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Table 3 Summary of techniques discussed for assessment of morphological changes in archaeological wood

From: A review of analytical methods for assessing preservation in waterlogged archaeological wood and their application in practice

  Technique Information yielded Advantages Disadvantages Sample requirements
Physical evaluation Visual assessment (descriptive approach) Surface detail; initial assessment of quality Done during excavation; fast; cheap; accessible Difficult to standardise; difficult to accurately describe appearance; reveals only superficial (surface) preservation Dry, waterlogged or conserved sample; non-destructive
Scoring systems Standardised list of visual preservation indicators; comparative data on surface quality Can be done in the field; increased level of standardisation; allows comparison across studies Can still be subjective; requires an experienced wood specialist; reveals only superficial (surface) preservation Dry, waterlogged or conserved sample; non-destructive
Advanced visualisation techniques Advanced photography 3D model from digital photographs; surface detail; shape; surface texture Widely available; cheap; easy to use Appearance may not reflect preservation; can still be open to interpretation Dry, waterlogged or conserved sample; non-destructive
Laser scanning Surface quality; shrinkage if done more than once; enhanced surface detail (e.g. cut marks) Provides a long-term digital record (ideal if artefacts will not be conserved); more detail than photography and/or illustration Requires specialist equipment and expertise; time consuming; reveals only superficial (surface) preservation Dry, waterlogged or conserved sample; non-destructive
Loss of wood substance MWC Potential behaviour upon conservation; loss of original material (assumedly cellulose); broad indicator of decay Calculated from easily measured parameters; gives numerical value allowing comparison between studies; cheap; accessible Lack of consistency between analysts/laboratories; lack of detail on nature of decay; can vary with depth through sample Waterlogged sample; destructive; ~ 0.5 g waterlogged sample is recommended
Density
Shrinkage
Porosity Waterlogged sample; non-destructive
Physical resistance (density) Pin test Measures resistance as a proxy for density Accounts for variation with depth; cheap; widely available Lack of consistency between analysts Waterlogged sample; minimally destructive (a hole is made in sample)
Mechanised probe (Pilodyn/Sibert) Gives numerical values, allowing comparison between studies Requires specialist equipment; requires data transformation
Non-invasive methods for assessing physical structure X-ray imaging Density through the whole structure; can show characteristic decay patterns; can be done using synchrotron radiation for higher resolution Non-destructive; techniques penetrate into a sample, providing a better analysis of the bulk; portable versions available; easy data interpretation Appropriate calibration required to obtain quantitative analysis; techniques not commonly used to assess state of preservation Dry, waterlogged or conserved sample; non-destructive (but size of instrument may demand that sample is cut)
Computed tomography
Ultrasonic testing Provides analysis of wood density through the entire structure Fast; portable; non-destructive; suitable for use in water Complex data interpretation; signal is affected by multiple factors that require calibrating Dry, waterlogged or conserved sample; non-destructive
Microscopic analysis of wood structure Optical (light) microscopy Nature of deterioration; wood species; collapse of cell walls; loss of cellulose Accounts for spatial variations; readily available; cheap Non-quantitative; requires specialist input; difficulty in preparation of degraded samples Waterlogged sample; destructive; at least 2 mm x 2 mm section required (larger sample usually necessary)
UV/Fluorescence microscopy Lignin content (in addition to above information) As above (and provides additional information) Less widely available; sample preparation required
Scanning electron microscopy (SEM) Cell wall loss; nature of attack; inclusions; separation of cell walls; fungal spores Advantages of LM, but with much higher degree of detail; easier on degraded samples than LM Samples usually must be dry; samples must be coated; expensive; not as widely available as LM Dry or conserved sample; destructive; approx. 3 mm3 sample required (larger sample usually necessary)
Environmental SEM Not under vacuum so sample can be waterlogged and does not require coating Reduced quality of images compared to normal SEM; less widely available than SEM Dry, waterlogged or conserved sample; non-destructive (but size of instrument may demand that sample is cut)
Transmission electron microscopy (TEM) Examines internal structure Very high-resolution images Expensive; complex sample preparation; limited access to instruments and expertise Dry, waterlogged or conserved sample; destructive; < 1 mm3 sample required (larger sample usually necessary)
SEM–EDX (or EDS) As for SEM, but includes elemental composition map; can obtain lignin distribution map if pre-treated Quantitative; simultaneous structural analysis Less widely available than normal SEM; more expensive instrumentation; more complex data analysis; needs smooth sample surface Dry or conserved sample; destructive; approx. 5 mm3 sample required (larger sample usually necessary)