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

High, Kirsty E.; Penkman, Kirsty E. H.

doi:10.1186/s40494-020-00422-y

Heritage Science

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
Physical evaluation	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
Advanced visualisation techniques	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)
Physical resistance (density)	Mechanised probe (Pilodyn/Sibert)	Measures resistance as a proxy for density	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)

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