Midwest Conservation

© Copyright 2002 by Lois Fruen

Neoprene shoe by Daniel Oates

This article accompanies the textbook The Real World of Chemistry 6th ed by Lois Fruen Kendall/Hunt Publishing ISBN 0-7872-9677-5

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Kristin Cheronis an object conservator at the Upper Midwest Conservation Association at the Minneapolis Institute of Arts. Cheronis conserves objects from public and private institutions from all over Minnesota. She loves her work, because every day she becomes part detective, part chemist, and part magician as she repairs priceless museum pieces. During my visits to the conservation department, Cheronis was working on five pieces, all of which require her expertise in chemistry.

Immediately, I noticed the two huge black neoprene shoes by Daniel Oates, waiting for Cheronis’ touch. They have suffered the attentions of curious viewers. Cheronis must remove chewing gum and oily fingerprints and repair scratches and gouges. Neoprene is polychloroprene (monomer is CH2=CClCH=CH2). Many of the glues, which Cheronis normally uses, would dissolve the neoprene. She has never worked with neoprene before and will spend time learning about the chemistry before making any repairs. But, that is what makes Cheronis’ job so interesting.

Next, she showed me an elaborate wrought-iron treasure chest from the 15th century, which she calls an "Armada chest." Over the years, the chest has been painted and varnished so many times, the intricacies of the bas-relief have been masked. In fact, Cheronis says that the chest has been painted with several coats of lead paint and oil paint. Then it was gilded, then painted with acrylic paint, and finally coated with pigmented shellac.

Conserving the Armada chest

For five days, Cheronis worked under the microscope to determine if any of the lowest layers of paint were original. None were, and since the paint was fragmenting and in poor condition, the curator and conservator decided all the layers should be removed to reveal the intricate details of the wrought iron and expose the surface of the iron to stabilize the corrosion. But, removing paint at the Minneapolis Institute of Art is not like stripping woodwork in your home.

Cheronis cannot use commercial strippers, because they contain methylene chloride, which corrodes metal. So, she has prepared a gel containing methylcellulose and ammonium hydroxide. She applies this mixture to a very small area of the chest and covers it with MylarTM for 15-30 minutes. Then she wipes away the top few layers of solubilized paint and repeats the application. Once the paint is loosened, she uses bamboo tools to scrape it out of the bas-relief. She estimates this process will take between 220-250 hours.

Once she has removed all of the paint, she will treat the iron with a tannic acid-based solution. The tannic acid will react with the iron oxide (rust) on the chest, and the resulting ferric tannate will give the chest a dark patina and will provide a stable oxidation layer that will help to prevent the chest from rusting.

The next piece she showed me was a large bowl made by Edwin and Mary Schieir. It is a valued piece in the collection of a private institution. The bowl was loaned to another institution and was discovered to have several hairline cracks when it came back. Cheronis was asked to repair it.

Cheronis says that the bowl is a particular challenge because small fragments of glaze and clay body have wedged the cracks apart, creating gaps. She cannot just add glue because there is no access to the breaks.

Edwin and Mary Schieir bowl

Cheronis will stay away from large-molecule adhesives, because they form very viscous liquids that do not easily penetrate cracks. Instead she will use a consolidant, and she has a choice of over 50 different types from which to choose. Consolidants are polymers, known as thermoplastics, which are applied as liquids, emulsions, or dispersions and set in situ. They can be liquefied by being melted or dispersed in a water-organic solvent mixture. If melted, the consolidants set by cooling. Solvent consolidants set when the water-solvent mixture evaporates (Cronyn 85-91). Both types can be reversed by either heating or dissolving in a solvent. Cheronis explained that reversibility is very important in her work. All repairs must be done so another conservator can reverse the repair in the future.

The type of consolidant she will use will depend on the chemical and physical properties of the bowl. Cheronis says that ceramic is a stable material, but different clays and firing conditions affect the porosity of ceramic pieces. Firing at a high temperature reduces the porosity of the clay. The porosity of the ceramic will affect her choice of consolidant. If the ceramic is non-porous, she will use a consolidant with high capillary action (non-viscous or runny adhesive), which will easily run into the hairline cracks. However, if she dissolves the consolidant in a low surface-tension organic solvent to increase penetration, the dilute solution may not contain enough polymer to adhere. If the bowl is very non-porous, the solvent may not be able to evaporate, so the polymer will not adhere. In addition, if she uses a solvent-based consolidant, she must determine just the right dilution because a too-dilute solution can drag the consolidant out of the crack when it evaporates, leaving adhesive on the surface. She must also be sure to use a consolidant that is strong enough to hold the piece together. If the surfaces are smooth, a stronger join is required and the consolidant must be matched in molecular charge to the ceramic. Finally, she must choose a consolidant with a refractive index that will be clear and non-yellowing.

When Cheronis first examined the bowl, she determined it was glazed with an alkaline glaze that left a matte surface with pinholes. The bowl looked vitreous (glassy), which she thought came from firing at a high temperature. But, when Cheronis put the bowl under a microscope, she discovered the artists had used a red clay, which contains iron. When the bowl was fired, the iron acted as a flux, reducing the temperature needed to create the vitreous effect. (It was probably fired at cone 7-8.) This means that the bowl is somewhat porous, so if Cheronis uses a high-capillary action adhesive, it will run into the pores but could stain the cracks. If she uses a thick (viscous) adhesive, it will not go into the cracks.

To repair the bowl, Cheronis thinks she will tread the middle ground. She is considering a consolidant called HxtalTM, which is a strong, non-yellowing epoxy adhesive. She can heat it to decrease its viscosity slightly, which will enable it to run into the cracks, but it will not be so runny that it will enter the pores. When it is thoroughly dry, she will remove residual adhesive from the surface using tiny swabs and solvent.

Cheronis says the consolidants and adhesives she uses most often are reversible. These include acryloid resin copolymers, which are dispersions, and polyvinyl acetate resins, which are dispersions and emulsions. Emulsion and dispersions are very useful in conservation. The dispersions have smaller particle sizes than emulsions. They both are milky in appearance, because they contain the non-polar polymer mixed with water and solvent. Once set, they are reversible when solvent is added. Cheronis says that many household glues, such as ElmersTM, crosslink and become nonreversible. (See chemical formulas at the end of the report.)

The last pieces she showed me also will involve gluing along with polishing and restablizing. These include a wood and leather carrying case and matching silver inkstand that were created in 1792 by Vincenzo Coaci for Pope Pius VI.

Wormholes on the leather case warn of the damage beneath. The worms ate the hide glue, which was used to attach the leather to the wood, causing the leather to lift off the wood. To reattach the leather, a polyvinyl acetate emulsion would work best, but there are reversibility issues, so Cheronis thinks she may use a polyvinyl acetate resin instead. The emulsion is water based, which creates crosslinking with wood. She might even use an acryloid resin, because the mend does not need a lot of strength.

There are also places where the worms undermined the structure of the wood. In those areas, Cheronis will inject a dilute solution of acryloid resin dissolved in ethanol or xylene, so the solution will penetrate into the wood. Cheronis will then fill the larger losses with acid-free paper pulp mixed with a little methylcellulose. Finally, she will make a loose foam insert for the case out of EthafoamTM, which is an inert polyethylene foam. Since the case will no longer be used to protect the inkstand, this insert will support the case and help to realign the wood.

 

Inside of inkstand case

She won’t fill and inpaint the wormholes because the curator wants to keep them as part of the history of the piece and are not causing any structural insecurities. She says the curator makes the decisions on what is to be filled or recreated, because the curator has the art-historical and stylistic knowledge. Repairing a piece is a partnership between the conservator and the curator.

Once the case is repaired, Cheronis will clean and repair the inkstand itself. The inkstand is a recreation of the obelisk and sculpture, standing in front of the Quirinale Palace in Rome. It has mechanical devices with moving parts, which caused one of the doves to come loose.

Cheronis will remodel the base of the dove and use a silver nut to reattach it. She will then remove the tarnish on the silver with a precipitate of gilder’s-grade calcium carbonate (CaCO3). This CaCO3 is much finer and will not scratch like the CaCO3 found in toothpaste. Then Cheronis will use 0.05µm micro-alumina water paste (Al2O3) to polish the silver.

Loose piece on the inkstand

Cheronis says she must be sure to remove all the powders from the inkstand, because they are hydroscopic and would attract moisture that would corrode the piece over time. To clean the gold, she will use petroleum benzene or xylene. She says that ethanol and acetone work well to remove paint remnants and gummy spots. Finally, she will clean the lapis lazuli and jasper with a very mild solution of triammonium citrate, clean it with water, and then apply a coat of synthetic microcrystalline wax.

So, next time you break and damage something valuable, consider that a conservator would make a chemistry experiment out of repairing it.

Chemical Formulas of Some Consolidants and Adhesives

Sources

Cheronis, Kristin. Personal interviews. Upper Midwest Conservation Institute. 8 and 14 Dec. 1999.

Conforte, M. "Inkstand, 1792 by Vincenzo Coaci." Unpublished paper. Minneapolis Institute of Art. Feb 25, 1985.

Cronyn, J. M. The Elements of Archaeological Conservation. London: Routledge, 1990.

Down, Jane L., Maureen A. MacDonald, Jean Tereault, and R. Scott Williams. Adhesive Testing at the Canadian Conservation Institute–An Evaluation of Selected Poly(vinyl acetate) and Acrylic Adhesives. Environment and Deterioration Research Report no. 1603. Ottawa: Canadian Conservation Institute.