Conservation and restoration of herbarium

Pressed and dried specimen

The conservation and restoration of herbarium includes the preventative care, repair, and restoration of herbarium specimens. This is performed by registrars, curators, and conservators on herbarium collections in universities and museums. Herbarium specimens may be susceptible to water damage, mold, pests, unattached specimens, dust and dirt, and damage from improper storage conditions. Preventative conservation can prevent much of the damage that could occur.

Herbarium

A herbarium is a collection of preserved plant specimens and associated data used for scientific study.^[1] The specimens may be whole plants or plant parts. These will usually be dried and pressed and mounted on a sheet of paper but, depending upon the material, may also be stored in boxes or kept in alcohol or other preservative.^[2]

Types of specimens

Pressed and dried: Vascular plant (flowering plants, conifers, ferns) specimens are pressed and dried plants that are mounted on herbarium sheets. Various techniques are used to attach the plants with the most common method using archival adhesive and, if necessary, heavier portions of the plant are supported additionally by linen thread or narrow strips of gum-backed linen tape or polyester film (Mylar). If there are loose seeds or fruits, these are placed in a small fragment packet, which also is glued to the sheet. A label with collection information is glued on the bottom right corner.
Dried: Small bryophytes (mosses, hepatics or liverworts, and hornworts) are dried and placed loose in folded packets. The label is glued on the front of the packet and the packets are filed loose in boxes, glued to sheets of mounting paper, or placed loose in folders.^[3]

Causes of deterioration

Pests

Common herbarium pests include:

Silverfish

A reoccurring threat to the longevity of herbarium specimens is insects, a number of which find dried plants palatable. Historically, various methods have been used to kill insects, which either come in with the plants when they are collected or are in the building where the plants are stored.^[3]

Pests are commonly treated with two different methods:

Freezing: the specimen is placed inside a clear polyester bag with excess air pushed out and heat sealed, or place inside polythene bags and sealed with parcel tape. Then it is placed into a normal domestic freezer for at least 14 days at a temperature of −18°C, or for 72 hours if freezing at -30°C.
Anoxia: this method starves the pests of oxygen. Small anoxic environments are created using sealed barrier films and placing oxygen scavengers and RH buffers inside before sealing.^[4]

An integrated pest management program is cost-effective over time and the best preventative measure against pests.

Fungal attack

The primary risk factor for fungal attack is incomplete drying of specimens, caused either during the specimen preparation process or afterwards, or in collections that become wet later through flood, other water damage or improper storage conditions, especially in the tropics. Properly dried plant specimens will not suffer from fungal attack if stored in the correct conditions. During the drying process specimens are particularly at risk if they dry slowly. This happens through poor drying conditions or specimens being wet before being pressed or having water-retaining or succulent parts. Specimens with sugary exudations or large quantities of nectar are also particularly attractive to fungi and need special care during drying to ensure that they dry fast enough to prevent mold growth. If fungal growth occurs on specimens, it can be brushed with 95% ethanol or methylated spirits (denatured alcohol). However, this may alter the specimen for chemical and other investigative research and only kills the fungus present on the specimen, not preventing further problems of fungal growth.^[5]

Light

Herbarium specimens are sensitive to visible light and ultraviolet radiation, which can cause fading of biological pigments (fading or shifts in color) and/or damage to chemical bonds (weakened or embrittled).^[6]

Temperature and RH

High temperatures contribute to desiccation. The rate of any chemical reaction that damages specimens will double with each 10°C rise in temperature. Excessive moisture in the air (above 65% relative humidity) encourages pest infestation and/or the softening of some adhesives.^[6]

Preventative care

Environment

Specimens are stored in environmentally controlled storage rooms away from direct sunlight with limited exposure to other light sources to minimize causes of deterioration. Storage rooms are kept at a relative humidity (RH) level of between 45% and 55%. Temperature levels are kept between 18°C and 22°C. Store rooms are never completely immune to pests. Pest infestations are reduced by the regular vacuuming of storage areas and not allowing any food or drink in the storage rooms.^[4]

Specimens in front of metal herbarium cabinets.

Storage and handling

Specimens are stored in standard herbarium cabinets, as metal shelves can be easily cleaned and a well-sealed cabinet will provide a stable micro-climate for the specimens. Metal cabinets do not release volatile organic compounds, like wooden cabinets do.^[4] Plants are thoroughly dry and free of pests before placing in cabinets to prevent deterioration. Specimens mounted on sheets are kept flat and placed on stiff cardboard or in a box when moving them from cabinets to work spaces to minimize damage during examination. The weight from folders stacked too high can cause damage to lower specimens. Folders are kept specimen-side up and never turned over, so the specimens are never face down. During examinations, handlers review for potential maintenance (loose specimens, unattached specimens, etc.).^[2] Leaning on herbarium sheets, writing notes on top of them, or placing heavy items or elbows on them can cause serious damage.^[4]

Older herbaria specimens may have been treated with toxic chemicals including lead, mercury, and arsenic to prevent pests. Handlers wear nitrile gloves and work in a well ventilated room when working with specimens prepared earlier than 1989. The card on which the specimen is mounted and the specimen itself may contain the chemicals. The dust on herbaria sheets may be contaminated, so conservators wear a dust mask and use a dust extractor or fume cupboard when cleaning.^[4]

Repair and restoration

Cleaning

Dust and dirt are removed from herbarium sheets by using a smoke sponge. Conservators gently rub the place where the dirt is and then softly remove any excess with a fine brush.

Broken specimens

Broken specimens are reattached to the herbaria sheet using thinly cut strips of archival pre-gummed linen tape. Detached materials such as seeds, leaves, etc., are placed in an acid-free card fragment packet, which is secured onto the sheet with the original specimen.^[4]

Pressed and dried specimen with linen tape supports.

Removal from mounting sheet

Unattached specimens are removed from the herbaria sheet by humidifying it to make them pliable and removing the plant with a paper lifter (smooth wooden spatula). They are then placed to dry and flatten along with the original mounting sheet and written documentation (label and annotations). The specimen and original documents are then remounted to a new sheet using the original sheet as reference.^[3]

Disaster recovery

Unforeseen disasters can occur at any time and disaster recovery planning and materials for herbaria are similar to museums and libraries. Little can be done with burnt specimens and fragmented specimens except to preserve and protect what is left intact. The damage most likely to occur is water damage through natural or man made flooding, such as roof leakage or fire sprinkler malfunction. Damaged of waterlogged specimens are frozen to delay deterioration and prevent fungal attack, which gives curators time to evaluate the situation and decide how best to approach the problem.^[5]

References

↑ "What is a herbarium". Royal Botanic Garden Edinburgh. Retrieved 10 April 2017.
1 2 3 "Conserve O Gram - Preparing and Storing Herbarium Specimens" (PDF). National Park Service. November 2009. Retrieved 9 April 2017.
1 2 3 Horton, Diana. "Herbaria and Specimens: What Are They?". Retrieved 9 April 2017.
1 2 3 4 5 6 "Care and conservation of botanical specimens" (PDF). The Institute of Conservation (Icon) and Natural Sciences Collections Association (NatSCA). 2013. Retrieved 9 April 2017.
1 2 Bedford, David J. (1999). "Vascular plants" (PDF). Retrieved 9 April 2017.
1 2 "Appendix T: Curatorial Care of Biological Collections" (PDF). National Park Service. 2005. Retrieved 14 April 2017.

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