On the surface, critical cleaning/contamination control and art conservation seem worlds apart. Scratch the surface (or, better still, avoid scratching the surface) and you find parallels between critical cleaning and art conservation. As a point of clarification, conservation encompasses actions taken toward the long-term preservation of cultural property. In contrast, restoration is a type of conservation treatment. It specifically refers to an attempt to bring cultural property closer to its original appearance. A primary task in the conservation of paintings is to remove accumulated contamination (surface dirt, discolored varnish, or non-original paint) without undesirable change in the aesthetics. Manufacturers would say that the goal is to clean without negatively affecting the original surface. Those concerned with contamination on or embedded in coated surfaces and manufacturers of components containing plastics or composites, can benefit from the approaches of the art conservator.
In contrast with critical cleaning or industrial cleaning, where commercial cleaning agents are used, the conservator acts as a formulator, preparing cleaning agents customized to the task at hand. One of the tools that conservators use to manage the chemistry of their cleaning solutions is the Modular Cleaning Program (MCP), developed by Chris Stavroudis, our co-author for this month’s column. The MCP is an interactive database computer program that incorporates chemical and physical properties of constituent materials. The MCP can be used to “fashion” a solution that is optimally suited for soil removal. It can be used for cleaning with solvent, solvent gels, or water-based systems. The system is complex. For example, parameters of the current MCP for aqueous cleaners include pH, ionic strength, gelling agents, surfactants, and chelators (and the next version will include osmotic modifiers and organic co-solvents).
One of Chris’ latest projects is to modify the MCP to assist the conservator in the cleaning of acrylic paint surfaces. Acrylic paints, emulsions of pigmented acrylic plastic particles in a water-based carrier, are the basis of most water-based paints for architectural coatings and have been used in the art world for about 50 years. After half a century, some works of art are candidates for conservation. Since many industrial products also have acrylic coatings, or coatings with similar properties or are constructed from plastics, the techniques of the art conservator are of global value.
THE YIN AND YANG OF ACRYLIC PAINTS
Acrylic paints are chemically and physically complex. Simple paints consist of a pigment to provide color and a binder to provide both cohesion and adhesion. In contrast, most acrylic paints are complex mixtures containing a number of chemical modifiers. The modifiers impart some of the visual and physical properties such as viscosity, improved film formation, pigment wetting, and extenders to reduce the amount of pigment necessary. One reason that acrylic coatings are valued is because, once the water carrier evaporates, the coating is stable and is relatively impervious to water. The key word here is “relatively” because water can affect the acrylic.
WATCHING PAINT DRY
Watching acrylic paint dry is actually fascinating. In terms of surface area, the polymerized emulsion spheres in 1.5 ml of acrylic emulsion have a surface area of 37 square meters. There needs to be sufficient surfactant, one of the modifiers, in the emulsion to stabilize the 37 square meters to form and maintain the emulsion. In actual practice, additional surfactant is added to the base emulsion to make the paint. As the paint dries and coalesces, the surfactants have less and less surface area to stabilize and must go somewhere. Therefore the surfactants are forced into the interstitial spaces. After the paint dries, the surfactants and other modifiers remain in the interstitial spaces between the coalesced spheres of acrylic medium. This reservoir of surfactant can be mobile and exude to the surface, sometimes causing visible changes to the acrylic surface. The surfactant can also reduce the glass transition temperature (Tg) of the surface of the acrylic film causing the film to become soft enough at room temperature to increase the possibility of dirt particles irreversibly migrating into the paint layer.
Some of the soluble modifiers in acrylics (salts, semi-soluble pigments or extenders, anionic surfactants, or ionic thickeners) cause the paint film to have an ionic quality. The ionic environment within the paint can cause the paint to interact with an aqueous cleaning system. If the paint is considered a porous surface, the movement of the ionic species is driven by entropy. If the acrylic paint is considered to be a semipermiable membrane, osmotic pressure will need to be accounted for.
Due to ionic or osmotic pressure, water can be drawn from a hypotonic solution (lower solute concentration) to a hypertonic one (higher solute concentration) and be retained, causing swelling.
The cutting edge of cleaning practice has the conservator paying attention to solution conductivity to minimize ionic pressure. Chris is currently exploring non-ionic osmotic effects on the grounds that while consideration of ionic strength is useful, consideration of osmotic effects may allow more subtle control of the conservation process. Ionic and osmotic pressures are related, both referring to the effect of differing concentrations of solutes on different sides of a surface or membrane. Osmotic pressure is a term used to characterize transport across a semi-permeable membrane. An example is reverse osmosis, a process to remove solute from water that requires input of energy (pressure) to overwhelm the osmotic pressure, reverse the flow, and cause water to flow from the hypertonic side to the hypotonic side. Ionic pressure describes the stored energy in a hypertonic solution of ionic solute, such as the energy that could theoretically be harnessed when fresh water flows into salt water at the mouth of a river.
To counter these effects, the conservator adjusts the cleaning agents. The MCP provides a tool to facilitate such adjustments. Chris finds that by controlling the pH and the ionic strength of the water used in cleaning acrylic paintings, it is possible to achieve cleaning while controlling the degree of surface modification. An isotonic cleaning agent (one with balanced solute concentrations that is therefore neither hypotonic nor hypertonic) combined with a low pH minimizes surface swelling. Combining isotonicity with a relatively low pH, approximately five to six, provides effective removal of surface soils. Most cleaning agents used in manufacturing are high pH. However, a low pH is needed to prevent deprotonation of the polyacrylic acid thickeners in the paint.
Even more adjustments to the cleaning agent may be needed. Remember that surfactants are effective because they have a love/hate relationship with water and with soils. One end of a surfactant likes water; it is hydrophilic or lipophobic. The other end likes oils and hates water; it is hydrophobic or lipophilic. The property allows surfactants to trap soils, most of which are oily, and to hold soils in suspension, eventually in a micelle.
One of the parameters that characterize surfactants is the hydrophilic/lipophilic balance (HLB) number. The higher the HLB, the higher the solubility in water. Most detergents have an HLB in the range of 13-15. A solution with a high HLB can help to suspend lightly bound pigment particles in the acrylic paint. A conservator wants to remove accumulated grime from a surface but not the artist applied pigment. Therefore the conservator might add 1% ethanol, with an HLB of 8, to lower the effective HLB, and therefore the detergency, of the cleaning solution.
THE FUTURE OF ART AND MANUFACTURING
The MCP is a work in progress. It is an active area of research. As theories are put into practice, and as feedback is obtained from users, new approaches to conservation will evolve. The cleaning of acrylics in the 21st century will begin on a very strong footing. Fine art has many parallels to manufacturing. Both may have stringent demands on performance and durability. Both may involve complex materials of construction. While custom formulations for each manufacturing process may not be realistic, it is reasonable to expect approaches to cleaning developed by conservators of fine art to be of value to manufacturers who are concerned with the quality and preservation of their product.