SCIENCE/TECHNOLOGY
er organized the first such program for the fall 1992 ACS national meeting, which was also held in Washington, D.C. She hopes that these sessions will become regular events at two-year inter vals. The 1994 affair was devoted specif so bad in some states that conservators ically to coatings. 208 ACS are resorting to smuggling the materials Coatings are crucial to the well-being National Meeting they need for their work. of objects in museums because they On a brighter note, studies of the ef make up the surfaces that are exposed to Washington, D.C. fects of temperature and humidity on the environment. And in the case of paintings suggest that museums do not paintings, the coating is the artwork. Yet have to control conditions as tightly as the fate of an artwork depends on the Stephen C. Stinson, was once thought. If true, the findings paint, stain, or other coating that the artist C&EN Northeast News Bureau would be good news for museums, used to create it. The museum curator can any of the questions polymer many of which face increasingly tight only deal with that choice after the fact. chemistry addresses in its tra budgets. Stringent control of tempera It was Donald C. Williams, senior fur ditional academic and indus ture and humidity consumes a signifi niture conservator at the Smithsonian's trial settings also turn out to be rele cant portion of museum budgets. CAL, who warned his audience that vant in a sphere not usually associated In the coatings industry, researchers clean air laws threaten the organic sol with the discipline—that of museums. are bringing such arcane techniques as vent-based coatings that conservators A day-long symposium sponsored by fractal geometry to bear on development depend on to care for objects. Williams the Division of Polymer Chemistry of products to protect metal from corro says state laws have made the situation made clear that there is a close associa sion. Such coatings are important to mu so severe in California and New Jersey tion between research on and environ seums also. The Smithsonian Institution, that curators there are smuggling in the mental issues associated with paints for example, must care for such diverse needed products from Nevada and and coatings and preserving artwork metal objects as the B-29 bomber Enola Pennsylvania. and artifacts in museums. Gay, which dropped the atomic bomb on The solvent system of a coating is im For example, federal and local clean Hiroshima, and a stainless steel sculp portant because the user needs a certain air laws are threatening conservators in ture, "Cubi ΧΠ," by the late U.S. artist range of solubility to apply and remove American museums with the loss of the David Smith. coatings. And the evaporation rate of the paint and coating formulations that they The symposium, called Polymers in solvent affects the quality of the film. need to protect and restore objects in Museums '94, was organized by Mary The waterborne coatings that commer their collections, according to a furniture T. Baker, a polymer chemist at the cial producers are switching to are inad conservator at the Smithsonian Institu Smithsonian's Conservation Analytical equate to the tasks, Williams maintains. tion in Washington, D.C. The situation is Laboratory (CAL) in Suitland, Md. BakCommercial waterborne coatings are tightly formulated around certain resins, surfactants, and other additives, so con| servators cannot reformulate and adapt | them to a specific use. Another problem | is that the surfactants that keep the res^ ins in water suspension remain in the 1 coating films where they spark crossI linking reactions in the resins. Crosslinking of polymer chains renders resins hard and brittle. And surfactants can mi grate into the body of the object, where they may do harm. "What we need and what industry wants aren't always the same thing," Williams says, coming to the heart of the problem. "For starters, our time ref erence is very different. When was the last time an industrial project depend ed on a polymeric material's maintain ing its stability and properties for a century or two? It might not be a part of the equation when you are painting highway stripes, but it's critical to us." National Air & Space Museum crew prepares a Grumman Goose for exhibit All of the Some observers suggest that musepaints and hard protective coatings used in the restoration of this craft will come under urns will probably receive waivere to use strict regulation and restriction as a result of their high volatiles contents. organic solvent-borne coatings. But Wil-
Polymer chemists seek solutions to museum restoration problems
M
28
SEPTEMBER 5,1994 C&EN
swings of temperature and humidity without developing damaging stresses. Also surprising, Tumosa finds, is that researchers can predict how a painting will respond even if they don't know the physical-chemical composition of the paint used to create it. TTiis is important because the curator must meet artworks on their own terms. Artists have used many different media ranging from the homemade linseed oil-dried paints of the 15th century to the store-bought waterborne acrylics of today. Modern artists have even experimented with blending mayonnaise into the acrylic. And a painting begins life in a stressed condition. This is because the artist starts by stretching the canvas tautly in a frame. For his own work, Tumosa uses commercial acrylic paint films laid down on canvas at the Smithsonian 13 years ago. Smithsonian Institution furniture conservator Williams examines the varnish on a cello He equilibrates these samples at the as part of a course onfinishesfor violin restorers; several types of polymeric coatings 23 °C and 50% relative humidity that cumay be used in the cello's conservation. rators have considered mandatory in museums. Tumosa then mounts the films in liams says, "While it's unlikely that the the present conservator's "documenta- stretching instruments. materials we use will be completely out- tion." Even if no written record survives, Tumosa determines the coefficient of lawed for preservation purposes, if reg- the future worker will reach the isolation thermal expansion at 5% relative huulations make it difficult or unprofitable layer with knowledge of what the pre- midity while varying the temperature to produce and use certain polymer for- decessor had done. between -10 and 30 °C. He also meamulations, [the materials] will disappear Inpainting is a kind of translucent or sures the moisture coefficient of expanfrom the market, and we won't be able colored organic "spackle" that fills in sion at 23 °C while changing the relato get them, legal or not. those gaps in the original coating that go tive humidity between 5 and 60%. "Our industry uses only 2,000 to 3,000 down to the bare surface of the object itSome of Tumosa's studies take up to [kg] of resin per year," Williams contin- self. Consolidants are used to treat flak- 18 months per sample. During this time, ues. "We're not only not the tail that ing surfaces. Consolidants get down to, changes in stress are so gradual that wags the dog, we're not even a hair on wet, envelop, and adhere to all accessible strains of up to 0.4 inch of stretch per the dog." surfaces and flakes to arrest further loss. inch of canvas length can occur. This is Ironically, one of the most important The final finish, Williams observes, "is the kind of patient, time-consuming, funproperties of a conservation coating is more than a mere shrink wrapping of damental research that the Smithsonian removability. Today's conservators the object. It is the final step to a some- has time for, says organizer Baker. She know that their successors of several times complex process, unifying the vi- wonders whether the paint and coatings decades or a few centuries from now sual interpretation of the object. This fi- industry could benefit from such data, may want to repair new damage to an nal step is every bit as important as the and whether such sharing could be the object or to try a new approach to redo- previous ones, for a poorly applied or ill- beginning of a collaboration between ing earlier restorations. chosen coating can substantially detract museums and industry. This points up Williams' emphasis from the appearance of the artifact." Tumosa finds that the samples can on solvents and solubilities as imporThe performance of the final finish undergo fairly rapid (within hours) tant issues in conservation coatings. brings Williams to the question, "What changes of 40% of relative humidity or And it leads into Williams' classifica- is the final function of an object?" And 28 °C of temperature without develoption of coatings types as used by con- the answer: 'To look a certain way. We ing unacceptable strains. These results servators: Isolation layers, inpainting can't restore the object to its original ap- could change the thinking of museum media, consolidants, and final finishes. pearance. We don't know what that was. managers about how closely to maintain Isolation layers are those coatings ap- Damage is permanent. You can only dis- environments in picture galleries. plied to the object with the express pur- guise damage. You can't make it go Barry S. Snyder, a chemist at the pose of preventing interaction of subse- away." Rohm and Haas research center in quent coatings with the object. Future Charles S. Tumosa, a chemist at CAL, Spring House, Pa., works on the probworkers can remove everything down to has studied the effects of changes in lem of how best to apply waterborne the isolation layer without scraping the temperature and humidity on paintings. acrylic paint to canvas or other subobject or any original painted decoration It is his work that has established that strates. His work also addresses the itself. The isolation layer is also part of paintings can withstand surprisingly wide question of the behavior of solvents SEPTEMBER 5,1994 C&EN 29
SCIENCE/TECHNOLOGY and additives in coatings. Snyder's research suggests that stress failure of an artwork can result from applying paint under improper conditions. Snyder says that paint film formation occurs in phases—concentration, compaction, and postcompaction. In the concentration phase, water and other volatiles evaporate at rates influenced by temperature and humidity until individual resin particles are closely packed together. In compaction, the particles coalesce as the force of resin-resin and water-water attractions overcome the competing process of continued separation of particles by the capillary movements of water. Temperature is critical during compaction, and there exists a minimum film formation temperature (MFT). Above the MFT, compaction and film formation occur. Below MFT, the result is a brittle powder. Finally, in postcompaction, polymer chains diffuse beyond boundaries of former particles to erase those boundaries by entangling one another. Temperature also governs successful thermal migration and entanglement of polymer chains. Snyder's
30
SEPTEMBER 5,1994 C&EN
Tumosa of the Smithsonian's Conservation Analytical Laboratory uses equipment to study the effects of relative humidity on the strength and stiffness of cultural materials. conclusion is that artists must work at humidities and temperatures that will result in films of good integrity so that their works will survive future environmental stresses. Molly Moon, a research chemist at Sherwin-Williams Co., Cleveland, has developed an electrochemical technique to gauge protectiveness of metal coatings against corrosion. The problem is that corrosion may go on under an apparendy intact coating without being noticed. With Moon's technique, a researcher can evaluate a coating's effectiveness soon after its application without waiting the thousands of hours of laboratory time that would be required for corrosion to become evident. The key to the technique is to use the same formulation to coat two identical metal sheets, which are then coupled to one another through an outside circuit that con-
tains voltage and current measuring equipment. The coated sheets are next exposed to a corrosive sodium chloride solution. In practice, Moon cements glass cylinders to each coated metal sheet and fills the cylinders with saline solution. She arranges a salt bridge (an inverted U-tube filled with saline-soaked agar) between the two cylinders. And she wires the sheets through the data acquisition instruments. In addition to all the other electrical effects owing to junctions of the different parts of the array, there is a minute coupling current and a coupling potential between the two metal sheets. As the salt solution penetrates the coatings and the metal surfaces respond, random noise variations appear in both the coupling current and voltage. Moon follows the electrochemical behavior of paired sheets for 2,000 hours. She then examines the plates for blistering and pitting of the coating and the condition of the metal under it. Her findings indicate that a coating's effectiveness can be gauged from current and voltage patterns in the earliest stages of the experiment. Coatings that are most effective against corrosion also give rise to wildly jagged swings in coupling currents and Projected crack pattern of an potentials, Moon finds. Those that are oil painting subjected to less effective yield undulating curves. cooling at low relative Moon uses fractal geometry to determine humidity is generated by a whether the seemingly random patterns computer model (top) and compared with a 19th-century are "persistent" or "antipersistent." The painting (bottom) that shows more active the corrosion, the more fractal analysis reveals a persistent pattern. • a similar crack pattern.
