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Journal of Cultural Heritage 13S (2012) S5–S9

Available online at

www.sciencedirect.com

istorical wood – structure and properties

homas Nilssona, Roger Rowell a,∗,b

Faculty of Forest Sciences, SLU, c/o SP Technical Research Institute of Sweden, Stockholm, SwedenUniversity of Wisconsin, Madison, WI, USA

r t i c l e i n f o

rticle history:eceived 27 March 2012ccepted 27 March 2012vailable online 9 May 2012

a b s t r a c t

To understand what has happened to a historic wooden object, it is important to understand the woodstructure and the chemistry of the original wood as well as the structure and chemistry of the historicwood. With this information, it is possible to understand the degradation that has occurred over time.With this knowledge, it is also possible to describe a possible method of conservation and preservation.

eywords:rchaeological woodistoric woodood structureood properties

© 2012 Elsevier Masson SAS. All rights reserved.

ood decay

. Research aims

The aim of this paper is to present a basic understanding of woodtructure and chemistry and to relate this to a better understandingf changes in structure and chemistry that occur in Historic andrchaeological wood.

. Introduction

Wood has been used by humans for thousands of years asools, fuel, weapons, structures and for recreation. Wooden artifactsemaining from human use not only provide us with an interest-ng picture of the skills and ingenuity of past generations but alsohe changes in wood structure tell us a lot about the environmenthat was present when the wood was in use and the environmentalhanges in the passage of time [1].

Wood and wooden artifacts are organic materials and are, there-ore, subject to the recycling chemistries of Mother Nature. In broaderms, wood is produced from the basic building blocks of car-on dioxide and water and it is recycled by Nature back to carbonioxide and water. In order for artifacts to survive from past civ-

lizations, the artifact must survive Natures recycling chemistries.any of the historical wood artifacts we have today have under-

one some chemical and physical changes. Most of the historicalood artifacts have been lost due to the biodegradable nature of

he material.

∗ Corresponding author.E-mail addresses: nilssonbielke@gmail.com (T. Nilsson),

mrowell@wisc.edu (R. Rowell).

296-2074/$ – see front matter © 2012 Elsevier Masson SAS. All rights reserved.oi:10.1016/j.culher.2012.03.016

To study and understand the changes that wooden artifactshave undergone with the passage of time, changes that may beof vital importance for preservation, it is important to understandthe structure and chemistry of wood [2].

3. Understanding wood

The study of wood is best done by looking at it at different levelsof detail and there are several levels of details to consider: macro,sub-macro, micro, sub-micro, and molecular. We recognize woodat the macro level as a tree and this level can be broken down intotwo sub-categories, softwood and hardwoods [3] (Fig. 1).

At the sub-macro level, we recognize wood as a solid board (inthe rough) or as wood furniture, windows, doors, etc. (Fig. 2). Atthe micro level, we study the wood cell wall and identify differentelements such as heartwood, sapwood, annual rings, etc. (Fig. 3a, b).

At the sub-micro level, we see cell structure such as lumens, pits,vessels, ray cells, etc. (Fig. 4a, b).

Finally, at the molecular level we can study the cell wallpolymers (cellulose, lignin and hemicelluloses) and their build-ing blocks of simple sugars, phenolic units as well as extractivesstructure and inorganic compounds (Fig. 5).

A general chemical analysis of hardwoods and softwoods isgiven in Table 1. Note that softwoods have a higher content of ligninand a lower content of pentosans.

Lignin offers some protection to the wood from being degradedby common cellulolytic micro-organisms. There are two main

forms of lignin: guaiacyl and syringyl lignin. Guaiacyl lignin ismore resistant than syringyl lignin to microbial degradation.This explains why softwoods, which almost exclusively containguaiacyl lignin, are more durable than hardwoods, where the lignin

S6 T. Nilsson, R. Rowell / Journal of Cultural Heritage 13S (2012) S5–S9

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Fig. 3. a: micro structure of the wood cell wall; b: more detailed micro structure ofthe wood cell wall.

Fig. 1. Macro level of wood.

s a mixture of guaiacyl and syringyl lignins. Extractives in cer-ain heartwoods increase the durability considerably. Sapwood islways susceptible to decay. Reaction wood, tension and compres-ion wood, has a structure that differs anatomically and chemicallyrom “normal” wood. These differences have to be considered innalyses of historical wood.

For some, the identification of tree species is the level neededo understand wood. For others, types of wood are important todentify a potential product or performance requirement. As you goeeper into the substructures, you start to understand that ‘wood’

s a very common name for a very complex structure and, at theseevels, wood is best understood as a porous three-dimensional,ydroscopic, viscoelastic, anisotropic bio-polymer composite com-osed of an interconnecting matrix of cellulose, hemicellulosesnd lignin with minor amounts of inorganic elements and organicxtractives.

There are several major factors that determine if wood will sur-ive over time. One important factor is the chemical structure ofood, which interacts with other major factors such as oxygen,

oisture, pH, temperature, ultraviolet energy, micro-organisms

nd contaminates. Wood that has not been exposed to oxygen canurvive thousands of years without major changes. Wood that has

Fig. 2. Sub-macro level of wood. Fig. 4. Sub-micro level of wood.

T. Nilsson, R. Rowell / Journal of Cultural Heritage 13S (2012) S5–S9 S7

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little strength or cell wall structure remaining.Many micro-organisms recognize that they can attack wood

by some chemical element in the cell wall [5]. In most cases,recognition comes from part of the hemicellulose polymers,

Fig. 5. Chemical structure of

een kept dry over centuries survives with little change in structurer properties. Without oxygen and moisture, few micro-organismsan survive, and, wood that is free of inorganic salts and other con-aminates can also survive with little change. In some cases, theontaminate may, in fact, help in the preservation of a woodenrtifact. An ancient tree preserved after being buried in the ash of

long past volcano shows us that wood without oxygen surviveshe passage of time unaffected.

Wood from a 10,000-year-old tomb in China that was kept dryll the years survived with very little change. Wood from shipsecovered from the bottom of lakes and seas is found to maintainost physical properties and recoverable since it was maintained

n a cold water-logged state. A bowl, made from a wood speciesot native to that region of the world tells us something about theigration of populations.A totem pole from the Alaskan tundra that was kept very cold

or hundreds of years can remain brightly colored and sound. structure built from solid timbers will last for years if thenly degradation is due to exposure from ultra violate radiationweathering). Weathering is a very slow process (about 2\3 cm perentury) due to ultraviolet energy and rain.

When the degrading elements work together, the wooden arti-act is not likely to survive [4]. For example, wood above its fiberaturation point (the point where all of the water in the wood isn the cell wall and it cannot hold any more), in a warm environ-

ent in contact with micro-organisms, is not likely to survive morehan a few months. The SEM image below shows wood that haseen attacked by brown-rot fungi and has lost a lot of weight andtrength (Fig. 6). Wood in contact with limited water and in contact

se (top) and lignin (bottom).

with micro-organisms may survive in a badly degraded state with

Fig. 6. Solfwood cell wall degraded by brown-rot fungus.

S8 T. Nilsson, R. Rowell / Journal of Cultural Heritage 13S (2012) S5–S9

Table 1General chemical composition of hardwoods and softwoods.

Species Holo-cellulose Alpha cellulose Pentosans Klason lignin Ash

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Hardwoods 71.7 ± 5.7 45.4 ± 3.5

Softwoods 64.5 ± 4.6 43.7 ± 2.6

erhaps from the arabinose sugar as it is the only sugar in thearbohydrate polymers that exists in a strained five-membereding. The three figures below show an intact cell wall, a cell wallhat has had part of the hemicellulose polymers removed and a cellall contining no hemicellulose (Fig. 7). As the hemicellulose isegraded, the cell wall starts to collapse and strength is lost due toatrix failure. More strength is lost as more of the hemicellulose-

ignin matrix is destroyed. Decrease in mechanical properties cane determined using an analytical sugar analysis [2]. The lossf a hemicellulose sugar, such as galactose, is directly corelated

o a loss in modulus of elasticity (MOE) and modulus of ruptureMOR). So, a quick and easy way of looking at losses in mechanicalroperties is to do a carbohydrate analysis on the degradedood.

Fig. 7. Different stages of cell wall degradation, no degradation (top), loss of so

19.3 ± 2.2 23.0 ± 3.0 0.5 ± 0.39.8 ± 2.2 28.8 ± 2.6 0.3 ± 0.1

Wood from a ship that has been in the ocean may be badlydegraded by ocean organisms, like shipworm and Limnoria (Fig. 8).While the damage is not due to micro-organisms, it is commonlyseen degradation that occurs in salty waters.

Mother Nature has five basic chemistries to convert a woodenartifact back to carbon dioxide and water. The five chemistriesare oxidation, hydrolysis, dehydration, reduction and free rad-ical reactions. Table 2 shows the major degradation pathwaysand the chemistries involved in those pathways. The chemistriesshown in Table 1 may result in a badly damaged wooden

artifact and from the type of degradation it may be possi-ble to distinguish the mechanism of degradation. This analysismay help provide valuable information on a possible conser-vation treatment for the artifact. A great number of analytical

me hemicelluloses (center), loss of all carbohydrate polymers (bottom).

T. Nilsson, R. Rowell / Journal of Cultu

Fig. 8. Wood degraded by Limnoria.

Table 2Major degradation pathways and chemistries.

Biological degradation Fungi, bacteria, insects, termitesEnzymatic reactions oxidation, hydrolysis,reduction, free radical

Chemical reactions Oxidation, hydrolysis, reduction, free radicalMechanical degradation Chewing, dust, wind, hail, snow, sand, stress,

cracks, fracture, abrasion, erosion, compressionThermal degradation Lightning, fire, sunPyrolysis reactions Dehydration, hydrolysis, oxidation, free radicalWater degradation Rain, sea, ice, acid rain, dewWater interactions Swelling, shrinking, freezing, cracking, erosion

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Weather degradation Ultraviolet radiation, water, heat

echniques exist and selecting the best-suited technique may be challenge.

For some artifacts, analyses should be based on more than oneechnique, in order to gain a better understanding of the degrada-ion processes. An example: a traditional chemical analysis will tellou that various components have been lost over time, but not the

ause of degradation. Microscopy can be used to detect or excludeicrobial decay.Degradation of wood is an irreversible process. Despite various

laims, degraded wood cannot be restored to its original state.

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ral Heritage 13S (2012) S5–S9 S9

4. Preservation and conservation

Preservation in situ or reburial requires knowledge of the actualstructure and durability of the wood and biological factors that maycontribute to further degradation and the environmental factors.Here, cooperation between wood scientists, wood biologists andconservators is required. For large structures, precautions must betaken to avoid mechanical damage. In certain situations, the dete-riorating factors can be monitored by installing fresh wood to thehistorical wood. It is extremely difficult to assess further deteri-oration in wood that already is decayed. Once the historical woodhas been recovered from its environment, it becomes a question onhow to preserve or conserve the artifact. Some method is needed torestore or stabilize the artifact from further degradation. Analysesof the wood before conservation will help to improve the conserva-tion. They will also provide facts of importance for the selection ofthe most suitable environment for future storage. Size and weightafter treatment decide if supportive structures are required. Here,the mechanical strength of the conserved wood has to be estimated.Long-term effects, such as creep and cell collapse, have to be takeninto account.

In the case of badly degraded artifacts where cell wall structureis damaged and the strength is very low, the method of restorationmight be to impregnate the wood with an epoxy resin or an acrylicmonomer that is later polymerized.

5. Storage

Once the restoration or stabilization is complete, the next con-cern is how to store the artifact. Should it be kept wet, dry, cold,out of sunlight, etc. The recent problems relating to sulphur andiron in the Vasa timbers demonstrate that we need a lot moreknowledge for designing optimal storage conditions. There is a con-flict between display, storage and accessibility of the artifacts. Ifthe item is to be displayed, how is that done in such a way thatthe artifact is not damaged by ultraviolet radiation or moisture.If the artifact is to be studied, how should it be stored for readyaccess. Some of these decisions are easy to make if there is a largequantity of similar artifacts but very hard if the quantity if verysmall.

References

1] R.M. Rowell, J. Barbour (Eds.), Archaeological wood: properties, chemistry, andpreservation. American Chemical Society Advances in Chemistry Series 225,Washington, DC, 1980, 472 pp.

2] R.M. Rowell, Handbook of wood chemistry and wood composites, Taylor andFrancis, Boca Raton, FL, 2005, 487 pp.

3] A.J. Panshin, C. de Zeeuw, Textbook of wood technology. Structure, identification,properties and uses of the commercial woods of the United States and Canada,fourth ed, McGraw-Hill Book Company, New York, USA, 1980.

4] R.A. Eaton, M.D.C. Hale, Wood decay, pests and protection, Chapman & Hall,London, 1993.

5] R.A. Blanchette, A review of microbial deterioration found in archaeologicalwood from different environments, International Biodeterioration & Biodegra-dation 46 (2000) 189–204.