Kiln and damper

A cutaway brick kiln with a fuel lever, an air damper and a draw rod: how hot you fire and how much air you allow decide what the clay becomes.

Trial tile and rod handle: draw the tile to judge itTile rack: each drawn tile, red or grey, porous or glossyBand on the quadrant: beyond the updraft's 1100 °CDrag the firing lever (Up and Down): its rock shaft, crank and link push the fuel carriage in, so more billets lie in the fire. Slide the damper over the air port (Left and Right) to choke the air; click the chimney lid or the flue slide (M) to convert to downdraft; click the rod (D) to draw a test tile onto the rack. The guide's tiers: pit firing at 600–900 °C gives porous earthenware; an updraft kiln reaches 900–1100 °C, so here it stops at 1100 °C; downdraft kilns reach 1200–1300 °C, where stoneware vitrifies. Plenty of air fires the iron red, a starved, smoky fire black or grey. Shell or limestone temper turns to quicklime above about 800 °C and blows the pot apart later: watch the flecks in the bowl by the cut turn chalk-white. The temperature is the model's value, not a reading: there is no pyrometer, and the lever, quadrant and damper scale are bench-model controls. The 600–1300 °C span, the chamber's 1.4 s lag, the damper's 35% threshold and the carriage itself are illustrative. Ware must be bone dry and heated gradually.

What you're looking at

This is a bench model of an updraft kiln, the brick firing chamber of the Roman and medieval potter: a firebox below, a perforated clay floor, and a chamber of pots under a barrel vault with a short chimney on top. The near side is cut away, so you see the glowing firebox and the ware; behind on the left a flue duct with a sliding plate runs to a tall stack, and on the right wall sit the stoke arch with a sliding iron damper over the air port beside it, and a spy-hole higher up with a draw rod and its T-handle. In front, a firing lever on a marked quadrant turns a shaft, crank and link that push a carriage of billets into the fire; to the right a rack of six pegs holds drawn test tiles. You drag the lever and the damper knob and click the rod, the chimney lid or the flue slide; the carriage, damper, lid, flue plate, rod, tiles, glow and smoke all move in response.

What it means

The guide's tiers are set by the kiln: pit firing reaches 600 to 900 °C and gives porous earthenware, an updraft kiln reaches 900 to 1100 °C, and only a downdraft kiln, which pulls the heat down through the ware to a chimney fed from the floor, reaches 1200 to 1300 °C, where stoneware vitrifies and holds water with no glaze. Air is the second lever: plenty of air fires the iron in clay red, a starved, smoky fire turns it black or grey. Shell or limestone temper turns to quicklime above about 800 °C and blows the pot apart later, and test tiles drawn and kept on a rack are how you learn what your own kiln does. The lever's 600 to 1300 °C span, the chamber's lag, the damper's switch point and the fuel carriage are illustrative, and the temperature is the model's value, not a reading.

Try this

Drag the firing lever's grip up and to the left, toward the kiln, or press Up (Down to go back) in 50 °C steps; End jumps to 1300 °C, and Home returns to 750 °C with the air port open and the kiln back in updraft. The crank and link push the carriage of billets further into the firebox, the glow brightens from dull red toward yellow, and the readout climbs with an arrow to the temperature it is heading for; in updraft it stops at 1100 °C and says updraft cap, and past 800 °C the flecks in the shell-tempered bowl turn chalk-white. More fuel means more heat, but an updraft kiln cannot pass its 1100 °C ceiling however hard you stoke it, and shell temper is ruined well before that.
Click the chimney lid on top of the vault or the flue slide beside the stack, or press M. The lid slides over the chimney, the flue plate lifts, the draught wisps move from the chimney to the tall stack, and with the lever high the temperature climbs on past 1100 °C to stoneware. Converting to downdraft is what lets the same kiln reach the 1200 to 1300 °C stoneware range.
Drag the damper knob on the right wall to close the air port, or press Left (Right opens it again). The iron plate slides across the air port, and once it is mostly shut the readout changes from oxidising to reducing and the smoke thickens and darkens. Choking the air intake is how a potter makes a reducing fire, which turns the clay's iron grey or black instead of red.
Click the rod's T-handle at the spy-hole, or press D. The rod pulls the test tile out through the spy-hole, it glows and cools, then hangs on the next peg of the rack, and the readout's last words name it: red or grey, porous or vitrified. A drawn tile records the atmosphere at the moment it came out and the hottest point it reached, the guide's habit of firing test tiles and keeping records.

Reading the readout

The line starts with the chamber's model temperature in degrees Celsius, rounded to 10, followed by an arrow and the temperature it is heading for while it is still changing. Next comes the tier: earthenware below 900 °C, updraft from 900 to 1100 °C, updraft cap when the lever asks for more than an updraft kiln can give, past updraft between 1100 and 1200 °C in downdraft, and stoneware from 1200 °C. Then the atmosphere, oxidising (plenty of air) or reducing (air choked by the damper), and last the most recently drawn tile, red or grey for its iron and porous or vitrified for its body.

What the model uses

Pit or bonfire firing reaches 600–900 °C; the result is porous earthenware, usually mottled black and orange, and a 20% breakage rate is normal even for skilled potters (guide text, uncited).
An updraft kiln, a chamber with a firebox below and a chimney above separated by a perforated clay floor, is the standard Roman/medieval design and reaches 900–1100 °C; it lets you hold a temperature and control the atmosphere (guide text, uncited).
Downdraft and dragon kilns reach 1200–1300 °C (stoneware): heat is drawn up, then pulled down through the ware by a chimney taking its draft from the floor (guide text, uncited).
Oxidising (plenty of air): iron in the clay becomes red/orange ferric oxide. Reducing (starved of air, smoky): iron becomes black/grey ferrous oxide. Control it by choking the air intake (guide text, uncited).
Avoid crushed shell or limestone temper if firing above ~800 °C: it converts to quicklime and blows the pot apart later; a pot that crumbles after weeks had limestone temper that turned to quicklime and expanded (guide text, uncited).
Stoneware (1200 °C+) vitrifies and is waterproof with no glaze at all; it requires the right clay and a good kiln (guide text, uncited).
Always fire test tiles alongside the real work and keep records; the rack of drawn tiles is that record (guide text, uncited).
A pot explodes in the fire if it is not bone dry or is heated too fast: dry thoroughly, preheat, ramp slowly (guide text, uncited).

The kiln's temperature follows the lever with an illustrative first-order lag, the damper does not change the temperature, the atmosphere switches at an illustrative damper opening, and a drawn tile's iron is read from the atmosphere when it is drawn and its body from its hottest point while it hung in the kiln; real firings depend on the clay, the fuel and the whole firing history. This is a code-drawn explanatory model for orientation, not an equipment drawing or a safety procedure; follow the guide's warnings.