Art Conservation

Mechanical Properties and Testing of Materials

W. (Bill) Wei

• Publisher: Jenny Stanford Publishing
• Year: 2021
• Print ISBN: 9789814877688
• eBook ISBN: 9781000371550
• Topic: Art
• Subtopic: Art General

Conservators and other museum professionals face a large number of issues involving the mechanical behavior of materials, including questions on craquelure, restoring physically damaged objects, art in transport, or the selection of adhesives. However, science in conservation and museum studies curricula focusses mostly on chemistry. This book fills this important gap in conservation training. It is the first such book written specifically for the conservation community and professionals with little or no background in (mechanical) engineering. It introduces the basics of mechanical properties and behavior of materials and objects with examples and exercises based on conservation practice. More complex issues of mechanical loading and advanced solutions are also introduced.

Chapter 1

Introduction

The scientific and technical world of the conservation and restoration of art and other objects of cultural heritage is largely based in chemistry. This is evident in the majority of questions which must be answered when making conservation decisions, such as

• what is the chemical composition of the materials in an object,
• what is the state of aging of the object,
• what kinds of chemicals and materials can be safely used for a conservation treatment, and
• what climate conditions, e.g., light, temperature and relative humidity, can an object be displayed and stored in?

However, conservators face a large number of questions in their work which involve the mechanical behavior of materials. Examples of problems which they have to deal with include

• bending and cracking in wooden objects such as panel paintings and furniture,
• reinforcing or repairing joints in statues,
• hanging of textile wall tapestries (Fig. 1.1a),

  

  Figure 1.1 Examples of mechanical issues which face conservators: (a) Hanging of a wall tapestry, (b) craquelure in an oil painting, (c) failure of bonds holding copper coils to the metal frame of an outdoor sculpture (“Tong van Lucifer” (1993) by Ruud van de Wint, located on the Knardijk near the A6 highway, The Netherlands), and (d) vibration and shock during the transport of works of art.
  

• craquelure in oil paintings (Fig. 1.1b),
• deformation and wear of metal objects,
• effects of (cyclic) changes in temperature and relative humidity on (outdoor) sculpture (Fig. 1.1c),
• paint loss,
• repair of tears and cuts in canvas paintings,
• strength of adhesives and consolidants,
• tension in painting canvasses, and
• vibrations and shock during the transport of objects, or during construction and other events in and around museums (Fig. 1.1d).

There are, however, other situations where it is not as obvious that mechanical stresses play an important role in the long-term condition of objects. Many such situations are related to the climate conditions noted above, including

• the effects of cyclic changes in temperature and relative humidity on canvasses,
• the effects of cyclic changes in temperature and relative humidity on wood panel paintings or wooden furniture,
• cracking of reinforced concrete art and architecture due to corrosion of the steel reinforcement bars,
• the concept of internal stresses, and
• dimensional changes (expansion or shrinkage) of adhesives.

The scientific curricula of virtually all conservation training programs around the world concentrate on the chemistry and physical properties of materials. Mechanical properties are almost never taught to any extent. It is therefore not surprising that there is much confusion, misinformation and uncertainty in the conservation world when it comes to dealing with mechanical aspects of restoration treatments.

In spite of this, when asked what terms they can think of that have to do with mechanical properties, most conservation students surprise themselves with how many words they can come up with. An alphabetical list of words compiled by the author in eight years of teaching daylong workshops on mechanical properties is shown in Table 1.1. If one now reorganizes these words into related groups, one sees that there are four main categories related in some way to the words “force,” “weight,” “deformation” and “failure,” (see Table 1.2). These terms lie at the core of the engineering concepts of the mechanical properties of materials in this book.

Table 1.1 Words related to mechanical properties familiar to conservation students

Break	Energy	Shrinkage
Brittle	Expansion (thermal]	Strength
Contraction	Force	Stress
Crack	Friction	Stretch
Damping	Gravity	Swelling
Deformation	Kilogram	Tear
Density	Loose	Tension
Dent	Mass	Tight
Elastic	Plastic	Wear
Elongation	Pressure	Weight

Table 1.2 Categorization of the mechanical property terms in Table 1.1

Force	Weight	Deformation	Failure
Damping	Density	Brittle	Break
Energy	Kilogram	Contraction	Crack
Force	Mass	Deformation	Tear
Friction	Weight	Dent
Gravity		Elastic
Loose		Elongation
Pressure		Expansion (thermal]
Shock		Plastic
Strength		Shrinkage
Stress		Stretch
Tension		Swelling
Tight

The objective of this book is to teach the basics of the mechanical properties and testing of materials. While learning these basics, it is, however, important to remember that every profession, be it art conservation, chemistry, history or engineering, has its own language and vocabulary. As with any foreign language, certain words mean certain things. They may look like something in another language, but if they are used in the wrong way, misunderstandings and confusion will certainly be a result. This should be taken into consideration when an engineer with no background in art or conservation is consulted on some aspect of mechanical testing.

This book begins with two chapters on the most important concepts and terminology of the mechanical properties of materials. Key terms are presented in bold-faced type the first time they are introduced. The most commonly used methods for mechanical testing are then described, and the book concludes with a discussion of a number of advanced concepts which conservators may come across in their work. It is hoped that the minimum which the reader comes away with after reading this book is the proper use of the terminology and language of this subject.

It is impossible to completely avoid mathematics in a field such as this, but the mathematics in this book has been limited to basic equations for the understanding and calculation of the mechanical properties of materials. Furthermore, all units for the various quantities given in this book are expressed in standard international (SI) units, in line with standard international scientific practice. Appendix 1 provides conversions between the SI units used in this book and their English (Imperial) counterparts which are still commonly used in industry, among others, in the United States of America.

Before continuing, two wor...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Preface
• 1. Introduction
• 2. Static Mechanical Properties
• 3. Dynamic Mechanical Properties
• 4. Mechanical Testing
• 5. Advanced Concepts
• Epilogue
• Appendix
• Index