Most people would expect a freshly cleaned oven floor to bake better than a blackened one. Modernist Pizza (2021), by Nathan Myhrvold and Francisco Migoya, argues the opposite for wood- and gas-fired pizza ovens, and calls the result “counterintuitive.” This page explains their reasoning, the limits of it, and the parts that do carry over to a kitchen oven.
What emissivity is
Hot surfaces give off infrared light, and how much they give off at a given temperature depends on the surface. Modernist Pizza defines emissivity as the ratio of the thermal radiation from a surface to the radiation from an ideal “blackbody” at the same temperature. Dark, dull surfaces have high emissivity; shiny ones have low emissivity.
The book stresses that absorbing and emitting are “two manifestations of the same fundamental phenomenon”: a surface that absorbs radiant heat well also gives it off well. Its everyday examples are a black T-shirt in the sun, a cast-iron pan that radiates more heat than a shiny one heated to the same temperature, and Japanese teppanyaki grills, which customers can sit around because their chrome surfaces radiate less heat than a darker grill would.
The numbers Modernist Pizza gives
| Surface | Approximate emissivity |
|---|---|
| Black soot or carbon | 0.95 |
| Firebrick and refractory stone, new and clean | 0.68 |
| Raw, white pizza dough | about 0.1, rising toward 0.9 as it darkens |
| Unbaked tomato sauce | about 0.5, rising as it darkens |
| White sauce | about 0.1 |
All values are from Modernist Pizza, volume 1, pages 352, 353 and 372. The dough figures explain why browning speeds up once it starts: a dark spot absorbs far more of the oven’s radiant heat than the pale dough around it, so it darkens faster still. That’s part of how a Neapolitan crust leopards.
Clean floor against black floor
Modernist Pizza defines a “dirty” oven as a sooty one, usually because it wasn’t swept. It works through two stages.
Right after a pizza comes out, the floor has been cooled by the bake and starts absorbing radiant heat from the ceiling. A black floor absorbs it faster: by the ratio of the emissivities, 0.95 to 0.68, about 1.4 times as fast at first, so it recovers more quickly.
Left to settle, the two floors reach different temperatures, and the clean one ends up hotter. The book’s reasoning is that the black surface absorbs more heat but, as it warms, also radiates more away. In a simplified example with the ceiling at a uniform 700°C (1,300°F), its figures are:
| Floor | Ceiling | Floor settles at about |
|---|---|---|
| Clean (0.68) | Clean | 460°C (860°F) |
| Clean (0.68) | Black | 520°C (970°F) |
| Black (0.95) | Clean | 150°C (300°F) |
| Black (0.95) | Black | 190°C (375°F) |
The authors note that a real ceiling isn’t uniform, and the example is there to simplify the math. Their conclusion is that a clean floor gives the dough a higher chance of burning.
A pizzaiolo’s view. They put the theory to a prominent pizzaiolo in Italy, who agreed: on a black floor, a pizza bakes slowly and uniformly, underneath as well as in the middle; launched straight onto a freshly cleaned stone, the bottom cooks unevenly and the middle can stay raw. The book adds that the pattern of scorch marks under a pizza follows the steam escaping from it: where steam kept the floor near 212°F (100°C), the base stays white, and where it didn’t, the floor reached about 320°F (160°C) and browned it.
Where the finding stops
It’s about masonry floors under a very hot dome. The example assumes a 700°C ceiling radiating onto a firebrick floor, the situation in a wood- or gas-fired pizza oven. A kitchen oven’s walls are far cooler, and a steel or stone in it is heated by the oven’s air and by radiation from its elements and walls.
Soot isn’t a goal in its own right. Modernist Pizza reports that pizzaioli told them a wood-fired oven is ready when the soot from lighting it has burned off and the blackness of the walls is gone. The book’s point is that a darkened floor recovers and bakes differently from a scrubbed one, not that you should let carbon build up.
It isn’t a cleaning schedule. Nothing in the book’s analysis tells a home baker to stop cleaning a pizza stone or steel. Follow the maker’s care and seasoning instructions for yours; for steel, the baking steel thickness guide covers Baking Steel’s.
What carries over to a kitchen oven
Dark steel. Modernist Pizza’s conclusion on baking steels is that a dark, not shiny, plate gives the best crust. In one experiment the team hung a black steel above Neapolitan pizzas in an electric deck oven, choosing steel over stone because steel radiates heat better than masonry. With the door closed, the pizzas baked more evenly without turning, in a shorter time, and the last one gained 29% in volume. That was a commercial deck oven, but it shows a dark metal surface acting as a strong radiator.
Infrared readings. An infrared thermometer works out a temperature from the radiation a surface gives off, so emissivity changes what it reads. Bare or shiny metal reads low on most handheld thermometers, and a dark, well-seasoned surface reads more reliably. Check your thermometer’s instructions for its emissivity setting, use readings to compare one session with the next, and never turn up the oven to make a reading match the dial. The home pizza oven temperature guide explains the difference between an oven setting and a surface reading.
Heat from above. The same physics explains why a home oven needs its broiler for top heat: hotter surfaces radiate much more strongly, as the pizza oven thermodynamics page explains, with the fourth-power law worked in kelvins.
Frequently Asked Questions
- What is emissivity?
- It's how well a surface gives off radiant heat compared with an ideal blackbody at the same temperature, on a scale from 0 to 1. A surface that emits well also absorbs well. Modernist Pizza gives about 0.95 for black soot and 0.68 for clean firebrick.
- Why would a clean pizza-oven floor burn pizzas more than a sooty one?
- In Modernist Pizza's worked example, a black floor absorbs radiant heat from the dome faster but also radiates more away, so it settles at a lower temperature than a clean floor under the same ceiling. A pizzaiolo they consulted agreed: pizzas bake slowly and evenly on a black floor and can scorch unevenly on a freshly cleaned one.
- Should I stop cleaning my pizza stone?
- No. The finding is about firebrick floors under a very hot dome in a wood- or gas-fired oven. Modernist Pizza even reports that pizzaioli consider an oven ready once the soot from lighting it has burned off. Follow your stone or steel maker's care instructions.
- Is a dark baking steel better than a shiny one?
- Modernist Pizza concludes that a dark, not shiny, steel plate gives the best crust, and it chose a black steel as a radiator in its deck-oven experiments. A dark surface also reads more reliably on an infrared thermometer.
- Why does my infrared thermometer read my steel lower than the oven setting?
- Partly emissivity: bare or shiny metal gives off less radiation than a dark surface at the same temperature, so most handheld thermometers read it low. Check the thermometer's emissivity setting, and use readings to compare sessions rather than to match the dial.
Sources
- Nathan Myhrvold and Francisco Migoya, Modernist Pizza (2021), volume 1, printed pages 351–354 (radiation, emissivity, clean and dirty wood-fired oven floors, the equilibrium example and the pizzaiolo’s account), 355 (the black baking steel experiment), 366 (leoparding), 372 (emissivity of dough and sauces) and 393 (dark steel plate).