IAP 1998, Presentation 16 :


The National Trust


This presentation focussed on the pollution monitoring project at the National Trust property Cragside, which a number of attendees at this meeting have worked on. The original pollution problem, results of trials to test different scavengers and the issues raised by this project will be discussed.

Cragside, a property in Northumberland, was built in the 1860's and 1870's by Norman Shaw for the 1st Lord Armstrong, a great Victorian engineer, gun maker and inventory. Indeed using Lord Armstrong's inventions, Cragside became the first house in the world to be lit by water-generated electricity. In his later years, William Armstrong became an avid collector of scientific, geological and natural history specimens, as well as works of art by contemporary artists, mainly from the Aesthetic movement. The museum in Connections with Hancock collection is now in the Natural History Museum, Newcastle. Some of his natural history collection is now displayed inside Cragside within four cabinets in the Gallery.

Cabinet Construction
The four cabinets, containing some of the natural history collection, are identical in construction. They have a mahogany frame, with an ebonised and gilt, lacquered exterior. There are glass shelves inside the cabinets, with wooden unlacquered trays (re-lined with black silk velvet), and enclosed drawers at the base.

The shells are displayed on the glass shelves mounted on glass slides, with paper labels. At some point in time, some of the shells were mounted on oak boards, which are 3 mm thick.

Pollution Problem
The cabinets are made of the same timbers as some cabinets at Hill Top, a National Trust property in the Lake District, where offgassing from the cabinet caused deterioration to a number of cameo portraits. When the deterioration occurred, around ten years ago, the cameos were removed from the case to prevent further deterioration.

More recently, we became aware of the possibility to measure the concentration of carbonyl pollutants within display cases using passive sampling tubes. This, together with a combination of other factors, allowed us to monitor the concentration of carbonyl pollutants the cabinets at Cragside.

At present, there is a whitish efflorescence on some of the shells. It has been observed on a variety of shell types, and in various location throughout the cabinets. Three potential sources of acidic vapours were identified:

  1. Mahogany carcase, and unlacquered wooden trays
  2. Oak boards
  3. Black silk velvet

An initial carbonyl pollution monitoring project was set up, in conjunction with the University of Strathclyde. The levels of formic and acetic acids were measured in the four cabinets by passive sampling tubes. Two tubes were placed in each cabinet, and one control tube in the Gallery, for fifteen days in February 1997.

Initial Results
It was established from these results that the levels of acetic acid were particularly high whereas the levels of formic acid were negligible. The acid levels were similar for each of the cabinets making them ideal for a comparison of pollutant control methods.
A trial using three different pollutant scavengers readily available on the market, was set up.

Products Tested
The three products tested were :

  1. Activated charcoal cloth. This product was described by the manufacturers as 'theoretically renewable, but this is never done, as it generally outlasts as long as the display case'.
  2. Purafil fibre filter. B850 which contains a blend of potassium permanganate and activated carbon. This is not a renewable cloth, there is a colour change test indicating when the cloth is spent. It was decided to use the cloth rather than pellets, as a direct comparison with the charcoal cloth.
  3. Corrosion Intercept. This is highly activated copper incorporated into a polymer matrix, polyethylene. It reacts with pollutants and neutralises them permanently. It is not renewable and changes colour as it becomes spent.

It is worth mentioning that my colleagues, who set up this trial, were disappointed by the lack of information provided by the suppliers of the materials. Their advice about quantities of material to use for given volumes and air flow rates, were very vague. Therefore the amounts used in the cabinets were determined by educated guesses.

The scavengers were placed in the three cabinets, in the wooden trays, so as not to be too visually obtrusive. The cloths were loosely pleated in the trays. The scavnegers were left for six weeks, to take effect, before the levels of carbonyl pollutants were again sampled. This set of analysis was undertaken by another institution. Three sampling tubes and two controls were placed in each cabinet, for one month, in October 1997 before being analysed. Sampling tubes were also placed in the Gallery, and in an office, to measure background levels. Again, formic and acetic acid concentrations were measured. Relative humidity and temperature were monitored during the course of this trial using the existing Hanwell radio-telemetric system.

The results were very interesting. Again, formic acid levels were negligible. The general level of acetic acid were found to be lower than before. However, they were lowest in Cabinet D, the cabinet containing no scavenger ! Therefore, how do we interpret these results ? There are a number of issues raised by these results.

1. Effectiveness of Scavengers
Although the pollution levels are lower in the cabinets after scavengers were deployed, the lowest levels were measured in the cabinet containing no scavenger. This rather suggests that the scavengers were not particularly effective.

2. Position of Scavengers
The scavengers were placed at the bottom of the wooden trays, but there was air flow between the shelves. Had we positioned the scavengers in the best position ? We did have to consider the aesthetics of the display case.

3. Quantity of Scavenger Used
Was enough scavenger placed in each cabinet ? During the trials, Purafil was tested, every two weeks, but found not to be exhausted and therefore theoretically, was working. The Purafil was tested again last month and WAS found to be exhausted. Therefore it was consuming acidic vapours from somewhere. Purafil had become exhausted after eight months. How can one test for exhaustion with charcoal cloth ? Would we have found the scavengers to have been more effective is measured six months down the line as opposed to six weeks.

4. Position of Monitoring Tubes
Were the monitoring tubes placed in the right positions ? Should they have been placed on different shelves in the cabinets ? Guidance had been given by the suppliers on their positioning. The results give an average from the five sampling tubes. A standard error of 15 % given with these results suggests that the tubes were sampling similar air in their respective positions.

5. Seasonal Effects
The two monitoring trials were undertaken at different times of the year. It has been noted before that seasonal fluctuations do occur in carbonyl pollution levels. They are greater when the temperature and RH are high. The first trials were undertaken in February, during the National Trust's closed season, the second trials were undertaken in October when the cabinets would have been uncovered for display and public access. The RH and temp data from the second trial show constant RH and T thoroughout the trial. We do not have data from the first trials, but we do have comparative results from this time this year, which show a similar, steady RH and T throughout the year. But we do have differences in air flow through the gallery, with visitors during the open period.

6. Interlaboratory Standardisation
A further complication is that the levels were analysed by two different institutes. If both trials had been analysed by both institutions, would be results be comparable ? The need for interlaboratory standardisation is essential, if these two results are to be compared.

Future Measures ?
What is our next step ? Air flow seems to be imporatant, so increasing ventilation is one option. We would like to try pellets, as these have a greater surface area than the cloths. We could combine the two, to give a form of active control rather than passive. We are looking closely at the relationship between the number of oak boards in each case compared to the acetic acid levels and visual ditermination of deterioration in the cases.

Interestingly, Cabinet D, which contains the highest number of oak boards, had the lowest acetic acid levels in both trials, yet contains the most number of shells showing visible signs of deterioration. Are the shells scavenging the pollutants better than our test absorbents ? Are the shells reacting with the acetic acid before it reaches the monitoring tube ?

This project has raised many interesting points and I would welcome comments.

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