UCL Institute for Sustainable Heritage


Collections Demography - On Dynamic Evolution of Populations of Objects

Large heritage collections are managed in the context of four aspects: (i) collection use, (ii) material properties, (iii) environmental considerations and (iv) resource requirements. In the context of these four aspects, informed decisions are possible if evidence is available. 

The four aspects are not independent, which is why gathering the required evidence is a long and research-intensive process. By focussing on reasonably well-researched library and archival collections, the Collections Demography project developed the terminology and the tools that are required to develop and visualise such evidence.

Collections Demography

Left: The National Archives (UK) store and right: the historic library at Brodsworth Hall (English Heritage) – two very different collections in terms of collection use and environmental considerations.

The developed collection model treats large collections as if they were populations, using principles of demography. Populations grow, are influenced by diseases and environmental agents and are assessed using census methods. Similarly, collection items are accessed due to their value and used, they degrade and may become unfit with time. The key elements in this appraisal cycle are how the objects are valued and by who, how the impact of endogenous and exogenous agents can be mathematically modelled, and at what point an object might be considered unfit for use, which could require a resource-intensive conservation intervention.

Collections Demography graph

 The cycle of collection appraisal. 

In heritage science literature, and particularly in preventive conservation practice, damage functions are used to model material behaviour and specifically damage (unacceptable change), as a result of an agent over time. For such functions to be of use, it is important to define:

  • The key stakeholders (e.g. the public, curators, researchers)
  • The principal mode of use is (e.g. display, storage, physical use)
  • The long-term planning horizon is (i.e. when in the future it is deemed acceptable for an item to become damaged or unfit for use)
  • The threshold of damage (i.e. extent of physical change assessed as damage).

Collections Demography - parcels

Objects that are no longer fit for use, can no longer be accessed by readers and the benefits of such a collection are diminished. 

By engaging with almost 550 volunteers in The National Archives (Kew), English Heritage, Library of Congress (Washington), and the Capitol Visitor Center (Washington), we explored how visitors value archival and library objects, what their attitudes to degradation are and when in the future they would accept that objects are no longer be available for reading or display in a satisfactory state. 90% of respondents found 500 years to be an acceptable long-term planning horizon.

With further 330 volunteers in The National Archives (Kew), the Wellcome Library (London), Library of Congress (Washington), Capitol Visitor Center (Washington) and the Dutch National Archives (The Hague), we explored the threshold fitness for display or reading in the context of access in reading rooms or exhibitions. Change was considered in the context of discolouration and mechanical deterioration such as tears and missing pieces: forms of physical deterioration that accumulate with time in libraries and archives. 


Typical forms of change that accumulate during storage and use of objects: discolouration, tears and missing pieces.

For objects of no particular pre-assigned value (e.g. a random archival or library object), missing pieces influenced readers/visitors’ judgements of fitness-for-use significantly more than discolouration and tears. It can be estimated that, in general, items become unfit when text is evidently missing. However, if the visitor/reader is prompted to think of a document in terms of its historic value, then change in a document has little impact on fitness for use.

As a result of physical use of library and archival documents, defined as reading in the context of general access, mechanical degradation (wear and tear) accumulates with the number of instances of reading. 

The degree of polymerisation (DP) of historic paper was shown to significantly affect the rate of accumulation of wear and tear. While at DP > 800, the accumulation process is random, a function was developed for documents with DP between 300 and 800. For objects with DP < 300, one large missing piece (i.e. such that contains text) developed on average with each instance of handling, and this DP value was defined as a threshold value for safe handling.

Collections Demography - paper

Model historical documents after 60 instances of reading: large pieces of text become detached and such objects would be considered unfit for use by the majority of archival and library users.

The developed model of accumulation of large missing pieces enables us to calculate the period of time required for an object to become unfit for use by readers in the context of general access. For example, in the context of the average frequency of document use at The National Archives (Kew), this period is 60 years for the category of very papers with DP 300, and 450 years for papers with DP 500. At higher DP values, this period of time increases beyond the long-term planning horizon of 500 years. For such papers, accumulation of wear and tear is not a significant collection management concern.

To model the decay of DP over time, we modelled 121 paper degradation experiments with known temperature, humidity of the environment, and acidity of paper (pH). New types of plots have been developed: isochrones, linking points of equal expected ‘lifetime’, i.e. time until an object is expected to reach the state of threshold fitness-for-use. 

Graph - Collections Demography

Isochrones for typical acidic paper from early 20th century, and for a typical pre-1850 rag paper. The choice of appropriate storage temperature and relative humidity to reach the required long-term planning horizon (e.g. 500 years) is straightforward.

We also modelled the demographic curves for a research collection of historic papers (SurveNIR collection), exploring the loss of fitness for use with time. The collection as a whole will degrade in two waves, the first wave (~30% of the collection) represents acidic papers, while the second wave will largely be able to survive 500 years at room conditions. The model also enables us to evaluate the effects of cold storage or of deacidification (a typical interventive conservation treatment).


Demographic curves for a research collection (SurveNIR) stored at room conditions, and consisting of 30% of acidic paper, which will degrade in the first wave (and are separately presented with the red dashed curve). In the usual Western library or archive, there is 70-80% of acidic paper. 

The new tools thus enable us to evaluate and visualise scenarios of management of the storage environment as well as levels of access, for different types of library and archival paper. In addition, the costs and benefits of conservation interventions can be evaluated. The limitations of the function are the context of use (dark storage), exclusive focus on properties of an average paper type, and de-prioritised effect of pollutants; however, the latter can be considered separately. 

This work also demonstrates that transparent and publically accountable collection management decisions can be informed, and challenged by, effective interaction with a variety of stakeholders including the lay public.

We are currently working on developing the model into an interactive online tool.

To read more about our research, please see the series of publications:

For the UCL press release see: http://www.ucl.ac.uk/news/news-articles/1215/231215-shelf-life-library 


Matija Strlic, m.strlic@ucl.ac.uk 


University College London

Principal Investigator:

Matija Strlic, Institute for Sustainable Heritage
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Kalliopi Fouseki, Institute for Sustainable Heritage 
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Jinghao Xue, Department of Statistical Science
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  • Dr Catherine Dillon
  • Dr Carlota Grossi
  • Ann Fenech 
  • Eva Menart

University of East Anglia


Peter Brimblecombe
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The National Archives, Kew


Nancy Bell
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  • Dr William Lindsay
  • Kostas Ntanos

English Heritage

Project Partner: 

David Thickett
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Dr Paul Lankester

Library of Congress

Project Partner: 

Fenella France
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Nationaal Archief

Project Partner:

Gerrit De Bruin

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The project wouldn’t have been possible without the generous support and help from:

British Library: Fiona McCarthy

City Binders: David Stevens

English Heritage: Richard Askew, Madeleine Hawkins, Tina Hornsby, Robin Matthews, Caroline Rawson, Richard Smith-Gore 

Library of Congress: Cindy Connelly Ryan, Meghan Hill, Matthew Kullman, Ray Privott, Mark Sweeney, Dianne van der Reyden, Hans Wang

Manchester Museum: Nick Merriman

Nationaal Archief: Gabriëlle Beentjes, Rene Gardien, Roberto Padoan 

The National Archives: Hannah Clare, Dinah Eastop, Richard Williams

University College London: Tiphaine Bardon, Matthew Chipping, Carolien Coon, Katherine Curran, Josep Grau, Andrej Kos, Pakhee Kumar, Julia Martinez, Keith Razey, Jane Robb 

University of Leicester: Lisanne Gibson

University of Strathclyde: Lorraine Gibson, Gemma Mitchell

Wellcome Library: Gillian Boal, Sharon Messenger, Stefania Signorello

As well as 874 volunteers.

We are grateful for financial support by the AHRC/EPSRC Science and Heritage Programme and Nationaal Archief