Firing a glaze higher or lower than its recommended cone range in a pottery Kiln

Firing a glaze higher or lower than its recommended cone range in a pottery Kiln can lead to various defects, changes in appearance, functionality issues, or even kiln damage (especially if mismatched with the clay body).

Glazes are formulated with specific fluxes, silica, and colorants that melt and mature within a narrow heatwork range (temperature + time). Deviating from that throws things off.

Here's a breakdown of the two scenarios you asked about:

Firing a Low-Fire Glaze Higher Than Recommended (e.g., Cone 06 glaze fired to Cone 5–6 or higher)

Low-fire glazes (typically Cone 022–04, ~950–1100°C) use higher amounts of flux (like boron or alkalis) to melt at lower temps. Pushing them hotter causes overfiring.

Common results:

- Running or dripping: The glaze becomes too fluid and flows off vertical surfaces, pooling at the bottom or base. It can drip onto kiln shelves, other pots, or elements—potentially ruining shelves, fusing pieces to them, or damaging the kiln (e.g., melted glaze shorts elements).

- Colour changes: Many colours darken, burn out, or shift dramatically (e.g., bright reds/oranges/yellows turn muddy brown or black; blues/greens may dull or change hue). Some vibrant low-fire effects (like certain commercial brands) are lost entirely.

- Bubbling, pinholing, or pitting: Over-melted glaze can trap or release gases, creating bubbles that burst and leave holes, or the surface can dimple/evaporate.

- Excessive gloss or texture issues: It might turn overly shiny or develop crazing (fine crack lines) due to poor fit with the body after over-melting.

- Worst case (especially on low-fire clay): If the clay is also low-fire and overfired to mid-range, the body itself can bloat (puff up from trapped gases), warp, slump, or partially melt into a blob—ruining the piece and potentially sticking to shelves/elements.

Some commercial low-fire glazes (e.g., certain Mayco Stroke & Coat or Jungle Gems) are tested to perform okay up to mid-fire with darker/muted results, but this is the exception—always check the label and test first. In community studios, accidentally including low-fire pieces in a mid-fire glaze load often causes major cleanup or damage.

Firing a Glaze Lower Than Recommended (e.g., Mid-Fire Cone 5–6 glaze fired to Cone 04 or low-fire)

This is underfiring—the glaze doesn't fully melt.

Common results:

- Dry, matte, or rough surface: The glaze stays powdery/chalky instead of glossy/smooth. It feels scratchy and doesn't seal properly.

- Poor adhesion or durability: The surface may flake, scratch easily, or absorb water/stains (not food-safe or wipe-clean).

- Colour issues: Colours often look washed-out, pale, or undeveloped (e.g., a vibrant blue might appear dull grayish). Some pigments don't develop at all.

- Crazing risk: If the body matures more than the glaze, or there's mismatch, fine cracks can appear (though crazing is more often from fit issues than just underfiring).

- Functional problems: For food ware, an underfired glaze won't be vitreous (non-porous), so the piece can leak, harbour bacteria, or stain over time.

Underfired glazes can sometimes be salvaged by refiring to the correct (higher) temperature—the glaze may melt properly on the second go. This is common advice for mid-fire glazes accidentally low-fired.

Quick Summary Table

Bottom line for beginners: Stick closely to the recommended cone on the glaze label and match it to your clay/studio firing (most NZ community spots fire mid-range Cone 6).

Test small pieces first if experimenting.

Mismatches are a top cause of ruined pots or kiln issues—ask your studio tech before firing anything non-standard!