If you've recently invested in a corten steel outdoor fireplace, fire bowl, or fire pit, you may be wondering why it's changing colour — or perhaps you're concerned about what looks like rust. Rest assured: this is entirely normal, and in fact, it's exactly what corten steel is designed to do.
In this guide, we'll walk you through the weathering stages of corten steel, what colours to expect and when, and how to care for your fire feature during the process. We'll also cover black-painted steel fire products — including popular brands like Bonfeu — and what to expect from those finishes over time.
What Is Corten Steel?
Corten (also written as COR-TEN®) is a group of steel alloys developed to eliminate the need for painting. When exposed to the elements, it forms a stable, tightly-adhering rust-like patina that actually protects the underlying steel from further corrosion — unlike ordinary mild steel, which rusts through.
This self-protecting layer is what makes corten so popular for outdoor fire features, garden sculptures, and architectural cladding. And crucially: once the patina is established, corten steel is entirely maintenance-free. No painting, no treating, no annual upkeep — ever. It's built to weather beautifully and look after itself for decades.
Pre-Rusted vs. Non-Pre-Rusted Corten: Know What You're Buying
One important distinction that's easy to miss: not all corten steel fire features look the same when they arrive. There are two types:
Pre-Rusted Corten (Already Patinated)
Most corten fireplaces and stoves are factory-treated to accelerate the initial weathering process before they leave the manufacturer. These arrive with a warm orange-brown patina already established — so you get the finished look from day one, with minimal run-off staining and no patchy "ugly duckling" phase.
On GardenHearth, you can identify pre-rusted corten products by their orange appearance in the product photography. Examples include:
- Barbecook corten fire pits and grills (e.g. the Jack and Nestor ranges)
- Bonfeu corten stoves and fireplaces (e.g. BonSolo, BonGiano)
- HEAT fire bowls in corten steel
- MOODZ Classic Fire Bowl in corten steel
Non-Pre-Rusted Corten (Raw / Mill Finish)
Some fire bowls — particularly the BonFeu BonBowl range — are supplied in their raw, unweathered state. These will go through the full visible weathering journey described below, starting with bright orange oxidation before settling into their mature patina. This is completely normal and part of the experience.
Examples on GardenHearth include:
- BonFeu BonBowl and BonBowl Plus corten fire bowls
- BonFeu BonBiza corten plancha grills and fire pits
If your product arrived looking dark or grey rather than orange, this is completely normal — the colour transformation is part of the experience. See the weathering stages below for what to expect.
A note from BonFeu on their fire bowls: To provide the necessary protection during transport, each fire bowl is carefully wrapped in plastic sealing foil before shipping. As the time from factory to the customer may differ, bowls may arrive in various stages of rusting, and with patches where the steel was in contact with the wrapping. This can be evened out with light scrubbing or even accelerated with a commercially available "rust accelerator". Eventually, the fire bowl will take on a full, even rust colour.
The BonFeu BonBowl at three stages: raw mill finish (left), early patina developing (centre), and mature corten colour (right).
The Corten Weathering Stages
Note: These stages apply primarily to non-pre-rusted corten products such as the BonFeu BonBowl. Pre-rusted products (most fireplaces and stoves) will have already passed through stages 1–2 and may begin at stage 3 or later.
Stage 1: Fresh from the Factory (Weeks 1–2)
Raw corten fire bowls typically arrive with a grey-blue or dark mill-scale finish — the steel surface as it left the factory. At this point, the patination process hasn't yet begun. Don't be alarmed: the transformation is coming.
Stage 2: Initial Oxidation — Orange Streaks & Rust Bloom (Weeks 2–6)
Once exposed to moisture and air, the steel begins to oxidise rapidly. You'll notice:
- Bright orange or reddish-brown streaks and patches
- Uneven colouration across the surface
- Orange run-off staining on patios or decking below — this is normal and temporary
This stage can look alarming, but it's simply the steel doing its job. The surface is not failing — it's forming its protective skin.
Minimising Run-Off Staining
During the first few weeks of weathering, orange-tinted water can run off the base of your fire feature and stain surrounding paving, decking, or gravel. To minimise this:
- Position your fire feature on gravel or a sacrificial surface during the initial weathering period if possible.
- Rinse surrounding surfaces promptly after rain — fresh run-off is much easier to remove than dried staining.
- Use a mild oxalic acid cleaner (available from most hardware stores) on stubborn staining on paving or stone.
- Avoid placing directly on light-coloured limestone or sandstone during the first 2–3 months if staining is a concern.
Run-off staining reduces significantly after the first 6–8 weeks and typically stops altogether once the patina is established.
Stage 3: Deepening Patina — Rich Amber & Copper Tones (Months 2–6)
As the oxidation layer stabilises, the colour deepens and becomes more uniform. Expect:
- Rich amber, copper, and burnt sienna tones
- A slightly textured, granular surface
- Reduced run-off staining as the patina sets
Stage 4: Mature Patina — Deep Chocolate & Espresso Browns (6–18 Months)
Over time, the patina continues to cure and darken. The fully mature finish is typically:
- Deep chocolate brown to near-black in sheltered areas
- Warm amber-brown on more exposed surfaces
- Stable, dense, and tightly bonded to the steel
At this stage, the patina is fully protective and the run-off staining will have ceased entirely.
Stage 5: Long-Term Stability (18 Months+)
A fully weathered corten fire feature is genuinely maintenance-free. No painting, no sealing, no annual treatment. The patina will continue to evolve subtly with the seasons — slightly lighter in dry summers, slightly darker and more textured after wet winters — but the steel beneath is comprehensively protected for the long term.
The corten patina process over time: from bright orange bloom to deep espresso brown.
Factors That Affect the Weathering Rate
The speed and appearance of patination varies depending on several factors:
- Climate: Wet/dry cycles accelerate patination. Consistently wet or consistently dry environments slow it down.
- Use: Regular fires accelerate the process significantly — heat and combustion gases speed up oxidation.
- Positioning: Sheltered areas (e.g., under a pergola) will weather more slowly and may retain lighter tones longer.
- Surface area and thickness: Thicker steel takes longer to fully patinate.
Will Corten Steel Rust Through?
No — corten's patina is chemically bonded and self-limiting. It does not continue to corrode beneath the surface the way ordinary rust does. Once established, the patina is the protection — and it only gets stronger over time.
Caring for Your Corten Fire Feature
The good news: corten requires very little from you. A few simple guidelines during the weathering phase:
- Do not apply oil, wax, or sealant during the weathering process — this can inhibit patination and cause uneven results.
- Manage run-off staining on surrounding surfaces during the first few months (see tips above).
- Use your fire feature regularly — the heat cycles help the patina develop evenly and quickly.
- Avoid prolonged standing water inside the bowl or base, which can cause uneven or accelerated corrosion in localised spots.
- Once the patina is established: do nothing. No painting, no treating, no maintenance required.
Black-Painted Steel Fire Features: What to Expect
Many of our fire features — including the popular Bonfeu range of outdoor stoves and fireplaces — have the finish option in a high-temperature black paint. These products have a slightly different set of expectations and care requirements.
The Steel Underneath
It's worth noting that the underlying steel in quality black-painted fire features is typically the same robust corten used across the range — the black finish is an aesthetic choice rather than a structural necessity. The paint provides a clean, contemporary look and adds a layer of surface protection, but the steel itself is inherently durable.
What's Normal: Surface Rust on Screws, Grills & Joints
Occasionally, some customers have expressed concern over the appearance of rust on:
- Chimney screws and fixings
- Door grills and hinges
- Weld seams and joints
Rusting in these areas can appear within just a couple of months of use. The reason is straightforward: small fixings and grill components are exposed to intense, repeated heat cycles and movement. The high-temperature coating on these components can burn or chip off more quickly than the stove's main body, and the exposed metal surface then oxidises in the presence of moisture.
This is a cosmetic issue — it does not affect the structural integrity or performance of your fire feature. But it should be understood that your black fire feature will not remain completely black forever without occasional maintenance.
What You Can Do to Retain a Black Look Longer
- Apply a high-temperature black paint (such as Rust-Oleum or Hammerite Heat Resistant) to affected areas once they begin to show rust. This is a simple, inexpensive fix that takes minutes.
- Store your stove or fireplace under cover when not in use, particularly during wet weather, to slow the process.
- Apply a thin coat of high-temp stove paint to the body of the stove annually to keep the finish looking its best.
When to Be Concerned
Surface rust on fixings and grills is cosmetic and manageable. However, if you notice:
- Rust blistering or flaking on the main body panels
- Structural weakening or perforation of the steel
- Rust appearing inside the firebox in a way that affects function
...please contact us. These issues may indicate a product fault and we'll work with you to find a resolution.
Summary: Corten vs. Black-Painted Steel at a Glance
| Feature | Corten Steel (Pre-Rusted) | Corten Steel (Raw) | Black-Painted Steel |
|---|---|---|---|
| Arrives looking | Orange-brown patina | Grey / dark mill finish | Matte or satin black |
| Colour change | Amber — deepens gradually | Grey steel → amber → deepens gradually | Black stable; fixings may discolour |
| Run-off staining | Minimal | Yes — temporary during weathering | None |
| Rust on fixings | Expected | Expected | Expected within weeks of use |
| Long-term maintenance | None required | None required | Occasional touch-up paint recommended |
| Long-term durability | Excellent — self-protecting | Excellent — self-protecting | Excellent with basic maintenance |
| Examples | Barbecook Jack, HEAT, MOODZ Classic, Bonfeu stoves | BonFeu BonBowl, BonBiza fire bowls | Bonfeu black stoves & fireplaces |
Still Have Questions?
Every fire feature is different, and weathering can look quite dramatic in the early stages — especially if you weren't expecting it. If you're ever unsure whether what you're seeing is normal, please don't hesitate to get in touch with the GardenHearth team. We're here to help you get the most from your outdoor fire feature for years to come.
Frequently Asked Questions
Is corten steel supposed to rust?
Yes — intentionally. Corten steel is engineered to form a stable, protective rust-like patina when exposed to the weather. This patina bonds tightly to the surface and prevents deeper corrosion, unlike ordinary mild steel which rusts through. The orange colour you see is the steel working exactly as designed.
How long does it take for corten steel to turn brown?
The initial orange bloom typically appears within 2–6 weeks of outdoor exposure. The patina deepens to rich amber and copper tones over 2–6 months, and reaches its mature chocolate-brown finish after 6–18 months. Regular use of your fire feature accelerates the process, as heat cycles speed up oxidation.
Will my corten fire pit stain my patio?
During the first 6–8 weeks of weathering, orange-tinted run-off can stain surrounding paving or decking. This is temporary and manageable: rinse surfaces promptly after rain, use a mild oxalic acid cleaner for stubborn marks, and consider positioning the fire feature on gravel during the initial weathering period. Staining stops once the patina is established.
Do I need to seal or treat my corten steel fire pit?
No — and you shouldn't. Applying oil, wax, or sealant during the weathering process can inhibit patination and cause uneven results. Once the patina is fully established, corten steel requires no maintenance whatsoever. No painting, no sealing, no annual treatment.
My corten fire bowl arrived grey, not orange. Is it faulty?
Not at all. Some corten products — particularly fire bowls like the BonFeu BonBowl — are supplied in their raw, unweathered mill-scale finish. They will begin to oxidise and turn orange within a few weeks of outdoor exposure. Pre-rusted products (most fireplaces and stoves) arrive already patinated; raw products go through the full weathering journey.
Why does my black Bonfeu stove have rust on the screws and door grill?
This is not unexpected. The small fixings, screws, and grill components on black-painted fire features are exposed to intense heat cycles that can cause the high-temperature coating to burn or chip off. The exposed metal then oxidises in the presence of moisture. This is a cosmetic issue only — it does not affect the structural integrity or performance of your stove. A quick application of high-temperature black paint (such as Rust-Oleum or Hammerite Heat Resistant) will restore the appearance.
Is the steel under a black-painted fire feature the same as corten?
Yes — in quality fire features like the Bonfeu range, the underlying steel is the same robust corten material used across the product line. The black finish is an aesthetic choice, not a structural requirement. The steel itself is inherently durable regardless of the surface coating. Should you never wish to repaint your black stove, you will eventually have a more typical corten look over the years.
How do I stop my corten fire pit from rusting through?
Genuine corten steel will not rust through — that's the key difference between corten and ordinary mild steel. The patina is self-limiting and chemically bonded to the surface. As long as your product is genuine corten (check the product specification if unsure), the rust you see is protective, not destructive.
Can I speed up the corten weathering process?
Yes. Using your fire feature regularly is the most effective way — heat cycles significantly accelerate patination. Positioning the fire feature where it receives natural wet/dry cycles (rain followed by drying sun) also helps. Some people use a commercially available rust accelerator solution for an even faster result, particularly on raw fire bowls. Or you can try a 1:1 solution of vinegar and water sprayed daily.
What's the difference between pre-rusted and raw corten steel?
Pre-rusted corten has been factory-treated to establish the initial patina before shipping — it arrives looking orange-brown and ready to use. Raw corten arrives in its grey mill-scale state and goes through the full weathering process after installation. Both are genuine corten steel and will reach the same mature finish over time; pre-rusted products simply skip the early stages.
