Reliability: analysis of failure
In engineering, the term reliability has been introduced to manage the frequency of failure in complex systems. Archival and library collections can be seen as complex systems due to the numerous interconnected components (use, value, material or design etc.). Reliability can be defined as “the probability that an item will perform a required function, under stated conditions” [33]. In this context, failure is the event of “non-conformance to some defined performance criteria” [33]. Therefore, in the context of archival objects failure is when mechanical degradation occurs because when paper is made it is not meant to tear and a book spine is not meant to break. However, it is important to bear in mind that, in reliability, time is always taken into account as a parameter. Therefore, the reliability of an object is “the probability of survival to time t” [33].
Reliability distinguishes different failure modes to describe the type of failure [26]. Two major types of failures are identified:‘intermittent’ and ‘extended’ failures. Intermittent failures are those which occur only for a short period of time. Looking at wear and tear in archival objects, only extended failures occur as this type of failure continues until some corrective action rectifies them: a tear, understood as a failure, will remain until a conservation treatment takes place. Within extended failures it is possible to distinguish between ‘complete’ failures, which result in a total loss of function, and ‘partial’ failures. In the case of an archival object, tears could be considered as ‘partial’ failure, while a missing piece of paper with text could be considered as ‘complete’ failure. Failures can also occur suddenly, without any warning signs, or be gradual. Wear and tear is a gradual failure, as a small tear can develop into a large tear and result in a missing piece and loss of text. However, sudden failures are also possible, for example, if a binding is overstressed and breaks.
In reliability, the severity of the failure mode is graded as catastrophic, critical or marginal, terms describing the event of a failure. A study among archival and library users has shown that loss of textual information is seen as critical, whereas other failures regarding the materiality of archival and library objects are graded as marginal [34]. In general, when archival objects are assessed during a survey, the severity of each failure is not individually graded, instead, the survey is used to grade the condition of the object as a whole.
In reliability, according to the IEEE Standard 1413.1-2002 [35], the causes of a failure are defined as the circumstances during design, manufacture or use which have led to failure (Fig. 1). Another way to divide the causes of failure is by using factors prior to use (design and manufacture) and factors during use (maintenance and usage) [26]. The causes of a failure can be classified as intrinsic (weaknesses in the item, wear-out) or extrinsic (errors, misuse or mishandling during design and production) [36].
Failure causes
In the previous section, general concepts of reliability were introduced and it was demonstrated that they can be translated to the studies of failure in collections. In this section, the applicability will be scrutinised. We will examine the factors and observations reported by surveys using the failure cause classification system as provided by reliability.
Factors prior to use
Design
In the context of archival and library collections, ‘design’ can be understood as the physical specifications of an object or, in other words, the material construction of the object: size, type of binding, thickness of paper etc.
The features of an object are closely related to the end-use that the object is meant to fulfil. Bindings are a good example. The purpose of a book will determine the type of binding, resulting for instance in a clear difference between archival and library bindings. Until the 19th century, register books, in contrast to printed books, tended to have a parchment cover rather than a leather one, were oversized and had their own types of binding, such as laced overband or the more modern spring-back binding [37, 38]. Different types of bindings led to different stresses to the materials and the structure on opening [39]. Hence, different types of bindings imply different rates of failure and, if failure occurs, book conservators are well aware that in order to prevent failure after repair, in some cases, the binding design has to be altered.
Economic reasons can also compromise the quality of bindings. In the Yale survey, it was observed that limp bindings were more fragile than rigid covers [18]. In 1906, Cockerell recognized that library bindings have to be cheap due to the large number of books to be bound; however “appearance must be sacrificed to strength and durability, and not, as is too often the case, strength and durability sacrificed to appearance” [40].
The descriptions of physical characteristics of objects provided by condition surveys could be a rich data source to be further analysed to help us to identify which design features (such as the size of the object and types of binding) contribute to the reliability of archival objects.
Manufacture
While design refers to the choices that can be made regarding the shape and construction of the objects, manufacture refers to the production of the materials and composite objects.
The history of the production of paper, leather and parchment shares in common the fact that the developments of production processes were always driven by increased demand. Increasing production and reducing the production time negatively affected the end product quality, which plunged dramatically after 1850 [41, 42].
In papermaking, although each step (beating, forming, pressing, drying and calendaring) affects the quality of the end product [43], the choice of raw materials and additives is seen as the major factor affecting paper durability [44]. The importance of the raw materials is illustrated, for instance, by the survey of 19th century paper conducted in Sweden [11]: 63.3% of the paper graded as being in good condition was made of rags and 6.1% of chemical pulp. On the other hand, 46.7% of chemical pulp paper and 13.3% of rag constituted paper was categorised as poor condition (based on the assessed mechanical degradation and manual fold test).
Similarly to experimental studies, surveys can also inform how raw materials affect the mechanical and chemical properties of paper. Several surveys report a steady decrease in paper stability between 1860 and 1890 [11, 18], whereas other surveys fail to detect any increase in stability until 1960 [14]. The surveys conducted more recently [13, 14] agree that pH was alarmingly low (pH < 5.0) until 1980 when neutral sizing was introduced on a large-scale [20]. New concerns have arisen in the last few decades with the introduction of recycled paper, particularly when used as permanent paper in governmental documents [45, 46].
Parchment manufacture has remained traditional. In general, parchment is considered as a durable product, composed of 95% collagen arranged in an oriented network, with pH between 6 and 8 [47]. The most critical stage is when the parchment is drying under tension. However, in the 19th century, lime baths with chemicals such as calcium oxide, calcium carbonate, and sodium sulphide were introduced to accelerate the unhairing of the skins.; the effect was that too much tissue material was removed, resulting in poorer quality parchment [48].
For leather, the tanning process determines the quality of the end-product. In the 19th century, the negative effect of sulphuric acid on leather, known as red rot, became apparent. Studies have shown that the sources of acid are sulphur dioxide present as an air pollutant, as well as the tanning process itself. It has been observed that leather tanned with hydrolysable tannins (such as sumac leaves and chestnuts) degraded less than condensed tannins, such as mimosa bark and gambier leaves [49]. In addition, from the 19th century onwards, the search for faster tanning methods resulted in the use of new techniques, negatively affecting the quality of leather. The traditional, natural dyestuffs combined with alum were replaced with synthetic dyes in combination with other mordants, which required an acidic environment obtained by the addition of sulphuric acid. By the 1870s chrome tanning had been fully adopted and is nowadays still the predominant tanning process [49].
Due to the poorer quality of end-products, manufacturing failures can be anticipated as an important cause of failure for specific groups of objects.
Factors during use
Maintenance
With the introduction of risk assessment, the preservation needs of collections also need to be assessed [15,16,17]. This includes a list of aspects, from protective folders to environmental control, from integrated pest management to disaster preparedness, and a self-assessment checklist of benchmarks in collection care has been developed [50]. Storage and physical protection practices are particularly well-embedded in preservation policies in archives and libraries. Measures, such as keeping documents unfolded, if possible with objects of similar size, in a folder and/or in a box [51], address the risk of mechanical degradation due to unsuitable storage. However, it has not been quantified yet how effective these measures are. It has been observed that boxes do not always prevent mechanical degradation. Losses along edges and corners cam occur due to ill-fitting boxes and permanent deformation of paper can be caused by sagging in boxes that are vertically stored [4].
It is interesting to note that several condition surveys report on folds and tears in chemically stable paper [11, 12]. In such cases, it is possible that such mechanical degradation is evidence of inappropriate storage practices.
Usage
Reliability has been defined as the probability of failure, under stated conditions, for a stated period of time [33]. Usage refers not only to mishandling, but also to an intensity of use, more than is reasonable for a certain object. However, what is reasonable for archival objects?
To increase the understanding of the effect of frequency of use on the rate of failure, condition survey results could be plotted against records of use [52]. If condition surveys record whether objects have been in circulation, but not the actual number of times it has been requested in a reading room, such information is not sufficient and conclusions can only partly be drawn for items that have not been requested. An example is the study of paperbacks in an academic library, the Georgia State University’s Pullen Library [25]. Because the condition of both circulated and non-circulated books was assessed, the authors were able to conclude that the damage to books which had not circulated was due to poor shelving practices. Interestingly, size was a major factor: taller and thinner volumes suffered more frequent damage than smaller, thicker books. From this example, it follows that the frequency of use could be a key factor in judging the relevance of other failure causes, such as design or storage.
Framework for the analysis of wear and tear
In the previous section, failure causes have been analysed individually, but they do not need to be seen as separate from each other. An analysis method frequently used to hypothesize root causes is the “fishbone diagram”. This is a “cause and effect diagram” where the main bone is the effect and the sub-bones are the causes. This diagram helps to “identify, explore, and display the relationship between an effect and all of its potential causes” [53].
Figure 2 shows a fishbone diagram that could be seen as a first approach to a systematic study of mechanical degradation in archival and library collections. In this diagram the classification of failure causes is based on information that can be collected during a survey.
Failure rate
During the lifetime of objects, different failure rates or, in other words, different reliability values can be observed. These differences are illustrated by the “bathtub curve” available in [54]. Three types of failure, with different failure rates, can be distinguished: early failures, useful life and wear-out failures. After a decrease in initial failures, the failure rate reaches a steady-state, called “useful life”, followed by a sharp rise at the end of the object’s lifetime during the wear-out period. This curve illustrates that, during most of the lifetime of an object, if failures occur, these are random, stochastic, and stress-related. However, during wear-out, failures occur notably more frequently, as the object deteriorates due to extensive hydrolysis, oxidation, fatigue etc.
When archival objects become part of a collection, the early failure stage, immediately after the manufacturing process, has already passed. Consequently, failures occur at a constant, low rate during the useful life of the objects, and the probability of failure increases during the wear-out period.
Let us consider the use of a book to illustrate the difference in failure rates. When a book reaches its wear-out period, because the materials, e.g. the sewing, have deteriorated (“aged”) or because the book had been intensively used (fatigue), the probability that the object will break increases notably. But if a book is still in its useful life period, perhaps due to good maintenance and/or low use, then the probability of such failure is considerably smaller. However, it can still occur, e.g. due to a design failure.
Different types of paper represent another useful example. Well-maintained and not misused rag paper will easily remain in the useful life period for the foreseeable future. On the contrary, ground-wood paper will rapidly reach the wear-out period due to faster chemical degradation. Failures will therefore occur and accumulate at a significantly higher rate in ground-wood paper than in rag paper, unless ground-wood paper has been maintained in a manner appropriate for this paper type (for instance, storage in a cool environment or deacidification treatment). In the wear-out period, ground-wood paper quickly accumulates mechanical degradation and its absence can only be explained by the absence of use.
It is also important to bear in mind that even poor quality paper has or has had a (short) useful life. Several surveys [18, 55] show that some younger paper was still flexible and usable despite its low pH, in contrast to similar but older paper, which was already brittle. This younger paper has probably not reached the critical degree of polymerisation (DP) value when the risk of failure upon use increases, and was therefore still in its useful life period. Recently, an experimental study has introduced actual actions of reading in the experimental design [28]. According to this study, the critical DP was found to be 300 and therefore for papers with a DP higher than 800, mechanical degradation occurred at a very low rate, independent of DP.