In August 2022, a century-old Musgo Gasoline sign sold at auction for $1.5 million, as Signs of the Times reported. Buyers paid that price for a slab of steel and glass that still glows after decades of sun and rain. Understanding how porcelain enamel signs are made explains why that sign survived. It also explains why a modern panel can carry a written 25-year fade warranty. Here’s the short answer: we fuse powdered glass to steel in a kiln, and the color becomes part of the glass itself.
We’ve built these panels in Tumwater, Washington, since 1983. Every step happens under one roof, from cutting raw steel to the final firing. Below, we walk through all nine steps in order. After each one, you’ll see exactly how it protects the warranty. Along the way you’ll also find a material comparison and a quick glossary. Collectors, stick around for the section on how the classic signs differ from today’s panels.
What Is a Porcelain Enamel Sign?
A porcelain enamel sign is a steel panel with a skin of glass fused to it in a kiln. The colors come from inorganic metal oxides, not inks or dyes. Because those pigments live inside glass, sunlight has nothing organic to break down. For a deeper primer, read our guide to what porcelain enamel is and how it works.
This chemistry has also held up in federal testing for generations. In 1939, the National Bureau of Standards set 192 enamel panels outdoors in Washington, D.C. Then researchers came back 30 years later. Their 30-year exposure report found that the most acid-resistant enamels also stood up best to weather. That finding still guides how we choose our glass today.
Step 1: Choose and Shape the Right Steel
Everything starts with special enameling steel made to ASTM A424 Type I, a standard maintained by ASTM’s porcelain enamel subcommittee. We use 16-gauge steel at minimum and step up to heavier gauges for big or high-wind panels. The chemistry matters too. Copper and aluminum stay below 0.002 percent, so the glass bonds cleanly.
Next, we cut, bend, weld, and drill each panel before applying any enamel. Our CNC laser and plasma cutters sit in the same building as our kilns. Shaping comes first for a simple reason: you can’t drill fired glass without cracking it. Our porcelain on steel panels also run up to 120 inches in a single piece, so large murals need fewer seams.
Warranty link: Heavy, clean steel stays flat through repeated firings. A flat panel holds an even coat of glass, with no thin spots where weather can sneak in.
Step 2: Clean and Degrease the Metal
Steel arrives with mill oil and shop grime on it. Glass won’t bond to oil, so we strip the surface completely clean before coating. Think of it like painting a house. Skip the prep, and the finish always fails first at the dirty spots. The Porcelain Enamel Institute publishes guidance on steel prep for exactly this reason.
Warranty link: Clean steel gives the glass full contact with the metal. Without that contact, you’d get bubbles and pinholes that let moisture reach the steel.
Step 3: Apply and Fire the Ground Coat
The first layer is the ground coat, a thin glass that serves as the bonding layer. We apply it to every surface, including the face, back, sides, and flanges, at 2 to 5 mils thick. Then the panel heads into the kiln. There, the ground coat fuses into the steel and locks on for good.
Coating the back and edges might seem like overkill. In fact, it’s why our panels don’t rust from behind. After all, a panel with bare edges is a panel waiting to corrode.
Warranty link: The ground coat creates the chemical bond that stops peeling and delamination. Every layer above it depends on this one.
Step 4: Add the Cover Coat
Next comes the cover coat, a separately fired layer that forms a smooth base for your graphics. It covers the face, sides, and flanges as well. We also add an extra cover coat to the back, which keeps the panel flat and sealed. Total enamel thickness lands between 0.004 and 0.020 inch, depending on the job. The cover coat also works as a primer for bright, highly saturated colors.
Our frits, the powdered glass we start with, reach Class A or Class AA acid resistance under ASTM C282. That’s the citric acid spot test. It measures the same property the federal studies tied to long outdoor life.
Warranty link: Acid-resistant glass shrugs off acid rain and harsh cleaners. As a result, the surface holding your color stays glossy and sealed for decades.
Step 5: Mix the Color in the Lab
Before we print anything, our in-house color lab builds each ceramic color. We draw on a library of more than 20,000 formulas. Then we match your target within 2 NBS units. Our warranty treats any shift under that line as invisible to the eye.
This is often where artists step in. When Pranav Sood created “I AM MORE THAN WHO I AM” for the NYC School Construction Authority, he visited our Tumwater lab. There, he approved physical color chips in person, hue by hue. The same lab work let us hit an exact “Yankee Blue” for commemorative medallion panels at Yankee Stadium.
Warranty link: Inorganic metal oxides don’t react to UV light. There is one tradeoff, though. Fluorescent neons and electric magentas aren’t possible with these pigments. Still, the colors we can make, we can make permanent.
Step 6: Apply the Graphics
Now the artwork goes on. We are going to be intentionally vague here because this is Winsor Fireform’s secret sauce and sets us apart from other porcelain enamel fabricators. We have several application methods, and sometimes multiple are used for a single design or art panel. Some of these application methods are techniques you will not find anywhere else. This allows the artists or designers we work with to have a full range of color and artistic control.
We also have the ability to layer on transparent colors. This is another proprietary method that stacks translucent colors across firings to build depth. Meanwhile, our Signature Matte finish gives graphics a soft, low-glare surface that no one else offers in graphic form. Designers in the Society for Experiential Graphic Design community often ask for it in bright, glare-prone spaces.
For a sense of scale, consider one of our larger art mural installations. It was Griffith Observatory’s “The Big Picture.” It spans 114 panels and 152 feet, with more than 1.5 million stars. Every one of those stars had to line up across panel edges.
Warranty link: These graphics don’t sit on top of the glass. Instead, they fuse into it in the next step, so there’s no film to peel and no ink to fade.
Step 7: Fire the Panel Above 1,400°F
This is the moment everything turns permanent. We fire each panel at temperatures above 1,400°F. The glass melts, the pigments dissolve into it, and the whole surface fuses to the steel. Layered, multi-color work may pass through the kiln several times.
That heat is why porcelain enamel acts like glass instead of paint. The finished surface rates 4.0 to 6.0 on the Mohs hardness scale. By contrast, vinyl, HPL and powder coat rate 3 or lower. So spray paint and marker sit on top, and standard cleaners wipe them away.
Warranty link: Firing turns separate layers into one solid material. With nothing organic left, sunlight has nothing to attack.
Step 8: Cool the Panel Into Compression
Cooling sounds like a waiting game, but we engineer it carefully. We formulate our enamel to expand less than steel does. So as the panel cools, the steel shrinks more than the glass. That squeezes the glass into constant compression.
Why does that matter? Glass under compression resists cracking, much like a stone arch that grows stronger under load. Our enamel stays in compression across the full range of outdoor temperatures. That’s why freeze-thaw cycles can’t pry it loose or damage the graphics.
Warranty link: Compression keeps the surface whole through thousands of freeze-thaw cycles. A whole surface keeps water away from the steel underneath.
Step 9: Inspect Every Panel Against a Control Reference
Finally, our team checks every panel against the approved proof or control panel. We look for breaks, gas bubbles, hairlines, and stress lines. Color must match the control within 2 NBS units under standard D65 daylight. A panel has to pass before it ships.
Then we pair the panels with frames and mounting systems built for the site. For park projects, that includes NPS-approved frames.
Warranty link: The control sample becomes the benchmark for your 25-year fade warranty. We define perceptible fading as a shift of Delta E 2.0 or more under ASTM D2244. The warranty even transfers if the property changes hands.
What the Process Delivers in the Field
Lab specs are one thing, but weather is the real test. While installing new trail signs at Valley of Fire State Park in Nevada, our team checked an original Winsor panel from 29 years earlier. It still held its gloss and color, with just a single surface chip. At Dry Tortugas National Park, two of our interpretive panels even sit underwater at an ocean dive site.
Now compare that with typical interpretive panels. The Center for Land Use Interpretation reports that standard wayside panels average only about five years in the field. Likewise, the National Park Service’s panel materials guide puts it plainly: a cheap panel that doesn’t last is no bargain. You can see more long-life work on our parks and trails signage page.
How Porcelain Enamel Compares to Other Sign Materials
Every sign material handles color differently, and that difference decides how long it lasts. For example, one Houston client lost two rounds of HPL signs to edge delamination in the humid heat before switching to porcelain enamel. The table below, drawn from our Specifier’s Guide, shows why.
| Feature | Porcelain Enamel | Vinyl Print | HPL Panels | Fiberglass | Powder Coat Dye-Sublimation |
|---|---|---|---|---|---|
| Color system | Pigment fused in glass | Surface ink | Printed laminate | Gel coat + print | Dye in aluminum |
| UV resistance | No fade; 25-year warranty | Fades in 2–5 years | Fades in 5–8 years | Yellows and fades | Fades in 5–8 years |
| Surface | Won’t peel or delaminate | Peels | Delaminates | Cracks | Scratches |
| Graffiti | Fully cleanable | Damaged by removal | Damaged | Damaged | Damaged by solvents |
| Hardness (Mohs) | 4.0–6.0 | Under 1 | 2–3 | 2–3 | 2–3 |
| Fade warranty | 25 years, written | None | None | None | None |
Then vs. Now: How Classic Porcelain Signs Were Made
If you collect vintage signs, much of this will sound familiar. Early makers also fused glass to steel, often one color per firing. They built each color layer with stencils, so a five-color sign might visit the kiln five times or more. That’s why you can often feel a faint ridge where colors meet on an original.
Modern production keeps that core chemistry and adds precision, with certified steel and lab-matched color. So the permanence collectors prize isn’t luck or nostalgia. It’s chemistry, and it still works. Today you’ll find it in public art and murals in transit stations, schools, and hotel facades.
Porcelain Enamel Glossary
Frit: Powdered glass that melts during firing to form the enamel.
Ground coat: The first enamel layer, which bonds chemically to the steel.
Cover coat: The fired layer over the ground coat that carries the graphics.
Control sample: An approved physical panel that every production panel must match.
NBS unit (Delta E): A measure of color difference. Our warranty treats a shift under 2.0 as imperceptible.
Mohs hardness: A scratch scale that runs from 1 (talc) to 10 (diamond).
Vitreous enamel: The common UK and European name for porcelain enamel.
Frequently Asked Questions
How long do porcelain enamel signs last? Our panels carry a 25-year fade warranty, and their documented service life runs 25 to 50+ years. Independent data backs this up, too. The NBS 15-year weathering inspection tracked enamel panels across several outdoor test sites.
What temperature do you fire porcelain enamel at? We fire every panel above 1,400°F. That heat fully melts the glass and fuses it to the steel.
Is porcelain enamel sustainable? Yes. It contains no petroleum or plastics, and it’s 100 percent recyclable at end of life. Our closed-loop plant also produces zero wastewater.
Who specifies porcelain enamel signs? Parks, transit agencies, campuses and public art programs all do. For instance, New York State’s interpretive sign spec calls for UV-resistant, graffiti-proof baked porcelain enamel. Browse our project portfolio for more examples.
Planning a Sign That Has to Last?
Now you know what happens inside the kiln, and why each step matters decades later. If you’re specifying signage, public art or an architectural surface that has to hold up, we’d be glad to help early. Talk with our Tumwater team about color samples, spec language or a control panel for your next project.
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Ready to Specify Porcelain Enamel?
Winsor Fireform manufactures handmade, bespoke porcelain enamel panels and graphic tile in Tumwater, Washington.
Every system is produced in-house and backed by a 25-year fade warranty. If you are ready to start your truly permanent project:
