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Humidity-controlled and warm-air pest eradication technology with a focus on museum collections and works of art

| May 8, 2018

Painting, 18th century, oil on wood, attacked by Anobium punctatum, front, back and detail with larvae.

There is hardly a more interesting, yet more challenging field of pest control, than in museums and art collections, when it comes to the actual artefacts. Consider the different composite materials found in museums; from paintings and frames, books and documents, gilded and painted wooden objects, tapestries, historic costumes, antique furniture, natural history collections – taxidermy or herbaria – all are prone to insect attack. Museum pests (Table 1) will eat wood, paper, leather, silk and wool, feathers and plant material. If permitted to continue undetected, they can turn almost any organic material into powder.

Elevated temperature kills insects

Most insects die when they are exposed to temperatures of around 50°C, provided the temperature is maintained for a certain period of time. The insect’s temperature sensitivity has been common knowledge for hundreds of years and companies have treated insect infestations in building structures using hot air for decades. Different insects die at different temperatures. Moths and silverfish die at temperatures under 50°C, while some beetles (for example some Cerambycidae and Lyctidae) need higher temperatures or prolonged exposure in order to kill the egg, larvae and adult.

Computer controlled humidity is essential

In any environment, if the air is warmed up, the relative humidity will drop. If the relative humidity drops, any hygroscopic material will shrink. The same will happen the other way around: when the temperature drops the relative humidity automatically rises, any hygroscopic material in that environment will swell. Shrinkage and swelling of an organic substrate, covered in paint for instance, results in movement and will consequently cause damage to an artwork.

A typical example of this phenomenon is the craquelure in paintings. The canvas underneath the paint has shrunken and swelled for decades, or even centuries, leading to countless micro cracks in the paint layer. In the long run, the formation of cracks can cause loss of original substances which is every conservator’s nightmare. In other words, an application of immediate, non-humidity controlled, heat as a method to eradicate heritage eating insects, is not suitable for sensitive museum objects.

The Thermo Lignum process differs by a number of parameters – room temperature, core temperature, Delta T (temperature differential between core and room temperature), which are constantly monitored and controlled by computer (Figure 1).

Figure 1

Figure 1: Thermo Lignum treatment cycle

While all of these parameters on their own are equally important, it is the humidity control which is the key feature. The method is unique and allows treatment of even very sensitive Boulle inlay or polychromatic objects.

The relative humidity inside the chamber is controlled, based on the Keylwerth Diagram (Figure 2). This ensures that during all phases of the treatment – warming/holding/cooling – there is no exchange of humidity between the object and the surrounding air. The EMC (equilibrium moisture content) is maintained. There is no movement in swelling or contracting and hence no damage.

Demanding customers

It is a feature of the industry, that museums and art collectors are extremely discerning and aware of material as well as historical value. Thermo Lignum’s regular clients are…

This is an extract of the full article published in International Pest Control – Mar/Apr 2018 issue.

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Authors: Nikolaus Wilke*, Rebecca Sawyer** and Oliver Junk***

* Managing Director, Thermo Lignum UK,
** Administrative Manager & Research, Thermo Lignum UK,
*** Operations Manager & Research, Thermo Lignum International, Salzburg/Austria

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Category: Public health

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