Testing residues: an experimental approach
Date
2009-09-08T12:48:25Z
Authors
Langejans, Geeske Henriette Janine
Journal Title
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Abstract
SUMMARY
I investigate the decay mechanisms for microscopic organic residues. Residue
analysis aims to identify microscopic remains or traces that are left on a tool’s surface
after use. Analysts have identified organic remains such as plant starch grains, fibres,
tissue cells, resin; animal blood films and red blood cells, muscle tissue, connective
fibres, bone fragments, hairs; fish scales, feathers and shell. Besides organic residues,
it is also possible to distinguish inorganic deposits such as ochre and ash. After the
processed materials are identified on the artefact, residue analysis can be used, for
example, to reconstruct tool use and assess site function.
Preservation of residues on artefacts is a debated topic and I decided to investigate
the following issues:
• Non-use-related residues, for example from unintentional handling or
sediments, might contaminate artefacts.
• Researchers are unable to describe the mechanism of residue preservation.
My study investigates what research biases might occur from extraneous, or “extra”,
remains and from deteriorated, or “missing” residues.
I use experimental archaeology and middle-range theory (Binford 1983, Raab and Goodyear 1984, Schiffer 1988, Reynolds 1999, Outram 2008) to bridge the gap
between the apparently static archaeological data (what one sees on the tools) to a
dynamic reconstruction of past processed materials (what tools were used on and for).
The aim of this study is to produce predictions about residue preservation, recognition
and possible misinterpretation.
In order to understand the decay of organic residues, I set out experimentally
made tools with modern residues at Sterkfontein Caves and Sibudu Cave in South
Africa and at Wilhelmina Polder and Zelhem in the Netherlands. The modern flakes
were of chert, hornfels and flint respectively and they were used to process muscle
tissue, bone, plant tissue, starch rich and plant material. The samples at the sites were
subdivided and deposited under different variables; these included time, precipitation,
temperature, burial circumstances, sediment pH, sediment constitution, rock type and
processed material. Practically this means that part of the sample was deposited inside
the caves and the other part outside and that part of the sample was deposited on top
of the soil and another part was buried. The sediment pH at Zelhem is low, whereas the pH at Wilhelmina Polder is high; at Sterkfontein and Sibudu the sediments are
neutral; the sediment at Sterkfontein consists of sandy clay, the sediment at Sibudu
consists of anthropogenic ash, at Wilhelmina Polder the sediment is clayey and at
Zelhem it is sandy. After four weeks I excavated and analysed some of these tools
from Sterkfontein and Sibudu. After one year I analysed the remaining set. The
Zelhem and Wilhelmima Polder experiments were all collected and analysed after six
months. At Zelhem and Wilhelmina Polder the experiments also included sets of
sandpaper of different grain sizes and glass slides with modern residues. Part of this
sample was buried with the residues facing up and the other part was facing down.
This was to test to influence of the raw material surface and microenvironment on
residue preservation. All experiments had a set of blank doubles; these are unused
flakes, which were deposited in the same settings. These blanks served as control
sample and they helped me to study the distribution of contaminants on stone
surfaces.
I also analysed ancient samples of stone tools from Sterkfontein and Sibudu.
This was to study the possible correlation in preservation between the experimental
and ancient residues.
I formulated predictions about the preservation of residues under different
circumstances, and in the course of this study I demonstrated that reliable predictions
can be made when the taphonomy and curation history of tools and preservation
optima of organic remains are carefully studied.
This study clearly demonstrates that all experimental and ancient artefacts contain
contaminants. Sediment remains adhere to tools and handling also leaves residues on
tools. Not only plant type residues are possible contaminants, I also found
contaminant animal type remains, such as connective fibres and muscle tissue. The
used samples showed more sediment contamination than the blanks. Because the
moist, use-related residues act as an adhesive it is probable that tools with dry
residues show less contamination.
It is possible to distinguish use-related residues from contaminants and
unintentional residues by quantifying the results and by implementing the contextual
approach. Use-related residues are more abundant than contaminants. In addition, userelated
residues form coherent distribution patterns with suites of associated residues.
For example, processing of animal material leaves deposits of muscle tissue and fat around the working edge. The distribution and association approach formulated by
Lombard and Wadley (Lombard and Wadley 2007, Wadley and Lombard 2007) is a
valuable tool in residue analysis.
Different residue types preserve differently. Plant tissue and bone preserve
better than muscle tissue, blood and starch grains. Although fat is a tough material, it
is often absorbed into the stone material and sediment and therefore lost for analysis.
In addition, what is a good environment for one residue can be destructive for
another. For example, at Sterkfontein (outside the cave), almost no starch residues
were preserved on the starch tools, but at Wilhelmina Polder plenty of starch grains
were present and, in addition, many use-related epidermal tissue deposits were also
preserved. Another example is muscle tissue, which preserved well inside Sibudu
Cave, but preserved badly outside the cave. Outside Sibudu Cave, bone and plant
tissue deteriorated less than muscle tissue.
Microbial activity is the main reason for deterioration. From the experiments I
gained insight into how the variables time, precipitation, temperature, burial
circumstances, sediment pH, sediment constitution, rock type and processed material
affect microbial activity. Consequently, precipitation leads to decay and therefore moist, humid and fluctuating environments are bad for preservation. Precipitation also
leads to mechanical weathering as residues can be washed off tools. When a soil is
rich in nutrients (organic matter) there are more microbes in the sediment and as a
result more residue decay. At moderate temperatures less residues are preserved than
at low (<5ºC) or high temperatures (>35ºC). However, chemical and mechanical
weathering can have a negative effect in more extreme temperatures. Because
microbes flourish in environments with a moderate pH (pH 6-7.5), these settings lead
to more decay. Residues are better preserved in acidic or alkaline environments, but
chemical and mechanical weathering sometimes benefit from more extreme pH
values and can then have a negative effect on residue preservation. Medium- and finegrained
rocks are better for preservation than large-grained or glass-like materials. At
this point it is unclear if direct burial protects or destroys residues. Direct burial
probably promotes preservation if the sediment shields residues from another harmful
environment, such a humid atmosphere (potentially the case at Sibudu), or
precipitation (as is the case at Wilhemina Polder). Prolonged surface exposure may
ensure preservation if it dehydrates the residues and if it keeps residues safe from
microbes. Residues always undergo decomposition. Depending on the circumstances, organic residues either decompose rapidly within a few years or extremely slowly.
With rapid decay, no residues are left for analysis even after only one year. With slow
decay it appears that residues are permanently preserved. However, chemically and
sometimes visually, these residues undergo change. Slow decay can take up to tens of
thousands of years, provided that the environments remain stable. In a stable
environment, residues preserve better than in an environment that is subjected to
reworking and extensive post-depositional processes.
The described results imply the following procedures for residue analysis: tools
should be selected from stable sites with low bioactivity. In practice this excludes
most open-air sites. Dry and desiccated sites, waterlogged, extremely acidic or
alkaline sites and cave sites potentially preserve residues best. On the basis of these
results it is possible to make predictions about residue preservation for each site. For
example, the prediction made in this study that ancient residues are preserved inside
Sibudu Cave proved correct.
Selected artefacts must be handled with care and residue analysis should be
the first analysis to be conducted on a tool sample. This means that no lithic or other
analysis should take place prior to residue analysis. Sediment samples and blank
flakes should be analysed to gain insight into possible contaminants and the raw
material.
The selected sample should include a substantial number of artefacts.
Because residual decay always occurs, some tools in a sample will not show the
expected patterning or preserve residues. To filter out these exceptions, and to ensure
that the overall signal becomes clear, more than 30 tools should be analysed. In
addition, these should be tools of the same type. It is only possible to recognise and
compare distribution patterns when dealing with a uniform sample. For example,
possible hafting of scrapers and the accompanying distribution pattern and residue
suite is easier to recognise in a scraper-only-sample than in a mixed sample including
retouched flakes and points.
The contextual approach should be applied and residues should be quantified.
The deductive list, which is discussed in detail in Chapter Six and Seven, is a practical
and heuristic device for conducting residue analysis and it follows the contextual
approach. Residues can be representative of past processed materials. However, not all residues
on tools are use-related and it essential to differentiate between use-related and postdepositional
and unintentional remains.
This study has shown that residue-analysis must be approached with caution.
Unfortunately the taphonomy of many sites precludes the preservation of ancient
residues. However, with the aid of the predictions regarding circumstances of
preservation of residues, the careful selection of sites and samples, and by
implementing the contextual approach, I have argued that residue-analysis can be a
valuable tool in the reconstruction of processed materials and past activities.