The case for Slow cooling
Down firing, or slow cooling, refers to controlling the rate at which your kiln cools. I’ve programmed my kiln to a slowly cooling rate for a variety of purposes, ranging from slow cooling large work to reduce dunting to cooling slowly and holding at certain temperatures to form glaze crystals. Slower cooling reduces stress on ceramic wares and is well worth the extra time. For glazes containing zinc, rutile, calcium, magnesium, lithium, and iron (to name just a few), slowing the cooling rate can result in some spectacular effects, ranging from feathering to small crystals and in some cases a fully-developed satin-matte surface. The combination of a short soak at peak temperature and then down firing can also eliminate pin holing in some glazes
(Ceramic Arts Network, Deanna Ranlett)



These three photos show examples of the microcrystalline effect in glazes when slow cooked giving an overall matte appearance.
Same high-iron glaze. One crystallizes and the other does not. Why?

Both mugs have the same cone 6 oxidation high-iron (9%), high-boron, glossy glaze. Iron silicate crystals have completely invaded the surface of the one on the right, turning the near-black glossy into a yellowy matte. Why? Susceptible glazes have a temperature at which crystals form the best and that temperature can be hundreds of degrees down from the firing cone.
Slow cooling is a technique used to make cone6 glazes look good. Electric kilns are made with 2.5 - 3 inch firebrick and have less insulation when compared to standard gas kiln and are usually smaller. All in all they have less mass in terms of pieces and shelves; with less mass and thinner walls they cool more quickly. Slow cooling is a way to overcome this. It encourages crystal growth in certain types of glazes and can combat glaze defects such as pin holes and dunting. During the cooldown in the 1800-1400F temp range glazes are most often still molten. By adding holds and slowing the cooldown crystalline and calcium glazes form crystals and iron and copper glazes are returning to oxide states and the slow cooling helps to allow them to fully mature. Below 1400F is when the glazes “freeze” and they reach their final appearance.
What to expect:
With most commercial glazes you don’t know what materials are in the glaze and therefore have no way of knowing if there will be any difference compared to a standard firing. Any glaze getting it’s color from mason stain will be unaffected, as well as underglazes, whites, blacks and clears also won’t be different. However, Coyote offers a line of glazes specifically made to take advantage of this process. Below is a chart taken from their website comparing the glazes normal run and slow cooled run side by side.

There is no reason to “not fire a piece” because of the slow cool schedule, just load pieces that people specifically want fired then load whatever else is on the shelves that would go in a normal glaze load.
The slow cooling will add a few hours to the glaze firing overall but the improvement in appearance to some glazes and potential for less flaws from the glaze hardening too quickly can make it worthwhile. Keep in mind that glazes you know have movement (runny glazes like Ancient Jasper or seaweed) may move even more so double the space you would leave at the bottom and keep layers thinner near the foot of you pieces.
Steven Hill incorporates this technique in his works as noted in John Britt’s book on mid-range glazes. Below is the firing schedule taken from that book for reference.

