White Surface Hazes
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The information presented on the Paintings Conservation Wiki is the opinion of the contributors and does not imply endorsement or approval, or recommendation of any treatments, methods, or techniques described.
Author: Mary Gridley
Contributors: Bonnie Rimer
- 1 Whitish Surface Appearance on Paintings
- 2 External and Mechanical Processes
- 3 Internal Processes and Chemical Processes
- 4 Discussion
- 5 Bibliography
Whitish Surface Appearance on Paintings
There are various phenomena which can result in the appearance of a whitish haze covering some or all of a painting. While the composition of some of these white films has been understood for a long time, the implementation of analytical techniques by scientists in the conservation field has allowed for the identification of various chemical reactions in and on the paint film which can explain others. So while the white films may appear relatively undifferentiated to the naked eye, they can be the end result of any one of a number of different situations.
While such hazes have traditionally been described by painters and conservators as blooming or blanching, the industry nomenclature has not yet caught up with the different causes. Other terms currently used are: efflorescence, exudation, fatty acid deposit or migration, saponification, crystallization, chalking, mold, and ghost images.
To date, the following physical, chemical and biological changes/processes have been identified as causes for the appearance of a white haze on a painting:
---Dust, atmospheric pollutants
---Physical break-up of the surface film, or light scattering
---Migration of free fatty acids
---Migration of wax
---Saponification and complex salt formation
---Formation of Epsomite/Magnesium
External and Mechanical Processes
Whitish films that form from external and mechanical processes include surface accretions, loss of cohesion in the paint or varnish film, and mycological activity.
Dust and Dirt
The most easily identifiable and treatable white haze on a paint film is a build-up of particulate matter: construction dust, cobwebs and the like can form a whitish and/or crystalline haze. Treatment involves dusting with a soft brush or cloth, vacuuming or cleaning with dry-cleaning sponges or wet cleaning with an appropriate aqueous solution.
Physical Break-up of the Surface Film, or Light Scattering
The physical deterioration of a surface film here refers to a break-up of the continuous, smooth layer of oil paint or varnish. As the film ages, or is altered by exposure to moisture, temperature extremes, solvents or mechanical action, it can lose its cohesiveness as a smooth film. The micro-fissures and/or moisture in the film(s) cause light to bounce off the surface in all directions, which the human eye perceives as a whitish haze. While natural aging alone of a film (particularly that of a natural resin varnish, which is often the layer at fault in an older painting) results in physical breakdown, exacerbating factors may include anomalies in an artist's materials or techniques such as underbound paint, poor mixing, excessive solvent added during the creation of the work, or the use of non-traditional materials.
Microscopic examination and wetting up a small area of the painting with a suitable solvent should help determine whether this is the cause of the haze. The solvent temporarily acts to fill the microfissures, restoring the appearance of a continuous film. As the solvent evaporates, the haze reappears.
If the haze is in the varnish, removing and replacing the varnish will solve the problem (although research indicates that repeated exposure to the solvents used in varnish removal will ultimately weaken the paint film through loss of small soluble components).(Erhardt 1990, Tumosa 1999) If it is the paint film which is affected, the cure is more difficult. Now discredited systems include 'oiling-out' (the application of more drying oil to the surface of the painting), Pettenkofer's process,(Schmitt 1990) or alcohol and flame to drive out residual moisture.(Lank 1978) Reforming the paint with slow-evaporating solvents has been successful, but this can be a tricky process with risks to both the painting and the conservator.(Lank 1972)
A whitish haze on a well cared-for oil painting is unlikely to be mold. While mold spores are ever-present, they require moisture (± 70% RH), warmth (± 70°F), food and preferably low light conditions for growth. The combination of temperature and humidity is the most critical factor. A traditional, unbroken oil or varnish film provides no nutrients for mold, even in high humidity. The painting support, however, which is often canvas, wood or paper, usually has the starch, cellulose and/or lignin which mold spores feed on. Additional mold nutrients may be in sizing layers or in additives introduced to the supports in their manufacturing process.
Mold can begin on the reverse of a painting, and in seriously damaged works, make its way to the face via cracks and fissures in the layer structure. This mold can be white, green, brown or black. The mold spores can be gently brushed or vacuumed off, or removed with dry cleaning sponges or a mild enzymatic or other aqueous solution, if appropriate for the surface.
An acrylic paint film, unlike traditional artist's oil paint, has additives from manufacture which--again, in the right conditions--can provide food for mold growth. This mold is generally also white, green, brown or black. In the case of acrylic paint, while the spores can be removed as detailed above, the mold usually causes staining.
Mold can also appear on cotton duck supports, both on the reverse and on the face of the artwork if there is exposed canvas. This mold (generally aspergillus) forms in spots, starting as a pale yellow and growing through pale green to light brown. The spores can be removed by vacuum and the mold killed with a misted/sprayed alcohol and water solution if adjacent painted areas can tolerate it. The staining has to be treated cosmetically.
Suitable precautions such as wearing gloves and respirators, and using HEPA filter vacuums should always be taken when treating mold.
Internal Processes and Chemical Processes
There are a number of chemical processes that occur within an oil paint film over its life span: some are reactions between molecules within the film, while others are between the film and the environment. In order to understand why these whitish layers appear, it is necessary to understand the physical formation and aging of an oil paint film.
A traditional artist's oil paint is composed of three main ingredients--oil medium, pigment particles and solvent. The curing and aging process of an oil paint film is well documented. (MOLART 2009, van Loon 2013) After the solvent evaporates from the paint, the oil, composed of triglycerides (a glycol molecule with three fatty acid molecules attached, most commonly oleic, linoleic, palmitic and stearic acids), coalesces and cross-links to form a hard and durable film encasing the pigment particles. During this polymerization process, some of the fatty acid molecules are liberated from the matrix. These small scission products can migrate through and out of the film as free fatty acids or can bond with other available metal elements within the film to form soaps. To date, analysis has identified these fatty acids as stearic, oleic, linoleic, palmitic, and azelaic acids. (Erhardt 2005) The volatility of these small molecules enables them to move within the film, to the surface of the film, or entirely out of the paint film (explaining the so-called 'ghost image' sometimes found on the inside of glazing). These acids may also react with salts, metals or other molecules they encounter--either existing within the painting's layer structure or in the environment--to create new substances which crystallize within the paint film or on its surface.
While some combinations of binder, pigment and application are more apt to develop a whitish haze than others, films of the same composition, even within a single painting, may react differently. While it is not known why similar combinations of materials produce a white film in some cases but not others, it is clear that elevated levels of humidity--either cyclical or maintained--aggravate the process.
The presence of a varnish layer seems to be irrelevant to many of these phenomena. Whitening can happen in the varnish only, in the paint only, or in both simultaneously.
Migration of Free Fatty Acids
As mentioned above, free fatty acids are formed during the curing process of an oil paint film. While the unsaturated fatty acids (such as oleic and linoleic acids) tend to be oxidized in the presence of oxygen and light, and either disappear or attach themselves to the inside of the glazing, the saturated fatty acids can remain on the surface of the artwork.(Schilling 1998) These can be palmitic, azelaic and stearic acids, volatile ketones, or carboxylate soaps (see Saponification, below). Once on the surface of a painting, the acids can crystallize and appear as a white haze. The slow shrinkage of the oil paint film as the cross-linking happens may also work to squeeze out these small degradation products.(Williams 1989)
This phenomenon has also been obverved in artworks made from egg tempera (egg yolk), alkyd, wax and oil stick.(Aufdermarsh 1988, Ordonez 1998, van Loon 2012)
Further research on the sources of free fatty acids indicates that while there can be a high percentage of them in the original artist's paint, both from the oils themselves (linseed, poppy, walnut, safflower) they may also come from wetting and handling additives such as aluminum stearate, ammonium stearate, zinc stearate, and beeswax. Other possible sources of fatty acids in paintings are previous consolidation, lining or patination conservation treatments done with wax-resin mixtures.(Ordonez 1998, Hinde 2011)
Additional factors in the increased production of free fatty acids through oxidation or hydrolysis (beside the driers and extenders mentioned above) are the presence of moisture, an acid/alkaline environment, and pigment type (those with a high oil content being most likely to produce free fatty acids).(Rimer 1999)
In one instance, fatty acids were found on the reverse as well as the face of a painting, demonstrating the unfocussed migratory nature of these molecules. It was also noted that thinly painted areas were more prone to fatty acid effloresence, suggesting that thicker layers of paint prevented or at least slowed migration, although this has not been the case in every instance.(Hinde 2011)
The pigment-media ratio also has an impact on the formation of free fatty acids, with oil rich paints such as alizarin, ochre, and vine black producing relatively more free fatty acids. By contrast, in pigment-rich paints--particularly those with metal components such as lead or copper, the fatty acids tend to attach themselves to elements within the film to produce soaps (see Saponification, below).
Research in this area is ongoing, but it has been hypothesized that several factors can affect the production of free fatty acids or metal soaps. In summary, the following have been identified as factors which INCREASE the likelihood of free fatty acid condensation on the paint surface:
- slow-drying paints (vine black, carbon black, ochres, alizarin crimson, titanium dioxide, cobalt blue)(Schilling 1998, 1999)
- walnut or poppy oil medium(Schilling 1998, 1999)
- pigment extenders and wetting agents (in modern paints) such as aluminum, ammonium and zinc stearates, which themselves contain high percentages of free fatty palmitic and stearic acids (Schilling 1998, 1999, Ordonez 1998)
- heat (Schilling 1998, Ordonez 1998, Rimer 1999)
- admixtures of linseed stand oil to the original paint (Koller 1990)
- zinc in the ground or paint layer, exacerbated by moisture (Koyano 1987)
- addition of wax to paint mixtures or wax introduced to the painting during conservation treatments (consolidation, lining, coatings) (Ordonez 1998)
- lack of absorbent materials within the artwork to consume free fatty acids (Singer 1995, Rimer 1999)
- lack of oxygen and air flow, such as paintings stored wrapped in plastic (Chan 2013)
Fatty acid films pose a tricky problem for the conservator. Although the crystals can be dissolved and removed with aliphatic hydrocarbons, treated with gentle heat to try and force them back into the film, or rendered invisible with a local varnish or wax application, they often reappear--sometimes in a matter of weeks. To date, no definitive treatment has been found. Current work indicates that the fatty acids exist in a crystalline phase before reaching the disfiguring crystal phase, and therefore current treatment protocols are aimed at returning them to the more fluid phase by using a dilute solution of microcrystalline wax in aliphatic hydrocarbons, or by adding oleic acid to disperse the crystals instead of removing them, or by removing them mechanically without the use of solvents. (Martin 2010)
Migration of paraffinic and wax crystals is similar to that of fatty acids. It can be seen in paintings where the artist has mixed wax into the oil paint, or in encaustic paints. The phenomenon may be related to unregulated heating and/or reheating of the medium during creation of the work. Critical temperatures or long exposure to heat are known to cause the molecular and intermolecular bonds in the wax to weaken.
Options for treatment can include burnishing with a soft cloth or cotton swabs, or removal with an aliphatic/aromatic hydrocarbon mixture, if the painting can tolerate it.
Saponification and Complex Salt Formation
The formation of soaps in paint films has been a subject of study for almost two decades. The soaps do not necessarily form a white haze on the surface, but they definitely disrupt both the physical integrity of the film and the viewer's reading of the painting. They can form bumps under the surface which scatter light, and the soaps themselves are transparent, changing the opacity of some layers to such an extent that there is an inversion of lights and darks. The formation and movement of the soap aggregates can cause flaking, pitting or a whitish surface film formation.(Maor 2008)
In saponification, the free fatty acids move through the layer structure of the painting and combine with other elements in the paint or ground layer to form aggregates. These appear as small protrusions or lumps, or, if they have been broken off, are evidenced by small craters in the paint film where they used to be. Since the fatty acids are now allied with heavier elements, they are too heavy to volatilize and therefore remain at or under the surface.
There are many metallic elements found in both the ground and paint layers with which these small acids can form soaps: white and red lead and lead-tin yellow (Higgitt 2003, Noble 2007, Centano 2009), calcium from ground layers, additives, or as the precipitate for red and yellow lakes (Ferriera 2011, Van Loon 2012), potassium from smalt (Van Loon 2012), zinc (Osmond 2004-5, Noble 2007), and copper from green pigments. (Van Loon 2012, Keune 2013)
There is evidence that elevated temperatures and levels of humidity increase soap production within the film (Higgitt 2003, Boon 2007, van Loon 2012), so a knowledge of the painting's display, storage and transport history may help in determining the likelihood of soap formation.
The translucent perimeters of these soap globules fluoresce strongly under ultraviolet radiation, so examination with a UV lamp of the surface or of cross-sections should be helpful in identification. (Townsend 2006/7) These rather soft, waxy soaps are not very soluble in the conservator's range of commonly used solvents, although they are susceptible to mechanical action during cleaning. In addition, their waxy nature makes them attractive to dirt and dust, which can cause localized color changes on the surface. (Higgitt 2003, 2005)
Furthermore, when these soaps encounter pollutants or moisture in the environment, they can chemically change from soaps to insoluble complex salt mixtures such as lead and zinc carbonate. (Van Loon, 2012) Efflorescing hazes made of re-mineralized metal soaps are generally insoluble, and are often intimately bound with the paint film, making them difficult to remove, (Noble, 2007) although identifying the compounds by analysis and then formulating a preferential chelating agent has been suggested as a course of treatment.
Epsomite can appear on the surface of a painting as crystalline rods and needles in a swirled pattern or, in one case, mixed in with other inorganic materials that are extruding from the paint surface. (Ordonez 1998) The crystallization is caused when magnesium carbonate, which can be found as an additive in many 20th century artists' paints, encounters a sulfurous environment under conditions of elevated humidity. This can result in increased sensitivity of the oil paint film to water, limiting traditional treatment options. (K. J. van den Berg 2008-2012, Silvester 2014) Whether the sulfates come from within the painting (degradation products, contaminants from the manufacturing process) or from the environment (polluted metropolitan areas), the result can be the formation of magnesium sulfate heptahydrate and water, commonly know as Epsom salts.
Now that so much more is known about the varied composition of these surface hazes, a more nuanced approach to their removal is starting to be formulated.
Whereas white hazes formed from dust or mold are straightforward to remove both practically and ethically, hazes comprised of elements migrating from with the painting require a more complex approach. Removal mechanically or with solvents permanently subtracts part of the lattice-work of the paint layer(s), and in many cases, aggravates the rate of migration as the painting struggles to reach equilibrium. To leave the hazes, however, is to render the painting unacceptable for viewing.
The appearance of surface hazes composed of elements migrating from within the paint film has raised some questions within the painting conservation field about the ethics of their removal. There is strong evidence that each cleaning of a paint film leaches some material from that film, with multiple cleanings/treatments over the centuries ultimately weakening and embrittling the paint. Yet these hazes appear as a result of the aging of the film, so do we consider them a dispensable by-product of aging? Or material that should be left in situ as part of the original artwork? While no one can deny that whitish hazes are disfiguring and can prohibit the visual appreciation of an artwork, there is worry that, because they constitute original components of the painting, removing them in some way alters the original.
In addition, there is evidence that removing fatty acid deposits from the surface of an oil film simply encourages more to travel to the surface, so that the haze regenerates over time. Frequent removal may embrittle the paint film over the long term, but this is speculative. It is not yet known what positive role--if any--these volatile components play in the dried paint film.
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