Porcelain enamel

Porcelain enamel is a term used to describe the fusion of a thin glass coating to metal when heated to over 800° Fahrenheit. Porcelain enamel is better known as vitreous enamel. It is a non-porous material that can be manufactured with a variety of physical and chemical properties. Architectural enamels are commonly applied to steel or iron substrates.

History
Porcelain enamel was originally used for jewelry and other decorative purposes but was first applied to sheet metal in Austria and Germany in the mid-nineteenth century. German enamel manufacturer, Theodor Bergmann, tested porcelain enamel’s ability to be applied to steel sheets and used for constructive purposes in 1890. In the later 19th century, enameling technology was introduced to the United States. It was common for German immigrants to work in the enamel factories or for the factories to use German equipment. In the early 20th century, the porcelain enamel industry had grown extensively. The Rolling Mill Company developed a product with low carbon and manganese for enameling. Porcelain enamel became a popular material in kitchenware, bathroom fixtures, and appliances in the 1920s.

Manufacturing Process
Porcelain enamel is the fusion of any glassy composition to a metal substrate. Enamel is a mixture of minerals, fluxes, and opacifiers. These materials are mixed together and smelted, then cooled and ground in a mill. Porcelain enamel was originally applied to a metal substrate in either a wet or dry method. The dry method meant that the frit (enamel) was applied to a heated substrate using a sieve. The wet process meant that the enamel was suspended in a emulsion of water, clay, and frit. This mix was then dipped or sprayed on. Ground coats were usually dipped to help form a bond with the metal substrate. Cover coats were usually sprayed on to provide color, opacity, and an acid resistance. Thickening agents could be added to change the flow of the enamel and thicken the mixture. Enamel substrates of iron and steel were dried and fired. Titanium steel was developed in the 1940s by Inland Steel, who marketed the product as Ti-Namel. Aluminum was later used because it was lighter and more corrosion resistant. Architectural enamels had a variety of finishes and textures to work with. A glossy finish was popular in the 1930s. Oatmeal mottle and stipples became popular after World War II and were found on many commercial chains. Corrugated, crimped, or embossed metal substrates also found popularity during this time. Matte finishes were seldom used on buildings before the 1960s.

Uses and Installation
Though enamel sheets and panels were produced in the 1920s, popularity of this product did not rise until the 1930s. White Castle restaurant chain was one of the first businesses to use porcelain enamel sheets in 1925. It opened its first porcelain enamel paneled building in Wichita, Kansas. The construction superintendent, L.W Ray invented a key-lock device for exterior panels that prompted the company to market other porcelain enamel type of buildings. The World’s Columbian Exposition in Chicago was the first time that porcelain enamel shingles had been introduced to the world. They were produced in 1924, for the first time, in the United States by the Columbian Enameling and Stamping Company. Vitrified Metallic Roofing Company also began similar production the same year. The shingles were shallow and flanged with a convex appearance and were made no longer than 12 square inches. Porcelain enamel tiles were introduced in 1925 by the Chicago Foundry Company that produced experimental tiles. Porcelain Tile company of Chicago introduced the Porstelain, which was flanged tiles of 20-gauge stamped steel. These tiles could be cemented in grooved insulation boards and were available in two sizes initially. After much success, and the purchasing of the company by Youngstown Pressed Steel Company, Porstelain became offered in larger sizes. The name was then changed to Veos. Porcelain enamel shingles were a staple in the restaurant business and soon found popularity with other business chains. Howard Johnson’s soon used orange porcelain enamel shingles as a signature look while White Castle, A&W, and Stuckey’s also found a use for the enamel shingles. Even with its popularity, porcelain enamel shingles struggled with competitive materials because they were expensive. Porcelain enamel was, however, noted for its color permanence, and resistance to weathering and abrasions. Expositions in the 1930s demonstrated porcelain enamel’s versatility for architectural purposes. Many found porcelain enamel to be ideal for light fixtures, roofing, and fireplace surrounds. The Century of Progress in Chicago introduced two porcelain enamel houses. Armco-Ferro House, with 13-inch wide porcelain enamel sheets, and the Stran-Steel House that was constructed of panels 2 feet wide by 10 feet long. Due to the overwhelming popularity of these two show houses, many manufacturers began producing architectural panels for storefronts, schools, and offices. Most flanged panels and spandrel panels were usually sized for installation on site. Simple piece of porcelain enamel, like flat sheets for exteriors or wainscoting, were usually held in place by screws and battens. By the 1930s, flanged veneer panels became the most popular type of paneling available. Panels were capable of being attached to walls in a variety of ways and attached to a variety of materials as well. Some installation techniques used molding or battens to cover joints, others relied on caulking. Architects eventually had up to 17 methods of installation to choose from. Some building codes limited or prevented the use of porcelain enamel for architectural purposes. The Porcelain Enamel Institute was the first to adopt standard specifications in 1946. World War II left the porcelain enamel companies with new ways for the material to be created which used less enamel on the metal base. Enamel wallpaper was one of several new materials introduced in this time frame. This product was called Mirawall and was a thin sheet of porcelain enamel backed with Masonite and adhesive. Diners, restaurants, and hospitals all used this material extensively. The first enamel for aluminum was introduced by the Kawneer Company in 1949. Steel was still the predominant material used for architectural purposes however. The Lustron Company introduced porcelain enamel prefabricated houses that were made almost entirely of porcelain enamel components. Porcelain enamel then gained popularity as spandrel panels for curtain wall systems. Curtain wall systems required thicker insulation for fire rating requirements. Composite panels were popular through the 1960s but facing panels are now more prevalent.

Conservation - architecture
Because porcelain enamel is made of glass with a variety of metal choices, its chemical properties will vary. The material’s resistance to corrosion or stress between the glass and the substrate, the manufacturing quality, and the amount of maintenance given to the piece over time will affect its durability. Porcelain enamels, regardless of composition, are known for being abrasion resistant.

Deterioration
Porcelain enamel is most likely to be effected by the corrosion of its base metal. Corrosion of the base metal is likely to cause the porcelain enamel to spall. When the chemical bond is broken between the glass and its substrate, the substrate will then be exposed. When thicker porcelain coatings are added, they are more likely to chip or crack. Chips and cracks to porcelain enamel can also be caused by careless installation or vandalism. Manufacturing defects are most likely inadequate enamel coverage. If the porcelain enamel does not cover the metal substrate, the metal will be prone to corrosion, which will lead to cracking of the enamel. Deteriorated joints are likely to allow water infiltration and can cause deteriorations well. Manufacturing defects may also cause warping. Corrosion of the glass can be caused by several factors including: water, acids, or atmospheric pollution. Porcelain enamel that has experienced some type of corrosion is likely to change both physically and chemically. The surface may become pitted or hazed.

Conservation Techniques
The first step in developing a conservation plan is to assess the condition of the porcelain enamel panels. The architects drawings and specifications can help to determine the type of substrate, how the materials are attached, and any caulking materials. Physically pressing on the panel should help determine if the piece is loose, but finding the missing fasteners may require the removal the panel by means of cutting with a reciprocating saw. Applying tape around the piece that is going to be cut should prevent the other pieces from chipping during the cutting. It is important to check for water infiltration. Traditional methods of caulking, such as extruded plastic or synthetic rubber, are outdated and may not work as well as modern sealers and caulking. If necessary, the old caulking can be replaced by new materials and matched to the existing color. There are no set acceptable techniques for conserving porcelain enamel but experimentation can be considered. Conservation techniques are usually adapted from conservation techniques that are used in museums, refinishing bathroom fixtures, or field experiments and practices. Small test areas should be used for treatments that have not been tested and may be irreversible. When the porcelain enamel seems in good condition, a cleaning may be all that is necessary to reveal the original finish. Cleaning is also necessary to prepare the surface for a coating for corroded areas. Testing is strongly suggested to check the effectiveness of cleaning methods and determine the best approach. Warm water and a clean cloth is the least abrasive way to clean porcelain enamel but may not be time efficient for larger pieces. Large pieces can usually be washed with low-pressure water. A solution with 1% trisodium phosphate should remove stubborn dirt without damaging the piece. Grease and oil tend to be more difficult to remove but should respond to an alcohol-based solvent. Caustic paint strippers are likely to etch the surface and should not be used to remove paints. Porcelain enamel is likely to also be damaged by exposure to a base metal. Intervention will usually save the piece as long as it appears in reasonably good condition. Intervention in this problem usually consists of removing the corrosion product and applying a protective cover. Protective covers have traditionally been epoxy, urethane, or lacquer; these should be applied sparingly to the damaged area. Lacquers can be pigmented and are a common medium for retouching porcelain or repairing minor defects. Urethanes were commonly used in the repairing of cast iron bathroom fixtures. Before applying either, it is important to understand the ability of each to retain color and adhere to the substrate. Before applying surface-tolerant epoxies, it is important to remove corrosion first. These epoxies are likely to last longer than lacquers and urethanes but must be applied correctly. Corrosion should be removed with a wire brush and the piece cleaned to remove sealant residue. Asphaltic urethane paints can also be matched and applied to the porcelain enamel. Polyester fillers are commonly used to repair bathroom fixtures or small chips in enamel, but only where base metal is not exposed. Polyester colors may be difficult to match to historic porcelain enamel pieces. After the filler has been added, it should be given time to dry then sanded (being careful to not scratch the surrounding area).

Replacement
It is possible to replace severely deteriorate pieces, especially if the piece is in a curtain wall. The curtain wall depends on all of the pieces being structurally sound and corrosion of pieces may cause the curtain wall to fail. Porcelain enamel repair requires personalized approaches. Flanged, concrete-backed, and spandrel panels should be replaced with pieces that match the existing color and finish. Solid colors can be readily matched while stippled may prove more difficult. When replacing a flanged piece, the caulking for the piece should also be removed. The clips that hold the piece in place can be cut with a reciprocating saw to free the material. Custom designed panels are usually used to replace these pieces and rest on stainless steel clips. Resealing the area and joints of the new panel should finish the job.

Conservation - historic objects

 * Cleaning
 * Consolidation
 * Stabilization