Firing is the stage that transforms shaped, dried clay into ceramic. Before entering the kiln, clay is fragile greenware or leather-hard material that dissolves in water. After a complete firing cycle, it is a permanently hardened material that water cannot reverse. Understanding what happens inside the kiln — and why the two standard firings exist — makes the process considerably less opaque for a beginner.
The Two Standard Firings
Most ceramic work goes through two kiln cycles: bisque firing and glaze firing. Some potters skip bisque firing entirely and fire once with glaze applied to dry greenware (single-firing), but two-fire sequences are the norm in community studios because they allow glaze application under more controlled conditions.
Bisque Firing
The bisque fire converts greenware into porous, hardened ceramic that can be handled, glazed, and re-fired without dissolving. It burns out organic materials — plant matter, paper from clay bodies, carbon from coloured slips — and begins the vitrification process. The clay physically transforms: quartz inversion occurs at 573°C, which causes a small but sharp volume change, and above 900°C feldspars begin to fuse into the clay matrix.
Bisque temperatures in most Polish studios range from 900°C to 1000°C for earthenware and stoneware bodies. The exact temperature depends on the clay body: too low and the bisqueware remains fragile; too high and it becomes dense enough to repel glaze.
Work must be completely dry before bisque firing. Any remaining moisture in the clay turns to steam during heating and causes pieces to explode inside the kiln. Standard practice is to allow at least 48 hours of room-temperature drying after the piece feels dry to the touch — particularly for thick-walled forms or joined pieces where interior moisture may persist.
Glaze Firing
The glaze fire takes bisqueware to the clay body's maturation temperature — 1200°C–1280°C for stoneware — where both the clay body and the glaze fully vitrify. The glaze melts, flows slightly, and bonds permanently to the ceramic surface as the kiln cools.
During glaze firing, pieces must not touch each other or the kiln shelves unless they are unglazed at the contact point. Glaze is liquid at peak temperature and will fuse two pieces together, or bond a piece to a shelf, permanently. Kiln wash — a refractory powder slurry applied to kiln shelves — prevents glaze drips from bonding to the shelf surface. Kiln furniture (stilts, posts, bat pins) raises pieces off shelves and allows free air circulation.
Temperature Cones: A Practical System
Kiln temperature is measured in degrees, but the relationship between temperature and clay maturity is not linear — it also depends on how quickly the kiln reaches a given temperature (firing rate) and how long it holds that temperature (soak time). The Orton cone system provides a standardised way to measure heat-work rather than temperature alone.
Cones are small pyrometric pyramids that bend at specific combinations of temperature and rate. Cone 06 (about 999°C) is a common bisque temperature. Cone 6 (about 1222°C) is a standard electric-kiln stoneware temperature. Cone 10 (about 1285°C) is used in gas kilns and high-fire reduction work. The difference in numbering — 06 and 6 — is a common source of confusion: cone numbers above zero run upward; cone numbers with a zero prefix run downward (cone 06 is lower than cone 6).
In Polish community studios, most electric kilns are programmed digitally and will fire to a set temperature with a soak period. But witness cones placed inside the kiln give a secondary, independent reading of what the clay actually experienced — useful when a digital controller gives an unexpected result.
Kiln Atmosphere: Oxidation and Reduction
Electric kilns fire in oxidation — there is sufficient oxygen in the kiln atmosphere throughout the firing. Gas kilns can be fired in reduction by restricting the air supply, creating a carbon-rich atmosphere that draws oxygen from metal oxides in the glaze and clay body. Reduction firing changes glaze chemistry significantly: iron oxide shifts from red-brown (oxidation) to grey-green (reduction); copper shifts from green to red.
Most beginners in Poland work in oxidation electric kilns. Reduction firing is available at some studio potteries and at the ceramics departments of art academies, but requires more experience to predict results. The glazes typically available in studio supply stores are formulated for oxidation firing unless specifically labelled otherwise.
Glaze Application Methods
Glaze is applied to bisqueware in liquid form. The liquid is absorbed into the porous bisque, leaving a dry layer of glaze materials that melt during the glaze fire. Application thickness matters: too thin and the glaze crawls or looks underfired; too thick and it runs down the piece and fuses it to the shelf.
Dipping
Dipping is the fastest method and produces the most even glaze layer. The piece is submerged in a glaze bucket for one to three seconds, removed, and allowed to dry. Overlap areas from multiple dips create thickness variations that affect the fired surface — intentionally or otherwise. Dipping requires a large-enough glaze bucket to accommodate the piece, which makes it impractical for large work without significant glaze volume.
Pouring
Pouring is used for interiors of bowls and cylinders that cannot be dipped efficiently. The piece is held over a catch basin, glaze is poured inside, swirled, and poured out. A second pour glazes the exterior. The method wastes more glaze than dipping but works well for large pieces and produces directional marks on the surface that some potters use intentionally.
Brushing
Brush application gives the most control over placement and is well suited to decorative work, multiple-colour surfaces, and small areas. It requires more coats than dipping — typically three to four passes — to build adequate glaze thickness. Brush marks can show in the fired surface depending on glaze viscosity; flat-bristle hake brushes produce a less textured application than round watercolour brushes.
Spraying
Spray application uses a spray gun and compressor to apply an even glaze layer and is the standard method for production pottery. It requires a ventilated spray booth and appropriate respiratory protection, since glaze slurry contains silica particles. Most community studios in Poland do not have spray booths available for student use; the method is more common in professional studio pottery settings.
Glaze Chemistry at a Basic Level
Glazes are composed of three functional groups: glass formers (primarily silica), fluxes that lower the melting point (feldspars, calcium carbonate, zinc oxide), and stabilisers that keep the melted glaze from running (alumina). Colourants — iron oxide for earthy browns and greens, cobalt carbonate for blue, copper carbonate for green — are added at 0.5–5% of the dry glaze weight depending on the intensity desired.
Most commercial glazes sold by Polish ceramic suppliers, including Keramik.pl and Ceramica Artigiana, are pre-formulated for specific cone ranges. Beginners do not need to mix glazes from raw materials to start. But understanding why a glaze runs (too much flux), crawls (too much clay), or looks dry (underfired or too much alumina) makes troubleshooting systematic rather than guesswork.
A useful external reference on glaze chemistry is the Digital Fire Glaze Glossary, maintained by Tony Hansen and widely cited in English-language ceramics literature. The Ceramics Arts Network publishes accessible overviews of glaze application suitable for beginners.
Cooling and Opening the Kiln
The cooling phase of a firing is as important as the heating phase. Rapid cooling causes thermal shock that cracks pieces — particularly large flat work and pieces with variations in wall thickness. The standard practice in electric kilns is to allow the kiln to cool with the lid closed until it reaches 100°C or below, then crack the lid for a further 30 minutes before opening fully.
Quartz inversion at 573°C occurs on cooling as well as heating. Kilns should not be opened or disturbed while above this temperature. Most electric kiln controllers include a cooling phase in their programs; studio members using shared kilns in Warsaw and Kraków are typically instructed not to open kilns until a safety thermocouple reading below 100°C is confirmed.
Last updated: 22 April 2026. Content reflects standard ceramic practice in Poland as of that date. Kiln safety recommendations should always be confirmed with the studio technician responsible for the specific kiln in use.