Fast firing

[vivk]

When I first started making pots quite  a long time ago, I was told that slow firing was the best way to fire. A bisque firing should last about 10 hours and a glaze firing about 8.

What is the current thinking on firing times- is it really necessary to go so slowly. Arent clay bodies more sophisticated now than they were then and can take more stress. I would like to know from the experts what is the quickest firing they have done and with what clay body?
 

[Kelly in AK]

There are so many variables it’s difficult to answer the question in a succinct way.

The quickest firing I’ve done is a wet pot off the wheel into a red hot kiln. Bisque ware in 30 minutes. The next was bone dry clay, specially compounded for fast firing. Room temperature to bisque ware in two hours. (The clay was awful to work with, and good for nothing but decorative pottery, but did have that one redeeming quality). Plenty of people have done “Raku” firings, glazed ware in an hour. 

Here goes, an attempt to address the meat of your question: You should fire bisque as fast as you can without the pots blowing up, cracking, the clay carbon coring, or leaving organics in it that will spoil the glaze. You should glaze fire as fast as you can without introducing or creating problems in your glazes and still have them come out looking awesome. 

The nuances here include, but are not limited to, your clay body and glaze repertoire, your kiln (gas, electric, digital controls, its capability to heat evenly, the speed it can rise in temperature...), how you produce your ware, your workflow, etc.

Industrial producers fire very quickly. Kilns are often conveyer systems where work is fed in and out at a set rate. The part of that process that we would consider “firing” proper (say, from 300°-2000° F) can be measured in minutes, not hours. Fast firing is not just relied on to save time and fuel, it can prevent some glaze defects occurring. The clay bodies often contain far less clay than plastic bodies made to form by hand. 

In my world, yeah, bisque firing takes around 10 hours and glaze firing takes around 8, not including cool down time. Soda firing takes between 10-12 hours because the kiln has to recover temperature after introducing the soda. If I’m reduction cooling it also adds a couple hours because I’m essentially down-firing. Some potters quick cool their glaze firings for the first 500,° others slow cool for matts and crystalline effects. The best speed is the one that makes the best pots, it varies. 

Generally, in terms of people making and firing their own pots, I don’t think much has changed in the physics of that. Bottlenecks are water smoking (200-250° F), quartz inversion (1063° F), and cristobalite (428° F, on cooling). Important consideration should be given to the burnout of organic matter (1000°-1600° F).

Something that has been revolutionary is the advent of digital kiln controllers. To me, the time it takes to fire is not really about the time the kiln takes firing pots, it’s the time I have to spend with the kiln. Pressing the buttons takes about a minute. My bisque firings take about a minute. 

[Callie Beller Diesel]

Going fast or slow will give you different results. Whether or not those results are desirable depends on what your end goal is. This is complicated by the fact that there’s more than one path to a given end. 

Understanding the changes your clay and glazes go through at different stages in the firing cycle can help you decide what speed is ideal for your situation. When considering speed, you also need to look at the thermal mass you’re trying to heat/cool. Each clay body/glaze combo is different, so there’s not any universal rule that everyone should use at all times with no exceptions. 

As far as clay bodies being sophisticated, there’s a limit to what you can do with materials that are dug out of the ground. We always have to deal with some LOI (Loss On Ignition) values of materials, and those vary according to your clay’s composition.  Whatever clay you’re using, quartz inversion needs to be eased through to some degree or other, unless you like cracked work. 

I know from experience that the clay body I use (a red stoneware that matures around cone 6-7), that it will more than tolerate a fast bisque cycle, and nothing comes out cracked or broken. But my glazes will pinhole or have a bunch of micro bubbles in them. If I go slow and allow more time for the organics (chemically bonded carbon, sulphur, etc) to burn off, the glazes look way better without having to resort to a multiple step glaze cycle.

In contrast, a friend of mine uses a white earthenware, but fires it multiple times. Because each firing increases the amount of mullite crystals in her clay body, she has to be increasingly conscious of quartz inversion. She can fast bisque with no problems, but she has to ease her final decal and china paint firings very slowly through that 500*C (ish) zone.

[oldlady]

have noticed the increasing interest in the speed of firing.   potters now talk about time instead of cones.   i will stick to what i know works, when i do not screw something up.  single firing is so simple, a firing may take 12-14 hours but that is the whole thing.   i can spray glaze a load  that fills  my very large L and L kiln in an afternoon.  fire it next day and it is done.  fire it that same day and it is done faster.  spraying glaze allows the work to be touched as fast as you put the spraygun down.  no waiting for drips to dry.

[neilestrick]

Far more glaze issues come from firing too fast vs firing too slow. Speeding up a bisque can prevent the burnout of organics, and speeding up a glaze firing can prevent good melt. If you were to speed up a firing, the final 200F degrees should still be kept slow to ensure good burnout and good melt, but below that you can really do whatever your clay can handle. I don't think crystobalite is an issue with most commercial bodies. I never slow down during quartz inversion and never have any issues with any of the stoneware and porcelain bodies we use in my studio. In an electric kiln, speeding up the firing will not save you much, if anything, in terms of firing costs. Yes, the firing will be shorter, but the elements will be using more electricity to fire faster. Plus the cost per pot of firing is really very low in most cases. Unless you have a small kiln, cooling times are where the real slowdown happens. Saving an hour in my big kiln won't make enough of a difference in the total cycle time to matter, because cooling can take 30+ hours.

[irenepots]

On 11/19/2022 at 12:19 PM, neilestrick said:

Far more glaze issues come from firing too fast vs firing too slow. Speeding up a bisque can prevent the burnout of organics, and speeding up a glaze firing can prevent good melt. If you were to speed up a firing, the final 200F degrees should still be kept slow to ensure good burnout and good melt, but below that you can really do whatever your clay can handle. I don't think crystobalite is an issue with most commercial bodies. I never slow down during quartz inversion and never have any issues with any of the stoneware and porcelain bodies we use in my studio. In an electric kiln, speeding up the firing will not save you much, if anything, in terms of firing costs. Yes, the firing will be shorter, but the elements will be using more electricity to fire faster. Plus the cost per pot of firing is really very low in most cases. Unless you have a small kiln, cooling times are where the real slowdown happens. Saving an hour in my big kiln won't make enough of a difference in the total cycle time to matter, because cooling can take 30+ hours.

Neil, what size kiln do you consider to be small?

[neilestrick]

On 11/20/2022 at 7:56 PM, irenepots said:

Neil, what size kiln do you consider to be small?

In electric kilns, anything under 7 cubic feet, so 18" wide kilns and short 23" wide kilns. They cool pretty quickly compared to the 7 and 10 cubic foot kilns.