6.2 Glazes, Kiln Firings, Pyrometric Cones, and Ceramic Studio Safety

Glaze Chemistry and Firing Decisions

A glaze is a glassy ceramic coating formed by firing, rather than an ordinary paint film that dries at room temperature. Choose a clay body, glaze, and firing program as a compatible system. “Ceramic,” “lead-free,” and “fired” do not by themselves mean a vessel is non-porous, food-safe, or suitable for thermal shock. The manufacturer's instructions, firing results, glaze fit, and intended use all matter.

The Three Main Glaze Functions

A useful introductory model identifies three functions in a glaze recipe: a glass former, usually silica; fluxes, which enable melting at a practical firing range; and a stabilizer, commonly alumina, which helps control the melt's viscosity and durability. Actual recipes are more complex, and ingredients can supply several oxides at once. Feldspar, for example, contributes more than a single “flux” ingredient.

Cobalt commonly produces blue. Copper can yield greens in oxidation and reds in suitable reduction glazes; iron can yield amber, brown, black, or celadon effects. These are families of results, not guaranteed colors at every concentration and temperature. Avoid mixing raw glaze chemicals in an ordinary student workspace. Prepared, age-appropriate products reduce powder exposure, but their labels and SDS still need review.

Oxidation and Reduction

An oxidation firing has oxygen available for reactions. Electric kilns commonly operate in oxidation. A reduction firing uses conditions that remove oxygen from some metal oxides, changing their chemistry and often their color. Fuel-fired kilns commonly achieve this by controlling fuel and air. Reduction is not a setting to improvise in an electric classroom kiln; follow the particular kiln's approved use.

For example, a photograph of a red copper glaze does not prove that brushing that glaze onto any body and using any firing will produce red. The atmosphere, heat work, cooling, and recipe interact. An exam question may ask which variable explains a color change between oxidation and reduction. Reason from the firing conditions instead of memorizing a single universal oxide-color pairing.

Cones Measure Heat Work

A pyrometric cone responds to time and temperature together, often called heat work. A thermocouple reports temperature at its location; a witness cone helps show what the ware experienced. Orton publishes equivalent-temperature charts for different cone types and heating rates. A cone number therefore should not be treated as an exact temperature independent of the firing schedule.

The leading zero is crucial. Cone 06 is cooler than cone 04; cone 04 is cooler than cone 01. Beyond cone 01, cone 1 is cooler than cone 6. Cone 06 and cone 6 are substantially different firing ranges. Confusing them can badly overfire a low-fire clay or glaze and damage ware and kiln furniture. The actual result depends on the materials; it is not correct to promise that every earthenware body will turn into liquid at a single temperature.

A common diagnostic arrangement uses a guide cone below the target, a firing cone at the target, and a guard cone above it. For a cone-04 target these could be 05, 04, and 03. Interpret deformation using Orton's directions for the cone and mounting method. A nominal 90-degree bend is the usual endpoint; the tip need not touch the shelf. Use witness cones at suitable locations to identify uneven heat work across the load.

Drying, Bisque, and Glaze Firing

Greenware means unfired clay at any drying stage. Before bisque firing, pieces must be thoroughly dry and the schedule must accommodate residual moisture. Steam pressure in inadequately dried or thick work can cause breakage. An air pocket alone is not proof that dry clay will explode. Even wall thickness, appropriate hollow construction, and careful drying help moisture escape.

A bisque firing converts greenware to durable ceramic, commonly leaving it absorbent enough for glaze application. Chemically bound water is lost during heating, and the clay undergoes irreversible changes. Quartz inversion near 573°C is one reason to avoid damaging thermal gradients, particularly in thick or quartz-rich work. Clay bodies differ, so neither a single ramp rate nor a single bisque cone suits all work.

A glaze firing matures the chosen glaze and body to the specified heat work. Some work is single-fired; two firings are a common teaching sequence rather than an absolute definition of ceramics. Glazed earthenware can retain a porous body under a glassy surface. Stoneware and porcelain bodies also differ in absorption. Never assume every glazed pot is fully vitrified.

Follow manufacturer instructions for drying, loading, ramps, holds, cooling, and opening. Do not crack a lid or remove peephole plugs as a generic “candling” recipe when the kiln's ventilation system and manual call for another arrangement. Keep unsupervised students away from firing equipment and allow adequate cooling before unloading.

Reading Glaze Defects

For crazing, reducing glaze thermal expansion can be appropriate, but simply adding silica or alumina is not a guaranteed repair for every commercial product. Changing a formula can affect melting and durability. In a school studio, consult the manufacturer and test a documented clay-glaze-firing combination. Do not repair an unsafe mug by painting over its defects or sell it as food-safe because the glaze label once said “non-toxic.”

Studio Dust and Kiln Safety

Clay and glaze dust may contain respirable crystalline silica. Work damp, buy prepared materials where practical, wet-wipe tables and equipment, and wet-mop floors. Use suitable HEPA-filtered equipment where vacuuming is approved. Avoid dry sweeping, compressed-air cleaning, and dusty sanding that disperse contamination. Employee exposure rules and any respirator requirements need the district's safety assessment; OSHA does not certify N95 respirators, and an N95 does not remove kiln gases.

Kiln installation requires the actual manual, listed clearances, electrical requirements, suitable surfaces, and local fire and building approval. Exhaust must control emissions outside occupied breathing zones. Electric kilns can release hazardous gases from the ware even without a fuel burner. There is no universal 36-inch clearance or one required room construction for every model. Use eye protection appropriate to the kiln and radiant exposure if viewing a hot chamber; ordinary tinted or didymium glasses are not a universal substitute for task-rated protection.

Keep glaze off surfaces that contact shelves as the product and support method require. Use compatible kiln wash on approved shelf surfaces, and keep it away from elements. Supports and stilts must be rated for the ware and firing, rather than chosen by a blanket low-fire/high-fire rule. A competent safety decision matches the particular system and avoids improvising from a memorized number.

Sources checked 2026-10-15: Orton cone resources, Orton cone FAQs, and Princeton EHS ceramics guidance.