Firing Schedule - no program, no kiln sitter

[SaigeDruid]

Hello all, I tried searching for a similar post and have found some asking about firing but most have to do with kiln sitters or using a programmer. I just purchased an older test size kiln that I can't find any information on. It has a tag that says "PRO-CRAFT" but I can't find a serial or model number anywhere. It has a pyrometer that goes up to 2500F and a dial that looks like one from an old stove. It has OFF, LO, HI then numbers 2-6 with a midpoint mark between numbers and between the LO and 2, and 6 and HI. I have been trying to find information on how to use this to fire my pottery. The only peephole is maybe 1/2" and doesn't seem likely I'd be able to make out a cone through it, but I haven't tried yet. Later today I am hoping to test the kiln and see what temp each mark on the dial reaches and how long it takes to get there so I have some idea of how to get the kiln to reach the temperatures I want.

What I need to know is how long to fire for. Everything I'm finding refers to programmable kilns that don't make sense or 3 tier manual kilns that say something like 'put on low 2-3 hours, med 2-3, then high til the cone melts'. I've seen a couple posts suggesting to refer to older pottery books about firing, but they don't list names of the books and the searches I do are all coming up with recent texts. This is a very small kiln that I was told heats up VERY quickly which is why I plan to check how fast the temp moves when adjusting the dial because especially for bisque firing, I don't want to explode things.

Other things of note regarding the kiln, it is a front loader, the interior is 8.5"W x 5.5"H x 8"D. Best I can tell, the brick is 3" thick. The clay I have is cone 6 stoneware and I do have a few cone 6 glazes as well as some underglazes. Any assistance on how to go about firing with this kiln would be greatly appreciated!

[Min]

Hi and welcome to the forum. 
Post some pictures of your kiln,  including the dials etc, someone might recognize it and it will help with suggestions.

[Bill Kielb]

Since it displays temperature I think it would be helpful to understand firing rates. If you understand basic firing in ceramics, you can apply it to any kiln, anywhere.
 

First segment

I would suggest the first segment to be the “make sure everything is dry”:  So room temperature to boiling, we usually go very slow so any moisture can slowly leave the ware rather than converting a lot to steam suddenly which is the explosive part. So slow climb or a hold at 180 f for instance can allow this off gassing. This is particularly true for greenware, but also can be a factor for freshly glazed items.

Now that out of the way, bisque firings generally rely on time and temperature so they are slower. Normally not more than 200f degrees per hour and range from approximately 10-12 hours.

Glaze fires can go faster so often up to 550f degrees per hour and can be approximately in the 4-8: hour range.

Approximating a cone and the last 200-250f. degrees of firing. (The final segment)

With Orton cones, you will see an end of firing rate. The middle column is 108 f degrees per hour. This is a nice speed to mature glaze and clay so in the last 200 - 250f degrees make your kiln go 108 degrees per hour and it should heat to the appropriate cone. This is true for whatever cone you are firing, bisque temperatures as well.

So as complicated as it sounds, most manual kilns are ramped up slowly to try and get the approximate 10-12 hour bisque as well as something like the 4-8 hour glaze. Kiln size and power all play into it which is why folks figure out for their kiln some schedule like  2 hrs on low, 2 hrs on medium, then high until shutoff but with a sitter their task is easier as the kiln will shutoff at a specific cone.

It appears this kiln you will need to supervise and manually shutoff.

So when you fire empty, I would suggest taking readings every half hour at your settings and observe the rates as you go. Record these. Toward the end of firing you will need a higher amount of power for a given rate of climb. Figuring out what gives you about 108-110 degrees per hour near cone six or 2232 - 200 or 2032.  So from let’s say approximately 2000 degrees onward you want to be in that 108 - 110 degree per hour range and fire to 2232 for cone six.

If you understand the why part, you will be able to craft your own best schedule for your kiln. Some folks do not like firing rates so they simplify it to a low medium high schedule. Both work effectively and take some observation.

A change of 1 degree per minute = 60 degrees per hour, so 2 degrees per minute is 120 degrees per hour or almost perfect to get your cone to bend whether shooting for cone 04 or cone 6.

Edited August 14, 2022 by Bill Kielb

[Kelly in AK]

Strongly agree with Bill’s post. Older elements take longer to heat than new ones, but then again a small kiln is less to heat. You’ll have to discover how long it takes your particular kiln to heat up. 

Definitely check to see if you can see a cone through that peep. By arranging the distance and height of the cone(s) you may be able to see it, or even  part of it, through a small hole. No matter what you need some cones in there to check that the pyrometer is accurate. 

I had a similar kiln, tiny, three switches, low medium and high. I dried things out with the lid propped 1/2” and one switch on low for several hours. If it  heats too much too fast, turn off the switch and shut the lid a couple hours then repeat. 210° F is the magical place things begin to explode. My experience is 180° for 6-8 hours keeps things from blowing up. 

It may be helpful to be familiar with the color to expect at various temperatures too. 

[Hulk]

Please add kiln glasses safeguarding against harmful rays and any superheated flying bits, anytime when facing a hot kiln, particularly when looking into the peep(s).

Excepting the rare occasions when I load pieces that are still wet, I run the kiln up to about 200F the night before, leaving out one peep plug, then go full gas (all three switches on high) the next morning, as the kiln is typically ~140-160F, hence was very warm all night and everything is bone dry.
When I do load wet or damp ware, I run up to 180F or so, leave a few peeps out and run the kiln fan, then double back after several hours and run up to 200F, and leave the fan on all night.

I'd fully avoid the fumes coming from any kiln, particularly an old/used kiln.

[SaigeDruid]

Thanks for the responses so far...I am still looking for an actual firing schedule. When I search online, I only find programmable schedules or a couple times where the instructions are for larger kilns and don't tell me what temps to get to. The only thing I've managed to figure out is to hold the low temp to dry everything first. So what next? It's dry, what temp do I bring it to, does it need to happen over a certain period of time? Do I hold temp at any point?  Do I hold temp once I hit the bisque temp or glaze temp or just shut it down? Do I just shut it off or slowly bring the temp down over time? This is the information I'm looking for and appreciate any help. If you have a book to reference, I'm happy to look at one that isn't regarding newer kilns.

[Bill Kielb]

2 hours ago, SaigeDruid said:

So what next? It's dry, what temp do I bring it to, does it need to happen over a certain period of time? Do I hold temp at any point?  Do I hold temp once I hit the bisque temp or glaze temp or just shut it down? Do I just shut it off or slowly bring the temp down over time? This is the information I'm looking for and appreciate any help. If you have a book to reference, I'm happy to look at one that isn't regarding newer kilns.

Yes of course …. It really has nothing to do about old or new kilns. Kilns can go different speeds based on their size and how much power was built into them. The important point is speed. So dry everything thoroughly first. Next segment for bisque go 100-200 degrees per hour right up to your bisque temp minus 200 degrees. No waiting, no holding. Then 200 degrees below your bisque temp go about 100 degrees per hour til you hit that temperature and shut the kiln off.

so from above

So for bisque: cone 04= 1945 degrees - last segment begins 1945-200 or 1745 degrees

• Start: Room temp => 180 degrees hold until dry (skip this if everything is absolutely bone dry)
• 2nd segment: 180 => 1745 - go 100-200 degrees per hour as you kiln allows, don’t go faster. ( at 200 degrees per hour this takes about 7.5 - 8 hours)
• final segment: 1745 => 1945 - go 100-110 degrees per hour, shut off at 1945. (At 100 degrees per hour this takes 2 hours)

Rates are a bit difficult but maybe think of them as driving a car, drive down Main Street at 20 mph, turn on the highway QQ  and go 50 mph until exit A…….. try not to exceed the speed limit, it’s radar and camera monitored all the way.

 

 

Edited August 16, 2022 by Bill Kielb

[Hulk]

I'm holding at ~1500F for half an hour to forty-five minutes, which seems to help drive off gassy/bubbly stuff in the red and black clays, particularly.
My kiln is set up with a downdraft vent, which, in theory, helps extend the life of the elements (less caustic gasses) and burn off the organic stuff (supply oxygen).

[Min]

6 hours ago, SaigeDruid said:

When I search online, I only find programmable schedules or a couple times where the instructions are for larger kilns and don't tell me what temps to get to. The only thing I've managed to figure out is to hold the low temp to dry everything first. So what next? It's dry, what temp do I bring it to, does it need to happen over a certain period of time? Do I hold temp at any point?  Do I hold temp once I hit the bisque temp or glaze temp or just shut it down? Do I just shut it off or slowly bring the temp down over time? This is the information I'm looking for and appreciate any help.

For the bisque firing: Start out with bone dry pots, if not then candle the pots to dry them out. Usual practice with candleing is to keep the temp below 180F for as long as it takes to dry the clay. Organic carbon burns out between 300-600F so don't blow through that range too quickly. Inorganic carbons burn out between 1290F through 1650F, again don't go too fast, especially if you are using a "dirty" clay, that is one of the darker claybodies. Sulphur starts burning out at the same time the inorganic carbons do but it can take up to 2100F for all of the sulphur to burn out. The most critical range is the 1290F - 1650F so go slow through this range. If you have a nice clean bisque firing you can often avoid glaze problems. Once you hit your desired temp for bisque shut off the kiln, no hold necessary.

For the glaze firing:  You want the body to be mature so don't go super fast even though your kiln will be able to. (given the element is okay) For glaze firing the glaze being used might have particular requirements, there isn't a one size fits all glaze firing schedule. Generally speaking matte glazes that are matte because of their chemical composition (and not just an underfired gloss glaze as some of them are) require a slow cooling period. (look up matte glaze firing schedules) Clear glossy glazes are fine without the slow cool to develop their attributes and the kiln can just be shut off after reaching desired cone / temp. There is a caveat to this though, often glazes will benefit from either a short soak near the top temp / cone or dropping the temperature by approx 100F and then doing a soak to smooth out pinholes. Length of soak varies but 15 - 20 minutes often works. There is some trial and error insofar as firing schedules to fire glazes without flaws. 

The colour chart that Kelly posted has good information as to what happens at certain temps to the clay.

BTW those analog pyrometers are not super accurate at higher temps so the colour chart will be helpful to use also. (wearing kiln glasses for eye protection)

[Pres]

On 8/14/2022 at 9:05 PM, Hulk said:

Please add kiln glasses safeguarding against harmful rays and any superheated flying bits, anytime when facing a hot kiln, particularly when looking into the peep(s).

Excepting the rare occasions when I load pieces that are still wet, I run the kiln up to about 200F the night before, leaving out one peep plug, then go full gas (all three switches on high) the next morning, as the kiln is typically ~140-160F, hence was very warm all night and everything is bone dry.
When I do load wet or damp ware, I run up to 180F or so, leave a few peeps out and run the kiln fan, then double back after several hours and run up to 200F, and leave the fan on all night.

I'd fully avoid the fumes coming from any kiln, particularly an old/used kiln.

After firing my fully manual kiln for years, I find that I have a slight bit of macular degeneration. Even though I tried to use kiln goggles while firing and peeking in at the cones or the color temperature. Now when I go out anywhere I use clip on sun glasses on my glasses. I heartily recommend either a welders shield or goggles.

 

best,

Pres