The Raku ceramic glazing relies on a short and violent thermal cycle that distinguishes this firing from all other pottery practices. The piece, heated to around 1000 °C in a dedicated kiln, is extracted glowing and then plunged into a reducing environment (sawdust, wood chips, newspaper) before a rapid cooling. This thermal shock produces the characteristic cracks, metallic reflections, and smoke traces.
Understanding what happens at the glaze level during these few minutes allows for anticipating results and minimizing failures.
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Behavior of Raku glazes in response to thermal shock: what changes compared to conventional firing
In standard electric firing, a glaze slowly rises in temperature, reaches its maturity, and then cools down over several hours in a closed kiln. The glaze has time to smooth out, degas, and solidify uniformly.
In Raku, the temperature drop is replaced by a hot extraction followed by smoking. The glaze solidifies within seconds upon contact with air, which profoundly alters its texture and color. Combustion gases penetrate the micro-cracks of the glassy layer before it solidifies. It is this penetration that blackens the bare clay and creates the typical dark crackle network of Raku.
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A detailed guide presents these techniques on Les Embellies Déco, specifying the precautions related to each step of the process.
| Parameter | Conventional firing (electric/gas) | Raku firing |
|---|---|---|
| Glaze temperature | Variable depending on type (stoneware: over 1200 °C) | Around 1000 °C |
| Duration of complete cycle | Several hours of slow rise + fall | Rapid rise, hot extraction, brutal cooling |
| Atmosphere | Oxidizing (electric kiln) or controlled reducing (gas) | Oxidizing then highly reducing (smoking) |
| Behavior of glaze upon cooling | Gradual smoothing, few cracks | Instant solidification, crackle network, metallic effects |
| Reproducibility | High | Low (each piece is unique) |
Choice and application of glazes for Raku firing
Glazes formulated for Raku are low-temperature glazes, rich in fluxes that allow them to melt quickly. Their composition differs from high-temperature stoneware or earthenware glazes.
Metal oxide glazes and desired effects
Copper remains the most used oxide in Raku. In a reducing atmosphere, it produces deep red copper metallic reflections that are impossible to achieve in conventional oxidizing firing. Cobalt yields stable blues, less sensitive to atmospheric variations. Iron, depending on concentration and duration of smoking, ranges from warm browns to dense blacks.
On the other hand, glazes containing lead, still present in some old recipes, pose a toxicity problem during hot extraction. The fumes from a lead glaze heated to 1000 °C are particularly harmful. Almost all contemporary studios have abandoned these formulations.
Thickness and uniformity of the glaze layer
Dipping remains the most common method for Raku. It ensures a relatively constant layer thickness, which limits areas of detachment during thermal shock. Spraying is suitable for larger pieces or complex shapes, provided the flow is controlled to avoid excessive thickness.
- Dipping: immersion of the bisque piece for a few seconds, quick draining, complete drying before firing. A still damp piece risks exploding in the kiln.
- Spraying: fine crossed layers, drying time between each pass. Allows for overlapping two different glazes for fusion effects.
- Brush: reserved for details or areas intentionally left unglazed. Less suitable for large surfaces as the thickness varies too much.
The bare clay (without glaze) absorbs the reduction smoke and blackens. Playing with glazed and unglazed areas is an integral part of the Raku aesthetic vocabulary.
Safety in Raku studio: specific risks and appropriate protections
Raku firing concentrates several risks that are absent or lesser in conventional ceramic practice: handling incandescent pieces, release of reduction fumes, possible projections during contact between hot glaze and organic material.
Personal protection during extraction
Insulating gloves that extend to the elbows are necessary for handling the long tongs used to remove the piece from the kiln. A leather or heat-resistant fiber apron protects the torso. Safety glasses filter the intense infrared radiation from the open kiln, a point often overlooked by beginners.
Ventilation and management of reduction fumes
The contact between the incandescent glaze and sawdust or newspaper produces dense smoke, laden with fine particles and volatile organic compounds. Practicing outdoors or under a widely open canopy remains the safest configuration.
In recent years, several studios and communities have begun to regulate Raku firings due to the local impact of these fumes on air quality. The choice of reducing materials (untreated wood sawdust rather than printed paper or inked cardboard) and limiting volumes per session are among the recommended practices.
- Use exclusively raw wood sawdust, without chemical treatment or paint residue.
- Inform the neighborhood during extended sessions, especially in suburban areas.
- Provide a closed metal container to smother embers after smoking and limit emission duration.
- Never use plastic materials as reducing fuel, even in small quantities.
Silica dust: a risk shared with all ceramics
Sanding the bisque before glazing and handling glaze powders expose one to inhalation of crystalline silica. A minimum FFP2 mask, worn during each handling of dry powder, limits this risk. Wet cleaning of the studio (mop, sponge) is preferable to a standard vacuum cleaner that re-suspends fine particles.
Raku glazing produces results that no other ceramic firing can replicate, precisely because the process partially escapes the potter’s control. Adapting glazes, mastering layer thickness, and adhering to a rigorous safety protocol during extraction and smoking are the three axes that separate a productive session from a risky one.