DATA BULLETIN PEI 501
Porcelain enamel is defined as a substantially vitreous or glassy inorganic coating bonded to metal by fusion at a temperature above 800°f.
The distinctive aspects of porcelain enamel's appearance properties are durability and variety. The wide range of colors possesses outstanding fidelity. The glass-hard surface assures a long lasting finish, whether it is high or low gloss, that is easily cleaned and maintained.
Porcelain enamel provides a permanent lifetime finish in an unusually broad spectrum of colors and hues. Pigments used are primarily inorganic compounds fused into the glass matrix and are extremely stable during aging. The colors are highly resistant to deterioration or change as shown by long-term outdoor exposure tests conducted by the National Bureau of Standards (NBS) and the Porcelain Enamel Institute (PEI) at a variety of sites over a period of 30 or more years. Neither colored or white porcelain enamels are photosensitive and they remain stable in the presence of ultraviolet and infrared radiation. Porcelain enamel colors are unaffected by rain, snow, dust, sunlight, heat, oxidizing agents or corrosive fumes. The porcelain enamel coating will not peel, blister or delaminate from its metal substrate or underlying porcelain enamel coating. Color permanence of porcelain enamel is affirmed by its use as physical color reference standards for the printing ink, paint, plastics and textile industries.
A virtually unlimited array of permanent colors is available in porcelain enamel. While most architectural applications utilize colors from the Porcelain Enamel Institute's color guide or from individual company color reference charts, specially formulated colors are also available-particularly for large volume production such as major appliances, cookware and plumbing fixtures. Most porcelain enamel colors are available in finishes from high to low gloss or muted nature tones. Stippled or pebbled effects and the application to textured or embossed metal give additional dimensions to the color effects and permit unusual design versatility together with the desirable performance properties. Uniformity of most colors can be controlled closely, but some slight variations should be expected in a few, such as purples and scarlets.
Gloss is the shine or luster of porcelain enamel-its specular reflectance. A wide range of glossiness is available in porcelain enamel. High gloss is usually desirable for appliance and plumbingware surfaces. Lower gloss is often specified for architectural and decorative applications where less reflection of distinct images is sought. Gloss can be scientifically scaled in tests such as the 45 degree specular gloss test. On this scale, a polished black porcelain enamel has a value of about 55. Most glossy porcelain enamels on steel will be in the 50 to 60 span. However, values can range from below 10 for specially formulated matte finishes up to 85 for some applications where extremely glossy surfaces are desired. Test to measure gloss: ASTM C346 Gloss of Ceramic Materials, 45 degree Specular (used primarily for appliances); ASTM C540 Image Gloss of Porcelain Enamel Surfaces (used primarily for architectural porcelain enamel).
Light reflectance is a measure of "whiteness." A porcelain enamel ordinarily must have reflectance of at least 65% to be considered white. Most white porcelain enamels have a reflectance of at least 75%. White porcelain enamels for light reflectors usually have a total reflectance of at least 80%. It is measured as the luminous daylight 45 degree- 0 degree directional reflectance.
Porcelain enamel is available in a wide variety of textures or degrees of smoothness to suit different appearance or functional requirements. Ordinarily, porcelain enamel has a smooth, fire-polished surface with slight surface waviness barely visible to the eye. Coatings applied by the electrostatic powder method are visually smoother without surface waviness. Unique textural effects may be obtained by applying porcelain enamel to embossed or rigidized metal. Other examples: a pebbly surface that breaks up a reflected image may be specified for architectural applications; a smooth yet minutely gritty surface that will take chalk evenly and erase easily is produced for chalkboards; a stipple surface may be desirable for appliance interiors.
The ability of a specific porcelain enamel coating to mask or hide the metal substrate or, more likely, another porcelain enamel undercoat is its hiding power. This property will vary with the thickness and composition of the coating. The best hiding power is exhibited by superopaque, titania-bearing porcelain enamels which provide satisfactory masking at thickness as low as 3-mils. A system can be deliberately designed to have poor hiding power. The clear frit used in colored porcelain enamels applied over a white undercoat for decorative effect is an example.
Smooth and glossy porcelain enamel is easy to clean, usually with only a mild detergent and very little rubbing effort. No matter what its texture, the unique hardness and abrasion resistance of porcelain enamel permits the use of cleaning methods that would be harmful to other coatings. Sanitation is an inherent property of porcelain enamel. The hard dense surface is a barrier against odor and bacteria retention. The porcelain enamel itself is odorless, tasteless and non-toxic. Porcelain enamels are also highly resistant to radiological and biological contamination. Surface contamination can be removed quickly and easily by simply washing. Smooth surfaces especially resist most common germicidal solutions and can be sterilized with live steam without damage. Porcelain enamels also resist corrosive industrial atmospheres, salt, air, wind-driven gases and smoke.