What Can Conservators Do?

Passive Conservation

The main philosophy behind the work of conservators and restorers today has shifted in emphasis away from purely interventive work to what can be first achieved by passive conservation. This term describes the action which can be taken to ensure a painting’s long term stability without actually having to work on the painting itself. Today it is generally felt that if a painting is in good enough condition then it should be left alone.

The scientific knowledge behind the principles which are applied in passive conservation is the result of much long and careful research. It is based on the detailed observation of art in its environment both at a macroscopic and a microscopic level. Conservators have been accused of applying too much science in their work, but this has brought them a long way toward an understanding of the forces which degrade paintings. Passive conservation involves a number of principles. The painting should be maintained in a stable environment, minimising fluctuations in relative humidity and keeping the painting in an RH between 40% and 70% (museums use a very much more narrow range: 55% ± 5% but this is difficult to achieve in a domestic setting). Levels of ultra-violet light should be reduced as much as possible, keeping the general light level down to 200 lux or reducing the total time that the painting is exposed to light (see below). The transport of paintings has also become something of a science and conservators should be able to advise on compliance with the relevant principles.

Where a painting’s true aesthetic qualities are obscured by subsequent or altered layers (darkened varnish, restoration and dirt) or it has actual physical problems then it may require active or what is often called remedial conservation.

Examination and Analysis

The first thing a conservator will do is to examine the painting thoroughly. For this there is a battery of scientific methods which can be used, from a plain microscope to a scanning electron microscope, x-rays, ultra-violet and infra-red light to gas chromatography/mass spectrometry. Just as important, however, is the information which an experienced conservator will be able to gather with his own eyes. The collector should appreciate the insight which modern scientific methods can give. Some examples are listed below. It must be noted that much of the equipment for this sort of analysis is very expensive and such an in-depth study of paintings is not usually necessary. Most conservators outside major museum conservation departments will only occasionally need this depth of analysis.

Ultra-violet light: can show previous restoration or aged varnish on a painting. It is a technique in general use by conservators.

Infra-red light: can show the drawing which lies underneath the paint layers if the drawing is executed in a carbon based material. This can be achieved with basic equipment but the information is better quality when the light is at a specific wavelength which requires quite elaborate equipment.

X-rays: can show paint lying underneath the top layers and can indicate the presence of a different image or significant changes possibly made by the artist beneath the visible layers.

Polarising microscopy: can be a simple method of identifying the pigments used. It is not in general use by private conservators. It is particularly useful when trying to establish date or authorship.

Cross-sections and thin sections: basically a “core sample” through the paint layers. They can show much of the construction of the painting and its subsequent history. They are a very useful tool and are coming into greater use.

Ultra-violet microscopy and staining: allow the conservator to gain more information from the cross-sections, such as the type of medium used. These require quite elaborate equipment not in general use.

Scanning electron microscopy: allows the conservator to look into the paint at several thousand times magnification. It can give information such as whether the paint is degraded after cleaning with solvents by showing voids in the paint structure itself. This method is only in use in major scientific conservation departments.

Gas chromatography/mass spectrometry can indicate what medium the artist used. This method is only in use in major conservation science departments.

After the conservator has examined the painting she will decide what treatment is necessary.

Remedial conservation

Some paintings will require “hands on” treatment in addition to passive restoration in order to ensure their long-term survival and to bring them back to a state in which they can be enjoyed.

It has already been mentioned that the cleaning of paintings is the most controversial of treatments, partly because it is an inherently irreversible process and partly because it has the greatest potential to alter radically the appearance of a painting. Appearance is not always an objective concept, it can have emotive connotations. A well known image, such as the Sistine Chapel or Titian’s Bacchus and Ariadne, will be very much loved by the public “just as it was” even though this was not as the artist painted it. The decision whether or not to restore therefore involves balancing the present state of the work and its perceived antiquity against what can be established as the artist’s original vision. It may be informative to read the comments made by David Bomford in The National Gallery’s (London) Pocket Guide to The Conservation of Paintings on the cleaning controversy relating to Bacchus and Ariadne.

Sometimes it has to be accepted that a painting is too damaged or “too far gone” to be worth cleaning and that the work of previous restorers should be left undisturbed. This may be a financial judgement or it may be that the work of the previous conservator is considered to be of a very high quality. It is important to appreciate that such a clinical judgement is not incompatible with a love of the painting as it stands. Given the age of some paintings the image must be allowed to prevail over condition where it is not possible to turn the clock back.

The owner may decide against cleaning for various reasons, particularly the risk involved. The painting may be in too fragile a state to be cleaned, or the paint itself may be too sensitive to withstand the cleaning process. For example, some painters used varnish as the medium for the glazes and therefore any solvent used to remove the varnish layer will inevitably remove these glazes. It must be remembered that the accusations of overcleaning aimed at restorers are sometimes quite unfair as the damage will already have occurred in previous cleaning, sometimes centuries ago. Some painters, particularly in the 19th century, used experimental materials which remain soft today and therefore do not allow the conservator to separate the subsequent varnish and dirt layers from the original. Some of the 19th century painters were trying to recreate the “golden glow” or “gallery tone” of the old masters, a tone created by the subsequent yellowing of their original varnish. Where this led the later painters to apply a tinted varnish to their paintings it would therefore be wrong to remove their intentionally yellowed varnish.

Normally, however, the effect of dirt and old varnish is to cause a flattening effect on the painting, evening out its tones. This greatly disrupts the spatial qualities in the painting and is a good reason for removal. Dirt and old varnish also disguise the subtle cool colours on a painting. Look at the back of your hand and see the soft greenish grey of the veins. Painters who have carefully depicted these cannot have intended to have them lost by covering them with a yellow varnish. Similarly a painter who used ultramarine pigment (ground lapis lazuli and very expensive) to depict the Virgin’s cloak in a painting of the Madonna cannot have intended it to be a greenish brown, as it will appear under old varnish and dirt.

It may also be necessary to clean a painting which requires structural treatment involving moisture or heat. Such processes can soften the paint layers and if dirt is left on the painting this can be impressed into the paint.

Conservators usually remove the added layers one at a time and it may sometimes be enough just to remove the dirt layer. This is sometimes referred to as surface cleaning. The decision as to how far to go in removing layers is both aesthetic, requiring the conservator to have a good understanding of the work of art, the artist and his period, and one based on the safety of the painting. Where it is not possible to separate layers without causing damage, the responsible decision may be to leave the work to future conservators for whom science may then have an answer.

Having removed the old varnish the conservator may then remove the old restoration using either solvents or mechanical action.


The next stage is restoration. The level to which this is carried out is also an aesthetic judgement. Damages are generally very disruptive to the unity of the composition and pull the eye so that the painting cannot be properly enjoyed. For this reason paintings are generally retouched or inpainted to disguise the damages. This disguise is not intended to deceive. The ethics of the modern conservator dictate that any additions should be carefully documented, and aside from photographing the painting before retouching, the conservator can mark his work in a number of ways. Where there would be too much conjecture to reconstruct the missing details he may retouch a large area of loss using a neutral tone. A technique called trattegio or rigatino may also be used in this instance. This is where the conservator in fills the loss with several colours of paint in short brush strokes all in the same direction which at a distance blend and meld with the painting (this is a sort of pointillism but in little lines). Other ways of showing additions are sometimes employed. The craquelure can be left out, the filling can be lower than the original, the additions can be slightly different in tone or an indistinct outline can be placed around the restoration.

Before retouching is carried out the losses are filled or puttied using a reversible material. The retouching is then executed on top of this, often using a synthetic varnish to separate the additions from original paint. Retouching is carried out in paints which are stable and also reversible. The ethics of conservation state that wherever possible any process should be reversible. This is because we must never assume that the work carried out today cannot be bettered. Reversibility implies that the additions should be easily separable from the original material.

Once a painting has been retouched it will then be varnished (unless it was the artist’s intention to leave the painting unvarnished). The varnishes used should be easily removable and stable (i.e. not yellowing, leading to the cycle of cleaning paintings every 20 years or so).

Paintings which are damaged structurally require further treatment. Where paint is becoming detached in any way from the support it is generally reaffixed using an adhesive which can be run under the paint and then set using heat. The heat and moisture in this process softens the paint enough to allow it to be pushed gently back into place. This process is sometimes referred to as blister or flake laying.

Lining canvas paintings used to be carried out almost routinely when paintings came to the restorer. Today, however, this is no longer undertaken unless it is absolutely necessary, particularly if the painting has not previously been lined. The process of lining involves removing the painting from the stretcher and adhering a new canvas, today quite often made of polyester, to the back. Modern adhesives no longer darken the original canvas as did some of the adhesives of the past (wax/resin) and need not penetrate the original canvas. This is in keeping with the principle of reversibility.

Where lining can be avoided but small damages require reinforcement, thin gossamer like fabrics can be used to support the canvas. Weakness along the bend in the tacking edge can be reinforced using thin strips of canvas. This process is called strip lining.

Stretchers which are no longer strong enough to function properly are replaced. There is, however, much historical information in the stretcher and therefore the conservator should document the stretcher carefully.

It is very rare that a painting will have to be removed from its original support and placed on an entirely new support. This process is called transferring, and is so drastic as to be necessary only in exceptional circumstances. An example of its justification is to be found at the National Gallery in London. The Incredulity of Saint Thomas by Cima da Conegliano required this treatment. A description of this can be found in the National Gallery Technical Bulletin volume 10, 1986.

Panel paintings pose other problems. Where members of the panel have split the panel should be glued together and this join may be supported in some way. Unstable panels are often treated by placing them in a micro-climate chamber. This will allow the painting to be kept in a particularly stable climate by building up the frame to incorporate sealed glazing, sealed backboard and possibly also conditioned silica gel. Conservators generally consider that warped panels should not be forced flat and that the frame should be built to accommodate the warp. Where a planed down, thin panel has had to have cradling removed conservators will devise a method for supporting the whole panel. Such supports are tailored to the specific problems of the individual panel and are often unique.

It can be difficult for the collector or owner of paintings to know how to look after paintings and to find a good conservator. The following article should help to address this.

Some comments on terminology…
People sometimes appear confused about the terminology de-line and re-line so just to explain the painting on its original canvas is referred to as UNLINED. When the canvas becomes weak and no longer supports the paint, it can be STRIP-LINED. This involves the addition of new canvas around the edges particularly to re-enforce the turnover point. Or the painting could be LINED especially if there are tears in the canvas. This involves adhering a new canvas overall at the reverse of the painting with a conservation grade adhesive.

RELINING refers to a painting that has been lined in the past, having the lining removed (DELINED) and a new lining (or canvas) to replace it.

Finally some paintings are treated by LOOSE LINING which refers to the provision of a secondary canvas behind the original but which is not adhered. This adds support.

The attached Tate Paper No 2 gives some detail into the processes of lining: Paintings on Canvas: Lining and Alternatives | Tate