Crust colour is the part of baking that home bakers judge by eye and adjust by instinct. It is also one of the few parts with published kinetic parameters, and those parameters explain why the instinct is so hard to train.
What a z-value is
Thermal process kinetics describes temperature sensitivity with a z-value: the temperature increase required to change the reaction rate tenfold. A small z-value means a steeply temperature-dependent reaction. A large one means a reaction that responds gently.
A 2023 study of baking on a wood-fired oven floor reported z-values for two products at their upper surfaces. For browning, it gives 19 plus or minus 3 degrees Celsius for a white pizza and 8 plus or minus 3 degrees for a tomato-topped one. For charring, 16 plus or minus 5 and 9 plus or minus 4 respectively. Reference temperatures were 100 and 50 degrees.
Read the tomato figure carefully. A z-value of 8 degrees means that raising the surface temperature by eight degrees multiplies the browning rate by ten. Sixteen degrees multiplies it by a hundred.
That is the arithmetic behind the experience of a top going from pale to acceptable to ruined in under a minute. It is not carelessness. It is a reaction with a very small z-value operating on a surface whose temperature is climbing.
The two products are not sensitive in the same way
The contrast between 19 and 8 degrees is as interesting as either number alone.
The white product's browning is comparatively forgiving: nearly twenty degrees for a tenfold change gives a workable margin. The tomato-topped product's browning is roughly twice as temperature sensitive, meaning half the temperature error produces the same rate change.
Charring follows the same ordering, at 16 and 9 degrees. So for the more sensitive product, browning and charring have similar sensitivities, which means the window between well browned and burnt is narrow in temperature terms as well as in time.
A caution: these are values from one study on two products, with uncertainties of 3 to 5 degrees. They should be read as establishing an order of magnitude for the sensitivity of surface browning, not as constants to design a process around.
Surfaces at very different temperatures in the same oven
The same study measured upper surface temperatures for three products baked simultaneously and found 182 degrees Celsius for a white pizza, 84 for a tomato one, and 67 for a margherita. It attributes those differences to differing water content and emissivity.
More than a hundred degrees of spread, in one oven, at one time. Two mechanisms are at work. A wet surface cannot exceed the evaporation plateau while free water remains, so a moist topping is temperature-clamped near 100 degrees while a drier surface climbs freely. And emissivity differences change how much radiant energy each surface absorbs and how much it re-emits.
Combine that with the z-values and the whole picture assembles. A dry surface reaches 182 degrees, where the browning rate at a z-value of 19 is enormous relative to its rate at 100. A wet surface sits near 84, where even a z-value of 8 cannot produce much colour because the temperature never rises into the sensitive region. Wet surfaces do not brown, and then, once the water is gone and the temperature is released, they brown very fast.
What has to happen underneath first
The interior sets on its own schedule, and the 2025 study of temperature evolution gives it. Dough viscosity turns from fluid towards solid at around 50 degrees. Starch gelatinisation peaks at 76.0 degrees by calorimetry. Pores open and connect between 71 and 81 degrees. Full setting occurs around 110 degrees, while the interior itself holds at a 100 degree plateau.
So the crumb is decided in the 70s and the crust is decided in the high 100s. Those are different regions of the loaf passing through different temperature ranges at different times, which is why crust and crumb faults have separate causes and separate remedies.
Where the heat for browning comes from
The surface temperatures above are the outcome of a heat delivery budget. A 2020 review gives oven walls at 200 to 400 degrees Celsius with emissivity of 0.2 to 0.3, meaning they radiate weakly for their temperature, and reports condensation carrying about 15 per cent of transfer where steam is present.
Both matter for colour. Weak wall radiation means top browning in a domestic oven is often limited by how little radiant energy reaches the surface. And condensation actively suppresses browning while it lasts, because a condensing surface is a wet surface, clamped near the plateau.
The 2023 study's base measurements complete the account: a floor at 439 plus or minus 3 degrees drove the dough base upward at 2.7 plus or minus 0.2 degrees per second to reach 100 plus or minus 9 degrees in 80 seconds. Base colour and top colour are therefore governed by entirely separate delivery paths and separate temperature histories.
What to do with this
Four practical implications follow from the numbers rather than from custom.
Manage surface moisture as the primary colour control. Steam early suppresses browning by holding the surface at the plateau; venting releases it. That is a larger lever than oven temperature, because it decides whether the surface can enter the temperature range where the reaction is fast at all.
Expect the last minutes to be non-linear. With a z-value in the range of 8 to 19 degrees, colour development accelerates sharply as the surface dries and heats. Checking at fixed intervals late in a bake is more useful than trusting a total time.
Treat toppings and enrichments as separate colour problems. The measured spread of 67 to 182 degrees across three products in one oven is a warning that a bake time calibrated on one surface does not transfer.
And do not infer surface temperature from the oven setting. The gap between cavity temperature and surface temperature is large, variable, and dependent on moisture and emissivity, which is exactly what those three measurements demonstrate.
