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IMPACT OF MOISTURE SORPTION STABILIZATION AS A PREVENTIVE CONSERVATION APPROACH

IMPACT OF MOISTURE SORPTION STABILIZATION AS A PREVENTIVE CONSERVATION APPROACH Pavel Zítek, Tomáš Vyhlídal, Ivana Kopeck á Centre for Applied Cybernetics, Czech Technical University in Prague, Department of Technology, National Institute for the Preservation of Cultural Heritage.

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IMPACT OF MOISTURE SORPTION STABILIZATION AS A PREVENTIVE CONSERVATION APPROACH

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  1. IMPACT OF MOISTURE SORPTION STABILIZATION AS A PREVENTIVE CONSERVATION APPROACH Pavel Zítek, Tomáš Vyhlídal, Ivana Kopecká Centre for Applied Cybernetics,Czech Technical University in Prague, Department of Technology, National Institute for the Preservation of Cultural Heritage Outline of the presentation • Unheated historical buildingsand moisture originated damage of the deposited collections. • Modelling the equilibrium moisture content (EMC). • Compensating the impact of temperature drift on EMC by means of humidity adjustments • Equal-sorption humidity control, implementation experience

  2. Introductory Statements Motto of preventive conservation: Not to remedy the consequences but to remove the causes of damage as soon as they emerge. To remove the harmful impact of moistureconsist in averting themoisture content changes, i.e. in maintaining the equilibrium moisture content (EMC)constant. Main concern: Unheated historicalbuildings are peculiar with great temperature and humidity changes necessarily bringing heavy sorption transport phenomena.

  3. Primary Considerations Challenge: A great deal of European cultural heritage is preserved in unheated historical buildings where the air humidityimpact is a prime cause of deteriorating the deposited artistic or historical collections. A chance: Basically, not the steady state but first of all the changes of moisture content bring about the damaging impact on the artefacts. Therefore certain air humidity correction can counteract these changes and become a potential means to prevent the exhibits from this impact. Problem: EMC is not available as a measured variable – hence only an EMC model can provide information needed for EMC control. Moreover: EMC is not even a microclimate parameter but a property appropriate to a specific material.

  4. The Karlštejn Castle – the Main Tower

  5. The Holy Cross Chapel – collection of medieval paintings

  6. Three-parameter Henderson model of moisture content Henderson: EMC– relationshipto relative air humidity and temperature T Three parameters: distinct for various materials A in 1/°C, B in °C, C dimensionless Character of dependence: EMC increases with growing relative humidityφ and decreases with growing temperatureT. Saturation – apparently, the modelis not applicable in cases near the state of saturation (ln(0) does not exist). However, such conditions are inadmissible for the deposits.

  7. Equal-Sorption Humidity Adjustment • Henderson model suggests a principle to maintain EMC constantby means of humidity corrections : For a temperature change from a specific humidity change can be provided that both these changes cancel each other if they satisfy the following condition Hence: If an air state change has satisfied this condition the material with the sorption parameter Bdoes not change its EMC. • However: Condition (C) holds for only one material in fact. For another material this condition requires a distinct humidity change due to a distinct B.

  8. Crucial Find: Humidity Adjustments for Various Materials Take two materials M and N with sorption parameters and suppose an air state change from to a different temperature T. The desired new humidity values result as follows The temperature parameter B takes on relatively high values compared to the temperature range supposed: Result: The humidity adjustments for M and N are rather low and therefore, in fact, cannot differ significantly from each other.

  9. Implementation in Třeboň Archives – Historica Collection

  10. The environment yearly course – before implementation

  11. The environment yearly course – after implementation

  12. Equal Sorption Control Implementation Relative humidity controller adjusting the interior humidity towards the desired value provides permanently an interior air state which does not cause any change of EMC in the material with the sorption parameter B from its value appropriate to the reference state Wide validity of humidity adjustment:Due to negligible humidity differences required for various materials, the control according to (R) is valid for a wide class of preserved materials.

  13. Conclusions • Decisive experience: The humidity corrections inhibitting the EMC changes are only slight and for the most part of various materials their mutual differencies are negligible. • Equal-sorption humidity control:due to approximatelyequalhumidity corrections for various organic materials – thehumidityadjustment can be considered as microclimate control in fact. • Predictive character. Due to very slow response of the moisture content the humidity control is easily able to provide its adjustment in advance, i.e. actually to remove the causes of the EMC changes before their consequences emerge. • Power consumption. Due to minimizing the heating demands the proposed equal-sorption control considerably reduces the power consumption.

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