Slow Cooling

[Norm Stuart]

Gas kilns use hard fire-brick which cool far more slowly than the foam-core brick used in electric kilns. Many here have suggested this is one major difference in the "cone 10" look of a gas kiln.

Using our Cress E23 computerized kiln controller we slow the rate of cooling between 1,800F and 1,500F.

Over the past two years I have used three different cooling rates: 185F per hour, as suggested in "Mastering Cone 6 Glazes"; 100F per hour; and 50F per hour. We finally settled on 50F as the rate we preferred.

Some glazes become so crystallized and matte at 50F cooling I need to add 10% Ferro Frit 3268, or slightly more, to give them enough flux and silica to retain their look.

These are photos of Ron Roy's Sapphire cooled at 185F, 100F, and 50F.

[Norm Stuart]

107.0%	Ron Roy  Sapphire Blue
37.0%	Frit 3134
26.0%	EPK
17.0%	Silica
16.0%	Custer Feldspar
4.0%	Strontium Carbonate
4.5%	Rutile
2.5%	Cobalt Carbonate

[Norm Stuart]

Vee's Tenmoku Gold - adapted to eliminate the use of Cornwall Stone. More costly at $2.14 per pound due to the use of Lithium Fluoride to replace Lithium Carbonate. Lithium Fluoride provides more flux and comes in a very fine mesh and is more reliable for me. Cooling at 50 degrees F per hour between 1,800 and 1,500F makes all the difference.

100.6%	Vee's Tenmoku Gold ^6  $2.14
23.4%	Silica
18.6%	Custer Feldspar
14.5%	Nepheline Syenite
10.0%	Red Iron Oxide Precipitate
8.0%	Whiting
7.8%	Kaolin EPK
7.2%	Dolomite
5.5%	Lithium Fluoride  $1.43
2.3%	Ferro Frit 3134
2.0%	Wollastonite
0.9%	Borax
0.3%	Soda Ash
0.2%	Zinc Oxide
0.2%	Strontium Carbonate

Alisa Clausen's version of Vee's Tenmoku Gold using Cornwall Stone.

http://www.flickr.com/photos/glazes/1085550738/

[Norm Stuart]

Vee's Tenmoku Gold also slow-cooled at 50 degrees F per hour.

But EPK is replaced with Grolleg and Red Iron Oxide Precipitate from Standard Ceramic is replaced with Laguna Red Iron Oxide.

[Norm Stuart]

Another typical difference between slow-cooling at 185 F/hr and 50 F/hr with Jayne Schatz Blue Matte.

 

[Norm Stuart]

Another typical difference between slow-cooling at 185 F/hr and 50 F/hr with White Bird with 1% Red Iron Oxide, which I needed to adjust with 10% Ferro Frit 3269 to restore some of the original feel of the glaze at 50 degrees cool per hour. I'm still trying other adjustments.

http://www.flickr.com/photos/glazes/page39/

[Norm Stuart]

Clear Base Blue ^6   http://www.wpapotters.blogspot.com/2010/11/cone-6-oxidation-results...

Another glaze which required 10% Ferro Frit 3269 to prevent complete crystallization at 50 degrees F/hr cooling between 1,800 F and 1,500 F. In the process it became quite a different glaze.

120.0%	Clear Base Blue ^6
20.0%	Feldspar G-200
20.0%	Wollastonite
20.0%	EPK
18.5%	Flint
11.5%	Talc
10.0%	Ferro Frit 3134
10.0%	Ferro Frit 3269
6.0%	Rutile
3.0%	Red Iron Oxide Precipitate
1.0%	Cobalt Carbonate
0.8%	Copper Carbonate

[Tom Humphries]

Some really interesting stuff here Norm, thanks for sharing!

[Wyndham Dennison]

Hey Norm, any idea how Vee's Temoku Gold gets those (what appear to be) dolomite crystals? I've tried that glaze several times and still don't get anything close.

I've gotten that effect on a  temoku cone 10 reduction by adding 8-10 % dolomite. Could that be a cone 6 reduction?

As to the 50 deg/hr cooling, that would seem to be a heat work thing going on because the first hr of 50 deg drop is less than a degree a minute.

Just another thought for the fire.

Wyndham

[Norm Stuart]

Wyndham - Slow-cooling after a Cone 6 or Cone 10 firing, is quite different to heat-work, even though a very modest amount of additional heat-work might be accomplished at what are essentially temperatures cooler than bisque temperature.

The heat work of cone 6 or cone 10 creates a certain level of densification of the clay body.  Our kiln typically achieves a witness cone 6 at a peak temperature of 2,204 F with appropriate glazes melting but not running.

What happens if I achieve the same witness cone 6 at a higher temperature of 2,250 by raising the temperature more quickly?

While the clay body is equally densified, the same glazes have become more fluid and have run onto the shelf.  So while the "heat work" on the clay body was supposedly the same, the glazing results at the higher peak temperature is not the same at all.

Extremely long holds may produce the same heat work as one cone, or 1.5 cones, higher - but not more. You have no practical hope of achieving the densification of a Cone 10 by holding a Cone 6 firing at peak temperature for days at a time. You achieve the same densification of cone 10 at cone 6 by adding additional flux to the clay body and the glaze.

Crystallization in glazes generally happens in the temperature range of 1,800 F to 1,500 F.  Hotter than this, crystals melt rather than form.  So the longer the kiln lingers within this temperature range, the more crystallization occurs.  Spending 6 hours at this temperature range might possibly add a small fraction of one cone of heat work to your cone 6 or cone 10 firing - but probably not, and besides, that's not why you're doing this.

Gas or wood fired kilns are typically made with heavy fire-brick which absorb and retain heat for a very long period of time - unlike a soft fire-brick kiln which cools very quickly.

As a consequence, heavy fire-brick kilns fired to cone 2, or cone 6, or cone 10 can't avoid slow-cooling and remain in the 1,800 to 1,500 temperature range far longer than a soft fire-brick kiln does.  With a soft fire-brick kiln you have to add programmed heating to prolong the time the ware spends in the 1,800 to 1,500 F range.

What is the composition of the yellow/gold crystals in Vee's Tenmoku Gold?  I could make a guess but I'd probably have only a small chance of being right.  A mineralogist could make a better guess.  But if you let a soft fire brick kiln cool naturally, it will probably spend  less than 30 minutes in the 1,800 to 1,500 F temperature range - so you'd be lucky to see any of these yellow/gold crystals with the naked eye.  With magnification you'd probably see that these crystals do exist, but they'll be very small as they weren't given enough time to grow larger.

So that's how I program our kiln: 

I ask the software program to use the thermocouple data to accumulate a Cone 6 worth of heat-work;

I add a 20 minute hold to allow the glazes to finish reacting and bubbling so they lay down flat;

I add a slow-cool of 6 hours between 1,800 and 1,500 F to create more crystal formation.

A witness cone placed in this firing will indicate more than a cone 6 and less than a cone 7, primarily because of the 20 minute hold at the peak temperature - but this additional heat-work doesn't appear to create any obvious differences in the clay bodies we use.

Wyndham Dennison said:

Hey Norm, any idea how Vee's Temoku Gold gets those (what appear to be) dolomite crystals? I've tried that glaze several times and still don't get anything close.

I've gotten that effect on a  temoku cone 10 reduction by adding 8-10 % dolomite. Could that be a cone 6 reduction?

As to the 50 deg/hr cooling, that would seem to be a heat work thing going on because the first hr of 50 deg drop is less than a degree a minute.

Just another thought for the fire.

Wyndham

[Wyndham Dennison]

I agree about the crystallization range. When I fire my crystalline glazes to cone 10 I hold at 2000 deg for 5 hr then another several hours at 1880 deg f.

And yes heat work is applied within a narrow rages in and around the final set point so cone 6 for a day is not cone 10 but a whopping electric bill.

Will follow up on another point a bit latter.

Wyndham

[Wyndham Dennison]

The other point I wanted to share is that the mass of the kiln at 2232 might be compared to a 55 gal drum and the drop of 50/hr might be compared to a small hole because the heat mass is storing so much heat compared to the down firing schedule, that at first there is little change in the ware heat mass.

True that somewhat latter, it does effect the ware in a cooling cycle and the crystalline effect seems to show up in both reduction copper reds to oxidation crystalline as well as what you were referencing.

What I also noticed is that glazes start to melt at this same 1500 to 1800 deg range which would also affect the final lock down(freezing) of some glazes at that 1500 range

Wyndham

[Kathryn Reedy]

Hi,Norm,

I've searched to the end of the internet for this recipe (Shatz Blue Matte)with no luck! Can you share the recipe?

Thanks!


Norm Stuart said:

Another typical difference between slow-cooling at 185 F/hr and 50 F/hr with Jayne Schatz Blue Matte.