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WOOD
Contributors: Helen Alten, Kate Clothier, Allison Miller
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Copyright: 2011. The Objects Group Wiki pages are a publication of the Objects Specialty Group of the American Institute for Conservation of Historic and Artistic Works. The Objects Group Wiki pages are published for the members of the Objects Specialty Group. Publication does not endorse or recommend any treatments, methods, or techniques described herein.



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Publication does not endorse or recommend any treatments, methods, or techniques described herein.



Wood

Composition

Trees

Gymnosperms or softwoods, such as Red Pine, have longitudinal cells that are mostly tracheids and a few longitudinal resin canals. One of the two largest groups of plants in the plant kingdom (the other is the angiosperms, or flowering plants). Gymnosperms, which typically form their seeds in the open spaces of cones, include all the conifers as well as the cycads.

Angiosperms or hardwoods, such as Red Oak, have longitudinal cells that include tracheids, fibers and vessels and ray cells that are more common and larger than the softwoods. These are flowering plants, the largest group in the plant kingdom, with about 250,000 species. The name derives from the fact that the seeds are enclosed in an ovary. Angiosperms provide us with our flowers, the vegetables in our diet, and our hardwood trees.

If you cut a tree across its midsection, you see distinct two distinct areas. On the outside of the tree is the bark. Bark consists of three layers, the cork, phloem and vascular cambium. Bark is the outermost layer of stems, branches and roots too. Inside the bark is the xylem (wood) consisting of sapwood and heartwood

The Bark

Cork, sometimes inaccurately thought to be the entire bark, is the outermost layer of a woody stem, derived from the cork cambium. The waterproof cork cells protect a layer of food-conducting tissue – the phloem or inner bark, also called bast. Cork serves as protection against damage, parasites and diseases, as well as dehydration and extreme temperatures. Cork can contain antiseptics like tannins. Some cork is substantially thicker, providing further insulation and giving the bark a characteristic structure, in some cases thick enough to be harvested as cork product without killing the tree. Among the commercial products made from bark are cork, cinnamon and other flavorings, dyes, and drugs such as quinine (from the bark of Cinchona) and aspirin (from the bark of willow trees). Tannins from specific trees are important in preserving hides and skin, converting them to leather. The outer bark of the paper birch is used to make baskets and canoes. Wide strips of bark served as an early paper and art substrate.

The phloem, known as inner bark or bast, conducts sap downward from the leaves to be used for storage and to nourish other plant parts. In vascular plants, phloem is the living tissue that carries organic nutrients, particularly sucrose, to all parts of the plant where needed. In trees, the phloem is part of the bark, hence the name, derived from the Greek word for "bark". “Girdling” a tree, i.e., cutting through the phloem tubes, results in starvation of the roots and, ultimately, death of the tree; trees are sometimes girdled by animals that eat bark. Where bark is harvested, as in basket making operations, care is taken to take vertical strips, never completely circling the tree. The fiber cells that strengthen and protect the phloem ducts are a source of such textile fibers as hemp, flax, and jute. The phloem is the main material in the Pacific Islander’s Tapa cloth, made by pounding bark to make a continuous material similar to papyrus or paper. The phloem of Cedar trees is cut into strips to weave into Cedar bark baskets in the Pacific Northwest. The phloem, or inner bark, is often a different color from the cork or outer bark. This difference is used for dramatic decorative effect in Birch bark baskets, twig furniture, and teepee backrests.

Along with the xylem (woody tissue), the phloem is one of the two tissues inside a plant that are involved with fluid transport. The phloem transports organic molecules (particularly sugars) to wherever they are needed. Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. The sap is a water-based solution rich in sugars and other organic molecules made by the photosynthetic areas. These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such as tubers or bulbs.

The Cambium

Beneath the bark is the cambium, a thin layer of fragile cells containing the living protoplasm. The vascular cambium is the only part of a woody stem where cell division occurs. It contains undifferentiated cells that divide rapidly to produce secondary xylem to the inside and secondary phloem to the outside.

The Xylem (Woody Tissue)

Beneath the cambium is the woody tissue, or xylem. In vascular plants, xylem is one of the two types of transport tissue in plants, phloem being the other one. The word “xylem” is derived from classical Greek xúlon, "wood", and indeed the best known xylem tissue is wood. The xylem transports sap from the root up the plant: xylem sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well. It is the largest component of trees. The supporting and water-conducting tissue of vascular plants, xylem consists primarily of tracheids and vessels. In trees the xylem is separated into sapwood and heartwood. Sapwood is closest to the bark. Here the protoplasm found in the cambium cells is gone and the cell walls are stiffened and fortified with lignin. The cells conduct water and dissolved nutrients. The heartwood at the center of the tree is dead. Extractive deposits color the heartwood and help make it decay resistant.

Wood tissue, or xylem, is like a cluster of straws. The structure is designed to pull water up from the roots. These straws are known as tracheids and vessel elements. Tracheids are elongated cells in the xylem of vascular plants, serving in the transport of water. The build of tracheids vary according to where they occur. In softwoods tracheids are the major cell type. Tracheids give softwood its strength.

The xylem is a complex tissue of plants, which means that it includes more than one type of cell. In fact, xylem contains other kinds of cells in addition to those that serve to transport water, but they are of lesser importance in the overall properties of wood.

Growth Rings

Each year a tree grows one ring wider. These growth rings are made of early and late wood. The early wood is made in the beginning of the growing season and consists of wide diameter cells with thin walls. The late wood is made later in the growing season and consists of smaller cells with thicker walls. These cells are denser and thicker. Looking at the growth rings gives you a clue to the climate in which the tree grew. If the temperature is nearly the same throughout the year, there is little difference between rings and between early and late wood. If the summer and fall are wet, then the early and late wood are similarly open. If there is a big difference in the moisture of the different seasons, then the cells vary in size from early to late. The transition from late to early wood creates the growth rings. These growth rings are more distinct in hardwoods (Angiosperms) than in softwoods (Gymnosperms).

Properties

Wood Planes and Anisotropy

Wood has three different planes. The cross-section of the tree is known as the transverse plane. The transverse plane is perpendicular to the tree stem and what one sees in this plane is the end grain. The radial section is parallel to the tree stem, passing through the center of the tree. Quarter-sawn boards, with flat, parallel grain, are radial cut wood. The last section is the tangential. It is parallel to the tree stem, but not through the center of the tree. Tangential cut wood is known as flat-sawn boards. They have interlocked V or U figures.

Wood is anisotropic, meaning that its properties are not uniform in all directions. The wood shrinks between 5 percent and 10 percent in the tangential plane and less than 0.1 percent in the longitudinal plane.

The sponge-like ability of wood to absorb water and the fact that the water is absorbed differently between the three different planes causing the wood to move more in one direction than in others, results in many forms of damage. If wood is constricted and the climate fluctuates, then it will crack or distort. If a basket made of woody roots, or split wood, or branches or strips of bark, is dirty and the climate fluctuates, then the gritty dirt can abrade parts of the basket, cutting the basket fibers. The most shrinkage occurs around the circumference of the log, so cracks are most likely to start there, radiating from the center of the log out to the bark. The next most shrinkage occurs from the inside to the outside of the log, so cracks would occur at the growth rings, making wood separate like the layers of an onion. And the least movement occurs up and down the length of the log. We rarely see cracks across the width of telephone poles or building supports, because there is little movement in the up and down direction. Depending on where a board is taken from the tree, it may cup, kink, diamond, or twist. The direction and type of distortion is predictable based on what we know of which direction moves the most when humidity fluctuates.

Identification

Identification Method of Hard and Soft Wood

Wood is a common material found in archaeological sites. Given its many uses, from being a mast of a sailing vessel to a simple spoon, its multifunctional nature made it a fundamental material for different societies. While some wooden objects in the archaeological record are easily identified, such as a well-preserved spoon, other object functions are more mysterious. In order to figure out the purpose of the object the identification of the type of wood is a useful indicator. In addition to helping pinpoint the function of the object, identifying the type of wood, whether hard or soft, helps curators decide a course of action to take when conserving the artifact. This report will focus on how to identify hard and soft wood in the archaeological context. When wood enters the archaeological record it is already dead. This means that sap, which is made up of a ìsolution of sugars, salts and other metabolic materialsî, is no longer present (Cronyn1990). What is left is the wooden skeleton made up of cellulose fibrils and lignin, and occasionally the bark of the plant, which is the ìdead tissue containing more lignin and tannin than wood itselfî (Cronyn1990). While both hard and soft woods contain these characteristics, under a microscope, loupe, or even the naked eye, cellular structure differences can be noted. Hardwoods contain vessels and tracheids, known collectively as xylems, in addition to fibers, and rays. Soft woods also contain the xylemís but lack tyloses and perforation plates which are unique only to hardwoods. While these are important gauges in identifying whether the material is hard or soft wood, they require a more trained eye to identify. A simpler way to tell the difference between hard and soft wood is to look at the tracheidís surface pattern in cross section. Looking at the surface of some wooden artifacts the tracheid pattern can be noted with relative ease. The tracheids of softwood tend be formed in a linear pattern and are closer together. Hardwoods, on the other hand, have tracheidís that are more sporadic and spread out, not containing a systematic pattern. Depending on the pattern, or lack thereof, the wood can be identified as soft or hard. Once a pattern is identified it must be documented to help with further identification(Wheeler 1986). Plants that are softwoods are generally composed of coniferous plants such as pines, Douglas firs, and redwoods, while hardwoods are generally made of deciduous trees like the oak, poplar, and walnut (Cronyn1990). Knowing the general historic use of coniferous and deciduous plants, such as pine being used as masts and walnut as expensive furniture pieces, the identification of hard or soft wood is a vital clue. Further identification of the plant species requires a more in-depth analysis by a wood specialist. Computer databases, as described by the authors of Computer-Aided Wood Identification, are examples of the complex nature of wood identification and the importance of identifying as much information as possible (Wheeler 1986). Identification of the type of wood is necessary conservation efforts. This gives clues to the artifacts original use as well as considerations for storage and display for post excavation. Looking into the structure of the wood itself is a helpful and useful tool in learning its identity and function.

Outreach.png See the Microscopic Wood Anatomy of Central European Species website for further wood identification resources.

Environment

Water and Wood

Movement of water, the absorption and loss of moisture, all lead to the expansion and contraction, or movement of wood and other plant materials. Thus, it is most important for these materials to be stored and displayed at constant moisture levels, or non-fluctuating relative humidity.

Moisture content in wood is the weight of water in wood as a percentage of the overall weight of the oven-dried wood. Weight, shrinkage, strength and other properties depend on the moisture content of the wood. Softwoods (Gymnosperms) have more water in the sapwood than the heartwood. Hardwoods (Angiosperms) vary by species. When there is more water in the wood it is heavier, more flexible and bigger than when there is less water in the wood. Brittle wood, such as dry veneer, can be bent without breaking it if it is put in a steam box overnight. The wood swells and softens, making it easy to bend into contorted shapes. This is the principle used to bend Aleut hunting hats, make bentwood boxes, and construct bentwood rockers. Twigs, roots, grasses and leaves are either used green, or dried and then soaked overnight in water, before weaving them into baskets. Same principle. Brittle, aged newspaper, made of wood pulp (small wood fibers), softens in a humidity chamber (RH 90 percent) overnight to the point that it can be unfolded without breaking. In an Aleut gut parka, seams are sewn with grasses. In the damp and wet of the Aleutian ocean climate the grasses swell, closing the seams tightly shut so water can’t penetrate through the gut raingear.

Wood can hold free water (moisture or water vapor in the cell), and bound water (moisture chemically bound within the cell walls). Bound water is the water held between cellulose microfibrils. Green wood is freshly sawn wood in which cell walls are completely saturated with water and there is often extra water in the voids and cavities, the lumens, of the cells. The fiber saturation point is the point at which the cell walls are full of bound water but there is no free water in the cells. Wood will not have any dimensional changes above the fiber saturation point (FSP). Below the fiber saturation point, when bound water begins to be lost from the structure, the wood will begin to move, showing dimensional changes. The process of losing free water from cut wood is called seasoning. In most woods the maximum bound water, or fiber saturation point, is about 30 percent, plus or minus a few percentage points depending on the species. Above the FSP, the wood is still considered green. A freshly cut, or green, tree can have over 100 percent water in it, but the average is 75 percent water. To reiterate, as water leaves the wood, there is no physical change in size until the moisture content drops below 30 percent.

There are different ways of seasoning wood. The oldest method, air drying, means stacking boards so air passes around them. Most air-dried wood has about 14 percent water. It can take 12 months for air dried wood to reach equilibrium. To speed up the process, modern wood may be dried in a kiln or oven. Kiln dried wood has 7 percent moisture and oven dried wood has none. Both will swell and may move dimensionally (twist, kink, bend) when they are back in the air.

Equilibrium Moisture Content (EMC) is that point at which wood is neither losing nor gaining moisture. It is dependent on the type of wood, the relative humidity and the temperature.

Wood in our homes is exposed to long-term (seasonal) and short-term (daily) relative humidity (RH) fluctuations, so there are continual slight changes in moisture content. The short-term RH changes affect primarily the surface of the wood, unless the wood object is very thin (such as paper or veneer). The long-term RH changes are usually gradual, allowing the wood to move and adjust slowly over time. RH fluctuations that are not gradual, like shipping a piece from moist Florida to dry Arizona for an exhibit, can have dramatic effects on the wood. Cracking can occur immediately, or appear over a period of months. In some experiments, wood objects showed damage six months to a year after the abrupt change in environment. Waterlogged objects removed from the water may take years, or decades if it is large (like a ship), before it reaches equilibrium and stops moving and cracking.

The Canadian Conservation Institute has done research in the types of wood and how susceptible they are to different relative humidity fluctuations. Here is a synopsis of their results :

Wooden Artifacts and Humidity Fluctuations

Very High Vulnerability (coating over right angled grain joint or crack)

5% RH change, gradual fatigue fracture
10% RH change, fracture possible each cycle
20% RH change, fracture definite first cycle

High Vulnerability (veneer over right angled grain joint, lacquer over knot-free wood)

5% RH change, zero fatigue fracture
10% RH change, gradual fatigue fracture or plastic deformation
20% RH change, fracture possible each cycle
40% RH change, fracture definite first cycle

Medium Vulnerability (wood with little or no coating)

10% RH change, zero fatigue fracture
20% RH change, gradual fatigue fracture or plastic deformation
40% RH change, fracture possible each cycle

Low Vulnerability (loose wood panels, single component tool handles)

40% RH change, possible accumulation of fatigue fracture or plastic deformation if the freedom to move or the coatings or the slowness of the fluctuation are less than perfect.

Moisture content changes can be retarded, but not prevented, by protective coatings, such as varnish, lacquer, gilding, waxes and paints.

Waterlogged Wood

There are several approaches to treating wooden materials. Bulking and impregnation agents are both commonly used in the treatment of waterlogged wood. They are necessary to prevent the cellular collapse that would occur as the artifact dried, which would cause the wood to distort or disintegrate. Bulking treatments refer to methods that fill the cell walls but not the lumens of the wood cells. The intent is to prevent cell wall shrinkage. This can include treatments utilizing sugar, or sucrose, and alkylene oxides. In contrast to bulking agents, impregnation treatments are intended to provide support and prevent cellular collapse by filling the lumens of the wood cells. Treatments with acetone rosin, camphor alcohol, and silicone oil are impregnation treatments. Polyethylene Glycol (PEG), an alkylene oxide, can fall into both categories as either a bulking or impregnation agent. This is due to the fact that low molecular weights of PEG fill only the cell walls making it a bulking treatment, whereas higher molecular weights will fill the lumens and make it an impregnation treatment. A consolidant may be applied on extremely dry or brittle artifacts that may otherwise flake off (Jones and Eaton, 2006).

“With the active conservation of waterlogged archaeological wood, the major problem is not that we do not have adequate treatment methods available, it is in recognizing the serious limitations of these methods” (Jones and Eaton, 2006). While the ultimate goal of most wood conservation treatments may be to retain the original shape of the object as much as possible, many other factors also come into play when choosing the best treatment option. This can include details such as the type of wood, whether it is part of a composite material, what color of the wood is desired, or the durability needed to withstand future storage environments.

Decoration and Finishes

Wood has been decorated by carving, pressing, inlay, overlay (veneer), marquetry, joining and coatings. Colorants include dyes, paints and dry pigments applied with minimal binder (such as Inuit dance masks).

Some finishes add problems to a wood piece. For example, cellulose nitrate, an early synthetic varnish first used around 1920, yellows, shrinks and cracks with age. It can be identified by looking at a wood object under black (ultraviolet) light. This hard, brittle coating fluoresces white in black light. Compare this with shellac, a resinous secretion of the lac beetle dissolved in ethanol known historically as spirit varnish, the historical varnish used for high gloss finishes such as French Polish. Shellac fluoresces orange in black light. Shellac, a soft coating, is highly susceptible to water damage, best known for the white rings left by drinking glasses.

Materials that use parts of plants and trees include paper, books, baskets, Tapa cloth, bark paintings, birch bark canoes, lashings, rope, grass skirts, and textiles. All objects made from plants swell and shrink from changes in relative humidity and moisture. They also soften when humidity is high and become more brittle when humidity is low. Because of their affinity for water, they will wick water. Some materials, such as cotton, wick water well. Cotton dust covers should be 6 inches above the floor, so they don’t wick water up if the storage area floor becomes wet. Cotton thread shouldn’t be used for tags attached to objects with liquid exhudation, such as grease dishes or bird skins. The oils will wick up to the paper tag and dissolve inks.

Waterlogged material will lose lignin from the cells. The lignin is the stiffener within the cell walls. With its loss, the cells collapse or deform under pressure. If the wood is air-dried without adding a bulking agent, the cells will collapse when the water bulking them out is removed. This is why waterlogged wood will distort or disintegrate when dried.

Deterioration

Types of Damage

Plant materials burn, leaving a black ash echoing the shape of the material. A burnt log still looks like a log until you kick it. A burned plant doesn’t shrink or melt with heat.

Another common form of deterioration is damage from pests. If the moisture content in the wood is high, fungi, lichen or bacteria will grow on it and then attract insects. This is the natural breakdown process for wood. Signs of fungal deterioration are tessellated cracking (little squares) or staining. Signs of pest damage are small flight holes and piles of frass, which look like sawdust.

Other forms of deterioration may be less obvious. Poor handling leads to gouges, scratches, breaks and losses. Wood surfaces abrade easily. (Sanding uses this fact to make smooth surfaces.) Handles on cabinets that are opened daily, such as historical furniture in a living history museum, will quickly show wear patterns around them. Fluctuating relative humidity cause cracking, de-lamination, deformation, varnish damage and mold. Flyspecks eat into surfaces, leaving permanent little gouges. Light damage includes blanching or bleaching of the surface. Extreme light damage makes the wood look dry. Wood darkens as lignin deteriorates. Higher temperatures increase the deterioration rate. The lignin deterioration products are water soluble and dark – they result in the yellowing of paper and the streaking stains seen on totem poles. Because they are water soluble, they move when an object is wet. Dark tide lines on paper or wood artifacts that have been wet in one area are the result of lignin deterioration products moving and concentrating at the edge of the wet area where moisture evaporates. Acids and alkalis cause chemical deterioration of cellulose. Acids causing more damage than alkalis. Lignin deterioration products are acid, weakening the structure as they deteriorate.

Treatment

As with many materials, the best means of preserving plant materials are:

Non-fluctuating relative humidity (keep away from exterior walls)
Low, non-fluctuating temperature (keep away from heat sources such as lights, heat grilles or fireplaces)
Fire prevention and quick extinguishing techniques
Reduce moisture and water sources
Use water alarms in high risk areas
Monitor for pest infestations. Develop and use an integrated pest management system (IPM).
Freeze incoming materials following accepted procedures
Cover to reduce dust accumulation (boxes, cabinets or dust covers on shelves)
Good air circulation to dilute or activated charcoal filters to absorb acid fumes from deterioration
Eliminate light in storage. Lowest possible lights in exhibits.
Wear non-powdered Nitrile gloves to handle artifacts.
Use handling supports and trays to minimize handling.
Do not add oil, wax or other lubricant to artifacts. Most yellow or cross-link or attract dust.
Clean by flicking dust off with a soft brush, into a covered (with netting) vacuum cleaner nozzle.

References

Cronyn, J. M., and Wendy S. Robinson 1990.†The elements of archaeological conservation. London: Routledge.

Jones, Mark, and Rod Eaton. 2006. “Conservation of Ancient Timbers from the Sea.” In Conservation Science: Heritage Materials, edited by Eric May and Mark Jones, 266—308. Cambridge: Royal Society of Chemistry Publishing.

Wheeler, E.A., Pearson, R.G., LaPasha, C.A., Zack , T., Hatley, W. 1986 Computer-Aided Wood Identification. North Carolina Agricultural Research Service: North Carolina State University. Raleigh [1]

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