Firing Considerations

Firing is where glaze chemistry stops being theoretical. The same glaze can change color, gloss, movement, and surface texture depending on atmosphere, kiln type, ramp rate, cooling curve, and even shelf position. For Glaze Library's audience, the most important baseline is that commercial brush-on glazes are usually developed and photographed in electric oxidation firings.

Oxidation means the kiln atmosphere has enough oxygen for fuel and glaze materials to oxidize fully. Reductionmeans there is not enough oxygen, so burning fuel and hot gases pull oxygen from metallic oxides in the clay and glaze.^[1][2]

That oxygen shift is why color changes so dramatically. Copper tends to stay green in oxidation but can move toward red in reduction. Iron stays more brown, tan, or amber in oxidation but can become celadon blue-green, tenmoku brown-black, or more fluid in reduction because reduced iron is also a stronger flux.^[2][3]

For commercial users, the practical rule is simple: most manufacturer sample chips assume oxidation, so reduction immediately makes the label less predictive.^[4][5] Reduction is not “wrong,” but it is a different test environment. If your glaze contains iron, copper, rutile, chrome, or a celadon-style chemistry, expect visible differences and test before you trust the jar photo.

Electric kilns are the most repeatable choice for commercial brush-on glazes because they naturally fire in oxidation and avoid ash, flame path, and fuel-air balancing variables.^[4][6] That is one reason most commercial glaze systems target electric users first.

Gas kilns add more atmosphere variability and can move between oxidation and reduction during the same firing. That creates richer traditional stoneware surfaces, but it also makes commercial brush-on samples less reproducible.^[2]

Some commercial lines deliberately try to mimic reduction looks in oxidation. AMACO describes Potter's Choice as offering mid-range reduction looks in oxidation, and its self-reducing copper-red family is designed to create a reduction-like red effect inside the glaze itself.^[7][8] These are powerful commercial tools, but they still depend heavily on thickness, cone, and clay body.

Wood, soda, and salt kilns do more than heat the glaze already on the pot. They also add surface material during the firing. Wood ash can flux the surface, while salt and soda vapors react with silica and alumina to create sodium-rich glaze effects directly on exposed clay and glaze surfaces.^[3][9]

This is why atmospheric firings can create orange-peel texture, flashing, darker gloss, or extra running even when the dipped or brushed glaze recipe was unchanged.^[3] For the public guide, that matters mostly as a caution: commercial glaze labels are not meant to predict wood, soda, or salt results with high accuracy.

Most firing mistakes happen because potters think only in peak temperature. Glazes do not mature at one number alone. They mature from the total combination of time, temperature, atmosphere, and cooling.

A pyrometric cone measures heatwork, not just temperature. That means time matters. Orton's own data shows that cone 6 bends at different temperatures depending on heating rate: approximately 1185C at 15C/hr, 1222C at 60C/hr, and 1243C at 150C/hr.^[10][11]

This explains one of the most confusing studio experiences: two firings can both say “cone 6” and still produce different glaze results. A faster firing often leaves glazes looking drier or less developed, while a longer firing can deepen color, increase gloss, or push a glaze into running.^[3]

Digital controllers are useful, but they do not replace witness cones. Controllers read thermocouples, and thermocouples drift over time. Orton explicitly warns that electronically controlled kilns still need cone verification because the controller only estimates what the ware experienced.^[12]

Manufacturers agree. Mayco tells users to always use witness cones, and AMACO product pages repeatedly note that their firing temperatures are based on normal medium-speed firings.^[4][5]

In practice, use cone packs on more than one shelf, especially when you are comparing test tiles, reactive glazes, or matte surfaces that are very sensitive to under- or over-fire. Orton considers a cone properly fired when its tip bends to roughly the 5-to-6-o'clock position.^[11]

Slow early firing is not just about being careful. It gives water, carbon, sulfur, and organic materials time to burn out before the glaze seals the body.^[13][14]

Steve Davis identifies the 700-900C zone as especially important for carbon and sulfur burnout. If that oxidation is incomplete, the later glaze firing is far more likely to show black coring, bloating, pinholing, or blistering.^[13]

Venting matters for the same reason. Skutt ties poor kiln venting not only to room fumes but also to weak warm colors, cloudy surfaces, pinholes, blisters, and uneven firing. Its downdraft vent guidance also says improved circulation can cut top-to-bottom temperature differences to about half normal.^[14]

For commercial brush-on users, the practical firing takeaway is: low-fire clears, reactive cone-6 glazes, and thick application all benefit from clean burnout and good oxygen supply early in the firing.

A short soak at maturity gives the glaze more time to smooth out and can help heal pinholes or small blisters. But there is an important difference between holding at peak and holding after dropping a little.^[15]

Digitalfire's drop-and-soak logic is that a glaze often keeps percolating bubbles during a top-temperature soak. Dropping the temperature by roughly 100-200F and holding there can help bubbles burst and heal when the melt is slightly more viscous.^[15][16]

This is especially useful in electric cone-6 firing for pinholing and blistering, but it is not a blanket rule. Long peak holds can also increase running, muddy layered overlaps, or over-mature a commercial glaze that was already close to its limit.^[16][3]

Cooling is part of firing, not something that happens after firing is “done.” Slow cooling gives crystals more time to grow, which is why it can improve rutile-blue response, reactive mattes, and some crystal-bearing commercial glazes.^[17][18][19]

Coyote publishes a concrete cone-5/6 slow-cool schedule as a starting point, and AMACO says its Cosmos line develops more crystal growth under controlled cooling.^[19][20] Those are good examples of when a special cooling curve is genuinely useful.

But slow cooling is not always better. Digitalfire documents boron-rich clears that cloud during slow cooling but stay clearer when cooled faster.^[21] Some glazes also become too matte, too crystalline, or too runny if the kiln spends too long in their crystal-growth window. The safest public rule is: use controlled cooling when the glaze line or your testing shows a real benefit, not by default for every glaze.

Even with the right cone and a good schedule, results still change if the kiln is loaded unevenly or if the glaze line itself is very schedule-sensitive. This is where practical kiln habits matter as much as chemistry.

Shelf position changes heatwork. Skutt notes that the center of the kiln is often hottest, while top and bottom shelves can underfire if packed too tightly. It also recommends about 5 inches below the lid for the top shelf, about 1 inch above the floor for the first shelf, and about 2 inches of clearance around the thermocouple tip.^[22][23]

Dense or unbalanced loads can therefore create the illusion that a glaze is inconsistent when the real issue is shelf position. If one side, one level, or one dense load always looks different, check witness cones on multiple shelves before changing the glaze itself.^[22]

Refiring can fix some problems, but it is not a neutral reset. Once the pot has already matured once, the body is denser and the glaze may remelt differently on the second firing.^[24]

Minor underfire, small pinholes, decals, lusters, and some overglaze work are reasonable refire situations. But reactive and highly fluid glazes are much riskier. Digitalfire notes that rutile-blue and blister-prone glazes can run or blister more on refire, and AMACO says that refiring will not rescue PC-70 Copper Red once its reduction materials have burned off.^[24][8]

The practical rule is cautious: refire only when you know what defect you are trying to fix, and test first if the glaze is known to be reactive, layered, or runny.

A glaze's printed cone range is a firing window, not a promise of one perfect appearance. Commercial glazes often have a range where they mature safely and a narrower zone where they look best.

AMACO's PC-70 Copper Red is a good example: the company says it is at its most red at cone 5, while cone 6 produces a more purple-red effect and more running risk.^[8] Mayco's SW-135 Wintergreen is another: it reads as a truer matte at cone 5 and becomes more satin by cone 6.^[25]

The same goes for special schedules. Some product lines respond strongly to controlled cooling. Others do not require it. AMACO says Cosmos gains more crystal growth with controlled cooling, while Mayco says its Stoneware Crystal glazes do not require a special firing schedule for their basic crystal effects.^[20][26]

That is why the best commercial-glaze advice is not “always fire cone 6” or “always slow cool.” It is: use witness cones, test your actual kiln, and treat the manufacturer's firing notes as product-specific data rather than optional marketing copy.