Fast Firing Cone 10 Reduction Test Glazes

[Mudfish1]

Folks;

I am developing some cone 10 gas fired reduction glazes, using test tiles in my gas kiln. I was reading Tichane's "Celedon Blues" and was amazed by the section where he talks about firing his test tiles in a small gas fired test kiln in only 2 hours! I was taught that reduction glazes need a good 8-12 hours to reach cone 10, so that everything can mature, all the glaze chemistry/body interactions have time to happen, gas bubbles can evolve and the glaze surface heal, etc.

Tichane seems to be saying that he can develop and test new code 10 reduction glazes with 2 hour firings, which seems crazy in light of what I had been taught.

Can anyone comment on this disconnect? For a gas fired reduction glaze, can a 2 hour test tile firing really look the same as a 10 hour full firing?

 

 

[Mark C.]

well maybe-off to bed now for tuna fishing all day in am-I'll post some details in a few days. I did a glaze fire to cone 11 in 5 hours last week  -but there is a story to tell.

[Bill Kielb]

2 hours ago, Mudfish1 said:

Can anyone comment on this disconnect? For a gas fired reduction glaze, can a 2 hour test tile firing really look the same as a 10 hour full firing?

 

Just my experience
Nothing miraculous about time other than it is the speed the wares and glaze can take without failure and the kiln can actually achieve that speed. So usually mostly for industry or ……. small kilns.  Even heating is very helpful as well.

Only a handful of metals react in reduction and maintaining a reduction atmosphere throughout is key for many results. Carbon trapping is achieved fairly easily and does not need to be a Smokey soot laden atmosphere, just one full of fine char.

If we could reach firing speeds of 600 degrees per hour we did and through countless firings - no issue so for our wares thermal conductivity  likely exceeded that speed. Quartz inversion for us was never an issue either, nor significant differences in top and bottom temperature of the kiln throughout much of the firing. . For us clay was tough and easily made it through these firings.

Unfortunately most kilns I have fired simply do not have enough power to maintain that rate in the desired atmosphere. Best reduction firing time I want to say was just under eight hours. These were all recorded every 15 minutes while testing and perfecting  a computer firing monitor to teach reduction essentials so we know exactly what speeds we were averaging and I still have all the data from the firings. To be clear though we were time limited mostly by the available power of the kiln and any timed periods used in the schedule to hopefully get a desired look.

Now would wares fire and look the same as those fired slower? Tough to say, the cooling rates likely differ a whole bunch so that could cause a significant difference in result. Also large kilns are limited in the atmosphere they can produce and maintain so I believe one would need to test and compare the difference with their larger kiln.

When I was reduction firing a bunch, each new firing contained a bunch of test tiles as folks refined their recipes and look. One can do a bunch with reduction but it’s not infinite. Often artful glaze strategy’s was where folks created unique looks. There are only a few metals that are reactive. Speeding up the test process probably would be useful though.

Edited July 30 by Bill Kielb

[Kelly in AK]

I think it’s the kiln and the mass of everything in it that needs 8-12 hours, not the glazes and clay per se. Most firing schedules I can think of call for reduction during a certain period of temperature rise, not necessarily for a certain period of time. “Between this cone and that cone.” Time is clearly important, cones are calibrated to rate of rise in the final 100°C, but I’m not surprised at all by the idea that glazes could be tested in a much shorter period of time than a typical firing takes. All those things you mention regarding chemical reactions and development happen in a small percentage of time relative to the overall length of firing. 

Sounds rather intense though. Like you would need a small well designed kiln to heat that fast evenly and undivided attention at the right times. Not the same throwing a few test tiles in your next firing. 

[davidh4976]

I do cone 10 reduction in 5-to-6 hours in my TDI downdraft conversion. It's a converted Skutt 1027 so not so big.  At the beginning of the firing, it's 600F per hour or more, slowing down a lot at the higher temps. If I had better insulation and a stronger burner, I believe it could be faster at the higher end although I would show it down anyway within the top 200F to let the glaze melt better. I let my glazed pieces dry for at least 24 hours in the New Mexico dry heat before firing to ensure there is minimal moisture in them, and they are typically no more than 1/4 inch thickness, all of which should help them survive a faster firing.

[neilestrick]

If the clay can handle it, then yes, you can fire that fast. The issue is whether or not the glazes will melt well with that fast of a firing, and if they will look the same as a slower firing. It totally depends on the glaze. As Bill said, the cooling cycle will also have a big effect on how the glazes look, because a lot of color/crystal development happens in the cooling. Little test kilns cool much faster than big kilns, so it's generally necessary to slow down the cooling to mimic the cooling cycle of the larger kiln you'll actually be firing work in. So yes, you can fire really fast, but IMO it's not worth it unless you know for sure how that glaze is going to respond compared to being fired more slowly in the big kiln, which would mean needing to fire a sample in the big kiln, which negates the benefit of of having a small test kiln...

[Mudfish1]

Clay Friends! Thanks for all the useful replies.

Bill, thanks for sharing your experience and thoughts - I suspect that test tiles can probably handle some very fast temperature ramp rates, but in the end you are right. The gas test kiln will be limited by how much heat gets generated, so unless it's way overpowered it will still take time to get there. Excellent point on the cooling rate - if the little test kiln can jump up to temperature fast, it's going to cool down just as fast, which may affect how the glazes look.

Kelly, thanks for your comments! Interesting that a lot of the glaze chemistry action happens fairly quickly anyway, so the heating rate may not be so important. However, your comment about cones made me wonder - even if I could build a small test kiln and get a 2 hour firing, what would my witness cones do? How would I know I was "done"? Would an Orton cone be accurate if I zipped from room temperature to 2300 F in 2 hours? 

David - thanks! I am in NM as well. My gas kiln is about the size of a 1027 but I haven't tried to push for a 5 hour cone 10 reduction firing. I've been on a 9-10 hour pace. If your glazes look good, I may try a shorter firing time and see what happens!

Neil, as always you are hitting the nail on the head - super quick firings are great for doing lots of glaze tests, but are sort of pointless if I find a great glaze and put it into a big 10 hour firing, and the glaze looks totally different. I think you are making the case for just putting a big set of test tiles in each full scale gas firing, and just accept that glaze development is going to be a bit slow!

Mark C - hope the tuna fishing is great, but you owe us a story about that glaze firing!

 

[Kelly in AK]

I can testify cones do wacky things when pushed to extreme speeds. Bending in random directions, defying gravity, sometimes just falling over. No bending, just melting at the at the base and falling over. Place lots of cones throughout the set, and see what happens. Early reduction really messes with them. Octopus tentacles.  Watch in conjunction with your pyrometer to see if any sense can be made of it. 

[Mark C.]

My story is a week ago I loaded my 12 cubic updraft and my 18 cubic Geil downdraft with glaze wares as always this past year (I'm now bisqueing in 35 cubic car kiln only. Been saving tons of gas $ doing this) . These two kilns take two days to cool . I usually light the pilot ring on both kilns late evening and get up at 5 am to light them both and go back to sleep or earlier if I wake up to pee. Then when I get up,  say 730  I turn them up again . Then the fire  get progesssing to a 12-hr+ fire from the light point at 5 am

Last week I did not turn on the ligfhts to see the geil gas guage and just cranked it (I have never done this) as I was 1/2 asleep . Went back to bed and got back to kiln at 8 am and it was near cone 10  so I turned it way down and nursed it till 10 am and then cone 11 was down. (that all in 5 hour not usual 12-13hrs) I down fired it another hour watching the digital pyro as it dropped about 20 degrees while with the oxy egen meter kept the redution the same no mnatter was the gas valve was at. then  about an hour at that I turned it a tad up and watrched it get almostr the the same temp as when cone 11 dropped as well as reducing bthe same by then it was 7 hr fire.The shortest I g=had done in about 4 decades. The kiln cooled two days and  at the opening I expected  chaos  in color and glaze depth. The foire was perfect in every way except I had a side crack in a large bowl form. I am thinking to fast for that form . all the wares looked great most ar smalls from meduim size serving bowls to  mugs all fine. so next time (this Saturday) I'll do it all over again and will fire a planned 7 hr fire  going thru quartzz inversion a bit slower than let it rip and slow oiot down at glaze melt. I have a friend who fire 6 hr geil fire all the time but his glazes are cooler and more matt than mine. Now as to your two hr fire and fast coil-it will afect color as Laguna clay does that every day of the week on test tiles for all tons of clay-the colors are washed out. The quick cool will do that. The deal is the glaze need a longer melt time at end to mature but that's all  it really needs unless its stoneware as it needs a body reduction mine stuff is all porcelain .