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A review of evaluation of cellulose ethers for conservation

Alan Derbyshire
Senior Conservator, Paper Section

Jane Rutherston
Contract Conservator, Books Section, Conservation Department

In 1990 The Getty Conservation Institute published Evaluation of Cellulose Ethers for Conservation by R. L. Feller & M. Wilt. This study was carried out by the Research Center on the Materials of the Artist & Conser-vator, Mellon Institute, Pittsburgh, through a contract from the GCI.

This book presents the results of a three year research programme into the ageing behaviour of cellulose ethers. The two objectives of the book are (1) to provide information about the chemical nature of cellulose ethers and (2) to provide a relative ranking of a number of generic chemical classes of cellulose ethers with respect to their long-term stability. The book succeeds in both these objectives.
The text is divided into seven specific chapters. Chapter 2 'Properties of Cellulose Ethers' gives the reader a clear under-standing of the manufacture and chemical properties of cellulose ethers. The previously unpublished investigation into cellulose ethers carried out by Dr J. Arney and his assistant Kate Novak, 1980-81, is discussed in Chapter 4.

There is an interesting chapter (3) on 'Standards of Stability' which needs to be carefully read in conjunction with the results and conclusions. As with the rest of the book, however, this chapter is highly technical and may initially seem inaccessible to the conservator.

A number of very useful appendices include a literature search in Art and Archaeology Technical Abstracts (AATA) and a questionnaire on the use of cellulose ethers by conservators. However, as this questionnaire was completed only by members of the Book and Paper Group of the AIC, it is, inevitably, biased towards the use of cellulose ethers in paper conservation.

There are also appendices which detail the availability of cellulose ethers and provide technical notes on methyl cellulose and carboxy methyl cellulose (CMC) - probably the two most widely used, and, fortunately, the two most stable of the cellulose ethers. These technical notes provide essential information on such things as how to prepare solutions along with safety, toxicity and handling requirements.

There are a large number of results, presented clearly in graph and table form, to sift through. The problem for the reader, however, is to be able to put all these results together, meaning-fully, and then come up with some definite conclusions.

The authors themselves conclude that only methyl cellulose, carboxy methyl cellulose and hydroxy propyl methyl cellulose (which is mainly of methyl cellulose composition) can be considered to have excellent long-term stability. There is still a question mark against Cellofas B-3500 (a CMC), however, which darkened on ageing when applied as a size, confirming previous findings. This highlights the problem that because of the unavoidable variation in the raw materials and chemical processing of cellulose ethers, the same generic chemical class can be found to be significantly different in terms of stability. In fact the authors describe a 'Simple Test of Thermal Aging' by which conservators, themselves, can test commercially available cellulose ethers.

Organic solvent-soluble cellulose ethers, ethyl cellulose, and the organic-soluble type ethyl hydroxy ethyl cellulose are considered, by the authors, to be of short-term stability and not suitable for long-term application in conservation. Similarly hydroxy ethyl cellulose is not encouraged for long-term application. Special attention is paid to hydroxy propyl cellulose (Klucel) because of its widespread use in conservation. Unfortunately the results are not that clear and are not that clearly presented. Some confusion arises because the authors have chosen to investigate Klucels L, G, H and M even though it is only Klucels L and G that are normally used in conservation. The high molecular weight Klucels (H and M) are definitely found to be unstable but the authors tentatively place Klucels L and G in the 'perhaps intermediate' category. For example, when applied as a thin film or as a low percentage size, Klucels L and G appear quite stable but on other thermal ageing tests they were found unstable. The authors themselves seem aware that this is an unsatisfactory conclusion and state that more testing is necessary.

The authors' final ranking with respect to the long-term stability of cellulose ethers, from most to least stable, is:
methyl cellulose,
carboxy methyl cellulose, water-soluble ethyl hydroxy ethyl cellulose,
hydroxy ethyl cellulose,
hydroxy propyl cellulose, organic-soluble ethyl hydroxy ethyl cellulose
ethyl cellulose.

Again the reader may find this confusing because hydroxy ethyl cellulose is ranked more stable than hydroxy propyl cellulose (Klucel) but hydroxy ethyl cellulose is, itself, not encouraged for long-term application.

To summarise, this study has obviously come up with some very useful information on cellulose ethers. It gives us confidence in continuing to use methyl cellulose and carboxy methyl cellulose but hopefully will discourage conservators from using ethyl cellulose, organic-solvent ethyl hydroxy ethyl cellulose and hydroxy ethyl cellulose. Perhaps the most disappointing aspect of the book is that the study still leaves conservators somewhat '` confused as to whether or not to continue using Klucels L and G.