Firing a basic kiln

Cones: what they measure

Follow your kiln's own manual first. This guide gathers what cone and kiln makers publish about basic electric kilns; it does not replace the instructions for your model, and where the two differ, the manual wins.

A pyrometric cone is a small, slender pyramid made from a standardized ceramic powder. When it has had enough heat, it softens and bends over in a repeatable way. You set cones in the kiln beside the ware and read how far they bent.

Cones measure heatwork, the combined effect of temperature and time. In the words of Orton, the main US cone maker, "Cones do not measure temperature alone." A cone heated slowly bends at a lower temperature than one heated fast, and a glaze behaves the same way. So Orton gives each cone's temperature for a stated heating rate over the last 100 °C (180 °F) of the firing. Once a cone starts to move, it bends over a narrow range, "usually less than 40° F" (22 °C).

Cone numbers

Cone numbers run from 022, the coolest, up to 42. A leading zero marks the cooler series and is read as "oh": cone 06 ("oh-six") is cooler than cone 04, and cone 01 needs less heat than cone 1. Orton warns that it is important not to mix the two series up. The gap is large: cone 06 bends more than 220 °C (400 °F) cooler than cone 6.

Suppliers in the UK, Europe, Australia and New Zealand often give a temperature in °C rather than a cone: find the nearest cone in the chart below and confirm it with a witness cone. Seger and Staffordshire cones, the older European series, should not be assumed to share Orton's numbers, so this guide uses Orton cones.

Holding adds heatwork

A hold (also called a soak) keeps the kiln at one temperature for a while. Because cones measure heatwork, a hold at the top can stand in for a hotter peak. Orton gives these rough equivalents:

Orton adds that a hold of 1–2 hours "may be sufficient to deform the next higher cone number".

So a hold is never free: if you add one, expect the cones to bend further, and lower the peak to match.

Kinds of cone

Orton makes four kinds. Two stand on the shelf to show what the ware received; two are for the kiln sitter.

Height matters: a ¼ in (6 mm) difference in the height of a large cone "can cause more than a half cone difference in bending". Cones do not go bad or age, and damp storage does not affect them.

What "cone 6" means in practice

The chart gives a self-supporting cone 6 as 1222 °C (2232 °F), but only when the last 100 °C (180 °F) climbs at 60 °C/h (108 °F/h). Climbing at 150 °C/h (270 °F/h), the same cone bends at 1243 °C (2269 °F). Real kilns differ again: Digitalfire reports that "Most kilns reach cone 6 at 2200°F, not 2232°F". So "cone 6" means the ware received the heatwork that bends a cone 6, whatever a thermometer read at the time.

Fire by the cone, not the clock or the dial. Set a controller by cone number, and let a witness cone on the shelf confirm every firing.

The cone chart

These figures are from Orton's cone chart (©2001) for self-supporting cones. Choose the column that matches how fast your kiln climbed over the last 100 °C (180 °F). Orton's rule of thumb is the 150 °C/h (270 °F/h) column for a fast firing, the 60 °C/h (108 °F/h) column for a medium one and the 15 °C/h (27 °F/h) column for a slow one. To work out a rate, divide the temperature climbed by the hours it took: a kiln that climbs 500 °C (900 °F) in 3 hours is heating at about 167 °C/h (300 °F/h).

A dash marks a figure not copied here; see Orton's full chart for it.

Large cones read within a few degrees of these figures: a large cone 6 at 60 °C/h (108 °F/h) is 1220 °C (2228 °F). Small cones, the kind used in a kiln sitter, are listed only at 300 °C/h (540 °F/h), so the chart gives a small cone 6 as 1255 °C (2291 °F). That is not the temperature at which a sitter turns the kiln off. The sitter cone's job is to match the shelf cone, and the shelf cone has the final say.

Two cautions. Orton's 2019 booklet gives cone 06 at the fast rate as 1024 °C (1875 °F), but the chart and Orton's FAQ both give 1013 °C (1855 °F), used here. And the large-cone table at the back of the Dawson kiln sitter manual has copying errors (cone 5 hotter than cone 6), so do not use it.

Setting up a cone pack

A witness cone is a cone set on a shelf beside the ware, where it gets the same heat. Orton is plain about its role: "The only true measurement of heatwork is from a Large or Self-supporting cone placed on the shelf next to the ware." A kiln sitter sits near the wall, and a controller's or pyrometer's probe reads the temperature near the wall; a witness cone reads where the pots are.

A cone pack is three witness cones set together, one number apart:

Self-supporting cones or a plaque

L&L, a US kiln maker, recommends self-supporting cones for the pack "because they are the easiest to use and provide the most consistent results": their height and lean are built in.

Large cones need a plaque (a ready-made holder) or a pat of soft, porous clay pricked with pencil holes. Set each with exactly 2 in (51 mm) showing, leaning 8° from upright toward its bending face. To find that face, stand the cone on its base on a flat surface, hold it by the tip and let go: it falls toward the bending face.

Where to put the packs

1. Put the pack "deep in the kiln, yet... visible through a peephole" (Orton), near the center of the load and away from drafts (L&L). Not right at the peephole: the cones should feel the heat the ware feels.
2. Before you close the lid, look through the peephole and check that you can see all three cones.
3. In a kiln with a sitter, keep the pack's shelf at least 1 in (25 mm) above or below the sitter tube, so a tilting shelf cannot jam the sitter.
4. For your first firings, put a pack on the top, middle and bottom shelves. L&L's calibration test goes further: two packs of cones 07, 06 and 05 on every shelf, one at the edge and one in the center.

Expect differences: Orton says "Typically, you will see at least a one cone difference from top to bottom in the kiln", and densely packed areas fire cooler. Packs you cannot see during the firing still give a record afterward.

The kiln sitter

A kiln sitter is the mechanical switch, on the outside of many basic and older kilns, that turns the kiln off when a small cone bends. The best known is the Dawson Kiln-Sitter. It makes firing easier, but it is a convenience, not a safety device.

How it works

A porcelain tube reaches into the kiln. At its end, a small cone or a bar lies across two metal supports, under a sensing rod that pivots like a seesaw. Outside the kiln, the other end of the rod holds up a weight by a small claw. When the cone softens and bends to about 90°, the rod drops far enough to release the weight, the weight falls, and the power to the elements is cut.

Small cones and bars "give equivalent heat treatment". Orton notes that many beginners choose bars, because their even shape makes it easy to place them the same way every time. A small cone tapers, so where it sits matters: with its thin part under the rod the kiln shuts off cooler, and with its thick part hotter, a shift of "as much as 1/2 cone".

Which cone goes in the sitter

Sources disagree. Dawson, who made the sitter, says to use the same number as the firing cone on the shelf, and to go one number hotter only if the shelf cone stays unbent. L&L says to use one number hotter than the shelf cones, because the sitter sits near the wall and the elements, gets more heat, and tends to shut off early. Orton settles the principle: "The shelf cone is the final judge of the firing process, not the Kiln-Sitter cone or bar."

Our starting point: put a small cone of the same number as your target in the sitter, with a cone pack on the shelf. If the shelf firing cone is under-bent when the kiln shuts off, go one number hotter in the sitter next time, or set the thicker part of the cone under the rod. If it is over-bent, do the opposite.

Setting it up, and checks before every firing

Dawson's procedure, which doubles as a check before every firing:

1. Turn all the switches off. Check that the sensing rod moves freely, sits centered in the tube and is not bent or corroded. If it is sluggish, Dawson says to replace the tube assembly.
2. Brush a thin coat of high-fire kiln wash (a coating that stops melted material sticking) onto the cone supports and the rod where it touches the cone. Never put wash on the cone or the porcelain tube.
3. Load shelves and ware at least 1 in (25 mm) above or below the tube, so nothing can touch, fall against or block it.
4. Lift the weight, press the claw down onto the trigger, and lay the cone or bar flat on the supports against the metal step, as your sitter's manual shows, the same way every time. Check it again before closing the lid: "AN IMPROPERLY PLACED CONE OR BAR COULD CAUSE AN OVERFIRING."
5. Set the limit timer and push in the plunger until it locks. Keep the kiln level and the space under the weight clear, so the weight can fall.

The limit timer

Many sitters, including Dawson's LT models, have a limit timer: a clock that turns the kiln off after a set time, as a backup "in case the KILN-SITTER fails". It should always be set longer than the firing is expected to take. Sources set the margin differently:

Our starting point: generous the first time (within the dial's 20 hours), then ½ to 1 hour longer than the time you recorded. The timer also tells you how long a firing took: set at 7 and ending at 1 means 6 hours.

When it shuts off early or late

After each firing, read the sitter cone beside the shelf cones.

To check the adjustment, Dawson uses a firing gauge that holds the rod where a fully bent cone would leave it; there should then be "1/16th-inch clearance" (about 1.6 mm) at the claw. Check it every 30 firings. Ceramics Monthly's rule: if the kiln shuts off early, move the trigger plate up slightly; if late, move it down slightly. Whatever the sitter does, "Always trust large witness cones over small sitter cones."

The sitter is built for firings "up to and including Cone 8". It cannot hold a temperature or cool slowly by itself. With practice and a log, though, you can "become the controller", as Digitalfire puts it: watch the cones and turn the switches by hand.

Reading cones

Cones are read twice: through the peephole while the kiln fires, and in your hand once it has cooled.

During the firing

A peephole is a small hole through the kiln wall, closed with a removable plug. Never look in without eye protection. Orton says "Welders glasses should be used to avoid possible eye injury. Sunglasses are not recommended." Shade 3 welding glasses, rated for infrared, are the most often recommended; go darker if you see spots afterward.

• The guide cone starting to bend means the end is near: watch closely from here.
• Once a cone starts, it takes 15 to 25 minutes to bend (Orton's newer booklet says 15 to 22), slowly at first, then quickly once it passes halfway, at 3 o'clock.
• The firing cone is done when its tip is level with its base.

On a manual kiln, the peephole is your only real-time signal, which is why the pack must sit where you can see it.

After the firing

Orton reads the bend as a clock face. An unbent cone points up, at 12 o'clock; a cone touching the shelf is at 6 o'clock. The end point is a 90° bend, the tip level with the base: the point the chart's temperatures are measured at, and Orton counts a cone anywhere from 5 to 6 o'clock as properly fired. Between 4 o'clock and touching the shelf the difference is small, "usually 1 or 2 degrees".

Adjusting next time

A cone offset is a controller setting that moves its temperature for a cone up or down to match what your witness cones show. Keep every fired pack and write down how each one bent (see After the firing, below).

Bisque firing

The bisque firing (biscuit in the UK) is the first, lower firing that turns dry clay into hard, porous ware ready to glaze. It is usually fired to cone 06–04, 998–1063 °C (1828–1945 °F) on Orton's chart; UK potters often fire to around 1000 °C (1832 °F), near cone 06. The bisque should stay porous, "generally more than 15%" (Digitalfire), so it soaks water out of the glaze. Too soft a bisque takes glaze on too thick, which then cracks and crawls (pulls back into islands, leaving bare clay). Under-fired bisque also "continues to shrink during glaze firing", which spoils the glaze fit. Loading is forgiving: pieces may touch, lids can be fired on their pots, and bowls go rim to rim rather than nested.

Why the early part goes slowly

Water expands more than a thousandfold as it turns to steam, so damp ware has to be dried slowly. L&L and Orton advise that "Special care should be taken below 1500°F (815°C) to heat the body evenly", and that carbon must be burned out before about 650 °C (1200 °F). Above 816 °C (1500 °F) the kiln can climb faster.

Quartz inversion is the sudden change in size of the silica in clay at 573 °C (1063 °F): it expands on the way up and contracts on the way down. L&L's Slow Bisque slows through it, between 538 °C (1000 °F) and 593 °C (1100 °F). Digitalfire found this "unnecessary for bisque firings", and Ceramics Monthly says it mainly matters when cooling dense ware. Slowing costs little, so our suggestion is to keep it for thick pieces.

Candling: drying in the kiln

Candling means holding the kiln at a low temperature, with the lid propped or the vent running, until the last moisture has left the ware. Sources disagree about the temperature:

Our starting point: climb slowly to no more than about 121 °C (250 °F), and hold until a mirror at the vent stays clear. Hold overnight for thick or damp ware.

The mirror test: hold a room-temperature mirror at a vent hole or at the gap under a propped lid. If it fogs, water is still leaving; wait, then test again.

Controller schedules

A controller is an electronic unit that fires the kiln through a schedule: a list of segments, each a rate of climb (the ramp), a target temperature and an optional hold. Below are two presets from L&L's DynaTrol and Bartlett controllers, and a schedule published by Digitalfire. "Peak" is the controller's own temperature for the cone you choose: for cone 04 it is 1052 °C (1926 °F), and for cone 06, 993 °C (1819 °F).

L&L's times are "for the fastest possible empty kilns"; a loaded kiln can take as much as four times longer, and any hold you add is extra heatwork the presets do not allow for. L&L advises Slow Bisque, with a preheat of 2–3 hours or longer, for large or damp pieces, and warns against fast firing, which "can cause defects like pinholes, cracking, and insufficient burnout".

BQ1000 takes about 7 hours 44 minutes. Its author finds up to 278 °C/h (500 °F/h) fine for thin, dry ware, and slows the last step to even out the temperature through the load.

Controller cones and Orton's chart

L&L and Bartlett controllers use 1052 °C (1926 °F) for cone 04 and 993 °C (1819 °F) for cone 06, a little below Orton's 1063 °C (1945 °F) and 998 °C (1828 °F); they agree at cone 6 and cone 10. Witness cones settle the difference: if they disagree with the controller, correct it with the cone offset.

On a manual kiln

A manual kiln follows the same plan by hand: dry slowly with the lid propped and the peepholes open, step the switches up from Low to Medium to High, and let the sitter end the firing. The published procedures are under Manual kilns in detail, below.

Glaze firing

The glaze firing melts the glaze into glass on the bisque. Most beginners fire low-fire glazes at cone 06–04 or mid-fire glazes at cone 5–6. Fire to the cone on your clay and glaze labels, with a cone pack around that number. For what happens inside the glaze as it melts, see Glazing from first principles.

Loading for a glaze firing

• Glazed pieces must not touch each other or the walls: leave about ½ in (13 mm).
• Keep glaze off the foot (dry-footing), or set low-fire ware on stilts, small stands with points. Stoneware and porcelain should not stand on stilts, which embed at those temperatures; use high-fire kiln wash or silica sand instead.
• Leave at least 1 in (25 mm) between the shelf edges and the wall or sitter.

Controller schedules

Slow Glaze is the safer default, given L&L's warning about fast firing. If you add a hold of more than 5–10 minutes at the peak, L&L says to lower the target or the cone offset to match, and notes that "Soaking at high temperatures accelerates element and thermocouple wear."

Cone 6: drop-and-hold

Mid-fire glazes sometimes come out with pinholes (tiny holes in the surface) or blisters, left by gas bubbling out of the melt. A drop-and-hold schedule fires to the top, cools quickly by about 56 °C (100 °F) and holds there. In Digitalfire's words, "Dropping 100°F and holding gives the melt enough viscosity to burst bubbles while still letting them heal."

PLC6DS takes about 9 hours 36 minutes. For more fluid glazes, Digitalfire drops about 111 °C (200 °F) instead; dropping too far "risks crystallization during the hold". Its peak of 1204 °C (2200 °F) is below the chart's 1222 °C (2232 °F) for cone 6; Digitalfire reports that most kilns reach cone 6 at about 1204 °C (2200 °F). Check it with a cone pack in your own kiln.

Slow cooling

Slow cooling means the controller cools the kiln at a set rate instead of letting it fall freely. Digitalfire's C6DHSC is PLC6DS followed by cooling at 83 °C/h (150 °F/h) from 1148 °C (2100 °F) down to 760 °C (1400 °F), then off. It "improves brilliance and surface quality" and helps rutile blues, but slow cooling "can increase matteness" (one matte glaze went dry) and can shorten the life of the kiln's relays. L&L ships a slow-cool program for cone 6 as well:

Skutt's controllers likewise offer a preheat and a one-step cooling option.

On a manual kiln

A manual kiln cannot follow these schedules exactly, but the plan is the same: gentle at first, steady through the middle, and a watched finish. Bracker's low-fire glaze schedule is under Manual kilns in detail, below; for hotter cones it adds "an average of 15–30 minutes per cone number". With a pyrometer, practice and a log, you can roughly copy a drop-and-hold or a slow cool by turning the switches down by hand, as Digitalfire suggests.

Manual kilns in detail

How the switches work

Older kilns (L&L says mostly those made before 1975) have four-position switches: Off, Low, Medium and High, with nothing in between. Later kilns use infinite switches, marked Off, then 1 to 11, then High on L&L's kilns. An infinite switch does not send less power. It turns the elements fully on and off in a repeating cycle: on Low, L&L says, they "may be on something like 7 seconds, and off 23 seconds in a 30-second cycle". The higher the setting, the longer they stay on, and on High they are on all the time. The numbers are not exact percentages, so learn your own kiln with cones and records.

L&L's general procedure

L&L's suggested procedure for a kiln with three switches, a sitter and no vent:

Bracker's schedules

Bracker's Good Earth Clays, a US clay supplier, publishes these for manual kilns without a vent:

Bracker's does not say which shelf cone its bisque sitter cone matches, so check with your own cone pack. With a downdraft vent, the steps are the same, but the vent runs through the firing and the cooling, and the lid stays closed. Bracker's also suggests staggering: bring the switches up one at a time, about 30 minutes apart.

When to close the lid and plug the peepholes

Our starting point: close the lid once a mirror held at the gap no longer fogs, and at dull red plug every peephole except the top one, which stays open for the whole firing.

Evening out the top and bottom

If your cone packs show the bottom firing cooler than the top, Orton suggests starting with the bottom switch on Medium and the rest on Low, then turning the bottom to High first. Or leave the top switch off until the others reach Medium, then bring the top up one step behind them, holding it on Medium for about an hour after the others go to High. Densely loaded areas fire cooler, and heavier loads take longer.

A starting point for your own kiln

If your kiln came without instructions, the table below is a starting point we put together from the published procedures above. It is not a published schedule. Calibrate it with witness cones and adjust it from your records; lengthen each stage for heavy loads or thick ware.

If the sitter trips before the shelf cones are down, go one cone hotter in the sitter next time, as L&L advises.

Using a pyrometer

A pyrometer is a temperature gauge read from a thermocouple, a heat probe through the kiln wall. It shows how fast the kiln is climbing, so you can pick the right column of the cone chart. But it reads near the wall, not at the ware, and it drifts. L&L notes that a type K thermocouple can be off by more than 14 °C (25 °F) after repeated cone 6 firings, "a difference greater than a full cone", while a self-supporting cone varies by no more than 2 °C (4 °F). Orton says pyrometers need recalibrating from time to time, and cones are how you check them.

Venting, peeking and safety

Venting

Early in a firing, clay and glaze give off water vapor and fumes, including carbon monoxide, and glazes can give off toxic fumes later in the firing too. L&L calls venting them "essential". With no vent system, that means the lid propped and the peepholes open in the early stages, as in the procedures above; slowing the heating during this time also helps the carbon burn out. In a kiln without a vent, Dawson, who made the sitter, also says to leave the top peephole open for the whole firing: the venting "will reduce deterioration of the kiln's metal parts and increase the life of the tube assembly". A downdraft vent (a powered vent fitted to the kiln) changes this. It does the venting, so Orton says the lid and peepholes stay closed for the whole firing except to check the cones, and L&L says to leave the vent on and plug the peepholes.

For venting the room, and for carbon monoxide alarms, see Safe mixing and ventilation and Don't poison your family.

While the kiln is firing

• Look through a peephole only with IR-rated shade 3 welding glasses; sunglasses are not enough. Skutt advises twisting the plugs into place so they do not fall out.
• Do not leave the kiln unattended beyond its expected firing time; a sitter is not a fail-safe.
• Never fire on or near anything that burns, such as a wood floor or carpet.
• Keep the clearances in your manual. Published figures range from 12 in (30 cm) to 3 ft (91 cm); where the manual is silent, 18 in (46 cm) from walls and 3 ft (91 cm) from anything that burns is the safest choice.
• Keep the space around the sitter's weight clear so it can drop.
• When the firing ends, turn the switches off and turn off the breaker as well.

Cooling and unloading

Cooling is part of the firing. L&L and Orton advise: "Let the kiln cool naturally with the lid closed", and "Never introduce cool air into a hot kiln." Opening too soon "can cause cracking", and the silica in the ware contracts suddenly as it cools past 573 °C (1063 °F). Without a controller, keep the kiln closed "until the temperature drops well below red heat (900°F / 482°C)", and for at least 8 hours. Never force-cool a kiln that is still glowing. Cracks with sharp edges in fired ware are a sign of cooling too fast.

When to open and unload

Our suggestion: open the lid only below about 93 °C (200 °F), and unload below about 52 °C (125 °F), or when you can hold the ware comfortably in bare hands. Glaze loads, dense and glassy, need more patience than bisque. Without a pyrometer, wait until the kiln has cooled completely.

Read the witness cones as soon as you open the kiln, before anything is moved, and write down what you see while it is fresh.

After the firing

Record it

Write down each firing: the date, the kiln, the load, the schedule or the switch times, the timer reading, and how every witness cone and the sitter cone bent, shelf by shelf. Keep the fired cones too. Orton notes that "Comparing the results of the last 6 to 10 firings will show gradual changes occurring in the kiln, such as heating elements aging." Glazecalc's firing logs can hold these records, with columns you choose such as time, temperature, cone, ramp and hold; glaze results can go in your notes.

Kiln sitter care

• Clean the cone supports "after every firing with a small wire brush". If wash or melted cone will not come off, turn the supports or replace them; Dawson suggests keeping two pairs.
• "NEVER USE LUBRICANTS OF ANY KIND ON THE KILN-SITTER."
• Inspect the pivot every 6–12 months, and replace a bent or worn rod.
• Check the adjustment with the firing gauge every 30 firings.

Shelves and kiln wash

Kiln wash protects shelves from dripping glaze and lets fired pots lift off. Put it on the top of each shelf and on the kiln floor only, "about the thickness of a postcard". L&L's best practice is three thin coats, fired between coats if possible. Many potters also set pieces with runny glazes on a cookie, a disc of bisque that catches drips. This is common practice, though we found no kiln maker's advice on it.

Elements

Elements wear: an oxide layer builds up, less current flows, and firings slow down. Long holds at high temperature wear elements and the thermocouple faster. Skutt advises a test firing for a new or repaired kiln, so the elements form a protective oxide layer. Follow your maker's instructions for testing and replacing elements.

Rings and bars

British industry has also used two other heatwork indicators. Bullers rings are flat discs that shrink in proportion to heatwork and are measured with a gauge after the firing. Holdcroft bars, from 1898, sag in the middle across two supports, the idea behind the kiln sitter's bar. We found no table converting either to Orton cone numbers, so this guide uses Orton cones; we plan to cover rings and bars in a later version.

Sources

• Orton: Using Orton Pyrometric Cones (cone chart, ©2001)
• Orton: Cones and Firing, a practical guide to successful firings (©2019)
• Orton: Pyrometric cones FAQ
• Dawson: Kiln-Sitter Operating Manual, models LT-3 and LT-3K
• L&L Kiln: Manual kiln operation with kiln sitters
• L&L Kiln: Orton firing tips
• L&L Kiln: Pyrometric cones
• L&L Kiln: DynaTrol controller manual
• L&L Kiln: The ceramic process
• Bracker's Good Earth Clays: Firing schedule for manual kilns
• Ceramics Monthly: Bisque Firing 101
• Ceramics Monthly: Tips and Tools: The Kiln Sitter
• Digitalfire: BQ1000 bisque firing schedule
• Digitalfire: PLC6DS cone 6 drop-and-soak schedule
• Digitalfire: C6DHSC cone 6 drop-and-hold slow-cool schedule
• Digitalfire: Bisque
• Glazy: Firing
• Skutt: Ceramic kilns, using the product
• Skutt: KilnMaster operating manual
• Boston University Environmental Health and Safety: Kiln awareness reminder
• Potclays: The difference between stoneware and earthenware
• Wikipedia: Pyrometric cone
• Wikipedia: Pyrometric device