Nasswettrová Andrea, Šmíra Pavel (Brno, Czeck Republic)
Thermal restoration of timber structures VkontakteFacebook

Fig. 1.: Simulation of the process of thermal restoration of an attacked roof structureFig. 2.: Treatment of monuments in the Czech Republic and Slovakia with mobile Nolting heaters – church of The Beheading of Saint John the Baptist in Dolní Chabry, St James's church in Brno and St Martin's cathedral in BratislavaFig. 3.: Measurement of air temperature and woodFig.4.: Measurement using thermal imaging camerasFig. 5.: Preventive chemical protection of all restored timber elements

Summary: Hot air treatment of timber structural elements can control the activity of wood-destroying insects. This method is non-destructive and suitable mainly for the treatment of timber structures in historic buildings, churches and valuable monuments, as well as in traditional timber houses, loghouses etc. The article describes the principle of the method and the technologies used for the preservation of cultural monuments (and not only) in the Czech Republic.

Keywords: thermal restoration; wood-destroying insects; historic wood;

Аннотация: Обработка горячим воздухом структурных элементов из древесины позволяет управлять активностью древоразрушающих насекомых. Этот метод является неразрушающим и подходит для обработки деревянных конструкций в исторических зданиях, церквях и цен-ных памятниках, а также в традиционных деревянных домах. В статье описывается принцип метода и технологии, используемые для сохранения памятников культуры (и не только) в Чешской Республике.

Ключевые слова: термическая обработка; дереворазрушающие насекомые; историческая древесина;

Термическая обработка деревянных конструкций

1. Introduction

Since the beginning of mankind, wood has been used for the construction of cultural monuments of many civilizations. [1] Unique historical monuments made of wood – from statues, altars or carved furniture to whole timber structures such as churches, campaniles or parts of castles – represent an artistic expression of their creators' imagination and aesthetic feeling. [2] Being the carriers of spiritual heritage, historical monuments are living witnesses of the nations' secular traditions in today's life. They tell stories of special civilizations, typical developments or historical events. It is therefore necessary to respect the original constructions and materials when restoring a historical monument, thus preserving its historical value. [3] Wood is a natural material with a great diversity of forms given by its growth conditions, environment, and hereditary characteristics. This variability of properties can be seen not only within each species, but also within each trunk, causing wood to behave anisotropically. [4] Due to its properties given by its vegetable origin, wood is permanently exposed to degradation processes, especially of biotic nature (wood-destroying fungi and insects), but also environmental (non-biotic). As a result of this degradation, significant changes of physical, mechanical and chemical properties occur. The scope of damage depends on the environment and processes the material is exposed to. [5] As regards structural elements, the change of wood strength is of importance. A long-term action, especially that of biotic factors, may finally be the cause of a structural element or the whole structure failing to fulfil its function. Therefore, assessment of the properties of wood used for historical objects and constructions remains an essential and practical question. The most active biodegradation occurs at wood humidity values of 25% to 30% [6] . In these conditions that are near the hygroscopicity limit, constructions are often attacked by insects and fungi at the same time. In such cases of attack, the combination of hot air treatment and subsequent use of chemical insecticides has proven to be effective. The destroying factor of this method is heat – it acts evenly in the whole area while preserving the historical value of the object. In 2010, this effective non-destructive method of control of wood-destroying insects was first presented in the Czech Republic by company Thermo Sanace s.r.o. The company successfully uses this method to protect the cultural heritage and historical value of old timber structures. [7]

1.1 Principle of the method

Hot air control of wood-destroying insects (Fig. 1) is a well-proven process recognized by DIN 68 800, Part 4. [8] [9] The principle of the heat process is to create a sufficiently high temperature in the whole cross-section of timber elements to kill the insects living in the timber in all their development stages (eggs, larvae, nymphs, and adults).

For successful killing of biotic pests, the temperature of the wood must reach 55 °C and remain at this level for 60 minutes. This temperature and duration ensure that coagulation of proteins in the insects occurs, leading to their destruction. [10] The denaturation of proteins manifests itself by disintegration of polypeptide chains, which then lose their characteristic structure. The destruction temperature and time of 55 °C / 60 min were defined based on a number of experiments, whose results and related practical recommendations are listed in directive 1-1,87 issued by Wissenschaftlich-technischer Arbeitskreis für Denkmalpflege und Bauwerksanierung e.V. (Scientific-Technical Study Group for Monument Preservation and Building Restoration), “Wood Protection” report. [11] The directive was elaborated by Dr. D. Grosser. The parameters of hot air control of wood-destroying insects as well as the method itself are considered a permissible method for treatment of constructions according to DIN 68 800, Part 4 (issued May 1974, suggested amendment July 1986). [12] In real-life treatment conditions, also the question of fire safety arises. For this reason, the temperature of the hot air output in the treated rooms must not exceed 120 °C. There is no risk of thermal degradation or fire as the air does not reach the flash point temperature of wood, which lies between 180−275 °C. At sterilization conditions (t = 55 °C), the duration of the treatment is approx. 7–17 hours, depending on the outside temperature, the size of the treated area, and the cross-section of the timber elements. If there are any structures sensitive to high temperatures in the treated rooms, the treatment temperatures are reduced to 80–100 °C.[текст с сайта музея-заповедника "Кижи": http://kizhi.karelia.ru]

2. Sanitation technology

Hot air with a temperature of 100–120 °C is generated by powerful mobile hot air generators: Nolting with an output of 7,500 [m3/h] (Fig. 2) and Heimer with an output of 22,500 [m3 /h]. The mobile heaters are equipped with environment-friendly three-phase LFO (light fuel oil) burners (380 V). Hot air is forced to the room to be treated via pipes, usually through roof openings or through the roof cladding, see Fig. 2. The piping consists of individual aluminium parts with a specific composition for each treated area. The roof area must be sealed to the outside as tightly as possible. Thermofoils that hermetically seal the area are used for this purpose. If the amount of heated air is sufficient, small leakage points improve air circulation and prevent the formation of air cushions with low conductivity. Materials and objects not resistant to high temperatures must be removed from the area or thermally insulated. The area to be treated must be cleared of all objects and any dust, wood pieces and other inflammable elements must be removed. Untight points of roof and technical openings must be sealed before hot air treatment (e.g. using natural sheep wool).

3. Measuring equipment

3.1 Temperature monitoring during treatment

The temperature of the structural elements undergoing treatment is measured, not calculated. The measurements are performed at regular intervals. The whole thermal restoration process is thus monitored and documented. Important are the air temperature and the temperature of the structural element being treated, see Fig. 3. For the treatment to be successful, thermoelectric sensors measuring the temperature inside the wood must be placed in the geometric centres of the elements. The central parts of the timber elements where the temperature is lower than on the surface are heated by means of thermal conduction, which is significantly dependent on wood dnsity. [13] [текст с сайта музея-заповедника "Кижи": http://kizhi.karelia.ru]

3.2 Thermovision camera for the sensing of temperature fields

The distribution of surface temperatures can be sensed by an IR thermovision camera, FLIR B425 (30 Hz, 2010 model), Fig. 4, with temperature sensitivity of 0.08 °C and image quality of 320 × 240 pixels. The thermal images, or thermograms, are stored by the camera as 14-bit images in the JPEG format. The camera is calibrated for direct sensing of temperatures on the sample surfaces. [14]

4. Checking the sterilization effect

The effectiveness of the hot air treatment is checked by monitoring the achieving of sterilization temperatures in the geometric centres of timber elements and also using control samples. According to ČSN EN 1390, the control samples with dimensions of 150 × 100 × 25 mm are infected with 6 larvae of the house longhorn beetle (Hylotrupes bajulus L.) with a specific weight. During treatment, the samples are placed in the most critical areas; after treatment, the samples are split in half and the condition of the larvae inside is checked. This control measurement is performed in cooperation with the Timber Research and Development Institute in Březnice, Prague, Czech Republic. [15]

5. Preventive chemical protection of the treated timber

As the thermal treatment is a mere sterilization process, a preventive chemical treatment is necessary to protect the timber against new attack. A concentrated, water soluble wide-spectrum fungicide and insecticide is applied by spraying, see Fig. 5.

6. Conclusion

The use of hot air enables comprehensive treatment of whole timber structures and elements of large cross sections. This method allows for more natural movement of the humidity field in the timber structure and prevents the formation of fissures due to surface condensation. The process is monitored, both on the surface and inside the elements. Thermal restoration cannot be used in areas where it is not possible to guarantee sufficient supply of hot air or in areas with materials not resistant to high temperatures (approx. 100 oC). It is also not advisable to perform the treatment in winter as the use of the method is dependent on outdoor air temperatures. Following the hot air treatment, the timber elements must undergo preventive chemical treatment.

// Системный подход к сохранению памятников деревянного зодчества
Составитель А.Е.Косканен
Интернет-публикация kizhi.karelia.ru. 2017.

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