2.4.1 Assessing Deterioration by Thermal Microscopy
The survey of ethnographic and archaeological skins and leathers has shown that a wide range occurs in the loss of thermal stability of artifacts. Temperature drops of 30 "C were noted and correlated visually and microscopically with obvious severe deterioration of fibre samples.
Deterioration was also quite variable within samples; the greater the variability, the broader the shrinkage temperature range; this increased heterogeneity of stabilities was
recently subjected to statistical analysis by Larsen et al 1993. At the level o f individual fibrils, some regions are apparently less thermally stable than others because of perturbation or deterioration of crystalline and ordered amorphous structure. The less stable regions begin shrinking at lower temperatures, whereas the more stable regions still denature at higher temperatures approaching more normal Tg values.
A similar description of variability applies to fibres. The total of fibril deterioration in each fibre is variable, and, therefore, some fibres begin shrinking at lower temperatures than do others. As a result, the shrinkage range of deteriorated samples can be broad, e.g.
10 or 15 °C, in additional to being depressed below normal values. On the other hand, uniform deterioration results in a narrow range, e.g. 5 °C, significantly below the temperature range for undeteriorated collagen.
The microscopical method is probably the most practicable o f the analytical
techniques examined for estimating thermal stability, and, therefore, it will likely continue to be the method of first choice for estimating collagen breakdown in artifacts despite the analytical developments reported in the following chapters. The advantages o f the
microscopical method over more conventional shrinkage temperature methods are its rapidity and the minute sample requirements which allows for essentially non-destructive testing o f artifacts. The method does suffer, however, from the current subjective manner of determining the onset and endpoint values, especially the former. Chapter six discusses a few options for improving precision in the measurements.
The very least of equipment needed to measure Tg microscopically would include a controllable and measurable heating element and a way to examine collagen fibres visually
at a magnification of between 5 and 100 times as they are heated. A melting point
apparatus can be used which includes a simple optical magnifier and contains fibre samples in water inside sealed capillary tubes or some other support vehicle. Even a hot plate with fibres contained between a microscopical glass slide and coverslip could be used, along with a hand held magnifying glass to see the fibres. Regardless of the specific apparatus, the experimenter must be able to - 1) measure precisely the temperature of the heating element, 2) calibrate temperature against substances with known melting points to determine the accuracy of the measurements, and 3) control the rate of heating. For microscopic sample sizes a polarizing microscope fitted with a microscopical hot stage is best used. The choice of apparatus will depend on the required level of precision and accuracy in the measurements and, of course, on the availability o f the equipment.
Precision requirements depend on the use to which the information inferred from Tg measurements is put. For example, a decision for or against the use of aqueous treatments might be required for a specific artifact. Even low precision (± 3°C) could be acceptable for this. Shrinkage in the low 40's (°C) or lower would indicate against aqueous treatments because of the possibility of dénaturation occurring during conservation. On the other hand, with shrinkage in the high 40's or higher, aqueous treatments might be safely used. Further study is required to confirm these predictions.
Other circumstances will require higher precision. For example, museum display conditions with continuous high levels of lighting during opening hours will warrant monitoring for even small changes in the thermal stability of artifacts—particularly those made of supple, semi-tanned hide, as from brain and smoke processes. A high degree of
precision (± 1°C) is necessary to ensure that any small measured change that occurs over a short time period (many weeks) reflects a real change in the artifact. Evidence of
deterioration would, of course, encourage changes to display and storage conditions to improve preservation.
Both the melting point apparatus and hot plate techniques suffer from a lack of precision, particularly because of difficulties with visually detecting the beginning of fibre shrinkage. Both may also suffer from slow sample preparation and slow cycling back to the starting temperature between measurements. The polarizing microscope/hot stage combination has the advantage of speed (5+ measurements per hour), greater precision and accuracy (if temperature is computer controlled), and ease o f use. This was accomplished partly because the polarizing microscope made the detection of shrinkage easier. Crossed or slightly uncrossed polarizers enhance detection by revealing the loss of birefringence (brightness) of the fibres as they shrink. This gave greater precision to the measurements by permitting a more exact definition of the shrinkage process.
One important conclusion from these experiments is that added deterioration of an object over time in storage or display can be monitored, in a visually non-destructive manner, by use of the microscopical technique.
2.4.2 Analysis and Treatment Design
When the extent and pattern of deterioration of an artifact are known, treatments can be designed to circumvent problems with anticipated responses of the artifact. As an example, analysis before treatment of a waterlogged parchment document (Logan and
Young 1987a) included separate Tg measurements of two delaminating strata. One, a very thin layer (<lmm), supported the writing of the document, and the other constituted the remaining thickness. The thin writing support had a lower Tg range compared to the
remaining thickness. Based in part on this difference, the two layers were expected to react differently to drying stresses. Treatment was designed to give the best possible
contact between the layers during drying to ensure similar amounts of area shrinkage and self-adhesion after treatment.
Tg measurements were also undertaken to identify archaeological Thule artifacts with particularly low hydrothermal stability, ones in the 30's Celsius (Segal and Newton
1990). As a result, SDS was substituted with the non-ionic detergent Synperonic N during the washing of the more deteriorated artifacts. Unlike the dramatic lowering effect o f SDS on stability (Figure 2.5), Synperonic N was found during pre-treatment trials to have virtually no effect on Tg, and thus it was safe to use on artifacts with stabilities (Tg) determined to be close to the temperature of the wash baths. In this way, dénaturation of the most deteriorated component of the fibrous collagen content o f these artifacts was avoided during washing.
2.4.2 Initial Conclusions
Often with ethnographic skins and semi-tanned leathers neither physical appearance nor handle gives any indication of the extent of deterioration. Routine measurements of Tg as part of pre-treatment assessments can identify those artifacts which are severely
As suggested earlier, such artifacts are susceptible to undergoing irreversible dénaturation in the presence of water, as, for example, from aqueous adhesives or even humidification during attempts to soften and reshape an artifact.
The hydrothermal stability of skin and semi-tanned leather artifacts reflects the physical and chemical integrity of collagenous components. Values of shrinkage temperature (Tg) below the normal range of 62 to 68 °C in water are consistent with deterioration in artifacts. As will be discussed in subsequent chapters, a drop in Tg indicates the disruption of fibrillar semi-crystallinity and of molecular integrity, as is suggested in cases of extreme deterioration by the association o f low Tg values with reduced birefringence observed by polarized light microscopy (Basu et al 1963).
Shrinkage temperatures below expected values when test fibres are in the presence o f selected conservation materials are consistent with the known destabilizing effects of at least one conservation material: aqueous SDS wash baths. Destabilization was reversible over the short term in the current investigation of SDS. Permanent damage due to long term contact was not investigated. The question of whether destabilized collagenous artifacts are more likely to deteriorate, and therefore degrade more rapidly in storage or display, deserves investigation.
CHAPTER THREE