IAQ 2003, Presentation 24:
A model for the degradation of textiles on open display and the risks of vacuum cleaning.
Owners of large textile objects cannot always give them the best protection against environmental pollution especially the deposition of particulates. The 'best' protection for objects on display is usually deemed to be a glazed enclosure within a filtered-air environment. This option is rarely used. This may be for reasons of cost, context or aesthetics.
It is widely held that the greatest risk comes from the need to remove the accumulated dirt by washing or by vacuum cleaning which inevitably leads to unnecessary loss of textile fibre. It has proved difficult to devise an experiment that demonstrates that the rate of deterioration is dependent on the rate of cleaning.
It is possible to construct a mathematical model for the degradation of textiles that enables a reasonable prediction of future life-time for that object. The model also demonstrates what common sense suggests; that a vacuum cleaner can cause loss of fibre that would otherwise remain in the textile. However it does not suggest that continued vacuum cleaning necessarily accelerates deterioration.
Since not all soiling would be removable by vacuuming, or even by washing, a case can be made for the concept of an 'acceptable rate of soiling' which would call for the definition of a 'just noticeable soil' in line with policies for lighting control in museums.
The paper is presented in order to elicit practical and anecdotal experience from the audience, as well as assistance in improving the mathematical model.
This paper is a cry for help. It is not unlike those letters that appear on Conservation On Line, "Will some kind person please tell me all I need to know to get my PhD in textiles degradation". The subject of the paper is 'textiles and dirt' and the question is "Is it safe to remove dirt from textiles?", a supplementary question is "Is this an important question?". The answer to the first is "It all depends". The answer to the second will guide the amount of time that is necessary to qualify the answer to the first.
I believe it is important to know how safe it is to remove dirt from textiles and also to know whether cleaning is just desirable or actually necessary . In my book on risk assessment, published in 1999, I made several statements about the subject that it would be wise to look at to see if they are still valid.
Had I actually gone onto the Cons Dist List on CoOL asking for help, I would have had an immediate response from a terrifying woman called Barbara Applebaum. She would tell me to read a few books before wasting people's time asking silly questions. Well I have read some books; the conservation literature and the scientific literature about conservation. I have particularly enjoyed reading technical books from between the two major twentieth century wars. This was a period when real men did real tests on real textiles in real time. Long before the invention of the scanning electron microscope and computer simulations.
I am also a great believer in using one's own experience to supplement, modify and cross-check what has been gained from reading. So my influences in this subject area also include.
Part of my experience of dirt has been lying in bed on a sunny Sunday morning watching the particles in their constant Brownian motion flashing in and out of the sunbeams. Some of these are textile fibres judging by their relative length and width. I lie there thinking "funny that they don't make clothes out of fibres that small, they must be the product of some deterioration reaction". This is the same notion that hits me when I inspect the fluff from the filter of my tumble-drier. These fibre fragments are very very small. Something must have happened to cause the long fibres used in clothing manufacture to break into much smaller pieces.
Anyone who has actually seen me will know that sartorially I am a walking experiment in textile degradation. This experience, coupled to a recent conversation with my knowledgeable wife, enables me to comment on Morten's observation that the prevalent particle type in his studies in the Viking ship museum derives from blue jeans material. Apparently traditional denim is a three end 2 to 1 twill. This means that on the outer surface the blue weft threads travel over two of the warps and then under one. This makes them more susceptible to abrasion than plain weave. Moreover the indigo dye sits on the surface of the fibres so that the concentration of dye in the abraded fragments would be high, possibly explaining the colour of his aqueous extracts. If the only degradation reaction is abrasion (in contrast to what I am about to propose) then one would expect all the fibres to be less than 1mm in length judging by the jeans I have on at the time of writing.
The area in which I need help does not immediately appear to fit in the programme of a conference on Indoor Air Quality. However you can see from Fig 1 that it follows logically from other work reported here . Moreover it gives purpose to some of the more abstract research and places it in the arena of real-world decision-making.
On the left hand side of the figure is the work of Nigel Blades and the IMPACT project , Bart Ankersmit and the SILPROT project and Andy Calver with his whipped cream. Moving across you pass through the past work of many Brimblecombe students and into the current Leverhulme project including the results given in Kathy Lithgow's excellent presentation. Centre right is the key decision - should this object go in a case? Or can it be subjected to a continuous cleaning regime? The supplementary work that is needed to help make this decision answers the questions - can I predict from its past history how weak an object is without physically testing it? Can I predict how long it will last? Bearing in mind the current rate of degradation how safely can I clean it and how often?
Its time to introduce the model (Fig 2). It is more a toy than a model. Playing with it increases one's coarse intellectual skills. It points as much to the absence of knowledge as it does to the desired answer. It concerns a hypothetical textile object in which all the fibres start with 100% of their original strength. A random event ( chemical, photochemical, biological, physical action) causes a change (eg breaking of a bond) causing the fibre to become weaker. The percentage of fibres with 100% of original strength decreases. Events of this type continue to happen randomly, but as susceptible sites are used up the possibility that the event causes a change decreases with time. Thus the rate of decay decreases with time . In degradation studies this is called ( in misleading reference to homogeneous chemical kinetics) a first order reaction. It is one where it takes the same length of time for the concentration of reactant to decrease to half as it does to decrease from half to a quarter, and the same time again to decrease from a quarter to an eighth and so on. The time is called the half life of the reaction.
As the number with 100% strength decreases so the number with only 90% strength increases and then begins to decease as further events lead to the formation of fibres with only 80% strength, then 70% , and so on down to the point where fibres have 0% strength. The numbers in this last category can only increase. Eventually 100% of all fibres have 0% strength. In the figure the time axis is in arbitrary units. For a historic object we have some idea of the start date and for a particular fibre and a specific type of degradation we may have some idea (within an order of magnitude) of how long after that the end point will come. There are some very ancient textiles with some strength left in them, so excluding the effect of ultraviolet radiation we may be talking of lifetimes in the order of 1000 years for cellulosics. For silk subjected to sunlight the end point is much nearer.
Sometimes we can deduce whereabouts we are along the horizontal axis at this point in time. For instance in the V&A there are silk embroideries which are not subject to abrasion where fragments of silk fibres can be seen in the bottom of the glazed frame. These fibres apparently have not the strength to support their own weight while some fibres remain integral to the object. This puts 'now' at about half way along the time axis at say 15 half lives. We now have a rough estimate of half life for a 10% change in strength and can use this to predict how weak the object will be at any point in the future.
If we study a similar graph (Fig 3), which is a little clearer as it deals with larger step changes of 25% loss in strength, we can start to think about what the dangers of vacuum cleaning might be. At about 2-3 half-lives most of the fibres are still pretty strong and you might expect that the only damage that the sucking power of a vacuum cleaner could do is remove whole fibres. This is actually quite unlikely especially with a closely-woven closely-plied textile with a long fibre length ( long staple). Careful cleaning is possible without damage.
At about 13 half lives all the fibres are very weak and subject to breaking from any stress such as movement. There may be many broken fibres loosely trapped in the remaining structure. Cleaning breaks many fibres and removes large quantities of already broken material. The use of the vacuum cleaner is not advised.
It is the intermediate stage at about 5-6 half-lives that is critical. The population of fibres with only 25% strength is growing ( red line). Since vacuum cleaners lift fibres upwards we can speculate that they exert a force greater than the force of gravity. Gravity breaks fibres of near 0% so the cleaner will break stronger fibres. Arbitrarily we will say that this critical strength is 25%. Thus cleaning appears to be doing damage and removing fibres that would have remained in the object had there been no need to clean. This loss of fibre is shown by the blue line. If you were to vacuum clean the textile shortly after the first occasion there would be nothing weak enough to break and nothing broken enough to pull out. So in this instance cleaning cannot be considered dangerous. In time stronger fibres decay and the population of 25% strength fibres increases again so a cleaning event after a 4-5 half-life period is dangerous. But at 13 half-lives there really isn't much difference in the strength of the whole textile whether it has been cleaned or not.
The removal of short ( broken) fibres may not greatly affect strength but will eventually affect structural integrity as the twisted and plied yarn and the weave structure become more open and less cohesive. It is a matter for debate ( or possibly experiment) whether the loss of broken fibres alters the image of the textile significantly. Given other work described at this conference it seems unlikely that the average museum or historic house visitor would be much bothered.
I only had time for one ( barely scientific) experiment while preparing this paper. I vigorously vacuum cleaned some double-ply woollen yarn and measured the length of the fibres that were removed and attempted to measure the weight loss. The longest fibre removed was 25mm long from a yarn made from fibre with an average staple of 100mm. A second cleaning, immediately after the first, did not remove so much fibre. The loss of fibre was well below 1% weight.
My interpretation of this single test is not incompatible with the model. No full staple fibres were removed. A very small percentage of fibres was sufficiently weak that vacuuming ( possibly ) broke a few and removed the fragments. For the second treatment the number of fibres susceptible to breaking appears to have been less than for the first. Obviously even this amount of interpretation is not scientifically justified but the experiment is easy and someone with more time could try it in a more systematic way.
At the molecular level
It is known that one mechanism of decay for polymeric materials is that the long polymer molecules break into smaller fragments. The population of different molecule lengths expressed as molecular weight forms an approximately normal distribution. As the chain length becomes shorter so the strength of the fibre that the molecules form becomes weaker. A highly idealised view of a number of polymer molecules in a fibre is shown in Fig. 4. If after a long series of random events breaks occurred at the points indicated by red dots then one imagines the fibre would fall apart. The this happy coincidence of a number of random events can itself be considered as a random event, one with a very low frequency. The same model can be used to describe the coincidence of breaks in a bundle of fibres or a bundle of yarns or even a woven structure. If the basic degradation mechanism consists of a continuing series of random events (eg photochemical interaction) that lead to a change (eg chain scission)at susceptible sites then the coincidence of weaker or broken elements to form detectable damage is a random event with low frequency.
If you model random breaks in fibres (which all start at the same length Fig 5) and allow several iterations where the resultant fragments are susceptible to breakage in proportion to their length you get a new distribution of fibre lengths- not surprising. What may be surprising is how soon you get a noticeable number of quite short fragments without invoking a physical mechanism such as abrasion. These lengths could be seen as the results of breaks or they could be seen merely as the potential to break given some additional physical stress. Viewing them that way gives an understandable picture of what it might mean to have 50% of original strength in a fibre.
It is surprising that with so many conservators worried about the effects of vacuum cleaning yet constrained to do it nonetheless that no-one I have asked has actually studied what it is they find in their vacuum cleaner bags. Is it the long fibres pulled from the woven fabric? Is it a range of sizes down to the very small as shown in Fig 5. Working from right to left you can see that there are still a lot of long fibres that would not easily be dislodged. There are some that are short enough not to provide strength to the yarn and there are some short fragments of the sort I see floating over my sunlit bed and some really tiny bits like the fluff in my tumble-drier.
Is unavoidable the same as acceptable?
The model, if not observation and common sense, shows that a textile object will eventually become too weak to suffer cleaning. It follows that the object will become soiled. Even if the object is put in a good quality case particulate matter will get to it. As a digression it should be noted that much of the discussion at this conference has been about fluff rather than sub-micron dirt. Sub-micron particles will get into a showcase almost as easily as air or a pollutant gas. There is an inevitable unavoidable rate of soiling if the object is going to be kept on display. By comparison with the treatment of light doses for light sensitive objects that has been the subject of much work at the V&A, one could think about defining an acceptable rate of soiling in line with the acceptable rate of fading.
It would be nice if someone would help me with the refinement or total redesign of the model. Or direct me to where this work has been already published. It would be nice to hear from someone who actually has inspected the contents of their vacuum cleaner bags.
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© November 2003