areas, and is relatively
inexpensive.

I understand that photogrammetry of petroglyphs has been done
subsequently in New Mexico, but have not been able to see a report.

My experience suggests that photography would probably only be
successful with carefully standardized lighting, such as the night
photographs taken of some petroglyphs. I would be interested to hear
from others about techniques that they have used.

Lisa Mibach
Heritage Resource Management
1-29 Cambridge St. North
Ottawa, ON K1R 7A4 CANADA


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                  Conservation DistList Instance 14:20
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