THE USE OF A THERMALLY APPLIED WAX COATING ON A LARGE-SCALE OUTDOOR BRONZE MONUMENT

By Joseph Sembrat

Abstract
Conservation Technical Associates LLC (CTA) was awarded a contract for Phase I for the conservation of the bronze statuary groups which compose a part of the Cuyahoga County Soldiers’ and Sailors’ Monument located in Cleveland, Ohio, USA. The monument consists of a central building with a large column rising out of the center. Surrounding the structure are four realistic bronze statuary groups which represent, in heroic size, the four principal branches of the military service: Each of the four sculptural groups measures approximately 5.8m wide by 2.1m deep with each of the individual figures measuring approximately 2.4m in height. The conservation treatment involved modeling, casting and reattaching missing bronze elements, performing superficial corrosion removal, locally patinating areas to help unify the surface coloration, and applying a protective barrier coating of microcrystalline wax to a heated bronze surface. It was determined that a modified hot-melt system performed best. Because of its self-contained features and ease of use, the hot-melt system appears to provide a considerable improvement over wax thinned with solvents to a consistency of yogurt.

Introduction
Wax has been used as a protective barrier coating on outdoor metal monuments for thousands of years and the techniques for its application have remained virtually unchanged over time. The term hot wax, which is generally considered to be the most effective form of protection for outdoor metal monuments, is often misused when it refers to traditional application techniques. Hot waxing usually refers to a procedure whereby the applicator brush-applies a paste mixture of wax and solvents to a heated metal surface. In this scenario, the wax is cold until it comes into contact with the hot metal.

The problems inherent in this procedure are that it is extremely tedious and time-consuming and, on certain surfaces, the application of the wax can be either too heavy or too light. Additionally, because a brush or rag is used in the application procedure, the final integrity of the finish can be compromised by brush hairs, lint residue from the rags, and streaks caused by the brush hairs; as a consequence, such application usually results in a considerable amount of discontinuities, or holidays, in the protective barrier coating (Sembrat 1998).

In his paper, “Development and implementation of a new method of wax application for outdoor sculpture using an airless sprayer,” Nick Veloz succeeds in overcoming the many problems associated with traditional application techniques of waxes through the use of an airless spray system; unfortunately, new problems are created with the use of large quantities of solvents (Veloz 1996). The environmental, safety, and technical problems associated with this technique greatly reduce its usefulness for the conservator of outdoor monuments. Building on the idea proposed by Veloz, this author sought out and developed a safer and more efficient means of applying wax to outdoor monuments. Additionally, it is one of the first known instances where the term “hot wax” is used correctly when referring to the application technique. This paper defines the techniques and equipment used to apply wax most efficiently to outdoor metal objects and discusses how the conservator of outdoor monuments can utilize these techniques and equipment to obtain protective barrier coatings of the highest quality.

The Function of Wax as a Protective Barrier Coating
There are numerous variations of wax recipes used by the conservator of outdoor monuments, but their basic function is the same, i.e., to protect the surface of the metal object from the harmful effects of the environment and to prevent the formation of deleterious corrosion products (Smith and Beale 1986). Typical ingredients of wax formulas usually include a blend of natural and synthetic waxes with small amounts of polyethylene added to them to provide additional hardness and greater durability. These ingredients are blended together by melting them in a large wax pot or double boiler. Solvents are usually added to this mixture at a rate of approximately 1 liter per 2 kilograms of wax to impart a workable consistency that is suitable for brush application. The wax mixture is then decanted into cups or small buckets and is allowed to cool.

Traditional Wax Application Techniques
Once a metal object has been properly cleaned and the desired surface appearance has been achieved, it is ready to receive a protective barrier coating. If hot wax is to be the coating of choice, the surface of the metal object must first be heated. The heating processes can range from burning a fire inside the monument, to conductively heating the metal with electricity, to heating the surface of the metal with a torch and compressed flammable gas. Whichever scenario is used, the surface temperature of the metal must be higher than the melting point of the wax (approximately 100°C) so that it can flow over the surface and saturate the corrosion products. Once this temperature is achieved, the wax formula is either brush-applied or rag-applied to the metal surface in a daubing or rubbing motion respectively (Fig. 1).

The process works well on small and uncomplicated forms, but depending upon the detail of the surface, weather conditions, the heat source that is being used, and the rate at which the metal loses heat, the process can be very slow and tedious. To compound this problem, as the wax solidifies within the bristles of the brush and the fibers of the rag, it tends to create a very stiff application tool which becomes almost unworkable in cooler weather.

Other negative factors inherent to the process and which directly affect the quality and integrity of the coating are usually related to the application tool. Because the size and shape of the brush restricts access to tight areas, those sections are usually left uncoated. Additionally, as a brush or a rag breaks down during the waxing process, it either loses bristles or lint, which become embedded in the coating. Not only is this scenario disfiguring to the final appearance, it creates holidays – or voids – in the wax which can lead to the premature failure of the protective coating. An additional concern in using a brush or rag as an application tool is that they both can leave streaks on smoother surfaces and either an excess or shortage of wax on rougher surfaces, such as raised letters and figures on plaques.

Rationale for Using Thermally Sprayed Wax Coatings
Because of the shortcomings of the traditional procedures described above and due to the highly competitive nature of the bid, a more efficient wax application technique was sought for the conservation of the bronze sculptural groups which compose a portion of the Soldiers’ and Sailors’ Monument located in Cleveland, Ohio (Fig. 2). The monument, which memorializes the men who served in the United States Civil War, consists of a central building with a large column rising out of the center. Surrounding the structure are four hyper-realistic bronze statuary groups which represent, in heroic size, the four principal branches of the military service: Infantry, Artillery, Cavalry, and Navy. Each of the four sculptural groups measures approximately 5.8m wide by 2.1m deep with each of the individual figures measuring approximately 2.4m in height (Fig. 3). The expense which was to be incurred in the modeling, casting, reattachment, and patination of the missing pieces did not provide a sufficient budget for the daunting task of hot-waxing the four sculptural groups. An additional requirement to complete all of this work within a four-month period convinced this author that an alternative approach to the traditional practices would have to be devised.

Thermal Spray-Applied Wax Coatings
The system devised by Nick Veloz was explored but was quickly abandoned because of the safety risks posed by the solvents in the presence of an open flame and the health risks for the applicators. In researching alternative methods of spray-applying waxes without the use of solvents, two similar pieces of equipment showed great potential. The first system tested was supplied by Global Corrosion Technologies. The principle of using airless sprays of the hot wax was good in theory. However, the system suffered from several design flaws all due to heat exchange problems causing lack of flow of the hot wax to the surface of the sculpture and the bulkiness of the equipment. Because of these problems, it was decided not to use the system. The second spray application system tested was a Slautterback Model AT-15 hot-melt unit (Fig. 4). The system was a completely self-contained unit originally designed to apply wood lamination adhesives, insulation, and bonding foam to various substrates. Because the principle of heating a solid mass of adhesive into a liquid for spray application is the same for the application of waxes, the author and the manufacturer of the equipment fine-tuned the system so that it could be used to apply protective barrier coatings of wax to outdoor monuments.

The unit consists of: an integral 7kg wax pot; a fully integrated electronic circuitry which can heat and maintain the wax up to 232°C; thoroughly heated fluid lines and handgun; and an electronic monitoring device which can display and control the temperature of the wax pot, the heat-traced lines, and the spray nozzle. The overall weight of the system was approximately 25kg, which made it relatively mobile.
The system utilizes an adjustable-flow electric screw pump to move the molten wax from the pot, through the material supply lines, and out through the spray nozzle (Fig. 5). A small external air compressor is used to atomize the wax as it leaves the nozzle. The atomized spray can be further refined by the addition of a heat-transfer pump which connects to the air compressor and heats the air leaving the nozzle to 232°C. When the system was field-tested there were several problems with the system which needed to be corrected before it was suitable for the project. Even with the very low flow rates that could be achieved with the pump, the system’s flow rate was still too high for our needs. To correct this problem, a smaller orifice fan-tip nozzle was installed which greatly helped reduce the flow rate. The second problem encountered was on breezy days when the system would cool down and the wax would solidify in the lines. This was corrected by wrapping the unit in an insulating blanket so that air could not cool down the uninsulated screw pump. The third problem encountered was that the spray nozzle was easily damaged if it was struck against a hard object. This problem was remedied by soldering a protective cage to the nozzle so that it would not be damaged. Once these items were successfully corrected and tested, the conservation treatment began.

The Conservation Treatment
Metal analyses were performed on samples of bronze removed from each of the monuments to provide information for the proper alloy to be used in the casting process. Missing sections from the monument were modeled directly on the sculpture in wax so that once they were approved by the client, they could be shipped to the foundry and prepped for the ceramic shell-casting process. Ceramic shell molds of all the missing pieces were made and an alloy that matched the original as closely as possible was used to cast the missing pieces.

The new pieces were then attached to the sculptures by means of TIG welding to help reduce the loss of zinc in the alloy during the welding process. Once the pieces were attached, the bare bronze was patinated using sulfured potash and cupric nitrate to match surrounding areas. The remainder of the areas on the sculptures were water-jetted at approximately 207bar (3000psi) to remove superficial corrosion products and airborne debris. Areas of light green corrosion were patinated using a solution of sulfured potash to help reduce the contrast between them and the darker regions. All patina residues were rinsed from the sculptures and the pieces were left until they dried. It was possible to have four conservators working on a group at one time. This was a great asset as it allowed greater areas of the sculpture to be heated simultaneously. One figure of the sculptural group at a time was evenly heated with propane torches to approximately 107°C. When the metal was sufficiently hot, the wax was spray-applied in a manner similar to spray-applying protective clear coatings (Fig. 6). Because of the speed at which the conservators were able to work, it was only necessary to re-heat the figures one or two more times after the initial heating so that the wax flowed evenly.

Once the entire sculptural group was coated, it received a second coat of wax using the same procedures explained above. The group was allowed to cool and the following day the surfaces were inspected for holidays or voids using a portable Tinker and Rasor M/1 holiday detector (Sembrat 1998). Any holidays were repaired by re-heating the surface and applying more wax. The sculptural group was allowed to cool once again and was then buffed out using clean lint-free cloth. The final appearance of the sculpture was a modulated green with areas of black and brown. The disfiguring streaking which had compromised the aesthetic integrity of the sculptures was either completely concealed or greatly reduced (Figs. 7 and 8). The addition of the recast pieces gave the sculptural groups more tension and conveyed a greater sense of energy to the overall composition. In addition to greatly improving the aesthetic qualities of the sculptures, the project was completed ahead of schedule and within budget. The owners of the monument were extremely pleased with the project and public support was overwhelming.

Conclusion
The hot-melt thermal spray system proved to be superior to traditional application techniques, such as brushing and wiping, because:

1) it allowed wax to be applied into previously inaccessible areas;
2) it applied the wax in a more even manner;
3) it reduced the amount of holidays or voids in the coating by an estimated 75%; and
4) it reduced the coating application time by over 50%. The time and money saved by thermal spray-applied wax coatings should offset the relatively high cost of the equipment (approximately $4,000 US) within a short period of time.

Additionally, it is hoped that the reduced labor costs of applying wax using this technique can either be passed on to the client or it will allow for more frequent maintenance intervals which would benefit the monument greatly. When used in conjunction with the proper coating inspection instruments, the thermal spray-applied wax system will help conservators achieve uniform and continuous coatings of the highest quality. This process should also help promote awareness in the field of outdoor monuments conservation that there is a need for better coatings application and monitoring techniques.

Acknowledgments
The author would like to thank the following individuals and companies for their invaluable assistance: Chris Blum, Linda Merk-Gould, Francis Miller, and Steve Servis of Conservation Technical Associates LLC; Dave Stella of Engineered Bonding Systems, Inc.; Berj Sharkarian the Cuyahoga County Architect, all the members of the Soldiers’ and Sailors’ Commission, and Julya Sembrat for her excellent editing work.

References
Sembrat J. 1998. Reliable methods for the measurement and inspection of protective barrier coatings for outdoor monuments. In: Postprints of Metals 98 of the ICOM Metals Working Group. London: James and James Ltd.

Smith R. and Beale A. 1986. An evaluation of the effectiveness of various plastic and wax coatings in protecting outdoor bronze sculpture exposed to acid deposition: a progress report. Conservation of metal statuary and architectural decoration in open-air exposure. Rome: ICCROM: 99–124.

Veloz N. 1996. Development and implementation of a new method of wax application for outdoor sculpture using an airless sprayer. In: Postprints of the Objects Specialty Group, Volume four of the American Institute for Conservation of Historic and Artistic Works 24th Annual Meeting, Norfolk, Virginia. Washington, DC: AIC: 23–33.

Materials
Hot-melt system – Model AT-15, Slautterback Corporation, PO Box 391, Monterey, CA 93942, USA.
Thermal spray system, Global Corrosion Technologies, Inc., 10655 Richmond Avenue, Suite 180, Houston, TX 77042, USA.

Waxes, Baker Petrolite Polymers Division, 1709 Industrial Boulevard, Kilgore, TX 75662-9309, USA.
Patination chemicals, Fisher Scientific, 711 Forbes Avenue, Pittsburgh, PA 15219-4785, USA.
Holiday detectors – Model M/1. Tinker and Rasor, 417 Agostino Road, PO Box 281, San Gabriel, CA 91778-0281, USA.