The number you were looking for
You typed a temperature into a search box and something handed back 350°F. That number is not wrong. It is just not a recipe, because grain does not have one roast. It has at least three, they sit in different parts of the dial, and each one is doing a chemically different thing to the kernel.
What follows is the honest version of the ranges, what each is for, and why every figure on this page is a place to start rather than a set of instructions to obey.
Grain roasts by product temperature, not air temperature
This is the part worth being stubborn about, because it is where most grain roasting advice quietly falls apart.
An oven dial reads the air in the oven. A grill thermometer reads the air under the lid. The grain is a separate object sitting in that air, with its own mass and its own water, absorbing heat at a rate that depends on how much of it there is, how wet it was going in, how long the metal around it has been heating, and how cold the day is. Early in a roast the grain lags a long way behind the air, because nearly every calorie going in is boiling water out of the kernel rather than raising its temperature. Later, once that water is mostly gone, the grain climbs fast on the same applied heat.
The gap between air and grain is not a constant you can subtract. It moves inside a single roast. There is no arithmetic that converts a dial setting into a kernel temperature, which is why two batches that both ran thirty minutes at the same dial setting can come out two shades apart.
A thermocouple riding in a turning drum reads the grain. A dial on the front of an oven reads the room the grain is in. Everything below assumes you can see the first number. If you cannot, the ranges still help, but you are aiming rather than measuring, which is most of what makes roasting grain in an oven a fair method with a hard ceiling.
The three neighbourhoods
Stewing: 210 to 230°F
Crystal and caramel malts are made in this window, and it barely looks like roasting at all. The grain is held wet, a little above the boiling point of water, long enough that the inside of each kernel converts — starch into sugar — while the husk keeps the moisture from leaving. Only after that does the heat come up to set colour.
Two things follow. Time here is long by the standards of the rest of this page, and it is not optional: shorten the stew and you get a toasted malt with none of the sweetness you were after. And the grain has to stay damp. If it dries out at 220°F you have not made crystal malt, you have made a pale toasted malt that took an hour. The mechanics of that hold are covered in more depth on the crystal and caramel malt page.
Amber and biscuit: around 350°F for roughly 30 minutes
This is the range most people mean when they ask what temperature to roast grain at. Take base malt to about 350°F and hold it there for something like half an hour and you land in amber territory: bread crust, biscuit, a light nutty edge, colour somewhere between straw and copper.
It is the friendliest part of the dial because it is slow enough to be steerable. Colour develops over minutes rather than seconds, so you can look, smell, pull a few kernels and cut them open, and still have time to act on what you found. A minute of inattention here costs you a shade. It does not cost you the batch. Most of what people call roasting malt at home happens in this neighbourhood.
The dark end: hotter and faster
Roasted barley, chocolate malt, black malt and the rest of the dark family run above the amber range and they run quicker. That combination is what catches people out. The reactions that make colour accelerate as temperature climbs, so the last stretch of a dark roast moves several times faster than the first, and the gap between good and burnt narrows to under a minute.
The other thing that changes at the dark end is smoke. There is real volume of it, it smells like a roastery rather than like baking, and it is one of the main reasons dark grain roasting belongs outdoors. The full account of that end of the dial is on the roasted barley page.