Porcelain enamel is a fused glass coating on a metallic substrate that provides a hard, durable, and extremely heat- and corrosion-resistant finish. While it has been used throughout history on gold, silver, bronze, and copper, enamel is also used on today’s ovens, clothes washers, water heaters, and bathtubs. Three new coatings have been developed which take today’s kitchen and bath finishes to a higher level, with performance that maintains beauty to build long-term brand popularity.
Porcelain enamel use remains strong on major appliances and in related industries. But, increasing conversion of much porcelain to stainless steel for aesthetics and to powder paint or plastics for cost reasons have made inroads in appliance applications. Here’s a look at the basics of the porcelain enamel process as well as some innovative porcelain materials and applications that may give applicators reason to consider a change back to porcelain enamel in some applications.
Porcelain Enamel Basics
Produced from natural, inorganic raw materials, porcelain enamel is not only a very durable finish, it is 100% recyclable and thus is inherently an environmentally friendly product.
Enamels begin as a blend of minerals smelted in much the same manner as common window or container glass. During this process the molten mixture is poured from a smelter and quenched between water-cooled rollers in a process known as “fritting.” This quick-cooled ribbon of glass is then shattered into flakes—frit. The chemical composition of the frit/glass is tailored to the end-product performance requirements and metal substrate used.
During processing, the frit—along with other minerals, which may include refractories, pigments, opacifiers, clays and surfactants—is mixed as an aqueous suspension or a dry powder to be applied to the metal substrate. The frit and raw materials are usually ball-milled to accomplish both complete mixing and proper particle size reduction and distribution.
Various methods are used to accomplish coating of the metallic substrate. For wet applications, the enamel can be sprayed or flow-coated on to the parts, or the metal may be simply dipped into a bath of liquid porcelain enamel slip.
For electrostatic dry powder application, porcelain enamel is usually sprayed in climate-controlled booths using corona guns. Considering the self-limiting nature of the electrostatic process, this method produces a very smooth and even coating of glass on metal with very good coverage of cut-outs and edges.
Porcelain enamel can also be applied to steel using an electrophoretic deposition process. The liquid enamel slip is specially prepared with appropriate electrical properties, rheology and particle size distribution. Parts are immersed in the bath and an electrical current is applied to causes the enamel particles to deposit on the metal part while water migrates away from the part via electro-osmosis. This process is frequently used by the major appliance industry in Europe. The resultant surface from electrophoretic deposition is perhaps the “best” quality porcelain enamel for smoothness and edge coverage, surpassing that of all other processes.
Cast iron is somewhat unique. It can be conventionally sprayed with liquid porcelain enamel slip for small parts. Today, small cast iron parts are also being successfully coated with electrostatic dry powder. For sinks and bathtubs, the coating is more commonly applied as a dry milled powder, “dredged” (sifted) on to a red-hot casting in three successive coats. As expected, this type of dry application produces a much thicker glass coating on the cast iron substrate.
Regardless of application method, the enamel must be fired to develop its final properties. Firing takes place at elevated temperature: 1,000–1,050°F for aluminum, 1,350–1,560°F for steel or copper and 1,500–1,800°F for cast iron. Desired finished product properties result from proper heating and cooling rates, considering the differences in thermal expansion between metal and glass. Generally, the finish is designed to put the enamel into slight compression when the substrate is cooled.
Final fusion occurs in this step, and adherence of the glass to the metal forms an inseparable interlayer between the frit and metal. This results when, during firing, the glass dissolves a portion of the metal at the interfacial boundary. This metal-rich layer is the key to the permanent bond of the glass to the substrate.
Originally, low-temperature frits for aluminum enamels contained about 25–40% lead oxide as flux. Recent increases in environmental and regulatory scrutiny such as the EU’s Restriction of Hazardous Substances (RoHS) directive are making the use of toxic substances unacceptable under any conditions, and consumers are demanding greener and safer products.
Initially, the lead was replaced with about 8–12% vanadium pentoxide as a flux, but this material also has toxicity, availability and cost issues. As a second step, vanadium pentoxide has been reduced to zero or near zero. Cadmium-free red colors as well as metallics have also been developed for cookware to catch the consumer’s eye on the shelf.
Another option for a single-coat colored enamel is to apply it over enameling-grade aluminized steel, which is used to make hollowware and architectural panels. This substrate is decarburized steel coated with 25–40 μm of a 90% aluminum/10% silicon alloy. It can be drawn and welded, and the only required surface preparation is an alkaline degrease. Furthermore, a vanadium-free, RoHS-compliant clear frit can be used to make colors and metallics.
A related application is high-end pan supports (stove grates), which are produced from cast iron because of the heat capacity, thickness, and weight of the metal. Top-of-the-line grates are enameled using electrophoresis, a water-based, nearly 100% efficient means of enameling complex cast iron parts with excellent coverage. Smooth, acid-resistant matte black enamels have been developed as well as durable single-coat gray and taupe colors.
Other types of pan supports currently produced are fabricated from steel wire and coated with wet spray or electrostatic powder application. These supports have uneven heat distribution. An alternative is using cast aluminum as a substrate to take advantage of the material’s high thermal conductivity (140–150 W/m°K) and relatively low density (2.71 g/cm3) compared to cast iron. Aluminum pan supports offer new market opportunities that are pleasing to consumers, such as a lighter weight stove grate.
Infrared-Reflective Enamel Finishes
The improved heat distribution of cast aluminum pan supports improves energy efficiency. An alternative route for porcelain enamel to improve energy efficiency is the incorporation of infrared-reflective technology.
For example, consider when sunlight, a form of electromagnetic radiation, strikes an object. Heat can either be transmitted, absorbed, or reflected. If the surface is opaque, transmission equals zero. Absorbed light is converted to heat energy and the remainder is reflected away from the object. Therefore, to keep an opaque object cooler, it is necessary to increase reflectivity and reduce absorption. In major appliances, if the object is an oven wall and the incident radiation is from the heating element, the increased reflected energy can go toward faster cooking.
Infrared-reflective porcelain coatings can be “tuned” to maximize reflection at a given wavelength—for example, 500 nm for sunlight or 2,250 nm for cooking. Roof coatings containing pigments reflective at 500 nm are most often used to keep structures cool.
In a comparison test of infrared-reflective and conventional enamels, an infrared-reflective material showed about 50% reflectivity at wavelength 2,250 nm versus a reflectivity of about 15% for a conventional black enamel. The same test showed that reflectivity of 90% or higher is possible using a specially formulated infrared-reflective glass, which is visually light colored and sufficient to decrease cooking times.
Metallics and Other Effects
Metallic surfaces, special effect colors, and reflective finishes have increasingly found their way into product design. The metallic trend shows no sign of slowing, but the growing demand for stainless steel and copper appliances, outdoor grills, fire bowls, and decorative items has been strained by volatile base metal prices.
Stainless steel typically used for appliances contains up to 10.5% nickel and 17–18.8% chrome. Both metals are subject to price volatility, making stainless steel have unpredictable economics.
Metallic porcelain enamels marketed under the Evolution name offer metallic looks with better durability at up to 60% lower cost than stainless steel. Colors developed to date include several shades of stainless-look enamel, mirror finishes, and copper. Oven interior pyrolytic ground coats with the copper color have also been created.
Metallic porcelain can be applied and fired like conventional colors at an application rate of about 30 g/ft2 on ground-coated, cleaned-only enameling grade steel. It provides all the benefits of conventional enamels, including sanitary qualities, easy cleaning and resistance to scratches, abrasion, chemicals and corrosion. It’s also flame proof, color-stable and contains zero solvents.
Additionally, studies have shown porcelain enamels to be significantly more durable than Type 304 stainless steel. Since austenitic stainless is more corrosion-resistant than ferritic stainless, one can conclude that enamels would be even more durable than the ferritics. Evolution enamels are suitable for all parts of the appliance, housewares, and architectural markets, and could also be used as accent pieces with stainless or for durable cabinetry.
Oven Cleaning Advances
Historically there have been three ways to clean ovens: using a self-cleaning pyrolytic ground coat, using a non-self-cleaning ground coat, and employing a catalytic continuous-clean enamel. The first method reduces foods to ash with exposure to temperatures of 900–1000°F, while the second requires harsh alkaline cleaners to remove soils. The third has largely fallen out of use and relies on high-metals, porous enamels to catalyze the reduction of soils to ash at normal cooking temperatures.
A new approach is steam-cleaning enamel, a porcelain material with a patented formulation that allows baked-on food residue to be released with exposure to moisture (as water or steam). It offers an option for cleaning ovens without harsh chemicals, high temperatures and the resulting fumes. It also requires no interlocks or extra insulation such as that needed for pyrolytic cleaning.
The steam-clean enamel is applied in a single coat over existing oven ground coats. It has the mechanical durability and thermal resistance of traditional enamel, can be applied in a single fire to steel, and fires at conventional ground coat enamel temperatures.
Although cleaning cycles may vary for various range manufacturers, steam exposure triggers the coating to shed baked-on soils in service. The steam can be generated with closed vents by heating 1 L of water in a pan to 350°F (177°C) for at least 30 min. Then, the oven is turned off and allowed to cool for 30 min. The sides of the cavity are wiped with the soft side of the sponge to allow water to run down the sides. After 20 min, soils can be wiped out of the oven.
RealEase enamel is a ceramic composite combining the cleanability of organic non-stick surfaces with the durability of vitreous enamel. It offers the hardness of enamel with the cleanability of PTFE. And, while there continue to be significant concerns about the chemical components of PTFE-based non-sticks, non-stick enamel offers a vitreous ceramic finish that meets EPA 2015 guidelines.
Some of the benefits of this coating include:
- Cleanability equal to PTFE and superior to stainless steel
- Pencil hardness of 8H
- Superior heat resistance compared to PTFE coatings
- Zero solvents.
The material can be applied to a wide variety of substrates, including aluminum, stainless steel, aluminized steel, cast iron, enameling steel and ceramics. For aluminum and stainless steel, roughening with blasting is required, while aluminized steel only needs an alkaline degrease. For cast iron and steel, a light primer application is necessary. Several vitreous primers called hard-bases are also available for use as base coats on ceramics.
Potential applications for non-stick enamel include cookware and bakeware (aluminum, aluminized steel, stainless steel, or ceramic), small appliances, toasters and microwave ovens, simmer plates, and outdoor/backyard grills and griddles. Because the coating is certified as safe for use in restaurant kitchens, it is also suitable for commercial kitchenware, cookware, bakeware and appliances.
Not all advances in porcelain enamel technology are aimed at the appliance industry. Corrosion of reinforcing steel in concrete significantly shortens the service life of structures such as bridge decks. Because of contact with the high pH of the surrounding concrete, the steel forms an iron oxide scale that expands into the concrete, putting it into tension and causing cracking.
|Pull-Out Test Results on Steel Rebar Embedded in Mortar|
Peak Pull-Out Force, N
Average Bond Strength, MPa
2,618.2 ± 466.2
3,497.9 ± 540.8
|Enamel/portland cement mix||
11,124.6 ± 235.3
While use of galvanized, stainless, or epoxy-coated rebar improves corrosion resistance, it does nothing to improve adhesion between the rebar and concrete.
Work done by the Army Corps of Engineers combining enamel coatings with calcium silicate, however, has shown that porcelain enamel can improve the performance and service life of reinforcing steel in concrete structures. In testing done to compare the pull-out strength of rebar subjected to different surface treatments, test rods were inserted into a 2-inch diameter × 4-inch long cylinder filled with fresh mortar. The rods were clamped so 2.5 inches of coated rod was under mortar, and test samples were placed in a 100% humidity cabinet at room temperature for a week before testing to measure the force required to pull out rebar.
As seen in the table, rebar coated with alkaline-resistant enamel mixed with Portland cement showed a very significant increase in pull-out strength versus both uncoated and enameled steel rebar, and work is continuing to field-test this technology.