Paintings Ageing Gracefully or Grievous Bodily Harm?
Paintings are remarkably tough creatures in many ways but in others they are remarkably fragile. They suffer deterioration both in their natural ageing process, which is generally regarded as acceptable and even enhancing the painting, and from environmental and physical forces, which we try to control or avoid wherever possible. Time has inflicted some sort of change on all paintings. In most cases they will also have suffered physical damage. It is pretty well a matter for suspicion if a 17th century painting is described as being in “mint condition”, as almost all paintings will have had some restoration carried out on them and commonly this restoration will have been at regular intervals because deterioration will not have been arrested, new varnishes would have continued to yellow and new restoration fashions will have come about.
All the elements in the construction of a painting exhibit some measure of decay. Paintings undergo the greatest change in the first few months of their life as the solvent or liquid part of the medium evaporates and the paint begins to dry. Then the chemical processes involved in the “drying” or hardening of paint take over. When paint dries a pattern of cracks (craquelure) generally forms. This is often distinctive, giving clues to the construction and therefore date and origin of the painting. These cracks are either ageing cracks, which form in most paintings in response to the stresses within the painting: or drying cracks which form as the paint dries and which, if very prominent, indicate that the artist used a faulty technique. Generally ageing cracks are thin, sharp and angular in comparison with drying cracks which are wide and rounded. If one looks very, very closely at the point at which the paint goes into the crack the difference is obvious as the ageing crack has a broken sharp edge and drying cracks have a smooth rounded edge.
Paintings in oil become more transparent as chemical changes take place, as mentioned above, revealing underlying layers. Other effects of ageing include the change in tone or fading of some pigments, for example copper resinate greens turn brown, smalt blue goes grey in oil, and red lakes fade. These effects are reduced if the painting has not been exposed to light but, unfortunately, paintings cannot be viewed in the dark.
The surface of a painting will commonly be covered with a layer of surface dirt, deposited over time from candles, fires, and general atmospheric pollution. Before cleaning it is difficult to say whether this has caused permanent damage. The dirt will penetrate into the paint layers on unvarnished paintings and will then be impossible to remove without damaging the paint. It is for this reason that it is recommended to glaze (incorporate glass or Perspex into the frame to protect the painting). A whitish haze can develop in paintings stored in damp conditions where moisture has penetrated the paint layers. This effect is known as bloom (blanching is also a term sometimes applied to this effect, but generally refers to the effect of too em a solvent having been used on the painting in cleaning, thus forming voids in the paint medium which reduces the saturation of the pigments).
Other effects of ageing include the darkening and yellowing of varnish layers. The varnish can also become more brittle than the paint itself and form its own crackle pattern or flaking independent of the paint.
Basically paintings are composed of incompatible elements, each having differing reactions to changes in relative humidity, temperature and light. Small changes are absorbed by the materials which are reasonably elastic. This elasticity, however, diminishes with age and eventually the painting cannot absorb the stresses caused by these fluctuations. The supports, too, are vulnerable to change and decay. Paintings on canvas suffer from weakening fibres which lose strength through oxidation and which eventually become too weak or brittle to support the paint layers. The tacks which hold the canvas to the stretcher can also oxidise (rust) and then further contribute to the weakening of the canvas. The wood of the stretcher is acidic and this produces more loss in strength in the canvas, especially at the angle where the canvas bends around the stretcher. Repeated fluctuations in relative humidity cause the canvas to slacken and then tighten and finally to go permanently slack. As the slack canvas sits in direct contact with the stretcher sharp cracks can form along the lines of the stretcher edge. There are called stretcher cracks or stretcher marks.
The movement in the canvas will eventually cause brittle and stiff paint to lift from the support. This flaking, where small areas of paint, or paint and ground, have sharp edges which are lifting away from the support, is a preliminary to losses of paint.
Another disfiguring effect of the changes in temperature and humidity is called cupping. This is where the paint has formed little areas within the crackle pattern which are slightly concave, or cup or dish shaped, and is generally caused because the size layer reacts more to changes in humidity than does the paint on top of it.
Panel paintings suffer to an even greater extent from the effects of fluctuations in relative humidity. The wood itself expands with an increase in relative humidity and as there is usually only one side which is painted the effects of humidity are greater on one side than the other. This leads to warping which over many cycles of expansion and contraction becomes set in the wood panel as the capillaries in the wood collapse, finally leading to a convex warp which cannot be reversed. If a panel is constrained when it is trying to warp it may seek release by splitting in the direction of the grain or along a join in the members of the panel. Flaking of paint layers on panels is common. The ground or gesso layer can also become detached from the panel and this can cause blisters to form. Where the humidity has been truly excessive the ground and paint layers can even lift away from the support (in both canvas and panel paintings) in long peaked blisters called tenting. This generally occurs when the support has shrunk and can no longer accommodate the paint.
All the organic elements of a painting are also vulnerable to attack by pests. Woodworm is a major cause of deterioration in panel paintings and will also attack stretchers. Mould can disfigure a painting badly, leaving a pitted surface or black spots on the paint surface. Black spots can also be the result of defecation by flies on the surface (fly specks or fly spots).
Paintings on other supports suffer from similar problems. Copper and other metal panels suffer particularly from detaching paint as the smooth surface of the metal gives very little key for the paint to hold itself onto. Reactions with moisture cause oxidation (e.g. rust). Oil paint and copper can set up a chemical reaction which causes a green layer to form. Paper, millboard and cardboard are even more responsive to humidity and become even more acidic than canvas.
All this without mentioning the damage which can be inflicted in the handling or mishandling of paintings.
Paintings can be protected to some extent from the effects of normal ageing processes by good preventative conservation measures and from physical damage by good handling procedures (both of these are discussed below).
Most paintings of any age have had some physical damage inflicted upon them. Paintings all run the risk of being scratched, knocked, and dented and furthermore canvas can be torn, punctured or just scratched. The paint layers generally show in time the effect of various sorts of impact on a canvas. A poke from the reverse can stretch the canvas to form a bulge and around the centre of this impact a spider web shaped crack generally forms. A scrape along the back of the canvas will often produce a centipede or fishbone sort of crack. A wedge lodged between the canvas and the stretcher will have stretched the canvas; knocking in wedges to tighten the painting can cause the hammer to rub against the canvas, ultimately forming fishbone cracks leading to the corner.
An assessment of the condition of an old painting is not only about the misfortunes which have befallen the painting in the ways mentioned above but also about the care and skill of conservators over the years. It is rare to find an old painting which has never been touched. Indeed, sometimes the neglect of attention when it was needed will have led to irreversible decay. Restoration as a term has implied that the painting would be returned to its original state. This is never truly possible as the effects of ageing described above are not necessarily reversible and the work also implies a certain amount of aesthetic judgement on the part of the restorer which cannot by nature be entirely objective. Restoration, when carried out by a skilled person has, however, ensured the continued long life and enjoyment of paintings and has only gained a bad name where the restoration work was overzealous or insensitive.
Sadly some of the restoration practices of the past have been quite ill conceived and along with lack of skill on the part of past restorers have led to permanent damage. This is most often in the form of overcleaning. This means that in the removal of dirt and aged varnish as well as old restoration a restorer has used too em a solvent and has removed some of the original paint. Glazes are the most vulnerable to this. A painting which has been overcleaned very badly is sometimes referred to as skinned. Restorers, in putting back paint where there has been a loss, often in the past did not confine the retouching or inpainting to the loss itself but extended the paint over original paint. This is said to be overpaint. If this retouching was carried out in an inappropriate medium which darkens with time the retouchings will eventually not match the original.
In the structural repair of paintings as well, past restorers have used methods which can damage paintings. Torn canvas, repaired with heavy patches can cause the canvas to bulge at the front showing exactly the position of the patch. Paintings with larger tears were lined, (backed with another canvas) and then often relined. Lining in itself can be a very good preserving process, but in the hands of some restorers led to the paint layer being squashed flat due to excessive heat and pressure being employed, or the introduction of incompatible material. Panel paintings were planed down or thinned before applying a cradle to control warping (see above). Cradling can cause further damage in a painting, and where it is overly constrained can result in splitting or a washboard (corrugated) surface Where there was an imperfect join in the members of a panel the step was sometimes planed down at the front removing more original paint.
Fashions in painting and contemporary taste have also led people to alter old paintings. For example, in the 19th century it was no longer acceptable to show certain parts of the anatomy. Offending parts were then quite simply overpainted with inventions of drapery. Ethics have now changed but in the past dealers or owners cut many paintings down to fit a smaller room or even a frame. Larger paintings may have been divided to make two or more images. It was also common to improve paintings to make them more saleable or acceptable by overpainting an unattractive feature (such as the head of St John the Baptist on a silver charger in paintings of Salome) or by adding figures to a landscape or even a signature to a previously unsigned work.
It is generally fair to say that these practices are not now acceptable and that restorers and conservators strive to interfere as little as possible to ensure the continued long life of a painting.