Back to the Paintings Page
Back to Main Catalogs Page
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: Samantha Skelton
Editors: Erica ESH James, Erin Stephenson
Raw canvas paintings (also called unprimed, ungrounded, or exposed canvas paintings) present challenges to safe and successful treatment protocol, requiring specialized knowledge in both textile and paintings conservation. Leaving large areas of exposed canvas as a primary feature of the design was a common practice of the Color Field painters, a group of mid-twentieth century artists who belonged to the larger category of Abstract Expressionism. Helen Frankenthaler, Morris Louis, Sam Gilliam, and Kenneth Noland, among others, worked extensively in the technique. While examples employing other fabrics are known, the majority of artists working in this technique used cotton canvas (often cotton duck). The paint applied to these canvases is often a stain rather than a bodied paint layer, with artists employing heavily thinned oil paint or solvent-borne acrylics like Magna, as well as waterborne acrylics. The canvas may or may not be sized, which will affect the way the paint settles into the fibers.
Raw canvas paintings have proven vulnerable to condition issues less common or problematic in traditionally primed and painted canvases. Cotton fabric is subject to darkening through natural degradation. Contact with acidic materials, such as those found in a wooden stretcher, can cause rapid oxidation of the cotton fibers resulting in permanent discoloration—a condition known as ‘stretcher burn'. Areas of exposed canvas are absorbent and especially susceptible to staining from contaminants such as oily fingerprints, liquids, food and drink, nicotine, and other foreign matter. Grime, dust, and airborne pollutants settling on the surface can become embedded in the fibers. Mold growth directly on canvas fibers is frequently observed in higher humidity environments and may result in permanent disfigurement. Tears or punctures are highly visible on these types of paintings due to the expansive solid fields and repetitive textural weave pattern. Additionally, these canvases are often subject to later coating with non-original materials.
Condition Issues and Treatment Strategies
Dust, dirt, and grime that accumulates on the surface of a raw canvas painting is hygroscopic and thus increases the likelihood of further accumulation; this hygroscopic nature can also encourage in higher humidity environments. For these reasons, in addition to aesthetic concerns, it is recommended that raw canvas paintings be dusted regularly with a clean, soft brush and a HEPA-filtered vacuum. Use of a new, clean brush prevents transfer of grime from previous dustings. Frequency of dusting is tailored to the rate of dust accumulation in each individual collection, but generally should occur anytime there is visible overall accumulation. Building and HVAC recommendations should be followed closely to reduce the overall rate of airborne accumulation (ASHRAE 2015). Glazing, which could protect the surface from many of these issues, can obscure textural qualities that are often important aesthetic components.
Bread cleaning is a common treatment for raw canvas paintings suffering from dirt and grime accumulation. A food-based treatment may seem counterintuitive but an in-depth 2004 Smithsonian study found no adverse effects, including no increased propensity for mold growth, when the treatment is executed properly (Ausema 2005). Bread made with unbleached, unenriched, and unbromated flour and free of eggs, oil, milk, and salt must be used while fresh; the soft interior of the bread is removed and rolled over the surface of the canvas until it breaks into small breadcrumbs and dries out. The crumbs are brushed off and followed by a final overall vacuuming. Other dry cleaning options, such as eraser crumbs, have been found to be less effective and sometimes more disruptive to the canvas fibers.
Other embedded particulate accumulations such as scuffs, acute dirt smears, and pencil marks require more specialized treatment. Depending on the morphology of the embedded particulate, more intensive dry cleaning methods or mechanical removal with a pin and vacuum under a microscope may be possible. Laser ablation is a viable option for raw canvas according to several studies (Sutcliffe et al. 2000, Skelton et al. 2014). Individual tests should be conducted for each object to determine the best combination of wavelength, pulse duration, beam profile, and energy density for safe and effective laser cleaning on canvas.
Mold easily propagates on raw canvas in humid environments, as cellulose provides an ideal substrate to support fungi. Mold releases digestive enzymes that weaken and degrade cellulose, and it can create a stain in the fibers that often results in permanent disfigurement (AIC, Book and Paper Group 1994). Preventive recommendations to avoid mold outbreaks should be followed closely. For more information, please see Mold.
Cotton is a vegetable fiber composed of 99% cellulose, a natural biopolymer with crystalline (70%) and amorphous (30%) regions. Cotton undergoes a process of oxidation as it ages, resulting in darkening and embrittlement. At the same time, the absence of lignin in cotton means it oxidizes at a relatively slow rate compared to plant fibers with a higher lignin content, such as jute. Oxidation is induced by high light exposure and ultraviolet radiation, which causes chain-scission in the cellulose polymer. Photolytic chain scission increases the number of short crystalline sections, resulting in a more brittle fiber (Balázsy and Eastop 2004). The color shift and darkening that accompanies this chemical process is a normal sign of ageing, though it often prompts treatment due to aesthetic concerns or when combined with other condition issues and preventive measures. Margaret Watherston advocated an overall washing procedure for these paintings in the 1970s (Watherston 1972). Jay and Holly Krueger introduced a modified version of this treatment, with a sun-bleaching component derived from paper conservation treatments, in the 1980s (Krueger et al. 2017).
Stains on canvas can occur when the fabric comes into close or direct contact with a material undergoing its own degradation process. In paintings this is often a wooden auxiliary support, such as a fixed strainer or expandable stretcher, on which the painting is attached and is the reason for the term ‘stretcher burn’. The acidic byproducts produced by the breakdown of the wood can cause accelerated canvas degradation through acid hydrolysis of the cellulose in the fibers. The resulting browning of the canvas is permanent. Cleaning or washing will often extract significant amounts of brown degradation products from these areas, but generally will not change the burned appearance of the canvas. Compensation with Solka-Floc® powdered cellulose is recommended for areas of raw canvas affected by stretcher burn.
Stretcher burn is a variable condition that does not always occur or appear to correlate with temperature or humidity conditions (Ausema 2005). However, it is recommended that all raw canvas paintings have their auxiliary supports coated to protect the canvas from potential future damage. A wood sealant has been used in the past but current recommendations suggest covering the auxiliary support with MarvelSeal® 360, a heat-sealable aluminized barrier film that easily conforms to the wood. The sealable side (the inside of the roll) should be applied to the any part of the wood that will come into contact with the canvas.
Staining from food, drink, cleaning products, condensation, or other liquid exposure can cause significant disfigurement to paintings with large areas of exposed canvas. Liquid staining creates a wet-dry interface in the canvas, where degradation products and other staining material will collect, creating a tideline. Tidelines around stains are often more intractable than the stain itself because they contain a higher volume of the staining material and displaced degradation products. These collected degradation products are often acidic in nature and can weaken the fabric along the tideline (Dupont 1996).
The nature of the staining material must be determined before treatment options can be considered. If the staining material is a food or drink product, textile conservation recommendations may be followed, with special attention to avoid the creation of tidelines (Skelton, Rogge, and Bomford 2016). Oily stains may require solvent-based treatment. If the staining material is water-soluble and the canvas is suitable for overall washing, the sun-bleaching treatment mentioned above (see 2.3 Oxidation) may be considered. Poulticing treatments are a common localized stain removal tool used in paper and textile conservation. Alpha cellulose, cotton pads, and rigid gels such as agarose and gellan gum have been employed as poulticing materials on textiles and raw canvas paintings. When poulticing under weight, care must be taken to feather out the edges of the poultice to avoid tidelines. It is highly recommended that the paintings conservator consult a textile conservator before attempting stain removal and that any treatment option be thoroughly tested before application - first to a mock-up and then to a hidden area of the tacking edge.
A toasted cellulose powder palette is recommended for diminishing the appearance of stains on raw canvas. This can be made by toasting Solka-Floc® powdered cellulose to progressively darker shades to match the tone of the canvas. While several grades are available, Solka-Floc® 200 is recommended for its desirable working properties and the natural appearance of its particle size. The color-matched powder is gently brushed on with a soft clean brush to mask stains and discoloration. The powder is not affixed with adhesive so it can be brushed or vacuumed off at any time if necessary. Compensation with powdered cellulose alters the textural appearance of the canvas to some degree.
Pigments mixed in funori are recommended for compensation over the painted areas of these works. If inpainting by this method is not matte enough, funori can be brushed on first and allowed to dry slightly before dry pigments are dusted over the area (Smithen 2017). Funori is a biological material so the treated painting must subsequently be kept in proper environmental conditions to prevent mold growth.
Losses to areas of bodied paint may require filling by carving or casting a dry fill material, which can then be attached to the canvas with an appropriate reversible adhesive.
Artists working with raw canvas typically chose sized or unsized canvases based on compatibility with their working technique. For instance, Morris Louis hand-applied additional sizing to his canvases before painting until 1960, when he ceased use of sizing in order to increase the absorption and saturation of his paints (Upright 1985). When original sizing materials are present, careful ethical consideration should take place before performing any wet treatment that may remove the original material.
In the past some conservators advocated the addition of coatings, such as Klucel (hydroxypropyl cellulose) and sodium carboxymethyl cellulose (Watherston 1972, Brenner 1974). Acrylic medium, polyvinyl alcohol, and polyvinyl acetate were also used as overall coating materials in an effort to protect these paintings. These coating materials often age poorly and have a tendency to darken and yellow. Removal methods can be difficult and depend on the solubility of the coating and the suitability of the canvas for overall wet treatment. Coatings of any kind are not recommended in any current or future treatment because of the risks mentioned above and because they are irreversible due to the porous nature of the canvas.
Tears and Punctures
Tears in raw canvas are often highly visible due to the expansive solid fields and repetitive textural weave pattern. The Heiber method, which makes all points of reattachment in the canvas on the verso, may be the least intrusive option for tear repair (Tomkiewicz, Scharff, and Levonson 2012). Some modification of the recommended adhesive mixture may be necessary to avoid darkening of the canvas near the tear (Waters 2006).
Many of the condition issues that occur in paintings with raw canvas can be partially or completely mitigated by proper preventive conservation measures, especially closely following environmental recommendations for storage and display.
- Current environmental recommendations are temperatures “between 68-72° F in the winter and 72-75° F in the summer […] allowing for a few degrees of fluctuation in either direction” and relative humidity “maintained at 40% ± 5% in the winter and 50% ± 5% in the summer” (Preventive Team, Conservation Department 2010). Large, sustained swings in temperature and humidity are the most problematic. Humidity below 30% can cause desiccation and excessive brittleness of the fibers, and humidity above 70% will likely result in mold growth.
- To discourage future discoloration of the canvas, light exposure should be kept below the maximums for paper (5 foot-candles or 50 lux) rather than paintings (15 foot-candles or 150 lux), and UV exposure should be eliminated.
- Raw canvas paintings should be treated as an extremely vulnerable surface and physical barriers, such as stanchions and platforms, should be used during display. Nitrile gloves should be used for handling to protect from skin oils that can discolor the canvas over time.
- HVAC filtration systems are recommended to reduce airborne pollutants, and a regular dusting schedule should be implemented.
- Barrier layers should be applied to auxiliary supports to protect the canvas from the wooden stretcher if possible. A loose lining of cotton duck can be added to further support the canvas.
AIC, Book and Paper Group. 1994. Mold. Paper conservation catalog. 9th ed. Washington, DC: AIC. 12: 1-39.
ASHRAE. 2015. ASHRAE position document on filtration and air cleaning. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, GA.
Ausema, T. Z. 2005. A wide open field of color: Caring for color field paintings at the Hirshhorn Museum and Sculpture Garden. Paintings Specialty Group Postprints. American Institute for Conservation 32nd Annual Meeting. Washington, DC: AIC. 17: 21–29.
Balázsy, Á., and D. Eastop. 2004. Chemical principles of textile conservation. Oxford: Butterworth-Heinemann. 23-29.
Brenner, A. 1974. High polymers for forming an invisible, soil- resistant coating on canvas. In The great decade of American Abstraction: Modernist art 1960–70, edited by E. A. Carmean. Houston: Museum of Fine Arts. 130–138.
Dupont, A.-L. 1996. Degradation of cellulose at the wet/dry interface: I. The effect of some conservation treatments on brown lines. Restaurator 17: 1–21.
Krueger, J., et al. 2017. Color field paintings and sun-bleaching: An approach for removing stains from unprimed canvas. Presented to the Paintings Specialty Group, 45th Annual Meeting of the American Institute for Conservation, Chicago, IL.
Preventive Team, Conservation Department. 2010. Guidelines and procedures for preventive conservation at Winterthur Museum. Winterthur, DE: Winterthur Museum and Country Estate.
Skelton, S., C. Rogge, and Z. V. Bomford. 2016. Testing the limits: The theoretical development and practical reality of a large-scale agarose gel treatment for a discolored Morris Louis. Studies in Conservation 61(sup2): 214–18.
Skelton, S., et al. 2014. Unmaking your mark: an investigation into the removal of pencil from unprimed cotton canvas. Presented to the Paintings Specialty Group, 42nd Annual Meeting of the American Institute for Conservation, San Francisco, CA.
Smithen, P. 2017. Personal communication. Tate Modern: London.
Sutcliffe, H., M. Cooper and J. Farnsworth. 2000. An initial investigation into the cleaning of new and naturally aged cotton textiles using laser radiation. In J. Cult. Heritage 1 Supplement 1, Proceedings of the International Conference LACONA III Lasers in the Conservation of Artworks, Florence, Italy, April 26-29 1999 , eds. R. Salimbeni and G. Bonsanti. S241-S246.
Tomkiewicz, C. M. Scharff, and R. Levenson. 2012. Tear mending and other structural treatments of canvas paintings, before or instead of lining. In Conservation of Easel Paintings, edited by J. H. Stoner and R. Rushfield. London: Routledge. 384–414.
Upright, D. 1985. Morris Louis: The complete paintings (a catalogue raisonné). New York: Harry N. Abrams.
Waters, L. 2006. Tear repair of cotton canvas: a variation of the Heiber technique. WAAC Newsletter 28 (2): 10-11.
Watherston, M. M. 1972. Problems presented by color field paintings. Cleaning of color field paintings. Studies in Conservation 17(sup1): 831–45.
- Mock-ups would most ideally be created using a naturally-aged sample of the artist’s preferred canvas, but artificially aged samples of similar canvas would also be helpful.