For more than ten years, the Science for Conservators series has been the key basic texts for conservators throughout the world. Scientific concepts are basic ot the conservation of artefacts of every type, yet many conservators have little or no scientific training. These introductory volumes provide non-scientists with the essential theoretical background to their work.
This chapter deals with some of the general features of dirt and points to some of the factors that need to be considered when carrying out cleaning in conservation work. Physical and chemical reasons for the adherence of dirt to objects are discussed and the basic principles behind cleaning techniques are outlined.
A What Dirt is and Why You Clean
Dirt can be defined as material which is in the wrong place, rather as a weed is thought of as a plant growing in the wrong place. The evidence of blood on textiles or the remains of food in a vessel may be considered âdirtâ on modern objects but may have to be carefully preserved on ancient artefacts. As a conservator, you often need to be able to remove material which is in the wrong place (for a variety of reasons) without removing material which is in the right place. This process is frequently complicated by the fact that the substance of the object may be very similar to the dirt.
A major objective of all conservation treatment is to increase the chemical stability of the object being treated. Cleaning often forms an important part of the stabilizing process. This is because dirt on an object can be a potent source of deterioration (as, for example, when chloride salts set up corrosion reactions on bronze, or moulds grow on organic materials like paper or textiles). At other times, cleaning may be a necessary preliminary to a further treatment, as when preparing a surface before coating or joining. In many instances cleaning requires delicate judgement and experience on the part of the conservator, in deciding what the final appearance of the object being cleaned should be and how much âdirtâ should be kept (patina, historical evidence, etc). It isnât easy to choose a cleaning method when you want to remove only part of the dirt.
cleaning
It is helpful to begin by classifying dirt into two categories:
Foreign matter which is not part of the original object.
Examples: soot, grease, stains, adhesives and fillings from old repairs.
Products of alteration of the original material of the object.
Examples: metal corrosion products, yellowed varnish, decayed timber or stone.
The scientific basis of this classification of dirt is to be seen in the distinction between physical mixtures and chemical compounds drawn in Book I (Chapter 2). Dirt which is foreign matter was not originally present in the substance of the object but has later become mixed with it. In contrast, dirt which is a product of alteration has formed through a chemical combination of the original material with chemicals from the environment such as gases in the air or salts in solution from soil or the sea.
foreign matter
product of alteration
âForeign matterâ, however, can give rise to âa product of alterationâ if a chemical reaction occurs between the dirt and the object. Nevertheless, even if nothing so harmful as this does occur, it is unwise to assume that loose foreign matter (dust) is innocuous or that it can be safely left as a thick layer on objects in store. Dust is commonly an amazing mixture of fragments of human skin, textile fibres, carbon particles (soot), and grease from unburned hydrocarbon fuels, from cooking and from the skin of people and animals. There are often many salts in dust, for example, sodium chloride (carried in from sea spray or on skin fragments), and sharp gritty silica crystals are often present. In this chemical mixture are the spores of countless moulds and fungi and micro-organisms which live on the organic material in the dust. These organisms are equally likely to attack objects made of organic material. Much of this dirt is hygroscopic (water-attracting) and this tendency can encourage the growth of moulds and increase the corrosiveness of salts. So even dust is damaging, although perhaps only slowly.
dust
If the dirt is a product of the alteration of part of an object you can see immediately that by removing it you are actually taking away some of the object itself. Thus the tarnish on silver is black silver sulphide formed by reactions of the silver in the object with hydrogen sulphide and moisture from the air. When you clean away the tarnish you not only remove the sulphur atoms, which are foreign matter, but also some silver atoms originally positioned by the silversmith. Hence, âcleanâ though it may be, the object is less than it was and the surface is not the original one. Perhaps the tarnish should have been left on? Perhaps it would be better to reverse the chemical reaction to recover the silver metal? Science cannot answer the ethical question whether the tarnish ought to be removed, but it may offer methods which avoid removal and get a clean surface. It gives you options beyond âtake it or leave itâ.
To remove dirt which is a product of deterioration necessarily involves taking away some of the original artefact. In principle, removing dirt composed of foreign matter does not imply this. In practice, however, it is rarely possible to separate the dirt from the object without taking some of the object too. The difficulties derive from the natural or corrosion-induced porosity of the surface and the extreme fineness of the dirt as it starts to coat the object. Soot particles in smoke may be as small as one micron (Ό) in diameter (
th of a millimetre) and will penetrate the finest crevices of a surface. In solution the foreign matter can reach an atomic scale of fineness, and it will be deposited when a solvent being used for cleaning evaporates. To remove only the dirt when it is so intimately mixed with the object is not easy and becomes more and more difficult the cleaner you want the final product to be. The penetration of crevices in a fragile porous surface during the cleaning process can easily cause the thin walls between the pores to break down. Some degree of damage to the object itself is likely, as illustrated in Figure 1.1.
Figure 1.1Different levels of cleanness showing how damage can occur through over-cleaning.
There are many questions that need to be considered when undertaking or deciding on a cleaning treatment. Some have already been raised in describing the nature of dirt itself but there are many others which relate to questions of conservation ethics and historical study. Science cannot take from you the responsibility of making these judgements but it can explain the actions and consequences of a wide variety of cleaning treatments which will help to inform your decisions. Faced with a dirty object you need to ask (and answer) the following questions:
Why clean?
Is it dirt? Should some or all of it remain?
Is the dirt doing damage?
Can the object tolerate being cleaned?
What are the physical and chemical properties of (a) the object; (b) the dirt?
What will affect the dirt without affecting the object?
What will be the effect of cleaning?
What will be the appearance of the object after cleaning?
Will the stability of the object be affected?
How often will the object need cleaning in future?
How can you clean the object?
Is there a suitable treatment?
How does the treatment work?
Is the treatment safe both for you and the object?
When do you stop?
B What Holds Dirt in Place?
Dust may adhere only lightly to a surface or may become mechanically trapped in the interstices of a porous solid (as shown in Figure 1.1) or, even more obviously, may get trapped in fibrous material such as textiles. The dirt may simply be entangled in fibres, or particles may be locked into crevices like pieces of a jigsaw fitting together. Dust may also be held on a surface by electrostatic attraction: the generation of static electricity by friction was described in Book I (Chapter 4). When two materials are rubbed together the friction forces at the point of contact can knock loosely held electrons off one surface onto another. The surface which gains electrons is said to be negatively charged and the one that has lost them becomes positively charged. Which surface becomes positively or negatively charged depends upon what is rubbed with what, but both types of charged surface act equally as dust collectors. When you polish a surface, mobile charged particles (electrons) are rubbed off the polishing cloth and deposited onto the surface being polished.
electrostatic attraction
negative and positive charges
If you are going to deal with the problem it is useful to know:
How the dust is attracted
Why the electric charge stays put, and
What can be done to disperse or avoid a build-up of static charge.
Charged surfaces attract dust because the dust particles themselves contain electrons which can be displaced. Figure 1.2 illustrates how this happens.
Figure 1.2
The attraction between the surface and the + end of the dust particle is stronger than the repulsion between the â end and the surface because the + end is nearer.
Once an electrically charged surface has been produced there is no way of preventing the dust moving on to it. Attempts to remove the dust by wiping will not work because more friction is created in the process and reinforces the static charge. The dust will jump straight back from the duster to the charged surface. The answer is to get rid of the charge, or if possible not to generate it at all. To understand these manoeuvres you need to know more about the contrasting electrical properties of materials; the consequences of their being either conductors or insulators.
conductors and insulators
It is a familiar fact that metals conduct electricity while most other materials do not. This observation is one of the pieces of evidence upon which the models of atomic bonding are based. In metals some electrons are shared among all atoms and are readily able to move throughout the solid (see Book I, Ch...
