Slow cooling a glaze firing for matte and crystal effects

To slow cool a glaze, program the kiln to drop about 100 to 150 F per hour through roughly 1900 to 1400 F (1038 to 760 C) instead of letting it free-fall. Crystals grow in that band, and they're what turn a glossy surface satin or matte, above all in glazes that carry iron, rutile, zinc, calcium, or magnesium.

Updated October 13, 2026 · Source: Ceramic Arts Network, slow cooling in an electric kiln

Why does cooling speed change a glaze?

A glaze at peak is a liquid. On the way down it freezes, and the speed of that freeze decides what's inside it. Cool fast and the melt locks into clear glass with a smooth, glossy face. Cool slowly through the right window and tiny crystals grow in the glass. They scatter light, so the surface reads as satin, matte, or mottled. Peak temperature sets how mature the glaze is. Cooling sets how it looks.

The effect is easy to see on one glaze. Deklan Ranlett, writing for Ceramic Arts Network, fired Dixie Teal on a stock medium-speed cone 6 program and got a glossy, dark result. On a slow-cool program the same glaze came out lighter and speckled. No recipe change. For the full set of cooling bands, including the dunting risk lower down, see how kiln cooling schedules work.

What is the best cooling rate for a matte glaze?

Start at 150 F (83 C) per hour from about 1900 F down to 1400 F, then let the kiln fall on its own. That's the rate in Digitalfire's tested cone 6 slow-cool schedule, and it sits in the middle of Ranlett's 125 to 175 F range. Go slower for a drier matte, faster for more shine.

The published schedules don't agree on every number, and that's useful to see side by side:

Digitalfire starts its slow cool high, at 2100 F (1148 C), right after a drop-and-hold. Ranlett waits until the kiln is about 300 F below shutoff. Both work. The upper start gives a few more minutes in the crystal band and a longer firing.

Below about 1400 F the glaze is too stiff for crystals to keep growing, so a slower cool there buys nothing for the surface. Switch the kiln off and let it coast.

What does a slow cool schedule look like?

Here's a full cone 6 program. Segments 1 to 6 are what the firing schedule builder writes for cone 6, glaze, medium speed, with drop-and-hold turned on. The builder doesn't write a slow cool, so segments 7 and 8 are the two lines you add by hand.

The builder's six segments run 8 hours 45 minutes. Segment 7 adds about 28 minutes and segment 8 adds 3 hours 20 minutes, so the kiln switches off at about 12 hours 33 minutes. Run segment 8 at 100 F (55 C) per hour instead and that becomes about 14 hours 13 minutes.

Two things to know before you trust those times. A controller can't cool faster than the kiln loses heat with the power off, so segment 7 may take longer than 28 minutes in a heavy, well-insulated kiln. And a packed kiln holds heat. Ranlett found his full loads on a stock cone 6 program finished closer to cone 7, so stand a witness cone in the first load and adjust the segment 5 hold from there. The base program is explained segment by segment in the cone 6 glaze firing schedule.

Which glazes change with a slow cool?

Glazes with crystal-forming oxides change the most. Ceramic Arts Network lists zinc, rutile, calcium, magnesium, lithium, and iron as the ones that feather, grow small crystals, or go satin-matte on a slow cool. A plain glossy clear barely moves.

Some named examples from tested sources:

• Rutile blues. Digitalfire built its C6DHSC schedule to develop the blue in its GA6-C rutile glaze.
• High-iron, high-boron glazes. A slow cool can grow iron silicate crystals that turn the gloss into a yellowish matte, which may or may not be what you want.
• Commercial reactive glazes. Krueger Pottery's listing for AMACO Cosmos CO-5 Dark Matter says it fires fine on a normal medium program, and a controlled cool gives more crystal growth.
• Glossy stoneware glazes. Digitalfire reports fewer pinholes and a smoother surface on a slow cool, even on coarse sculpture bodies.

And some that don't respond much. Digitalfire saw little difference with highly fritted commercial glazes on porcelain bodies. It also notes that some barium mattes keep the same surface whatever the cooling speed.

When does a matte glaze go too matte?

When it's already matte on a normal cool. Digitalfire's G2934Y is a magnesium matte that comes out silky on a free fall. On the 150 F per hour slow cool it turned into a dry, stony matte that looked worse. The fix there was the faster schedule, not a new recipe.

This matters for pots you eat from. Digitalfire points out that a rough matte can mark when cutlery scrapes it and holds stains, because the surface never leveled out. Ranlett's advice is to leach test every result you plan to use for food. A slow cool is a change to the glaze, so treat it like one.

How is a crystalline glaze firing different?

It holds instead of crawling down. A satin slow cool grows crystals too small to see. A crystalline glaze grows zinc silicate crystals large enough to see, and that takes a long soak a little below the melt.

The Ceramic Shop publishes two cone 6 crystal schedules. Both climb at 108 F per hour to 2232 F (1222 C). Schedule 1 then drops at full speed to 1950 F (1066 C), near where cone 04 bends, and holds for one hour. Schedule 2 drops to 2000 F (1093 C), holds 15 minutes, then cools at 50 F per hour to 1900 F, which the shop says gives a more matte crystal surface. The length of the hold changes crystal size.

These glazes run. The Ceramic Shop tells you to test first and fire on glaze catchers or sacrificial supports, because thick spots will flow onto the shelf. A thin coat near the foot slows the run.

Can you slow cool a kiln without a controller?

Yes, by hand, with a manual kiln and a log. Ranlett's method: after the kiln sitter drops, re-engage it and set the safety timer as a backup, leave the kiln off for 15 to 20 minutes, then switch between medium and low in blocks of one to two hours. The aim is a fall of about 125 to 175 F per hour.

Write down every switch time and the pyrometer reading. Your second firing will be closer than your first, and by the third you'll have a switching pattern for that kiln and that load size. Digitalfire suggests the same thing for kilns without a controller.

How should you test a slow cool before a full load?

Fire tiles, not pots. Dip two tiles from the same bisque batch in the same bucket. Fire one on your usual program with a free fall, and the other with segments 7 and 8 added. Lay them side by side and you'll know in one glance whether the slow cool helps that glaze.

Then check three things on the slow-cooled tile. Did the witness cone still bend to cone 6? Does a steel spoon leave grey marks? And does the surface still clean easily? If it passes, try a half load, since a full kiln cools slower than an empty one and can push the surface further than the tile showed. The guide to witness cones shows where to place them.

One cost to weigh. Digitalfire notes that long slow cools wear out the relays in an electric kiln faster, because the controller keeps switching the elements on and off for hours. Use the slow cool on the glazes that need it, not on every firing.

Sources

• Ceramic Arts Network, Super Cool: Slow Cooling in an Electric Kiln, by Deklan Ranlett (ceramicartsnetwork.org).
• Digitalfire, C6DHSC Plainsman cone 6 drop-and-hold slow-cool schedule (digitalfire.com).
• Digitalfire, matte glaze glossary entry and the G2934Y slow-cool versus free-fall comparison (digitalfire.com).
• The Ceramic Shop, Crystalline Glaze Guide, firing schedules 1 and 2 (theceramicshop.com).
• Krueger Pottery, AMACO Cosmos CO-5 Dark Matter product listing (kruegerpottery.com).
• Ron Roy and John Hesselberth, Mastering Cone 6 Glazes, appendix firing schedules, named by Ranlett as his starting point.