Herbarium Sheet Mounting: Adhesives Recommendations That Really Stick

In the summer of 2018 Jennifer Doubt, the museum’s botany curator, presented me with a challenge: find a better adhesive for mounting the museum’s botany specimens on herbarium sheets.

I was working in the museum’s Conservation lab as part of the Scientific Training Program, but my background is in fine arts, and conservation training with materials including metal, wood, textile, and paper conservation—not conserving ferns and flowers. Natural history conservation is specialized and rarely taught in conservation programs in Canada. So, I was excited for the opportunity to challenge myself in something new.

Having the best possible adhesive is critical because every year the botany team adds more than 5,000 specimens to the museum’s National Herbarium of Canada which includes more than 650,000 herbarium sheets in the vascular plant collection. At present, new specimens are attached to herbarium sheets using linen tape, a process that’s effective, but very time consuming. My task was to find a conservation glue that’s easy and quick to apply and keep specimens stuck for a long time.

Closeup image of a herbarium sheet with a spreading wood fern specimen that has a section missing.

Notice the missing section of this spreading wood fern (Dryopteris expansa). Over time the adhesive holding the specimen to the herbarium sheet aged and failed, resulting in the loss. Specimen collected in 1895. Catalogue number CAN 10005210. Image: © Canadian Museum of Nature.

Close-up image of a herbarium sheet with an alpine hedysarum specimen attached to the sheet with thin strips of white linen tape.

This alpine hedysarum (Hedysarum americanum) is attached to its herbarium sheet with linen tape. This technique allows the plant to bend with the sheet, decreasing the risk of breakage and loss. But applying linen tape is a time-consuming process, with each piece of tape carefully cut and placed by hand. Specimens collected in 2016. Catalogue number CAN 10042881. Image: © Canadian Museum of Nature.

My core challenge was that in the conservation literature there’s no single recommended adhesive for herbarium collections. So, in my search for the best adhesive I considered the museum’s needs while exploring new research in adhesives.

Three of the key herbarium conservation needs are strength, flexibility, and neutral pH. Herbarium specimens must be conserved for decades, if not centuries. So an adhesive must have enduring strength to hold a specimen on its sheet and also flexibility to keep it held while the sheet’s handled by researchers.

A neutral pH keeps the surrounding substrate from becoming acidic, turning yellow, and prematurely aging.

Close-up image of a herbarium sheet with a subalpine fir specimen that has some acidic, yellowed adhesive masking some of the specimen from view.

This closeup image of a subalpine fir (Abies lasiocarpa) herbarium sheet shows that the adhesive has browned with age masking some of the specimen from view. The browning also indicates that the adhesive’s become acidic, which could cause damage to the area over time. Specimen collected in 1975. Catalogue number CAN 10005767. Image: © Canadian Museum of Nature.

In addition to these archival requirements, the adhesive I was looking for also needed to have other practical properties. It needed to be water-based, high tack, and clear when dry.

The Canadian Conservation Institute’s long-term study of commercially available adhesives, Adhesive Compendium for Conservation was an invaluable resource.

My research led me to recommend three neutral pH polyvinyl acetate (PVA) adhesives or “white” glue.  These adhesives have both great workability properties in the lab and long-term preservation qualities. They have stood the test of time while remaining strong, flexible, neutral, and exhibiting minimal yellowing.

My adhesive research truly stuck with me. It enabled me to more deeply appreciate how in the selection of conservation materials every small detail must be considered to ensure the preservation of collections.

Understanding material characteristics and their deterioration was a core concept in my conservation training. I’m proud to know that what I contributed to the museum’s herbarium will increase the chance that in the 22nd century a museum botanist will hold a herbarium sheet prepared today—with its specimen still firmly attached.

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