DECEMBER 2008 » book review

The Stone Age

Stone Conservation Principles and Practice
edited by Alison Henry
Donhead Publishing, Ltd., Shaftesbury, UK; 2006
352 pp; hardcover; 122 b&w photographs; $84
ISBN 978-1-873394-78-6

Reviewed by Raymond M. Pepi

Buildings constructed with natural stone are subject to the relentless forces of environmental decay and weathering. It is shocking to witness the rate of stone decay and the devastating alterations to beloved monuments, especially outdoor statuary. The common public perception is that acid rain is the main culprit, but the real story is much more complex and nuanced, depending on chemical composition, geologic DNA, exposure, architectural detailing and maintenance.

The process of treating stone and halting or reversing the decay process is the subject of an important new book: Stone Conservation, Principles and Practice. This 14-chapter compendium by various experts, mostly from the UK, emerged out of an annual Stone Conservation course at the University of York. I think it is fair to say that it represents current attitudes and accepted practices in the United States as well as the UK. It is, on balance, a book aimed at conservators, architects, curators and contractors, but could be enjoyed by an interested general audience as well.

Consisting of a broad range of technical subjects, it is, with minor exceptions, more descriptive than analytical. The chapters about decay mechanisms contain no chemical formulas or quantitative measures, except for mentioning the chemical names of crusts and by-products. The passages describing chemical cleaners do not explain their basic properties or why they work or don't. Whether this is a shortcoming depends upon your point of view. It could be argued that a deeper understanding of the underlying processes would better inform those responsible for deciding upon a treatment, but this does not take away from the overall value of the book and should not dissuade anyone from reading it.

It will be observed that the term "restoration" is not used in the title. Professionals are careful about the term because it is often misleading and does not adequately capture the current attitude towards the preservation of historic buildings. Perhaps the main reason for this sensitivity is a well-documented record of destruction in the name of restoration. The informative chapter by Michael Drury titled "Restoration Versus Conservation" is a discussion that charts the philosophical evolution of the restoration and conservation field in the UK and, for example, the inherent conflicts facing professionals deciding upon a treatment for stones that are simply beyond saving.

Taken to the extreme does stone replacement qualify as a conservation treatment or is it an admission of defeat? Does replacement represent a loss of authenticity and the fruitless effort to return a building to some earlier appearance, negating the process of aging and cumulative alteration? As you might expect there is no simple answer. He repeats John Ruskin's famous invocation:

"But, it is said, there may come a necessity for restoration! Granted. Look the necessity in the face, and understand it on its own terms. It is a necessity for destruction. Accept it as such, pull the building down, throw its stones into neglected corners, make ballast of them, or mortar, if you will; but do it honestly, and do not set up a lie in their place."

Ruskin was a theorist and did not practice himself; can you imagine the earnest architect delivering such drastic news to the unfortunate owner of a decaying building? However, Ruskin also pronounced the central manifesto of conservation in The Seven Lamps of Architecture, published in 1849:

"Take proper care of your monuments, and you will not need to restore them. Watch an old building with an anxious care; count its stone as you would the jewels of a crown…do this tenderly, reverently, continually and many generations will be born to pass away beneath its shadow."

Drury makes the important distinction between restoration that is, on the one hand, based on documentation or sound evidence and, on the other, conjecture. The latter approach inflamed Ruskin: "Do not let us talk then of restoration…the thing is a lie from beginning to end." Conservation as it is practiced today aims for scientific documentation of the fabric to avoid any conjecture.

To fully appreciate the scope of this book it is helpful to know more about trends in the field. Although architects, scientists and artisans have been traditionally involved, architectural conservators are often responsible for making the many decisions regarding treatments. Conservators usually have graduate-level training in the science of historic building materials. They are committed individuals who advocate for the building fabric. Their tools are an interdisciplinary mix of geology, chemistry, physics, engineering, architecture, art history, contracting practice, stone carving, stone setting, rigging, waterproofing and more. Their working method is to review previous alterations and treatments, carefully document building conditions (stone-by-stone), test the performance characteristics of stone in the laboratory and in the field and then draw conclusions and recommendations based on this research.

A program of test treatments and full-scale mockups to test their decisions often follows this protocol. The time and expense required to carry out this level of investigation is often beyond the resources of the project or owner, but it must be advocated, whenever possible, to responsibly plan and specify treatments that won't inflict even more damage to fragile materials. Conservators are aware of the fact that, historically, much damage has resulted from well-intentioned treatments. Therefore the "Prime Directive" of conservators is to select treatments that are reversible or sacrificial.

This means that, for example, if a cement-based shelter coating is applied to failing stone it should be softer than the stone itself or capable of being removed if it is not performing as desired or if some future treatment has been found to take its place. Both the selection of the treatment and the ability to remove it are non-trivial matters that sometimes defy best efforts.

The chapter on the "The Role of Archeology" by Jerry Sampson reviews the process of collecting information about existing conditions. He describes the method of manually recording conditions on a drawing. Doing this manually certainly has its place, especially for small buildings. However there is no mention of the latest trend in the use of handheld personal digital assistants (PDA), mapping programs or electronic databases linked to CADD drawings. These new technologies are especially applicable for large buildings. They allow for greater portability, flexibility and manipulation of the data that can be organized or sorted by any attribute, condition or feature. With electronic databases, conservators have a greater role in the cost estimating process and are better prepared to formulate maintenance strategies, since many repairs need to be deferred or phased over time due to lack of resources. This chapter could have benefited from more legible illustrations, drawings and photographs to demonstrate the process of recording and interpretation.

Nicholas Durnan and Colin Muir authored a brief paper on "Principles and Practices." In addition to reviewing inspection protocols, they introduce elementary but important concepts such as plastic repairs and stone replacement. The section on non-destructive testing lists several processes (impulse radar, thermography, impact-echo, etc.) but does not describe how and where to use these techniques.

The chapter on "Sourcing and Selection of Stone For Repair," by Ewan Hyslop, a geologist, introduces rudimentary topics on the testing of stone and finding a suitable replacement stone. The use of petrographic thin sections is shown to be an important tool for sourcing stone, but other tests are simply listed without further explanation.

Clifford Price, a leading expert on the chemistry and conservation of building materials, authored the chapter on "Consolidation." Tampering with the internal composition of geologic stone millions of years old is notoriously difficult and unpredictable, and the history of this subject is replete with episodes of initial optimism based on newly discovered techniques followed by disappointment, when either the treatment or the stone begins to fail sooner than it should. Consolidation is the process of impregnating porous stone with resins or reagents that solidify once in the stone, shoring up the weak matrix.

Although Dr. Price manages to avoid chemical equations, the chapter is an excellent survey of consolidation using lime solutions, chemical reagents such as barium hydroxide and organic or inorganic resins to reconsolidate loose grains and structure. In addition to discussing currently preferred treatments such as acrylics and alkoxy silanes, Dr. Price describes controversial techniques such as calcium hydroxide, which was effective on 300 sculptural figures at the West front of Wells Cathedral between 1974 and 1986. Other important recent advancements have been made on this side of the Atlantic by Norman Weiss, who adapted an innovative use of tartaric acid to get silanes to work with limestone or marble (silanes work better on silicate-based stones such as sandstone), in a product marketed as HCT (hydroxylating conversion treatment).

The section on "Cleaning Techniques," by Kyle Normandin and Deborah Slaton, the only North American-based authors, is a very good review of cleaning technology and the criteria for choosing a cleaning regimen. There is a concise but welcome summary of soiling composition and stone properties as it might affect cleaning. The authors accurately summarize most of the pertinent cleaning methods (chemical, abrasive, laser, latex film, etc.) in formulaic charts matching methods with type of soiling and stone. One worries, however, that some enthusiastic reader might misunderstand that these charts are only a starting point for beginning a testing program and are not to be taken literally until a chosen technique is proven to be safe and effective.

Three chapters cover the main subject: limestone, sandstone and marble. They are ably written and each goes into further detail regarding geology, decay mechanisms, repair, consolidation and cleaning. Nicholas Durnan ably summarizes topics related to the conservation of limestone. He includes a heading of increasing use and interest in North America: the relative advantages and disadvantages of putty lime, pozzolanic putty lime and hydraulic lime for repairs, although the terminology may confuse the non-expert.

A timely chapter has been included on "Polychrome Stone." Christopher Weeks, the author, nicely summarizes the challenges of conserving painted stone-work. This subject involves the analysis of both paint and stone as well as their interaction.

An entire chapter is devoted to "Church Monuments," such as plaques and statuary, covering similar ground as earlier chapters.

The final chapter on "Graveyard Memorials" by Chris Daniels is a valuable review of the subject that will be of use to the many volunteers who inevitably wind up caring for decaying and damaged headstones.

The book is richly illustrated with 122 black-and-white photographs. Each chapter includes references and useful suggestions for further reading. There is one appendix prepared by Nicholas Durnan (Limestone) that is essentially a brief specification on the Practical Application Procedure for Putty Lime Mortar Repairs and Sheltercoat, and a detailed index.

Although there is still no standard textbook on the topic of stone conservation, the field has matured to the point where very competent and comprehensive practical guides such as Stone Conservation are being written by practicing professionals who share the same values and standards of practice. This book is very informative, and I recommend it, but it is not a rigorous manual or how-to book.

The coda is provided by Dr. Price who writes: "In focusing our attention on the consolidation and preservation of historic stonework, we should not ignore the function of buildings we seek to conserve, or the maintenance of the skills that went into their creation. These deserve just as much consideration as the stone itself and, if we are led to conclude that stone cannot always be made to last for perpetuity, we may be better able to redress the balance." TB

Raymond M. Pepi is the president and founder of Building Conservation Associates (BCA). With offices in New York City, Boston and Philadelphia, his firm has been involved in the restoration of hundreds of historic buildings. (See Profile, Traditional Building, July/August 2004, page 6.)

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