Soiling measurements are needed to address strategies for deposited dust control and to determine the source of the dust. There are many methods, which can be used to collect dust such as frisbee samplers, glass slides and sticky tape samplers. As we know, they have advantages and disadvantages. An important question related to this is what dust measurement technique provides the best proxy for soiling potential? At the moment, dust monitoring methods for museums and historic properties have no widely recognized standard method, although several methods could be developed in different situations. In our case, sticky samplers were used.
Our work has been concerned with COARSE PARTICLES because the particles deposited in dusty rooms are very large often in the 20-50 micron range. Such particles cannot be monitored as concentations in the air with conventional high volume samplers which are often unable to sample particles much above 15 microns. We have been interested in the way in which dust coveres objects and concerned with the area covered as a surrogate for soiling. Area proves relatively robust as an indicator because contrast change is dependent on area yet independent of particle brightness.
The sticky samplers were made with 1.3cm diameter circles of white sticky labels and attached to aluminum stubs using Blu-tack. The assembled samplers were attached to wall or panels in horizontal orientation with Blu-tack. These samplers were deployed ain three museums according to the sampling strategies such as vertical and spatial profile including different floor covering, different distance from visitor and different cleaning regimes. After exposing for about 2 months, these were analysed using two different approaches.
As you know, optical microscope is a manual method, which is available to identify the particle types, colour and shape but there is a disadvantage; it is very time consuming. Adobe Photo Shop is a semi-automatic method. Fraction area covered can be measured automatically and counting through the monitor screen also available, but it does not offer the single particle sizing. SEMPER image analysis program allows particle counting, sizing and fraction area covered measurement automatically. To prepare the image samples, photo were made with same size and scanned with gray scale at high resolution. These have adjusted to the same brightness and contrast and converted to negative images using a Photo Shop. After this further image analysis used the program SEMPER. An important thing is that white or light colour particles such as quartz, gypsum, calcite and the white cloured fibres do not get detected. This required that these particles were re-painted again in the Photo Shop.
Correlation between the manual counting through and eyepiece and SEMPER was found to be 0.87 (n=247), which is reasonably good agreement. As might be expected, the counts witnessed by eye through a monitor screen also obviously showed good correlation ( r2=0.81) also. These correlation are not perfect and some error might be caused by photographic quality, scanning bias or thresholding intensity in SEMPER, but perfect agreement is not unexpected given the way in which we perceive fine particles under a microscope.
Correlation between number and area
|There is a reasonable agreement (R2=0.56)between fraction area covered and number of particles||Fibres which have highly variable length and so agreement is poor|
Particles deposited on the sticky samplers could be identified under the microscope, as: soil dust, soot, fibres and more rarely dandruff, plant and paint fragments, human hair, and insect parts.
All the profiles seem to have a similar characteristic shape, with elevated rates at floor level and typically 0.8-1.5m height above the floor. Our studies suggest that dust stirred from the floor by walking is most important in the first 30 cm above the floor. This does not seem to rise to any great height, perhaps only 20-30cm. This upward flux from the floor is probably of limited importance in soiling in historic materials, because such items are frequently found at eye-level. This means dust from the floor is not a primary source of coarse particles at eye-level.
At eye level the deposit appears to contain fibres shed from visitor's clothing, suggesting that they are the principal source. However, carpet fibres are only found in the very lowest levels about 2-20 cm and would clearly not affect normal eye-level objects on display. We can see the flux of particles and fibres are very much lower in the unoccupied room. The result shows that soiling decreases a factor of 14 and 17, for non-fibrous and fibrous dust deposition, when the room is unoccupied. Note that only the occupied room shows a clear maxima over the 80-150cm height range.
Conservators have long worried about the importance of floor covering in controlling interior dust level. We examined dust deposition in rooms with different floor coverings such as carpet, wood, stone, and rubber tile. The soiling on a per capita basis shows very similar, although thel sampling sites have different floors covering. An exception is the case of Drawing Room (Fig. e), where samplers were 3-4m from visitors shows much lower covering rates than in the South Corridor (Fig. g), where samplers were within 30 cm at the same flow of visitors passed. Therefore, this result could prove that floor type is not a significant factor.
We also used SEMPER to class particle sizes on the sticky samplers in all the vertical profiles from the buildings studied. We can see that the smaller particles are most numerous in the highest elevation and the height of eye-level, although the very finest would not be observed in our light microscope. Close to the ground the most numerous particles are about 50 microns and also the coarse fibres almost a millimetre long predominate. In general it is fibres that make the largest contribution to covering the surface.
At the moment, we can have a question that where is the main contributor to increase the soiling degree at the eye level? To solve this question, we set samplers out at regular intervals on the floor in transects at right angles to the cordoned visitor pathway in the Felbrigg Hall.
This figure shows the covering rate as a function of distance from visitors. It is plotted semi-ogarithmically to emphasise an apparent exponential decrease with distance of about 1m. Transects from both the Drawing room and Red bedroom show that the covering rate at floor level, declines by at least a half with every half metre from the visitor path.
It was clear that visitor flow was a major contributor to soiling, such that soiling mechanisms in different museums could be compared on a per capita basis. The relationship between the average covering rate and particle flux at the three sites yields a satisfying relationship with visitor number. The high value in Norwich Castle Museum (NCM) seems to be an influence of the large number of visitors.
Dust from clothing
Clearly support for the source of dust that soiling derives from visitors comes from this close up photo of dust emerging from a woolen coat. (Scale about 10cm).
Sticky sampler - In this study, sticky samplers were used to measure the soiling potential in some museums and historic properties. Although sticky samplers have a problem if stickiness is lost, glass slides, which are frequently adopted in the museum environment, can lose larger particles. Furthermore sticky samplers have other advantages: 1) inexpensive 2) easy to make 3) high collection efficiency (stickiness of surface) 4) wide application (applicable for measuring particulate matter using SEM or image analysis) and 5) unobtrusive.
Visitor - It was clear that visitor flow was a major contributor to soiling, such that soiling mechanisms in different museums could be compared on a per capita basis. The proximity of visitors to objects was another important factor with the soiling declining which suggests soiling of objects on open display could be reduced by increasing the distance from visitors. It impact of visitors on coarse dust deposition declines rapidly by a half for each half metre from visitor pathways.
Vertical profile - Our studies suggest that dust stirred from the floor by walking is most important in the first 30 cm above the floor. At eye level the deposit appears to contain fibres shed from visitor's clothing, suggesting that they are the principal source. Thus carpetting may not necessarily be a critical source of dust for museums with eyelevel displays.
Soiling unit - The number of non-fibrous particles were always higher than fibrous particles, but of course they were much smaller, so simple particle counts expressed as particle number density and number flux are not necessarily a good estimate of soiling. Firstly, we adopt fractional area covered over the sampling period, but clearly this is better expressed as a covering rate with the units, s-1 (i.e. dividing the fraction covered by the number of seconds in the sampling period i.e. where eight weeks, 5.184x106s). Our early study indicated that the visitor is an important contributor to increase the dust level. Therefore, soiling could be expressed in final with fraction area covered, which is calculated per capita based.
Young Hun Yoon* and Peter Brimblecombe
School of Environmntal Sciences
University of Est Anglia
Norwich NR4 7TJ UK
e-mail * : firstname.lastname@example.org
(*) Author to whom correspondence may be addressed
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