WATERLOGGED WOOD DETERIORATION
Notes for investigating possible treatment of waterlogged archaeological spruce root basketry
Ellen Carrlee, March 2009
WOOD BASICS (especially conifers)
Gymnosperms (Softwoods) Seeds NOT in ovules. Usually have needles. Conifers in this group. They have sieve cells, a type of phloem characteristic of non-flowering vascular plants.
Angiosperms (Hardwoods) Seed in ovules. Flowering plants. Usually lose leaves seasonally. Divided into monocotyledons (grasses, palms, bamboo) and dicotyledons
Cell Wall has layers of:
1. cellulose (a polysaccharide aka complex carbohydrate aka complex sugar), cellulose will form H-bonds with about 36 other chains to form a microfibril, sort of like making a thick rope from thin fibers. Some regions of these microfibrils are crystalline and some are amorphous. They are 5-12nm wide and have tensile strength of steel. About 40-80% of the secondary cell wall is cellulose microfibrils (Bidlack et al 1992.)
2. hemicellulose a polysaccharide more soluble than cellulose…especially soluble in strong alkali. Hemicellulose is now called cross linking glycans by biologists. They don’t aggregate and don’t form microfibrils. Thought to cross link non-cellulosic and cellulosic polymers. About 10-40% of the secondary cell wall is hemicellulose (Bidlack et al 1992.)
3. lignin (a complex polysaccharide) is basically a polymer of phenolics, especially phenylpropanoids. About 5-25% of the secondary cell wall is lignin, and the cellulose microfibrils and hemicellulosic chains are embedded in lignin, which serves as a amorphous polymer “glue.” Lignin % is higher in conifers than hardwoods in general. Lignin is not easily soluble, and is probably bound chemically to the hemicellulose and not the cellulose. Lignin is the precursor to coal (Bidlack et al 1992.)
The outermost layer of the cell wall is the middle lamella, and has the glue that binds it to adjacent cells. The primary wall is thin, found in young, growing cells, and has pectic polysaccharides (30%), hemicellulose (25%) cellulose (15-30%) and protein (20%.) All plant cells have a middle lamella and a primary wall. Secondary wall is an additional deposit inside the primary wall. Mostly for support and made of cellulose and lignin. In the secondary cell wall, elongation stops during growth because of deposition of lignified secondary cell wall. There are often distinct layers (S1, S2, S3) that differ in orientation of the cellulose microfibrils. Lignin rarely occurs in the S3 lamella (Bidlack et al 1992.) The lumen is the space bound by the cell wall. The wall is made from the outside in, so as the wall gets thicker the lumen gets smaller.
Organelles are parts of the cell that do activities. Include nucleus, cytoplasm, mitochondria, plastids, ribosomes, dictysomes, microtubules, vacuoles (water-filled part of the plant), endoplasmic reticulum.
Ergastic Substances are passive products of organelles (?) Include starch, tannins, proteins, lipids, crystals/phytoliths (sometimes survive and aid ID.)
Cutin is the lipid in the cuticle layer. Suberin is the lipid in the Casparian strip, often in roots and periderm. Cutins and suberins are persistent in nature. Suberin is a physical barrier to water.
Plant tissues divide into
1) Simple, like pith (ground tissue in the center of a stem or root) which is made of parenchyma cells. Parenchyma are simple tissues, but come in a wide range of shapes and sizes with different functions.
2) Compound, like vascular tissue which is made of both parenchyma and sclerenchyma cells. Sclerenchyma are compound tissues divided into sclereids (ie stone cells) that are bigger diameter and have a thicker cell wall and fibers that are long and thin with pointed tips and a small lumen. Both of these have multilayered birefringent walls. Sclerenchyma are thick walled, dead at maturity and give rigid support. Note: Douglas fir sclereids are elongated and resemble fibers.
3) Complex, like the bark, which is made of parenchyma, sclerenchyma, and collenchyma cells
Vascular tissue is the xylem and phloem
Epidermis is cuticle has cutin, lipids, oils, resins.
Periderm is the outer protective and supportive secondary tissue, formed by the cork cambium and it replaces epidermis in the stems and roots, mostly. Bark is periderm. Some of its parenchyma cells are called phellem and they contain a lot of suberin.
Ground tissue is everything BUT the vascular tissues, epidermis, and periderm, like cortex and pith. AKA “fundamental tissue”
Xylem is the vascular tissue for water movement and has lignified secondary cell walls and pits in the cell wall. Secondary xylem is wood, and preserves better than phloem. Also sections nicely for detailed analysis. Xylem is usually larger than phloem. Sapwood is the living secondary xylem and has starch grains. Heartwood is dead, and contains extractives making it darker. It is less porous than sapwood because it includes tyloses, tannins, salts, resins, silica etc that block fluid movement. Growth rings are the result of early wood (larger tracheary elements and thinner walls. More holes.) and late wood (smaller tracheary elements and thicker walls. Denser.)
Xylem is made of cell types:
- tracheids Found only in softwood, transport water and dissolved minerals and have thickened lignified walls for support. Generally, loss of water through leaves drives the flow. No perforations in end wall, but lots of pits in the walls. Secondary thickenings of various kinds are diagnostic. Spiral thickenings (like springs) appear only on Douglas fir and yew on the nothwest coast (but many other confiers worldwide). Yew has steeper and less regular spirals and no resin canals. Douglas fir has resin canals, but is not a true “fir” despite its vernacular name. DON’T confuse with spiral thickenings with spiral checking, which is more common in other conifers too. The tangential diameter helps ID of conifers (called “texture”.) Big holes are “coarse” textured, like redwood. ID chart on p.17 of Hoadley. Nonliving cells.
- vessel members (aka trachea) Transport water in hardwoods. These do have perforations in common end walls.. Secondary thickenings are also diagnostic. Nonliving cells
- fibers in hardwoods. Nonliving cells
- parenchyma Alive for many years
Secondary xylem in roots: vascular rays, periderm, vascular cambrium, vessel distribution, primary xylem. Look carefully at ray anatomy, esp crossfield pitting and ray tracehids.
Phloem is the vascular tissue for food transport (sugars and amino acids) and has thin-walled cells with less lignin than the xylem. Secondary phloem is inner bark for food storage and transport. It contains layers of fibers alternating with sieve element cells and parenchyma cells. Parenchyma cells (12-38% of inner bark volume) are thin walled and weak next to the stronger fibers of sclerenchyma and this is why inner bark separates into sheets. Note: genus Pinus has no sclerenchyma. Phloem contains mostly sieve elements and neighboring regulatory cells (“Sieve elements”/ albuminous cells in gymnosperms, “sieve tube elements”/ companion cells in angiosperms.) Sieve elements don’t survive well for ID, but are at least half the cells of the inner bark. Sieve elements are a lot like tracheids but have sieve areas rather than pits. In the cedar bark used for weaving, tissue is made up of stong hair like fibers, delicate cuboidal parenchyma cells that store food, and porous food conducting sieve cells.
CONIFEROUS WOOD has
Axial tracheids with smooth walls or walls with spirals; circular bordered pits (CBP) with numerous rows. Are like 100X longer than their diameter. Almost all the longitudinal cells in conifers are this kind.
Ray tracheids that are smooth or dentate (in pines) Don’t show up in Western Red Cedar.
Axial ray parenchyma that are smooth or dentate
Axial parenchyma with smooth or nodular end walls. Shorter than the tracheids. Sometimes are filled up. Don’t show up in pinus and picea.
Ray parenchyma with smooth or nodular endwalls (nodular in spruce root.)
Resin canals that are thick or thin walled. Larger rounded opening that has thin-walled epithelial cells that secrete the resin (easily damaged in sampling.) If it bulges and blocks the canal that’s called a tylosoid. Pines have numerous, evenly distributed ones with thinner epithelial cells. Firs and spruces have fewer, sometimes grouped, and thicker epithelial cells. Can be hard to see in samples but important for ID. Spruce root has large coalescent resin canals in the central region.
A ray initiates in the cambrium and extends radially into the secondary phloem and xylem. (cross over the growth rings at 90degrees) Mostly made of parenchyma cells but also tracheids. It shows up in the secondary xylem of some conifers. Most other cells go parallel to the stem or trunk. A fusiform ray has a larger resin canal in the middle that makes it look like an eye in tangential view, diagnostic for pine. Rays are important in ID. Upper and lower rows of ray cells are ray tracehids, and the middle ones are ray parenchyma. Ray tracheids can be hard to see in hemlock.
Pits are voids in the secondary cell wall. It matches up with a pit in an adjacent cell, forming a pit pair. Pits in longitudinal or ray parenchyma cells are simple pits. Tracheids have more elaborate bordered pits, look like targets or donuts (they indicate you have a conifer.) The number across the radial wall of earlywood tracheids is important for ID (one for spruce, two for larch, up to four in redwood.)
Where ray parenchyma and tracheids intersect, you get field crossings and the pitting there has one of four shapes: piceoid (slit opening, as in spruce root,) cupressiod (oval, almond opening,) taxodioid (round, as in western red cedar,) fenestral (like a window, squarish)
ROOT: Similar to branches (withes) in the circular arrangement of cells. But cells are larger, thinner walled, often collapsed, and tissue can have open/disorganized appearance. Resin canals might join together. Rows of bordered pits may be more numerous than typically found in trunk wood for that species. Longitudinal parenchyma are very prominent, and the width of the ray parenchyma cells is bigger than in the rest of the tree.
* Loss of holocellulose in waterlogged wood, because it is so soluble. (Florian, 1990)
* Holocellulose degrades more rapidly than lignin, and hollocellulose in archaeological wooden arrows from Nyden Bog had decomposed completely. (Christiensen 2006)
* Holocellulose is the hemicellulose plus the cellulose, and it deteriorates first. Umax is a function of deterioration in the form of holocellulose loss, and thus the void space in the cell wall. Depends on the density of that species of wood. (Cook and Grattan, 1986)
* Holocellulose = cellulose + hemicellulose. As water content increases, carbohydrate decreases and you’re left with proportionally more and more lignin. Lignin is quite resistant to microbial and chemical attack. Cellulose and hemicellulose are lost at about the same rate, even though hemicellulose can be degraded much easier than the crystalline cellulose. (Hoffmann 1982)
* Degraded oak timbers show about half of the cellulose and hemicellulose is gone, with fissures and cracks criss-crossing the tissue. (Hoffmann 1986)
* Lignin bonds only to hemicelluloses, not to celluloses. Lignin is made of large 3_D crosslinked molecules. (Hoffmann 1982.)
* Wood shows zones of progressive degradation. Secondary wall loosens because of hydrolysis of carbohydrates. Cell walls lose fluorescence and birefringence. (Does this impact the cobalt thiocyanate staining?) Lignin skeleton eventually collapses, leaving only granular debris. Tertiary walls and compound middle lamellae keep the dimensions stable as long as they are filled with water (Hoffman & Jones 1988; Blanchette & Hoffmann 1994.)
* Double bonds in lignin are affected by warm treatments (Christensen et al 2006)
* Chemical degradation of the secondary cell wall starts at the lumen and progresses inward (Hoffmann 1982)
* Western red cedar inner bark cell walls very thick. Lumens in inner bark of Western Red Cedar were very small compared to those in wood. Some smaller thinner-walled cells contain resins. (Bilz et al 1998)
* In cedar bark (phloem) the fiber cells are usually intact, holes in sieve plates and pits are enlarged, pectin, starch bodies and inorganic crystals are gone, thin-walled parenchyma cells mostly gone, cellulose seems to be gone or changed. Cell walls have a crystalline look that might be due to impregnation with inorganic salts from burial, perhaps an early stage of fossilization. Some of the insoluble resins and tannins of the bark are still present in reduced amounts, and some of the color-inducing brown phlobaphenes are still there too. High lignin and tannin content in the call walls of fibers and sieve cells. (Florian 1977)
* Study of the Bremen Cog (oak, fresh water) indicates wood cell walls are thinned, erosion bacteria were a primary agent of degradation, and that non-degraded tissues were impermeable to PEG 3000 and only impregnated with PEG 200. (Hoffmann et al 2004)
* Higher cellulose content is correlated to greater risk of fungus development. (Grattan 1986)
* Mold and sap-staining fungi eat starch in ray cells. Spiral checking of tracheid and fiber cell walls often attacked by staining fungi. Don’t mix up spiral checking with spiral thickening. (Jagels, 1982)
* Mold and sap stain fungi only utilize the food stored in the wood and do not destroy the strength (Florian 1977)
* Egg-shaped voids are typical of holes left by fungal hyphae (Hoffmann 1982)
* Surface cuboidal cracking is often due to soft-rot. (Florian 1982)
* Pectin is the main chemical in the membranous valves of bordered pits, and bacteria that specifically attack pectin are often the first to enter the wood, making it more permeable. (Florian, 1977)
* Collapse of radial walls of tracheids when waterlogged in Thuja plicata is a weakness. (Florian 1982)
* Loss of pectin that normally glues cells together is often missing in deteriorated wood, which might contribute to cracks between the longitudinal cells and the stronger ray parenchyma cells. (Florian 1982)
* Tension wood is not characteristic of roots (Florian, 1982)
SHRINKAGE AND COLLAPSE
* Shrinkage from capillary tension happens when the free water is pulled by evaporation from the void structure of the wood. Shrinkage is also caused from desorption of bound water from the cell wall. Article gives extensive info about which cells collapse in which direction. (Barbour and Leney 1982)
* The meniscus of water (which has high surface tension) applies stress to those capillaries as it leaves and can collapse them. Forces are not as intense if there are air bubbles, as in green wood. This is why wood that is only damp and not waterlogged can slowly air dry successfully, especially if the cell walls are not too degraded. (Grattan 1986.)
* Hygroscopically bound water in the capillary network of the cell walls needs to be replaced by PEG for dimensional stability to occur (Young, 1982)
* “Second order space” is the term used to describe the volume of the microcapillaries in the cell wall that does not include the voids caused by deterioration. At the fiber saturation point, water exactly fills this space without filling the lumen in undeteriorated wood. (Cook & Grattan 1990)
* Areas like roots that have a lot of longitudinal or ray parenchyma will have more shrinkage (Florian 1982)
* Salinity differences between water inside cells and outside of cells can cause problems with osmotic pressure if you put waterlogged material found in saltwater directly into fresh water. Salt solution moving out of the cell moves faster than fresh water moving in and cell can collapse. Desalination is crucial. (Bradley 1992)
* Shrinkage in sound or slightly deteriorated wood is anisotropic, or varies in the three major directions: longitudinal 0.5%, radial 3-6%, and tangential 5-10%. More deteriorated wood ill show less well-defined shrinkage (but more shrinkage overall?) (McCawley 1977)
PENETRATION AND PERMEABILITY
* Capillaries make up about 40% of the volume of the cell wall. Capillaries seem to range between 10-80nm. Water is 0.2nm, PEG 400 molecule around 2nm x 0.25nm, PEG 1000 around 4.5nm and PEG 4000 around 18nm. (Hoffmann 1982.)
* Intracell connections, “plasmodesmata” or “cytoplastmic connections” have small diameters. Water and glucose (5-carbon ring) can pass, for example, but PEG and sucrose (6-carbon ring) cannot pass. Osmotic pressure can also cause those openings to collapse and make PEG penetration harder. Perhaps PEG 200 is the best for these spaces. (Grant et al 1997.)
* Low mw PEG in cell lumina and larger voids in cell wall will diffuse back out of the wood again, only remains in the capillary system. Larger mw needed for those larger voids (Hoffmann 1986.)
* Lower mw PEG penetrates the micro-capillaries of the cell wall, while higher mw penetrates the lumens, flows through the vascular system (Grattan 1986.)
* Normal anatomical wood characteristics that impede permeability: few/small hardwood vessels, many hardwood thick-walled fiber cells, short longitudinal tracheids in softwoods, the absence of ray tracheids, the absence of radial and longitudinal resin canals, aspirated and encrusted bordered pits, blind simple pits, high specific gravity, ray parenchyma containing resin or other material, reduced tangential wall pitting, extractives in heartwood. (Florian, 1982)
* Pits between the cells have valves (the torus) which can block penetration of PEG if closed (aka blocked or aspirated.) Must be viewed at 1000X to 3000X so requires SEM? (Bradley 1992)
* In white oak, which is hard to penetrate, the rate of degradation by microorganisms is faster than their speed of penetration, so you get areas of very deteriorated wood on the exterior of the timbers and less deteriorated zones towards the core. (Christensen 1970, Grattan 1986)
* Pine and oak are two species where the heartwood is only slightly permeable to liquids, and often less degraded than outer areas of the wood (Hoffmann 1986.)
* Western Red cedar (thuja plicata) is very difficult to penetrate, even when deteriorated. Similar to oak in this way. (Cook and Grattan 1990)
*Picea sp and Thuja plicata have no tangential permeability because there aren’t many wall pits, there are blind pits and there’s resin in the ray parenchyma cells. Picea will be more permeable than Thuja plicata because it has resin canals and ray tracheids that Thuja plicata does not. (Florian 1982)
* In many coniferous species, fast-growing wood is more permeable than slow growth wood.
* Softwood samples generally show open and enlarged tracheid bordered pits (probably from bacteria) which make them more porous (Florian 1982)
* Deterioration greatly improves the ability of PEG to penetrate and treat wood successfully, but most excavated waterlogged wood is only moderately deteriorated. White oak, various cedars, and white ash are hard to penetrate, while aspen, cottonwood, alder and spruce allow greater penetration. Caution with determining degree of deterioration from thin 3mm cross sectioned wafers of wood examined under the microscope, as the sample has a lot of disrupted wood cells and suggests more cell wall accessibility than there really is. (Young, 1990)
* Degree of deterioration in PEGCON determined by comparing the density of the deteriorated wood to the density expected for that species.
* Percent water content is often used to evaluate deterioration of waterlogged wood, but it varies with species and is measured in different ways which makes comparisons difficult (Grattan 1986.)
* Archaeological woods have a higher ash content, which means they have more minerals in them than fresh wood. Minerals are determined as oxides after their organic content has been burnt away in analysis. (Hoffmann 1982)
* Some waterlogged wood, Mary Rose, Brown’s Ferry Wreck, show elevated ash content. Also, metals and siliceous materials enter the wood and add to the ash content. (Richard Clarke comment in Singley, 1982.)
* A device called a Pilodyn has a spring loaded blunt pin and helps to determine degree of deterioration on large things like timbers from the Mary Rose. but it is far too large for basketry (Grattan 1986)
* Amount of water in waterlogged wood is calculated: weight of wet wood minus weight of oven dried wood divided by weight of the oven dried wood and multiplied by 100 to give % water. Anything over 200% is considered degraded. (Hamilton 1998)
Alden, Harry. Plant Anatomy and Morphology for Objects Conservators and Archaeologists. CD from a course offered by Smithsonian Center for Materials Research and Education, 2000.
Barbour, R.J. and L. Leney. “Shrinkage and Collapse in Waterlogged Archaeological Wood: Contribution III Hoko River Series. In book. Proceedings of the ICOM Waterlogged Wood Working Group conference: Ottawa, 15-18 September 1981. ICOM Waterlogged Wood Working Group (1982), pp. 209-225.
Barbour, James. “The Condition and Dimensional Stabilization of Highly Deteriorated Waterlogged Hardwoods.” Proceedings of the 2nd ICOM Waterlogged Wood Working Group Conference. Grenoble, 28-31 August 1984.
Lots of info about the layers of the cell wall
Bernick, Kathryn. Personal communication April 9, 2009.
Bidlack, Jim, Mike Malone, and Russel Benson. “Molecular Structure and Component Integration of Secondary Cell Walls in Plants.” Proceedings of the Oklahoma Academy of Science Vol 52 1992 pp 51-56.
Bilz, Malcolm, Tara Grant and Gregory S. Young. “Treating Waterlogged Basketry: A Study of Polyethylene Glycol Penetration Into the Inner Bark of Western Red Cedar.” Proceedings of the 7th ICOM-CC Working Group on Wet Organic Archaeological Materials Grenoble 1998. pp.249-253
Blanchette, Robert A.; Hoffmann, Per “Degradation processes in waterlogged archaeological wood” Proceedings of the fifth ICOM Group on Wet Organic Archaeological Materials conference, Portland, Maine, 16-20 August 1993 1994
Christensen, M, M. Frosch, P. Jense, U. Schnell, Y. Shahsoua, O.F. Nielsen. “Waterlogged Archaeological Wood, Chemical Changes by Conservation and Degradation.” Journal of Raman Spectroscopy. Vol. 37, issue 10. Special Issue: Raman Spectroscopy in Arch and Archaeology II. 2006. pp. 1171-1178.
Christensen, B. “The Conservation of Waterlogged Wood in the National Museum of Denmark.” Museums Tenniske Studier 1, National Museum of Copenhagen, Denmark. 1970.
Cook, Clifford and David Grattan. “A Method of Calculating the Concentration of PEG for waterlogged Wood.” Proceedings of the 4th ICOM Group on Wet Organic Archaeological Materials. Bremerhaven 1990.
Florian, Mary-Lou E., Dale Paul Kronkright, Ruth E. Norton. The Conservation of Artifacts Made from Plant Materials. J. Paul Getty Trust. 1990.
Florian, Mary-Lou. “Anomalous Wood Structure: A Reason for Failrure of PEG in Freezer-Drying Treatments of Some Waterlogged Wood from the Ozette Site.” In book. Proceedings of the ICOM Waterlogged Wood Working Group conference: Ottawa, 15-18 September 1981. ICOM Waterlogged Wood Working Group (1982), pp85-98
Florian, Mary-Lou. “Waterlogged Artifacts: the Nature of the Materials. Journal of the Canadian Conservation Institute. 1977
Grant, Tara and Malcolm Bilz. “Conservation of Waterlogged Cedar Basketry and Cordage.” Proceedings of the 6th ICOM Group on Wet Organic Archaeological Materials York, 1996. Pub 1997
Grattan, D. “Some Observations on the Conservation of Waterlogged Wooden Shipwrecks.” AICCM Bulletin, Vol. 12 No 3 and 4. 1986.
Hamilton, Donny L. Methods of Conserving Archaeological Material from Underwater Sites. Nautical Archaeology Program Department of Anthropology Texas A&M University. 1998.
Hoadley, R. Bruce. Identifying Wood. Accurate Results with Simple Tools. Taunton Press. Newtown, Connecticut. 1990.
Hoffmann, Per, Adya Singh, Yoon Soo Kim, Seung Gon Wi, Ik-Joo Kim, and Uwe Schmitt. “The Bremen Cog of 1380: An Electron Microscopic Study of its Degraded Wood Before and After Stabilization with PEG.” In Holzforschung Vol 58 No 3 2004 pp 211-218
Hoffmann, Per and Mark Jones. “Structure and Degradation Process for Waterlogged Archaeological Wood” in Archaeological Wood Properties, Chemistry, and Preservation. Developed from a symposium sponsored by the Cellulose Paper and Textile Division at the 196th National Meeting of the American Chemical Society, Los Angeles, California, September 25-September 30, 1988Advances in chemistry series 225. American Chemical Society. Washington 1990.
Hoffmann, Per. “ On the Stabilization of Waterlogged Oakwood with PEG II Designing a Two-Step Treatment for Multi-Quality Timbers.” Studies in Conservation 31 (1986) pp.103-113
Hoffmann, Per. “On the Stabilization of Waterlogged Oak with PEG – Molecular Size Versus Degree of Degradation.” Waterlogged Wood Study and Conservation, Proceedings of the 2nd ICOM Waterlogged Wood Working Group Conference, Grenoble, France. 1984. pp. 243-252.
Hoffmann, Per. “Chemical Wood Analysis as a Means of Characterizing Archaeological Wood” In book. Proceedings of the ICOM Waterlogged Wood Working Group conference: Ottawa, 15-18 September 1981. ICOM Waterlogged Wood Working Group (1982), pp.73-84.
Jagels, Richard. “A Deterioration Evaluation Procedure for Waterlogged Wood.” In book. Proceedings of the ICOM Waterlogged Wood Working Group conference: Ottawa, 15-18 September 1981. ICOM Waterlogged Wood Working Group (1982), pp. 69-72
Martin, Robert and John G. Christ. “Elements of Bark Structure and Terminology.” Wood and Fiber Vol 2 No 3 1970. pp 269-279.
McCawley, J.C. “Waterlogged Artifacts: The Challenge to Conservation.” In Journal of the Canadian Conservation Institute. Vol 2, 1977. pp17-26.
Rodgers, Bradley. ECU Conservator’s Cookbook: A Mthodological Approach to the Conservation of Water Soaked Artifacts. Chapter 2: Waterlogged Wood. Herbert P. Paschal Memorial Fund Publication. East Carolina University. 1992.
Saupe, Dr. Stephen G. “Cell Walls –Structure and Function” Plant Physiology (biology 327) College of St. Benedict/ St. john’s University. Collegeville, Minnesota. 2009. http://employees.csbsju.edu/ssaupe/biol327/Lecture/cell-wall.htm
Singley, Katherine R. “The recovery and conservation of the Brown’s Ferry vessel” In book. Proceedings of the ICOM Waterlogged Wood Working Group conference: Ottawa, 15-18 September 1981. ICOM Waterlogged Wood Working Group (1982), pp. 57-60
Young, Gregory S. “Microscopy and Archaeological Waterlogged Wood Conservation.” CCI Newsletter, No. 6, September 1990. pp 9-11.
Young, Gregory S. “Polyethylene Glycol Localization within the Structure of Waterlogged Wood.” 9th International Congress on Science and Technology in the Service of Conservation. 1982.