TL;DR
The brown black layer on an old sheet pan is polymerized fat, not dirt -- oil that has cross linked under heat into a hard varnish. It comes off with a long hot soak plus a baking soda paste and patience, in two or three rounds. Abrasives are the wrong answer on nonstick and on bare aluminum, where they cause permanent damage.
What carbon buildup actually is
Every time oil is heated on a pan, a fraction of it does not burn off. It reacts, forming long molecules that bond to the metal.
Repeat that a few hundred times and you get a layer that is chemically closer to a varnish than to grease. Dish soap does almost nothing to it because it is no longer a fat that soap can emulsify.
This is the same process that creates seasoning on cast iron, which is why the layer is stubborn. The difference is that on a cast iron pan the layer is smooth and useful, while on a sheet pan it is uneven, sticky in patches, and holds new residue.
Understanding the mechanism explains why every effective method involves either alkalinity, heat, time, or all three, which is a theme in the guide to cleaning kitchen tools without damaging them.
How do you remove carbon buildup from a baking sheet?
Soak the pan in very hot water with a generous amount of degreasing dish soap for an hour or more, then spread a thick paste of baking soda and water over the residue and leave it thirty minutes to a few hours. Scrub with a non scratch pad or a nylon brush, rinse hot, and repeat rather than escalating to harsher tools.
The method, in order
1. Identify the pan first. Nonstick, bare aluminum, anodized aluminum, and stainless steel all tolerate different treatment. Check the underside for markings.
2. Long hot soak. Fill the pan with water as hot as the tap runs and a generous squirt of degreasing dish soap. Leave it an hour, or overnight for heavy buildup. Refresh with hot water if it cools and nothing has shifted.
3. Baking soda paste. Mix baking soda with a little water to a spreadable paste. Spread it thickly over the residue and leave it thirty minutes to several hours. On stainless it can sit overnight.
4. Scrub with the softest tool that works. A non scratch nylon pad or a stiff nylon brush. Work in the direction of any grain on stainless.
5. Rinse with hot water and assess. Look at what came off rather than what remains. Progress is normal, completion in one pass is not.
6. Repeat. Two or three cycles is typical for a pan that has years of buildup on it. Each round removes a layer.
7. Dry completely. Especially at the rolled rim, where water sits.
What is safe on which pan
Nonstick pans. No abrasives at all -- no scouring powders, no metal, no steel wool, no melamine foam. A scratched coating is ruined permanently and food will stick in the scratch forever. Use hot soaks and a soft cloth only, and accept that a nonstick pan with heavy carbon is usually near the end of its service life.
Bare aluminum. Aluminum is soft and reactive. Baking soda and other alkaline products can darken and dull it, which is cosmetic rather than harmful. Abrasive pads leave visible scratches. Short baking soda contact and a hot soak are the safe approach.
Anodized aluminum. The anodized layer is a hard coating. Abrasives and strongly alkaline cleaners degrade it, so treat it like nonstick and stick to soaks.
Stainless steel. The most tolerant. It takes long baking soda pastes, firm nylon scrubbing, and a non scratch pad. Avoid chlorine bleach, because chloride bleach pits stainless. Avoid steel wool, which leaves particles that rust.
What not to use
Oven cleaner. Aggressive caustic products damage aluminum badly and give off fumes that need serious ventilation. They are formulated for the inside of an oven cavity, not for cookware.
Bleach. It does nothing to polymerized fat, it pits stainless, and it must never be combined with ammonia, with vinegar or any other acid, or with rubbing alcohol, because each combination releases a toxic gas. If you have used one product on a pan, rinse thoroughly with plain water before using another.
Steel wool on anything coated. It removes coating, not carbon.
A self cleaning oven cycle with the pan inside. The temperature warps sheet pans and destroys nonstick coatings.
Boiling water on a hot glass or ceramic dish. Thermal shock cracks it.
When to stop and let it be
A dark, smooth, evenly discolored sheet pan is not a problem. Discoloration on bakeware is cosmetic, and many cooks prefer a darkened pan because it browns more evenly.
The reasons to actually intervene are a sticky or tacky surface, flaking residue, an off smell when the pan heats up, or food sticking where it did not before.
The reasons to replace a pan are a warped base that rocks on the stovetop, a nonstick coating that is peeling or scratched through, and rust that has pitted the metal.
Muffin tins have all the same problems in a harder to reach shape, covered in how to clean a muffin tin, and racks that sit in the same oven are in how to clean a wire cooling rack.
Keeping buildup from coming back
Line the pan. Parchment, foil, or a silicone mat means the pan barely gets dirty. Mat care is in how to clean silicone baking mats.
Skip aerosol cooking spray. The emulsifiers and propellants in spray leave a residue that polymerizes into an especially stubborn film. Brush on oil instead.
Wash while warm. A pan put into hot soapy water before it cools sheds most of what would otherwise bake on next time.
Do not stack pans wet. Trapped moisture between pans marks them and, on any bare steel component, starts rust.
Match the pan to the job. A dark pan browns faster, so recipes may need a lower temperature or a shorter time.
Cleaning bakeware reduces residue and rancid flavor transfer. It is not a food safety program on its own.
Source: USDA Food Safety and Inspection Service guidance on cleaning food contact surfaces
FAQ
What is the brown buildup on my baking sheet?
Polymerized cooking oil. Heat causes a fraction of every oil application to cross link into a hard varnish bonded to the metal, rather than burning off. It is not dirt, and ordinary dish soap has almost no effect on it, which is why soaking and alkaline pastes are the standard approach.
How do you clean a badly burnt sheet pan?
Soak it in very hot water with degreasing dish soap for an hour or overnight, then spread a thick baking soda and water paste over the residue for thirty minutes to a few hours. Scrub with a non scratch nylon pad, rinse hot, and repeat two or three times.
Can you use abrasives on a baking sheet?
Only on stainless steel, and even there a nylon pad is preferable to steel wool. Nonstick coatings and anodized aluminum are permanently damaged by abrasives, and bare aluminum scratches easily. On those pans, soaking and time replace scrubbing entirely.
Is a discolored baking sheet still safe to use?
A dark, smooth, evenly discolored pan is fine and often browns food more evenly. Replace a pan when it is warped, when a nonstick coating is peeling or scratched through, when residue flakes off, or when it smells off as it heats.
Can you use oven cleaner on baking sheets?
It is not recommended. Caustic oven cleaners damage aluminum badly and require serious ventilation, and they are formulated for an oven cavity rather than for cookware. Hot soaks and baking soda pastes achieve the same result without the risk.
How do you stop carbon from building up?
Line pans with parchment, foil, or a silicone mat, avoid aerosol cooking spray in favor of brushed on oil, and wash pans while they are still warm. Most buildup comes from thin oil films left to bake repeatedly, so preventing the film prevents the layer.
Pans respond to soaking and patience. The oven and kitchen around them take real time. If you would rather hand that off, get a quote or read more about house cleaning services.