Plant Materials

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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.

THIS ENTRY IS A DRAFT

Plant materials are ubiquitous to the human experience; as such, they are found in a variety of forms--raw to heavily processed, utilitarian to decorative--within public and private collections around the world. In the fabrication of artifacts, plant materials have been used as architectural elements, furniture, basketry, cordage or lashings, textiles, substrate for written language, a variety of personal accessories, vehicles (i.e. boats, carts, airplanes, etc.), housing materials, musical instruments, religious icons, tools, and nearly every other aspect of human life (fig 1).

Figure 1. Siapo mamanu (tapa cloth).Samoa. c. 1890s. Collection of the Museum of New Zealand. Courtesy of WikimediaCommons.

Materials and technology[edit | edit source]

Materials[edit | edit source]

Macroscopic level[edit | edit source]

Kingdom Plantae is one of the eukaryotic kingdoms and is comprised of some 300,000 species (Judd, et al. 2008). Within Plantae, there are three main botanical divisions, Bryophytes, Pteridophytes, and Spermatophytes. Materials used to construct artifacts come from plants in all three divisions, though the majority are derived from spermatophytes.

Bryophytes, or non-vascular plants (mosses, liverworts, and hornworts) have leaves and stems, but no true roots and limited vascular tissue.

Pteridophytes (ferns and horsetails) have leaves, stems, root-like rhizomes, and well-developed vascular tissue, but no true flowers or seeds. Spermatophytes, (seed-bearing plants) have stems, leaves, and roots. They can be divided into two distinct groups: gymnosperms (non-flowering) and angiosperms (flowering). Gymnosperms include most coniferous trees and shrubs; angiosperms can be further divided into mangoliids, monocots, and dicots. Further information about plant taxonomy and anatomy can be found in most introductory biology textbooks and is abundantly available online. However, phylogenetic research has greatly changed the understanding of taxonomy both across and within domains and kingdoms in recent years; updated sources should be referenced.

The morphology (physical form and external, macroscopic structure) of plants often dictates the way that they are used to create objects (Florian 1990). For example, stems have great longitudinal strength but readily split in the radial or tangential directions; this makes them well-suited as weavers for basketry items. Both reproductive structures (e.g. seeds, flowers) and somatic structures (roots, stems, leaves) are represented in artifact production; commonly encountered examples are listed below.

• Seed/Husk: cotton, kapok, coir, milkweed, corn husk, seed pods (legume), gourds

• Stems: flax, hemp, jute, ramie, sunn, kenaf, urena, nettle, rosella, bamboo, cornstalk, sugar cane, bagasse, esparto, cereal straws (wheat, barley, etc.), willow, oak, hazelnut

• Bark: cedar bark, paper mulberry tree inner bark, breadfruit tree inner bark, birch bark

• Leaf Fibers: sisal, manila hemp, agave, henequen, cantala, maguey, Mauritius hemp, caboga, pineapple, pita, bromeliad, banana, palm, New Zealand flax, yucca

• Roots: spruce root, willow root, cedar root

Molecular level[edit | edit source]

An important structural component of plant cell walls is cellulose, a polysaccharide built up from ß-D-glucose units. D-glucose (C6H12O6) is a saccharide containing five hydroxyl functional groups and an aldehyde group on carbon-1 (fig. 2). Cellulose is a straight chain polymer; hydroxyl (-OH) groups form hydrogen bonds with atoms on neighboring chains to connect them, forming microfibrils. Cellulose exhibits both crystalline and amorphous regions. Many properties of cellulose depend on its degree of polymerization, the number of glucose units that make up one polymer molecule.

Figure 2. Cellulose chain. Courtesy of Wikimedia Commons

Plant materials feature cellulose with varying degrees of polymerization. For example, cellulose from wood pulp has typical chain lengths between 300 and 1700 units, while cotton and other plant fibers have chain lengths ranging from 800 to 10,000 units (Klemm 2005). Plant-derived cellulose almost always occurs in a mixture with hemicelluloses, lignin, and pectin (Mills and White 1994; Whitmore 1994). Hemicellulose is a branched polysaccharide related to cellulose. It is derived from several sugars in addition to glucose (including xylose, mannose, galactose, rhamnose, and arabinose) and consists of shorter chains. Lignin, found in the secondary cell walls of plants, is a complex polymer comprising various phenyl propane units. It provides strength, filling spaces between cellulose, hemicellulose, and pectin components. Pectin consists of a complex set of polysaccharides that are present in primary cell walls; they are both linear and branched. The relative amount and chemical composition of pectin is variable.

The structures of plant materials are extremely varied and certainly affect the way that different materials used in artifact production as well as the way that they will age. Different methods of processing and materials added to modify their properties (through retting, dyeing, softening, etc.) will further affect how these materials will behave over time.

Technology[edit | edit source]

Plant materials are often selected for their designated uses based on their inherent properties of tensile strength, flexibility, shape, density, surface morphology, and other physical and chemical properties. The manner and extent of processing affects a materials’ working properties and long-term stability. Tools and techniques for artifact production and specimen preparation are discussed below.

Processing[edit | edit source]

The processing of plant materials is often an integral part of the fabrication of artifacts; thus, the processing employed is specific to the individual type of object (i.e. basket, textile, herbarium specimen, musical instrument, etc.).

General examples of processing include retting (soaking the plant in water to promote microbial digestion of pectin and some cellular structure), wetting (soaking the plant in heated water or an alkaline solution to further promote the extraction of waxes, oils, and pectin), fiber extraction (separating both mechanically and chemically the fibrous matter of the plant from the epidermal and parenchymal tissue), bleaching (producing a lighter color to the plant material via exposure to natural light or chemicals), drying or seasoning (reducing the moisture content of the cellular structure of the plant material in a controlled manner to achieve equilibrium with ambient relative humidity),and dyeing (exposing the plant matter to natural or synthetic colorants and/or mordants in acidic or basic conditions to impart a permanent or semi-permanent color change) (Florian 1990).

Tools[edit | edit source]

Figure 3. Using a scraper to remove the soft tissue on the interior of a bottle gourd. The gourd had already been cut in half as it was being prepared to be used as a sound box for a replica of an early 20th century lute from North Africa. Photo by Anne Kingery-Schwartz, February 2004.

The type of plant material used and the type of object being constructed determine the types of tools and the extent of their use in processing and fabrication. For example, the creation of a woven textile garment versus a musical instrument will likely employ different processing techniques and tools for fabrication. In general, plant materials are worked with knives of various types and sizes ranging from surgical scalpel blades to stone blades and everything in between to effectively remove or separate unwanted material or reshape. Chisels, drills, saws, rasps, punches, awls, scissors, needles, gauges, pliers, and scrapes can all be similarly employed to physically remove or reshape plant material to create holes, notches, carvings, or other features to augment the aesthetics or structure of the object (fig. 3).

Processes such as applying steam or boiling water to the material for shaping and molding the object can also be employed. Such processes exploit the hygroscopic nature of plant materials and often require blocking and controlled drying to achieve the desired shape without breakage.

Manufacture and Construction[edit | edit source]

The techniques for manufacture and construction of plant-based objects are as variable as the objects themselves. The general manufacture of basketry and textiles creates a woven structure with varying degrees of structural strength and flexibility. Weave patterns of the two reflect utility as fabrics, netting, storage containers, and aesthetic objects; knotting, linking, looping, wrapping, coiling, twining, and plaiting are allmanufacturing techniques related to the construction of either textiles or basketry. Basic descriptions are included below:

• Knotting: 2 active elements (nets)

• Looping: single active element creates a loop over itself or another passive element (bind edges, button holes)

• Wrapping: active element(s) wrap around passive ones, elements are often of differing size

• Coiling: fine active element secures a foundation element (wrapped coiling, looped coiling, linked coiling)

• Twining: a pair of active elements interlock between foundation elements (countered, regular, open)

• Weaving: many variations including plain weave, twill, etc.

• Plaiting: weaving at a 45 degree angle to the selvedge or rim.

Plant materials can also be worked into sheets through the lamination of thin sections of the processed plant tissue together and beating it into an intertwined mass; this is the concept for papyrus or barkcloth production.

Decoration of plant materials can be structural (e.g. twill weave) or applied. Feathers, tassels, fringes, quill work, beading, shells, stitches, dyes, and paint have been used to adorn the surfaces of artifacts.

Further descriptions of these and other techniques can be found on the OSG Basketry Wiki and in sources that are listed at the bottom of this page.

Processing for Scientific Specimens / Herbaria[edit | edit source]

Botanical specimens collected for scientific use can undergo a variety of processing techniques that enhance or stabilize their structures and properties for specific scientific utility (i.e. histology, DNA extraction, taxonomic study, etc.). The modern herbarium (a collection of botanical specimens) is often comprised of specimens prepared in a variety of different ways to provide flexibility in how the specimens can be used and studied.

The most common preparation technique involves the controlled drying of the specimen (by pressing and drying) and mounting to a standard-sized sheet of paper that is then stored in a folder within the storage cabinet. Herbarium sheet is typically 100% cotton rag paper that is either buffered or unbuffered (the latter is preferred for future use of the specimen for DNA extraction). Mounting techniques and materials for applying the dried specimens to the sheets differ widely and can include adhering the specimen with natural glues or synthetic adhesives (either covering the entirety of the specimen verso or only in spots), sewing (with natural-fiber thread), or strapping (with gummed fabric tape). Oversized specimens that are bulky or more three-dimensional can be dried and stored in individual boxes or trays within storage cabinetry.

Some botanical specimens are not dried but rather preserved in alcohol to allow for study of their structures without the dimensional changes and cellular deformation that can occur with drying. Fluid preservation is a vast topic in the realm of organic materials and varies widely in its application. For botanical specimens, fluid preservation generally includes historically a mixture of denatured alcohol, water, and glycerin (For a thorough explanation, SEE: Simmons 2014). Fluid preservation often begins with the fixation of the plant in 10% formalin (most commonly 4% formaldehyde gas in water) in water (Simmons 2014). Once fixed, the specimen is often staged into a 70% ethanol solution for long-term storage in a glass jar. At moderate temperatures, fixation of the tissue is held in equilibrium, which also requires a moderate pH. Fixation can be reversed if temperatures fall or the pH shifts below 5 or above 7; thus, long-term storage requires the use of a buffer, such as dibasic sodium phosphate to maintain a stable pH around 7 (Simmons 2014).

Additional specialized preparations of botanical specimens include thin-sectioning and mounted slides and frozen or cryogenically preserved tissue samples (SEE: Metsger and Byers 1999).

Identification[edit | edit source]

Identification of plant fibers can provide an increased understanding of deterioration processes and inherent instabilities of an artifact. It may also aid in dating and determination of provenance and provide insight into artist’s methods and materials. It should also be noted that reporting a plant identification in conservation documentation or for publication, the scientific name of the plant should be included whenever possible to avoid confusion with common names. Common names are not consistent between geographic regions, languages, or time periods and are less reliable for creating true documentation of a plant.

Figure 4. Photomicrograph of a sweet grass cross section captured at 10x under cross polarized light. Photo andannotations by Lesley Day. UCLA/Getty Program in Archaeological and Ethnographic Conservation. 2014.

Transmitted light microscopy is the primary analytical method employed in fiber identification. It involves removing small samples from the artifact and viewing them at high magnification. Morphological features such as cellular structure, tissue organization, birefringence, and fiber shape, and size -coupled with contextual knowledge of the larger artifact- can assist with identification (fig. 4). Auxiliary techniques used in fiber identification include drying/twist tests for bast fibers, micro-chemical spot tests, and analytical techniques such as scanning electron microscopy. Common stains for plant materials include Safranin 0, Graff "C" Stain, and Hertzberg stain.

It is important to note that precise identification of some fibers is not always possible; often they can only be characterized as belonging to larger groups (e.g. stem from a monocot plant). The differences between the many species are not always sufficiently distinctive or well documented in existing literature (Goodway 1987).

The organization of common morphological features listed below has been adapted from the Fiber Identification section of the Paper Conservation Wiki, originally authored by M.L. Florian (SEE: Florian 1990). It is supplemented with information from other sources that are noted.

Woody fibers

• Gymnosperm (non flowering) conifers most important group, they are generally evergreen and called “softwood” by tradespeople [examples: pine, spruce, hemlock]; cell types: longitudinal tracheids (fibers), parenchyma; diagnostic features: bordered pits and ray cross field pitting on tracheids can be used for species identification

• Angiosperm (flowering) both monocotyledon and dicotyledon trees can be evergreen or deciduous, called “hardwood” by tradespeople [examples: oak, maple, poplar, red and black gum]cell types: tracheids, vessels, parenchyma, vasi-centric tracheids; diagnostic features: vessel size, shape, simple and scalariform perforation plates, spiral thickening and pitting patterns on vessels

Fruit fibers

• Cotton, coir, kapok, gourds; cell types: fibers; diagnostic features: cotton characteristically shows ribbon-like twists and birefringence. Cotton hairs are single cells that originate from the fruit or boll of the cotton flower. Other seed hairs such as kapok and cattail characteristically show few to no features, and lack of features can help in their identification (Florian 1990). The type and frequency of nodes and cross markings on fibers may also prove diagnostic.

Leaf fibers

• Abaca or manila hemp, agave, yucca, banana, sisal, pineapple, New Zealand flax; cell types: fibers, sometimes vessel segments, epidermal and parenchyma cells; diagnostic features: lumen and fiber width, fiber contour, sometimes fine crossmarkings on fiber, sometimes lumen contains granular material

Stem Fibers

• Cereal straws (wheat, rice, oat, rye, barley) sugar cane bagasse, cornstalk, bamboo, rattan, papyrus, esparto, rush, willow, grapevine, cactus; cell types: fibers, vascular bundles, sclerenchyma bundles, serrated epidermal cells, parenchyma, vessels, stomate (trichomes in esparto); diagnostic features: dimension of fibers, vessels and associated cells; pitting on vessels, annular thickening on vessels; shape of trichomesmacroscopic features: great longitudinal strength but split easily in radial and tangential directions, sometimes will see an epidermis (often water repellent) (Pouliot 2011)

Bast fibers (also stem fibers)

• Collected from the phloem or bast (inner bark) of stems

• Trees and shrubs: kozo, mitsumata, gampi

• Herbaceous dicotyledons: flax, hemp, ramie, sunn, jute, kenaf; cell types: fibers; can include parenchyma or shive at times; diagnostic features: Salient features used to identify bast fibers are dislocations (or kinks), long length, tapered ends and narrow lumens. The fibers usually occur in clusters.

Resources for Fiber Identification include the Fiber Reference Image Library, Natural Fibers and the Oriental Papermaking Fibers Library.

Deterioration[edit | edit source]

Plant materials are susceptible to chemical, physical, biological, and mechanical deterioration. Chemical degradation reactions are inherent to the material or are introduced through processing or the environment. Physical degradation encompasses stresses induced by light, relative humidity, and heat; chemical and physical degradation processes are closely linked. Biological deterioration includes damages caused by microorganisms and insects. Mechanical deterioration (breaks, abrasion, etc.) is the result of the interaction of loads and forces on a macroscopic scale and can be exacerbated by chemical, physical, or biological deterioration.

The rate and types of deterioration observed on plant material may be influenced by species type, processing, usage, previous storage and display conditions, quality of original fabrication, pesticide application, and previous restoration or conservation treatment, among other variables.

Physical Deterioration[edit | edit source]

Physical deterioration processes are induced by light (photo-oxidation), cycles of high and low humidity (mechano-sorptive creep), and heat (thermal degradation).

Photo-oxidation

Plant materials absorb electromagnetic radiation in the visible and ultraviolet range (300-700 nm), promoting photo-oxidative reactions that weaken fibers by breaking long-chain molecules.

Both cellulose (and its derivatives) and lignin can be oxidized. In cellulose, primary hydroxyl groups are oxidized to aldehydes, ketones and carboxylic acids. This can lead to discoloration and further promotes or initiates hydrolysis reactions. Lignin, however, is primarily responsible for photo-oxidative darkening. Lignin contains several chromophores with conjugated aromatic rings and carbonyl groups that absorb in the near UV spectrum (300-400 nm). When these chromophores absorb light they can decompose into yellow-colored ketones and quinones, turning the plant fiber yellow. Ketones and quinones in turn absorb visible light and react further, exacerbating the degradation process (Royal Society of Chemistry 2013).

Painted and/or dyed plant materials may be particularly susceptible to light damage, as light may cause discoloration or fading. Their presence may also catalyze other degradation processes of the plant fibers.

Thermal degradation

Heat accelerates the rate of chemical reactions and thus the degradation of polymers. Plant materials have relatively low thermal conductivity; they insulate. Therefore applied heat is not readily dispersed and is locally damaging, rapidly increasing the rate of degradation.

Mechano-sorptive creep

Mechano-sorptive creep occurs as a consequence of changes in the moisture content of the material while it is subjected to applied forces. The sorption characteristics of the amorphous and crystalline portions of the cellulosic structure are widely variable, as are those of other hydrophilic (hemicellulose) and hydrophobic materials within the matrix (Nordstrom 1994). With repeated moisture cycling, the primary and secondary structures of plant materials are gradually broken down and compacted. This process results in increased crystallinity at the microscopic level as microfibrils deform and re-align, leading to decreased flexibility and eventually to permanent deformation at the macroscopic level.

Chemical Deterioration[edit | edit source]

Chemical degradation reactions involve compounds and molecules that are inherent to the plant material, have evolved over time, or have been introduced from an external source (previous treatment, gaseous pollutants, etc.).

Acid Hydrolysis
Hydrolysis is perhaps the most notable chemical reaction that contributes to the degradation of cellulose and hemicellulose. It is certainly linked to physical deterioration processes, which cause repeated swelling and shrinking of cellulosic materials, photo-induced free radicals, and thermal degradation products. Acidic degradation products (including acetic acid, aldehydes, and carboxylic acids) are often directly linked to the latter processes or to environmental contamination (sulfur oxides, nitrogen oxides, etc.). These, coupled with a small amount of moisture, may lead to acid hydrolysis degradation of plant materials. In the acid hydrolysis of cellulose, a protic acid catalyzes the cleavage of an intermolecular bond via a nucleophilic substitution. Bond scission of the cellulose chain at glycosidic bonds results in smaller molecules and a lower degree of polymerization.

The rate of acid hydrolysis is increased by both heat and moisture. On a macroscopic level, acid hydrolysis leads to darkening and embrittlement of plant materials.

Previous Treatment
In the past, plant materials were sometimes treated with applied oils, glycerol, or wax coatings that were thought to “feed” the dry material, reviving shine and polish. These coatings may age unevenly, crosslinking to cause darkening and embrittlement of the surface. They are often difficult to remove.

Old adhesive repairs or consolidants may also prove detrimental to the artifact. For example, soluble nylon (N-methoxymethyl nylon) was used as a coating/consolidant for a period of approximately 25 years (1958-1980s) until its aging properties proved detrimental (Sease 1981).

Applied organic and inorganic pesticides are discussed in the Preventive Conservation: Handling section of this Wiki.

Biological Deterioration[edit | edit source]

Biological sources of deterioration for plant materials include insects, small mammals(rodents), fungi, and bacteria.

Figure 5. Damage caused by afurniture beetle. Courtesy of CSIRO Science, Wikimedia Commons.

Commonly encountered insect pests affecting cellulosic materials include:

• drug-store or ‘Herbarium’ beetle (Stegobium paniceum)

• cigarette or tobacco beetle (Lasioderma serricorne)

• booklice (Lyposcelis species)

• common furniture beetle (Anobium punctatum) (fig.5)

• powder post beetle (Lyctidae family)

• silverfish or fishmoths (Lepisma saccharina)

• cockroaches

• minute brown scavenger beetle (Cartodere filum)

• firebrats (Thermobia domestica)

Note that the above list is not comprehensive, as other insects can be damaging to plant fibers, particularly when they are associated with other materials.

Plant materials are also susceptible to damage by small mammals (primarily mice) that use them as nesting material. Damage from rodents often takes the form of chewing and gnawing marks with some loss of material. Staining related to rodent excrement and habitat nesting is also common.

Cellulose is prone to enzymatic deterioration caused by bacteria and fungi in high-humidity environments (Rivers 2003). These can be controlled by ensuring that relative humidity is below 60% (Kronkright 1990).

Further information regarding pest identification and management is included in the Preventive Conservation section below.

Mechanical Deterioration[edit | edit source]

Mechanical deterioration is a broad category that is often intimately linked to the other categories of deterioration; plant materials already compromised chemically and physically are more susceptible to mechanical damage. It can manifest as fractures in the structure, holes, tears, abrasion, folds/creases, deformation, soiling, and staining, among others. Improper handling or transport of plant materials with chemical and physical instability can easily promote mechanical deterioration. These damages often disrupt the overall design or change the way that the structure absorbs and distributes stress. Some mechanical damages serve as evidence of use of the artifact, while others are the result of physical forces, neglect, or improper care; at times this nuance can be difficult to discern (Florian 1990).

• Shape, Form: Macro- or micro-fractures in plant materials may cause sagging or deformation of the overall structure of the artifact. In turn,this may lead to the development of tears or holes. Similarly, stress caused byfolding may lead to creases and eventually tears.

• Surface: Plant materials are inherently porous. They may also have a naturally waxy/tacky surface (dependent upon plant type and parts used in construction). Therefore, they easily harbor dust, soot, grime, and other accretions. In addition to affecting the color and sheen of the artifact,particulate matter is hygroscopic, abrasive, and attractive to insects.Staining and tidelines may occur upon exposure to moisture.

The conservator’s challenge is to assess and determine the impact of mechanical deterioration on artifact stability. This topic is thoroughly discussed by Kronkright (1990). He notes that if damages interrupt the structural integrity, deterioration will progress until a new equilibrium is established.

Conservation and care[edit | edit source]

This information is intended to be used by conservators, museum professionals, and members of the public for educational purposes only. It is not designed to substitute for the consultation of a trained conservator.

To find a conservator, please visit AIC's Find a Conservator page.

To learn how you can care for your personal heritage, please visit AIC's Resource Center.

Preventive conservation[edit | edit source]

Handling[edit | edit source]

Figure 6. Example of a herbarium sheet with a specimen collected during the US Exploring Expedition(1838-42) that was treated with mercuric chloride as an anti-fungal and insecticide. This and other similarly treated sheets exhibit grey staining after exposure to light associated with “[...]mercuric sulfide (metacinnabar, probably from reaction with sulfur in the mounting paper) and a mercury sulfide/mercury oxide (2HgO⋅HgS, possibly from reactions with paper and air) as well as [metallic mercury and] unreacted mercuric chloride” (Hawks et. al 2004, 783). Less extreme staining can be misidentified as mold growth.

Collections care professionals should use caution when handling plant-based materials; many materials pose a significant health risk through inherent or acquired sources of toxic compounds. For example, it was common practice throughout the 19th and into the 20th century for natural history and anthropology collections to be treated with organic and heavy metal pesticides, antifungal agents, and tissue fixatives ([[#ref13|Pool 2005). The role of such compounds used by farmers, native peoples, and craftsmen prior to the object’s museum life should also not be overlooked (fig. 6) (Hawks 2001). These chemicals are often highly persistent and some are toxic to humans. To protect both the artifact and the handler, proper personal protective equipment should be utilized when handling plant materials that may have been treated with pesticides or stored in closed cabinets with materials that may have been treated (Hawks et al. 2004). Chemical spot testing and x-ray fluorescence spectroscopy are two methods that have been used to confirm the presence of heavy metals (Odegaard,et al 2000; Shugar 2012).

Non-powdered, nitrile gloves should be worn when handling plant materials. White cotton gloves are not as effective as barriers to potential toxic substances and also carry the potential to snag on components of the artifact. Handling trays and supports should be utilized whenever possible.

As with all objects, care should be taken to lift and/or manipulate the object by its most stable point.

Environmental recommendations for storage and display[edit | edit source]

Cellulosic materials are dimensionally reactive to moisture. Relative humidity above 70% encourages mold growth, which can cause staining and weakening of the structure. Low RH leads to stiffness, delamination, and distortion of the structure. Continual fluctuations in RH cause permanent damage to the cellular structure of plant materials, eventually leading to permanent deformation.Therefore, a stable relative humidity in the range of 40-60% is recommended (British Standards Institution 2012). As with other materials, deterioration of plant materials progresses at a faster rate at higher temperatures. Further information and resources can be found on the Environmental Guidelines page of the Preventive Care Wiki.

Recommended light levels for plant materials are at or below 50 lux and 75 uw/lm ultraviolet content (Michalski 1997). As outlined in the Photo-oxidation section of this page, damage from light exposure is cumulative and irreversible, including fading, yellowing, darkening, and loss of strength. If possible, plant materials should be stored in the dark.

To reduce dust accumulation, plant materials should be protected by dust covers or stored in closed cabinets. Dust covers may serve a dual purpose to block light.

Integrated Pest Management[edit | edit source]

A regular and diligent integrated pest management system is essential to the care of plant materials collections, as cellulosic materials are susceptible to damage from insects and other pests.

Resources for pest identification and management are widely available, including What's Eating Your Collection?, BugGuide.net, and Museumpests.net.

Supports and Mounts[edit | edit source]

Fragile objects or unusually-shaped materials should be supported with soft interior and/or exterior supports. --passive supports. There are many types of support, including ring-shaped supports, pillows, dome-shaped supports, and collars.

There are a wide variety of materials that can be used to create supports, including Ethafoam® polyethylene foam, Melinex® polyester film, Coroplast®, fluted polypropylene sheet, Tyvek®, polyester batting, unbleached cotton fabric, acid-free papers, and acid-free cardboard.

It is important that materials in contact with the artifact are acid-free and non-abrasive to avoid contributing to further deterioration.

Labeling[edit | edit source]

The goal of labeling museum artifacts is to provide a permanent number/tracking system without compromising the safety or integrity of the artifact. There are several ways to label plant-based artifacts; techniques should be selected on an object-specific basis.

If selecting a tag method for plant-based materials, the tags should be attached using soft cotton or polyester thread. Nylon monofilament or similar material should not be used for this purpose, as it can easily cut, scratch, or abrade plant fiber. The tag material itself should also be considered carefully, and a modern fabric such as Tyvek that offers a smooth, durable, soft material may be preferred to paper-based or stiff plastic tags. Metal tags should be avoided entirely, as corrosion products can easily stain plant materials. Historic labels made of non-archival materials that must be retained for research or scientific use can be encapsulated in polyester sleeves. To decrease the risk of future breaks, tags should be attached to the area of greatest structural integrity.

Depending on use and type of object, a direct-applied painted or written label can be appropriate. An isolating layer between the plant material and the labeling medium should be applied and selected to have differing solubility parameters than the writing medium and object. The label should be painted onto a carefully selected surface that is unlikely to obscure important features or aesthetics. Typical isolating layers often include conservation-grade synthetics such as Paraloid B-72, B-67, and other acrylic copolymers. A sufficient amount of time should be allowed between applying the base layer and writing the label. Labeling media should be pigment-based (rather than dye-based) to avoid bleeding and fading. Such media often include conservation-grade acrylic paints and pigment-based inks.

For specialized objects, a sewn-on fabric label can be appropriate.

Interventive treatments[edit | edit source]

Surface Cleaning[edit | edit source]

The nature of woven structures coupled with the porosity of some plant materials make them inherently susceptible to accumulated surface grime. Grime is abrasive, hygroscopic, and attractive to pests. In addition, it detracts from the luster of the surface and may be visually distracting. Mechanical methods of surface cleaning are preferred for plant materials, though localized solvent or aqueous cleaning methods can also prove successful. It is important to note that mechanical surface cleaning should be undertaken prior to solvent testing or humidification to avoid further penetration of the grime into the structure.

Mechanical Methods

Surface cleaning often begins with a HEPA-filtered, rheostat-controlled vacuum and soft brush. Vacuuming can effectively remove loose particulate matter, mold bodies, and insect materials from the surface and interstices. Hand-held air blowers and compressed air may also be used to remove loose surface grime (fig. 7)

Figure 7. Surface cleaning a basketwith a rheostat-controlled HEPA vacuum and soft brush. Cheesecloth is affixed to the end of the vacuum nozzle and the brush ferrule is wrapped with blue tapeto avoid scratching the object (Florian 1990). Note that the vacuum hose is angled behind the conservator to avoid interfering with the object. Copyright: National Museum of Natural History, Smithsonian Institution.

Other commonly used materials for mechanical cleaning include:

• Polyurethane cosmetic sponges

• Slightly moist polyvinyl alcohol sponges

• Vinyl erasers (blocks or crumbs)

• Polyurethane swabs

• Soot sponges (cis-1, 4-polyisoprene rubber heavily filled with calcium carbonate)

• Groomstick molecular trap (vulcanized cis-1,4-polyisoprene with titanium dioxide filler)

There is some recent evidence to suggest that Groomstick may leave a residue on the surface of artifacts, but the nature and impact over time of these residues is not understood (van Keulen 2012).

Further information about these and other surface cleaning materials can be found on the AIC Book and Paper Group Surface Cleaning page or in the Conservation & Art Materials Encyclopedia Online Database (CAMEO).

Laser ablation as a cleaning method for plant materials is an area of ongoing research (Elliot 2007; Szczepanowska 1994).

Solvent and Aqueous Methods
When choosing to utilize a solvent or aqueous cleaning technique, it is important to consider the surface and porosity of the artifact.

Waxy cuticles/epidermal layers, gums, resins, paints, oils, and waxes may be affected by the application of a solvent. In addition, ‘wet’ treatments may cause grime to migrate, forming tidelines. Soluble cell material from the deterioration of the plant structure can also be affected and extracted through contact with solvents. Additionally, all plant materials experience some degree of swelling when exposed to water or other polar solvents. For these reasons, solvent and aqueous treatment methods must be chosen carefully.

There are numerous techniques for applying solvents or aqueous solutions to the surface to control its degree of penetration into the plant structure and the overall exposure time. General techniques and materials can include the manipulation of solvent rheology (i.e. gels and poultices); dampened, hand-rolled swabs; and brushing with a blotting paper behind the plant element. In some cases, softening of an accretion with solvent followed by mechanical removal can be an effective compromise. Some success has also been achieved through experimental cleaning with liquid and supercritical carbon dioxide (Tello and Unger 2010). As with other conservation treatments, solubility testing should be carried out on each component of the object (including applied media) in discrete areas prior to treatment.

In the case of removing mold from plant materials, a hybrid response of mechanical and wet cleaning techniques may be appropriate and required. A HEPA vacuum should be used to remove the loose mold debris, in a similar manner to those outlined above. Additional removal of accretions may require the use of soft brush or gentle scalpel removal. Areas with potentially active mold growth can be treated with a lightly dampened swab of 70% ethanol (Szczepanowska 2014).

Consolidation[edit | edit source]

Plant materials are susceptible to cracking, flaking, delamination, and a host of other surface instabilities related to deterioration of the material. Consolidation of the object by impregnating the affected areas with a binder, although a permanent treatment, may be the most viable method for preventing loss either during additional treatment steps or subsequent intended use. As such, localized or superficial consolidation (for instance with a nebulizer) is often preferred to limit the application of an adhesive to only the affected area(s).

In general, the selection of consolidants is based on the long-term stability of the adhesive material and compatibility with the object. The introduction of consolidants to a plant substrate can cause physical and aesthetic changes to the object, including darkening, deformation, matting or glossing, and increased flexibility or rigidity. Thus, great care should be used when selecting the consolidant, viscosity, and application methodology to manipulate the desired effect. Preferred consolidants include those used for structural repairs, such as cellulose ethers, Aquazol, and acrylics (See Stabilization and Aesthetic Reintegration below), often at lower concentrations, or even waxes like polyethylene glycol. To achieve effective consolidation of affected areas, typical treatment methodologies can include direct brush application, Mist Consolidation (aerosolizing the selected consolidant), or even wholesale immersion of the object for overall consolidation. The latter can be logistically impractical for many plant-based material, and alternative stabilization treatments such as encapsulating or lining should also be considered.

Humidification and Reshaping[edit | edit source]

Plant materials become deformed or distorted if they are physically damaged and/or have been stored incorrectly. Water acts as a plasticizer for cellulose; as RH is increased to 60-80%, cellulose becomes quite flexible due to increased moisture content (Florian 1990). Humidification is used to temporarily increase the flexibility of the fibers, allowing reshaping to occur.

It is important to note that many variations of methodology exist and that each object should be individually considered prior to designing a treatment plan; severely degraded plant materials may not be strong enough for safe reshaping with humidification. Humidification methods for plant materials are most easily categorized as the following:

Figure 8. Basic humidity chamber setup for a small object. A folded paper towel acts as a buffer, and moisture is introduced via a glass jar filled partially with a water: ethanol mixture and cotton wool. A humidity indicator strip placed within the closed polyethylene bag measures the relative humidity of the space. Image courtesy L. Kaplan, 2015.

• Overall humidification in a humidity chamber

• Local Humidification through a semi-permeable membrane such as Gore-Tex

• Local Humidification using a micro-chamber

Direct application of moisture to the surface is generally not recommended, as it has the potential to cause staining and tidelines. However, the Solvent/Aqueous Methods section above outlines these methods and situations in which they may be appropriate. Humidification chambers range in size and complexity and can be created using a variety of materials (polyethylene, polypropylene, etc.) (fig.8). Moisture is introduced to the chamber by evaporation of the solvent from containers of liquid, ultrasonic humidifiers, or blotters saturated with the liquid. Ethanol or denatured alcohol is often added to the water; it acts as a mold inhibitor and encourages more uniform wetting of the plant material by lowering the surface tension.

Local humidification may be practical for large or composite objects. A spacer, such as spun polyester or Gore-Tex, should be placed between the artifact and the moisture source to prevent direct contact with the surface. Then, a moisture carrier (linen, cotton, or acid-free blotter paper) is dampened and placed behind the spacer. It is covered with plastic sheeting and gently fixed in place or weighted.

Local humidification via a micro-chamber is achieved by dampening a piece of fabric, natural sponge, or blotter paper and placing it in a shallow beaker or container so that it fits snugly and will not move or drip onto the artifact. The container is then placed above or beneath the area to be humidified, and plastic sheeting aids in controlling the movement of the moisture. In all cases, the humidity and flexibility of the plant materials are closely monitored. In an overall chamber, this is done using a hygrometer that is placed inside the chamber. For local humidification, flexibility may be monitored manually.

Once the object is conditioned, it can be gently manipulated into the correct alignment and ‘blocked,’ or held into place with active pressure. Blocking may be accomplished using a variety of materials, including twill tape, inert polyethylene foam, polyester batting, gentle clamps, blotters, glass plates, rare Earth magnets, weights, etc. The appropriate blocking system is entirely dependent upon the shape of the artifact. Care should be taken not to place too much weight upon the object and not to force a stubborn area into alignment. Difficult areas may be dealt with in stages, recognizing that they may not be entirely returned to their original orientation. The blocked object can be placed back into the humidity chamber (moisture source removed) and allowed to equilibrate slowly and evenly. Even equilibration is especially important for woody plant materials, as uneven drying can cause cracking and further distortion (Rivers and Umney 2003).

Alternatively, passive methods may be used to gradually re-shape objects over time. Storage supports are shaped to encourage gradual movement of the artifact; supports are periodically adjusted.

Repair, Stabilization, and Aesthetic Reintegration[edit | edit source]

A variety of materials have been successfully utilized to mend and/or reinforce breaks, folds, losses, and splits in plant materials. Some repairs are purely mechanical, while others require the use of an adhesive. In general, repair materials are sympathetic to the strength, sheen, and porosity of the cellulosic materials (Stone 1998). They can be toned to match the surrounding area. Some examples, including citations of successful use or additional information, are listed below:

Adhesives:

• Arrowroot/sodium alginate paste (Uden 2014)

• Methyl cellulose/arrowroot/sodium alginate (Brooks 2011)

• Wheat starch paste/methyl cellulose (Carrlee 2007)

• Wheat starch paste (Kite 1995)

• Hydroxypropylcellulose (Feller 1990)

• Methyl cellulose/PVA emulsion

• Methyl cellulose (Yamuachi 2009)

• Polyvinyl butyral (Butvar B-76 or Butvar B-98) (Porter 2010)

• Acrylic dispersions (Lascaux 360 HV, 303 HV, or 498HV) (Eagleston 2007)

• Berger’s ethylene vinyl acetate(BEVA)

• Poly(2-ethyl-2-oxazoline)(Aquazol)

Backing or repair materials:

• Nylon gossamer

• Tyvek (Agua Caliente 2013)

• Stabiltex

• Mulberry tissue papers (Wills 2002)

• Acid-free matboard or corrugated board

• Cotton, polyester, or linen threads

• Spun polyester (Reemay or Hollytex)

• Other rigid materials for structural repairs (e.g. balsa wood, bamboo skewers) (Griggs-Hakim 2003)

When choosing mending materials and techniques, it is important to consider the strength and sheen of the original material, the intended use of the artifact, and the removability of the chosen techniques. Commonly utilized repairs include sewn or tied repairs, splints, self-mends, patches, backing strips, ‘frankenstein’ mends, and linings.

Splints are often made of a rigid material and can be inserted into the structure to support, join, or replace elements (e.g. the missing passive element of a coiled basket). The ends of the splint are often tapered or cut/carved to conform to the structure.

Patches and backing strips involve the use of a secondary material, often mulberry paper or spun polyester. The strip or patch is adhered across the break, crease, or loss to support the area. The repair material should be sized such that it covers or supports the break, extending onto adjacent undamaged portions of the structure. Backing strips are often entirely structural, while patches may be aesthetic or structural.

‘Frankenstein’ mending is a technique often used with basketry or other woven structures. Broken or torn areas of the weave are bridged with small twisted strands of mulberry tissue paper. The strands are applied perpendicular to the break or tear.

Lining, securing a support fabric or paper to the reverse face of an artifact material, may be necessary for badly degraded artifacts that cannot be safely handled or displayed.

For further guidance on the interventive conservation treatment of plant materials, especially wooden artifacts, please see more specialized Wiki entries (i.e. Wooden Artifacts, Wood, Basketry, Paper, and Textiles)

References[edit | edit source]

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