General Points on Artefact Treatments
D. Gilroy and I. M. Godfrey
Objects from the past have been described as having the character of both historic documents and of aesthetic entities (Caple 2000). Objects are sources of important information but also provide aesthetic experiences for those, such as museum visitors, who interact with them. Before beginning any treatment of an artefact therefore, consider these four factors:
- the history of the artefact;
- the materials present;
- how the artefact is constructed; and
- the condition of the artefact.
Knowledge of an artefact’s history is important to prevent the removal of material that may provide information about its past. A treatment as simple as surface cleaning could conceivably remove traces of DNA or fingerprints that provide information about an object’s past. Before attempting to remove a stain from a textile, for example, assess the nature and significance of the stain. Does the stain pose a risk to the textile itself? Does it provide information about past usage of the textile? Is the stain associated with a significant part of the textile’s history?
Often a balance needs to be struck between the need to preserve the object and the information contained either within or on it and either cleaning or restoration work that is designed to enhance the aesthetic qualities of the same object. In these instances it is important to have a discussion with all stakeholders to ensure that the best possible decision is made.
If some form of treatment is necessary, examine the artefact thoroughly to determine the material type, mode of construction and its condition. If there is any doubt about either the type of treatment to apply or the possible effect of a particular treatment, consult a conservator. It is also very important to be aware of any potential risks posed by chemicals used to treat objects. Consult references and material safety data sheets to fully familiarise yourself with the chemicals and precautions needed in their application.
Do not treat objects unless they are likely to continue to deteriorate if not treated. Keep conservation treatments to the minimum necessary for stabilisation of the object.
The methods described in the following chapters are for artefacts constructed of one material type that are reasonably sound. Composite objects, made up of different materials, are often difficult to treat. In some cases the components can be separated and treated individually. If this path is taken, exercise great care as reassembly may be difficult following treatment.
If an object is fragile, badly deteriorated or very valuable, seek advice from a conservator before attempting any treatment.
If possible, try treatments on scrap or spare pieces of material to gain experience and to observe the effect of the treatment on the material itself. If no extra material is available, attempt the proposed treatment on an inconspicuous part of the object itself. Only apply treatment to the whole object after this has been done.
Note that if an object needs conservation treatment its environment should be examined as carefully as the object itself. There is no point in treating an object and then returning it to the environment that may have contributed to its deterioration in the first place.
Conservation or Restoration?
The aims of conservation have been expressed as a balance between the oft competing aims of revelation, investigation and preservation (Caple 2000). Thus, while it may be more important to enhance the aesthetic quality of an oil painting by cleaning the surface (revelation), such an action may remove evidence of the paintings past. Analysis and investigation of an object is necessary in order to gain historic information and also to enable informed treatment and preservation decisions to be made.
With every object to be treated, make a decision as to the extent of the conservation treatment needed to investigate, document, preserve and/or enhance its aesthetic qualities. The intended use of an object, its condition, composition and possibly even the availability of conservation resources may all affect the final treatment proposal. Conservation may be either preventive or interventive, with interventive treatment being the minimum needed to stabilise the object and to prevent it suffering further damage. Restoration involves conservation treatment plus the repair or replacement of missing or damaged sections and is a process that aims to give the object an appearance that more closely resembles its form and colour from an earlier, significant stage in its life.
Restoration, although often carried out to enhance the aesthetic value of objects, is really only necessary when the replacement of missing parts will help stabilise and protect the object from further damage.
With all restoration work it is essential to know exactly how the original object was constructed. This may involve much research as the key to ethical restoration is the quality of the evidence that is available. Useful evidence may arise from:
- the symmetry of an object;
- the presence of patterns or lines of continuation;
- old photographs; and
- scientific examinations of the object, which may reveal old paint fragments, paint layers and the like.
Do not carry out any restoration unless there is definitive evidence of what should be reinstated or replaced. Any restoration work should use techniques and materials that affect the object least. Ideally the materials incorporated into a restored object should also be detectable, though not necessarily conspicuous and be able to be removed should further work be needed in the future. In theory, nothing original should be taken away, but in practice (and often for the greater good) material from the centre of an object is sometimes removed to make way for dowels or other jointing methods that do not distort the surface.
Restoration techniques are beyond the scope of this book and are dealt with only briefly.
Whenever an object is treated it is essential to keep a detailed, permanent record of the treatment. This record should include details of the:
- condition of the object before the work was done;
- treatment carried out; and
- condition of the object following treatment.
The aim of the record is to provide as much information as possible about the object. Include photographs, drawings and written descriptions of the object where relevant. In many cases a sketch and written description will provide more information than a photograph.
Also add to the record any known history of the object, dimensions, the size of cracks (if any), missing parts, colours, component materials, treatment chemicals and procedures.
Basic Chemical Information
Many of the treatments described in this book involve the use of chemicals. Take the following precautions when mixing and handling these substances, especially acids:
- always wear rubber gloves and protective glasses;
- work in a well-ventilated area; and
- when mixing and diluting chemicals, always add the measured quantity of the chemical to the water, never the reverse. This is especially important with acids and alkalis.
Chemicals come in both solid and liquid forms. Citric acid, for example, is a solid whereas concentrated hydrochloric acid is a liquid. When solids are involved in a formulation the composition is usually expressed in parts by weight of solid, whereas liquid formulations are often expressed in parts by volume. For example, a solution of involving solid citric acid may be expressed as 5 g per litre of water whereas a solution involving liquid polyethylene glycol 400 may be expressed as 200 ml per litre of water.
For most of the formulations mentioned in this book kitchen scales are accurate enough for weighing purposes and any convenient volume measure can be used.
A solution made up of a solid in water may be specified as weight of solid per weight of water (w/w) or, more usually, as the weight of solid per volume of water (w/v). The two are almost equivalent as 1000 millilitres of water weighs about 1000 grams. Examples are given below.
citric acid (10%, w/w) = a solution in which 10g of citric acid is dissolved in 100g of water
citric acid (10%, w/v) = a solution in which 10g of citric acid is dissolved in 100ml of water
Acids and alkalis - Concentrated acids and alkalis in liquid form are sold by specific gravity (SG) as they are not necessarily 100% strength.
Specific gravities of some commonly used solutions are listed below:
The convention adopted in preparing some dilute solutions is to assume that the commercial chemical is 100% and to dilute it accordingly. For example:
10% sulfuric acid = 100ml of sulfuric acid (SG 1.84) dissolved in 900ml of distilled water
50% ammonia solution = equal volumes of ammonia (SG 0.88) and distilled water are mixed
There are exceptions however as concentrated hydrochloric acid (SG 1.18) is only a 32 % solution in water. It is most important therefore to read the labels associated with all chemicals so that solution concentrations are calculated correctly and any precautions associated with their use are also taken into account.
The strength of a hydrogen peroxide solution is indicated by the number of volumes of oxygen gas produced by the decomposition of one volume of solution. It is available commercially as follows:
100 volumes = about 30% hydrogen peroxide
20 volumes = about 6% hydrogen peroxide
10 volumes = about 3% hydrogen peroxide
These solutions are supplied in amber coloured bottles. Store them in a cool place, protected from light (a refrigerator is ideal).
Mix chemicals thoroughly in glass, stainless steel or enamel-coated containers. Do not use concrete water troughs, aluminium, copper or brass containers as the chemicals may damage them. Do not use polyethylene bowls as these may distort with the heat generated during mixing of certain chemicals. Sodium hydroxide for example, produces a lot of heat when it dissolves in water.
Once the chemicals are mixed and cold, store them in glass or polyethylene containers. The majority of treatment solutions can be re-used a number of times. Discard the solutions when they are no longer effective and prepare fresh ones.
Disposal of Chemicals
Local authorities restrict discharge of chemicals directly into a sewage system. Obtain copies of local by-laws and follow the regulations regarding chemical disposal.
All chemicals may be classified as being either acidic, neutral or alkaline. The pH value of a chemical gives an indication of its acidic or alkaline nature.
pH values range from 0 to 14. A neutral solution has a pH of 7 whereas an acidic solution has a pH less than 7. The more acidic a material the lower is its pH value. Alkaline solutions have pH values greater than 7. The more alkaline a solution is, the higher its pH value.
The pH value of a solution may be determined in a number of ways. These range from the use of sophisticated equipment to impregnated papers that give characteristic colours in acidic or alkaline solutions. In the latter case, the colour of the paper after it has been dipped in the solution is compared with standard colour charts to determine approximate pH values. Colours range from red (acidic) through pale orange (neutral) to dark blue (alkaline).
Vinegar (acetic acid) and lemon juice (citric and tartaric acid) have low pH values and turn the paper red, whereas household soap and cleaning fluids usually have high pH values (alkaline) and turn the paper blue. Tap, distilled or rainwater should have a neutral pH value close to 7.
Usually pH measurements are used to determine whether all the chemicals have been removed by rinsing following the treatment of an object. Moisten pH paper with distilled water and place it on the clean object. Any traces of acid or alkali from the cleaning solution will show up as blue or red respectively. Repeat washing until the pH paper does not change colour.
Caple, C., (2000), Conservation Skills: Judgement, Method and Decision Making, Routledge, London and New York, pp 256.