Acrylic paintings now make up a significant part of the permanent collections of museums and art galleries. Artists' acrylic paint was introduced in the 1950s and since then has dominated the arts and crafts market. In addition, it has been accepted by artists as a viable alternative to oil paint. The behavior of acrylics as a painting medium and their physical and chemical properties are different from oil paint which warrants distinct guidelines for acrylic paintings' care. Some traditional conservation methods can be harmful to the acrylic paintings. The aging characteristics of acrylic paintings are just beginning to be understood. At present, preventative care seems to be the best care for acrylic paintings.
The Behavior and Properties of Acrylic Paints
- There are two groups of acrylic paints used in the fine arts: solvent-based and water-based. The solvent based acrylic paints, such as Magna, are soluble in mineral spirits. Few artists have used solvent-based acrylics. Common water-based acrylic paint and acrylic emulsion paint are water-soluble.
- The dried acrylic paint is not water soluble. Acrylics dry in less than thirty minutes. In contrast, oil does not become dry to the touch for 48 hours.
- Acrylic paint dries differently from oil paint. Acrylic paint "dries" by evaporation of solvent of water. Oil paints dry by the interaction with oxygen from the air with their binder and evaporation of solvent.
- Acrylic films are not as hard as oil films. The surfaces of the soft acrylic films hold onto dust and dirt. The paint may even flow around the particles, over time, so that they are incorporated into the film.
- A variety of additives are added to the acrylic paint to achieve a desirable working. Examples of these additives are thickeners, stabilizers, preservatives, surfactants, coalescing solvents and defoamers. Some of the additives are water-soluble and some are solvent-soluble.
- Acrylic paintings are expected to develop cracks much less often than oil paintings because they are more flexible and can withstand much greater forces without breaking. However, cracks do form in acrylic paintings.
- Acrylics are good adhesives and can support many times their own weight. They should not, however, be expected to hold in place very dense materials or structures projecting far from the support.
- Some acrylic paintings may appear to form a grey veil on their surface or develop yellow discoloration with aging.
- When they are exposed to sub-zero temperatures, acrylics become increasingly brittle and they will crack.
Acrylic paintings attract and gather dirt easily. Acrylic emulsion paints used in the fine arts have glass-transition temperature (Tg) near or below room temperature. This means that acrylic emulsion films will always be soft at room temperature and that the paint surface will hold onto dust and dirt and even unite them into the film. The problem is further exacerbated by the fact that acrylic resins are non conductors and tend to have electrostatic charges on their surface which attracts dirt. Protective framing has been suggested as a good way to exclude dirt.
Removal of the top most dirt layer is perceived to be easier on a varnished painting. Unfortunately, varnishing an acrylic painting is problematic because the dried acrylic paint layer is soluble in the solvents used to make most resin solutions. Cleaning an emulsion paint with no varnish is also problematic because water may remove water-soluble additives and could make the pigment/polymer-binder interface less intimate causing colors to appear less saturated. Cleaning may also swell the thickener additives, disturbing the paint layer. Presently, there is no completely acceptable resolution to the problem of cleaning acrylic paintings.
Acrylic paint become soft around 60ºC. This heat sensitivity indicates that using the hot table, or any heat source, for lining is impossible. Lining is used in conservation to apply an additional fabric support to the painting's weakened structure. Commonly, it involves using heat, vacuum (negative pressure) and solvents (in conjunction with adhesives and adhesion). Temperatures above 60ºC are necessary to accomplish the procedure successfully. Lining methods involving heat, vacuum, and solvents will cause acrylic paint to deform. The artist's unique brush strokes and impasto (thicker paint build up) will flatten, resulting in a loss of aesthetic and monetary value.
The soft film formed by acrylic paint will easily abrade or dent with just fingernail pressure. This type of damage can ruin the nature of the image of a large abstract painting which muse display a perfect surface.
Acrylic paint is affected by most commonly used solvents. For example, xylene, a mild solvent used often in the removal of varnish in conservation, can soften acrylic emulsion paint. Magna (a solvent based acrylic resin) is immediately soluble in most common solvents except water and methanol.
Acrylic Paintings and Varnishing
Traditionally, varnishes provide surface protection from abrasion, dust and dirt. They also provide saturation to the paint they cover. There are concerns as to whether or not to varnish acrylic paintings and many artists insist that their acrylic paintings be unvarnished. Varnishing acrylic paintings has several problems: 1) Acrylic resin proprietary varnishes have similar solubilities to those of acrylic paint. This necessitates the use of solvents which might damage the paint layer for their removal. 2) Traditional natural varnishes, such as dammar, will yellow in time and the solvent used in their removal will dissolve or soften the acrylic paint layer. A water soluble varnish may be an answer. It is an issue that manufacturers might be able to address.
Mold growth has been noted on acrylic paintings and has become an increasing concern among artists and collectors. Unfortunately, there is no ideal treatment that does not cause some degree of damage to the original paint. Mold growth tends to become apparent when humidity and temperature rise. Prevention is the best care.
There are many trade acrylic paints that have been used by artists and the range in their quality is broad. The lesser quality paints tend to have cheaper colorants which fade easily under ultraviolet light. Thus, fading colors which change the tonal balance of the painting may be due to the intrinsic nature of the materials and can not be reversed by conservation.
Acrylic emulsion paintings have unique characteristics which require diligent preventative care. Their soft paint films attract and hold dirt and are difficult to clean and varnishing is not an ideal solution. It is important to store acrylic paintings in a dust free environment to reduce the amount of dirt deposited. It is also important to keep the display or storage temperature below standard room temperatures to reduce further softening of the paint film. Protective framing is one possible way to exclude dirt from the painting surface. One might have to accept that as time goes on, acrylic paintings will experience some visual change due to dirt deposition and that dirt removal will also cause visual damage.