The deleterious effects of indoor air pollutants, such as acetic acid, formic acid and formaldehyde, on museum artefacts have been recognised for many years. A project was set up which dealt with a number of aspects of this problem. The project involved the construction and validation of an atmospheric sampling chamber, the development of a novel sampling method for the detection of pollutants and the reduction of pollutant concentration using mesoporous scavengers. Also, in an attempt to understand the synergistic effects of carbonyl pollutant concentration and humidity on a range of materials, the project will assess the damage caused by different polluted environments on a variety of materials commonly found in museums.
Validation of atmospheric chamber
A series of chambers have been constructed in the University to permit atmospheric testing of enclosed polluted environments. The polluted environments are generated by flowing air over heated permeation tubes. The permeation rate of the tube, and thus eventual pollutant concentration, is controlled by selection of the desired oven temperature and gas flow rate. Thus, a wide range of accurate pollutant concentrations can be studied. The pollutants selected for inclusion in this study are formaldehyde, acetic acid and formic acid. Validation of the pollutant concentration generated inside the chamber is critical. In order to confirm the gas concentration inside the chamber, the pollutants are measured using passive sampling methods of analyses. Validation experiments for acetic acid, formic acid or formaldehyde contaminated environments will be performed over the range 100 ppb to 5 ppm.
Once the testing phase of the exposure system has been completed, environments containing mixed pollutant vapours will be generated and used to determine the effect of interferent gaseous molecules on the current sampling methods used. In addition, active sampling methods will be developed to permit rapid detection of pollutant vapours.
Novel sampling method
One of the main aims of the project is to investigate the potential of novel non-invasive sampling methods for use in a museum environment. The first method to be assessed involves the preparation of a sol-gel (a glass support) and impregnation of the sol-gel with a specific chemical. The choice of chemical depends on the target pollutant, but in any case, it is intended to react with the gaseous target molecule after it diffuses into the porous glass support. In the first study, the ability of formaldehyde to react with a hydrazine-impregnated sol-gel will be assessed. If successful the sol-gel will be coated onto an optical fibre and the colour change observed after reaction between the gas and the impregnated sol-gel using UV or Raman spectroscopy.
At present activated charcoal is often used to remove deleterious pollutants from contaminated museum enclosures. It is thought that mesoporous silicates offer a number of advantageous over the sorbents currently used. These substances have a larger pore size than zeolites, typically 2 to 50 nm, allowing them to physically scavenge pollutants more easily. In addition, at present, the sorbents are normally tested in active mode, but it is thought that mesoporous substances could also be used effectively in passive mode. With their larger pore size it is also possible to chemically modify them, e.g. make them basic, thus increasing their selectivity for acidic molecules. One advantage of a chemically modified scavenger would be the elimination of back-diffusion problems currently encountered with activated charcoal. Mesoporous silicates have been prepared and are currently being studied using XRD to determine their structure. Porosity testing and surface analysis will also be carried out to determine the nature of the substances prepared. The mesoporous silicates will then be tested both passively and actively using the atmospheric chambers.
Effects of long term exposure on artefacts
A number of materials commonly found in the museum environment (e.g. limestone, eggshells, copper, bronze and lead coupons) will be subjected to different polluted environments for approximately 2 1/2 years. Mixtures of formaldehyde, formic and/or acetic acid solution will be placed in sealed desiccators to provide a number of contaminated environments. All pollutants will be measured at approximately 1ppm and at either 40 or 100% RH. Experimental design has been used to define the experiments needed in order to obtain the most information from the study. The polluted atmospheres will be monitored regularly and the pollutants replenished when necessary. Material damage, which occurs as a result of exposure to the polluted environment, will be monitored by recording weight gain measurements over time. In addition optical microscopy and scanning electron microscopy will be used to study the surface of the materials. If necessary, XRD and chromatographic techniques will be used to identify corrosion products.
Claire Watt* and Lorraine Gibson
Dept. of Pure and Applied Chemistry
University of Strathclyde
295 Cathedral Street
Glasgow, G1 1XL, U.K.
(*): Author to whom correspondence may be addressed
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© January 16th, 2002