Why do objects deteriorate over time even though they have survived so long? The most common causes are:
- Accidental damage
- Atmospheric pollutants
- Exposure to too much light
- Fluctuating humidity and temperature
- Inappropriate methods of repair
- Poor handling
- Poor storage
Deterioration of Metals
Metals will react if exposed to water. Moisture, even that in the air, can accelerate corrosion, and over time will cause considerable damage. Archaeological metals may be contaminated with ions (salts) that accelerate corrosion, especially upon exposure to the open air after excavation. These objects continue to be very much at risk until they can be stored in a permanently dry environment. Decorative metals also deteriorate as a result of high humidity.
Another factor to consider is that of pollution. The mixture of pollutants around us in the 21st century poses a real threat to metals; these are produced by materials within the Museum, as well as from outside. Sulphur pollution will result in the tarnishing of silver, whereas acidic gases released from the wood of oak cupboards will eventually reduce lead objects to powder. The effects of pollution are often aggravated when the humidity rises.
Gloves should always be worn when handling metal objects, as chemicals given off by bare hands, however clean, leave fingerprints that etch into the surface.
Exposure to light is a less important factor; however, ultraviolet light has been shown to oxidize metals over a long period of time.
Some of the treatments used in the past are now considered to have been too aggressive. At one time archaeological copper alloy objects were routinely chemically 'stripped'. This involved removing all of the corrosion layers, often losing all the evidence of decorative inlay, mineralized textiles, etc., leaving the metal surface matt and pitted.
Deterioration of Textiles
and damaging repairs.
Textiles are very sensitive to light, which can quickly fade dye pigments. Exposure to light will also break down the chemical structure of textile fibres, making them weak and brittle.
Poor handling damages textiles. Even clean hands will leave traces of oil behind, so gloves should always be worn. Many historic textiles are so fragile that any touch will cause damage. Therefore textiles are stored in folders, or on trays or 3D mounts that allow the object to be examined without the need to touch it.
Materials used in cupboards, shelves and drawers (e.g. oak or chipboard) can be the source of atmospheric pollutants such as acidic gases, which damage textile fibres and dyes.
Textiles must be supported in storage. A robe decorated with heavy embroidery should not be hung as the weight is concentrated on the shoulder area where the fibres will be strained and will eventually break.
A textile object can be completely destroyed by an infestation of moth or carpet beetle. Storage areas should be kept clean and monitored for insects.
Previous treatments on textiles can cause damage to fabric weaves if stitching is too tight. Old adhesives can discolour and harden, making the textiles weak, brittle and prone to splitting.
Deterioration of Casts
Casts made of plaster are particularly vulnerable to damp conditions and accidental damage when they are touched or moved. Larger casts can become unstable if iron armatures inside them start to rust, staining and cracking the plaster from within. Mould can grow on surfaces that were originally painted with oils, waxes or even milk.
Light will not harm plaster but can age and discolour surface coatings.
Plaster is easily scratched and chipped by fingernails or hard dusting brushes. Fine dirt particles can make their way into the porous surface and their removal is very difficult and potentially damaging.
Deterioration of Ceramics
The main hazards for ceramics are soluble salts, pollution and poor handling. Wood and wood products give off pollutant gases, such as acetic acid, which can react with salts within the body of the ceramic. This results in crystals forming under the surface and their growth is accelerated by fluctuating humidity. Eventually the crystals break through the surface forcing it to split and flake.
Great care is needed when handling ceramics. However robust ceramics may appear, they become more fragile with age. Sometimes the joins between the handles and the body of the pot weaken and they may break if treated as though they were modern pottery.
Light is not harmful to ceramics as such, but pigments used in surface decoration could be damaged by over exposure.
Some old repair methods have caused damage in the long term. Very strong adhesives were used in the past, but in ageing they have been found to discolour and shrink, and in shrinking a layer of the ceramic can be pulled away from the body of the pot. Today's conservators have a wide range of adhesives from which to choose. Those used with ceramics will usually be weaker than the ceramic body to prevent too strong a join from causing further damage.
Deterioration of Paper
on a Japanese screen.
Paper is an incredibly strong material, despite its apparent fragility, provided it is treated properly. There are as many reasons for deterioration as there are kinds of paper. Works of art on paper are composed essentially of two layers: the paper itself and the pigment(s) lying on the surface.
Inherent instability - This may include fragments of metal from machinery dislodging and being formed in the sheet; with time this metallic fragment will deteriorate, particularly if exposed to moisture, resulting in deterioration and discolouration such as rust.
Composition - It is also important to understand the incompatibility of certain materials, and this is often a problem in the formation of a work of art on paper. Iron gall ink as it breaks down will degrade the support and 'eat' through it, leading to the complete deterioration of the work.
Humidity - Fluctuations of humidity are as damaging as extremes of humidity. An extremely dry atmosphere can lead to brittleness. A damp atmosphere can trigger the moulds, microscopic or in broad patches. Iron gall ink, the brown ink used by so many old masters, composers and many others deteriorates at a greater pace when the humidity fluctuates regularly.
Light - Too much light falling on a drawing will fade watercolour and pastel pigments, and will start breaking down the cellulose of which paper is made, resulting in a sheet so discoloured and brittle that it cannot be handled with safety.
Insects - Different insects are drawn to different components of a work on paper. The types of damage caused are varied too; some leave deposits on the surface, some eat all the way through the paper, some graze the surface, and some will eat only specific media resulting in just one colour being damaged.
Handling - Because paper readily picks up the natural oils from fingers (which then attracts dirt) it should not be handled without wearing gloves.
Storage - As paper absorbs pollutants readily, mounting, framing and boxing materials must be of a high quality and acid-free. Cheap mount-board is often acidic, causing discolouration and embrittlement. The primary purpose of a mount is to keep the artwork away from the glass of the frame; it also acts as a filter for atmospheric pollutants and as a buffer against sudden changes in humidity.
Tapes and adhesive - Probably the worst enemy of paper is the adhesive on pressure-sensitive tapes, which are oily and stain paper, sometimes irreversibly, and can only be removed by using toxic solvents. Even re-positionable sticky notes leave sufficient adhesive behind to pick up dirt for a long time afterwards.