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Cone 5 - Last 300 degrees to the TOP - Colors vs Even-out myths


JASON GANLEY

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I am firing to cone 5.  So many opinions on firing schedules for glaze firings. 

I have a 28 cu ft Geil, Gas, Downdraft, Fiber kiln. 

Orton says the cone will bend around 2187 F   if you go up 108 degrees / hr. 

I did that once, got great results.

I've been in the habit now of going up 100 degrees /hr from 2000 F to 2100, then slowing to 50 degrees / hr up to the cone bending

That means a few hours from 2100 F up, which is a long soak, trying to get Oxidation, abut afraid I will burn out colors like yellow and Pink. 

I'm torn to go back to 108 hr, but that slow 50 hr in the last few hours is essentially like a built in hold, keeping the gas from cranking fast. 

Opinions. 

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Jason:

Orton developed his firing schedules from research he did on firing clay bodies ( various types ) in lab controlled studies. So "myth" would not be an accurate term. The 108F per hour ( slow cycle) came directly from his research on organic and inorganic carbon burn out phases found in natural clay. InOrganics burn out by 1000F roughly, and in organics burn out from 1250-1750F. Inorganics include lignite coal particles and sulfides, primarily from iron disulfide. West, Brown, Ougland, Brindley, Lawrence (PhD's) all confirmed his findings from the 1950 to 1970's. Orton did most of his research in the 1920's. 

The Orton cone schedule is based on this research: it was developed to mature clay: not glaze. The focus on firing schedules to mature specific glazes is a modern trend of the last 20 years. The 2050F I recommend in my firing schedule also comes from studies done in the USA, France, Brazil, India, Germany, and China ( a University Studies) which all confirm an exothermic reaction (releases heat) at  2050F. Clay bodies absorb heat ( endothermic) up unto that temperature, the most notable being quartz inversion at 563C. The clay body is actually porous and expanding until it hits 2050F, and it begins to close up as glass/mullite develops. Better known as the metakaolin to spinel phase change. At 2050F up to 2190F when feldspars are most vigorous: both produce gas as they melt. The 125F that I recommend is to allow spars to off gas before the body becomes too compact, and to allow additional time for spars. Most modern pinhole issues come from clay body issues, not glazes.

If you are using dark or red bodied clays: then 108F an hour is advisable through the entire cycle. Porcelain or white stoneware can handle a medium cycle up to 2050F. The 125F after that is solely to allow spars to off gas.

myth? no... My recommendations are based in proven research.

Nerd

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 Amazing. Great stuff. I was wondering why the Orton chart didn’t say glaze or bisque. 

 

 So what do you think about the last hundred degrees of my firing for a glaze firing? 108° to 124°, or will 50° an hour give me a soak? It’s a soak necessary? I hear people talk about flattening out the glazes. I’ve got five minutes to make a decision or my kiln goes into 50° in our the last hundred degrees

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108f an hour from 2050F to peak is the soak. Intended to mature clay, but has the added benefit of flattening glazes. The glaze is in direct contact with the kiln temp, it can take an additional 30 minutes for that atmospheric temperature to reach the core of the clay body.  50F an hour is not necessary.

T

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I replied before I saw your second response...

I may have misread... Are you saying that 2050 up to 2190 in your recommended schedule is for Bisque or for Glaze?

I have been Bisque firing to Cone 04 (candle for 12 to 14 hrs, then start).  To be safe, I've been going 108* / hr all the way up to Cone 04 (1940 or until the cone bends) with a soak at 1200.

Well... I did that Cone 5 Glaze fire last night.  200* / hr up to 2000;     100* / hr up to 2150;     50* / hr up to 2250;   Cone bent at 2208

That's the 2nd time I've gotten the cone to bend at exactly 2208 with this 50* at the top schedule (Orton Self Supporting cones).  I think when I did 108* / hr the cone would bend at around 2178.

 

So in terms of glaze, the 108 IS, in itself, a soak. 

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Jason:

4 primary considerations when selecting a firing schedule.

1. Functional of non-functional use. You have a lot more freedom if non-functional although there are still limits.

2. Bisq fire or single fire. If starting from bisq, you can fast fire up to 2050F, then either 108 or 125F an hour to peak (cone 5-6). Assuming proper bisq firing has been observed. The slow ramp from .2050 to peak is allow spars to off gas and to allow heat to reach the core of the body. If single fire, some variations pending porcelain, white stoneware vs dark or red bodied clays. 

3. Large, heavy wall, or sculptural pieces need special consideration. Mugs, bowls, yarn holders etc can fire on medium up to 2050 without issue in a single fire. Wall thickness over 3/8", large foot rings, large surface contact, or just heavy needs to go on slow speed to allow for heat work. Quartz inversion becomes an issue (1063f) can crack or split large pieces. 

4. Speciality firings such as raku, salt, wood - all have their own schedules. Not going to address any of those: not my area. Marcia Selsor and several others could certainly expound. 

Rather you single fire or start from bisq (observing rules for dark/red bodies- high iron) and also observing quartz inversion ramp for large or heavy pieces: once you hit 2050F, slow ramp to 108 to 125 to peak. Potassium starts melting at 2112f and sodium at 2144F is an additional reason you slow down at 2050F to allow these spars to off gas completely.

Tom

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