Unvented Kiln

[MikeFaul]

So, I'm loading our glaze kiln (Skutt 1231 PK) 31 inches tall, 28 wide, 11.5 cubic foot firing chamber. And the kiln slides on the floor. Now it's on casters, but the casters are suppose to be locked down, and even if they aren't the Envirovent coupler is spring loaded and should drag on the floor... Weird!?!

 

So, I get down on the floor to inspect this oddity, and find... what do I find? The Envirovent coupler is disconnected, not engaged, laying on it's side far away from the vent holes in the floor of the kiln. Meaning we have been firing our kilns unvented. How long? Unknown. Did I say Kilns? Well, me rolls over to inspect Meshach (Our glaze kiln is called Shadrach). Meshach, who does most of our bisque firings is... What's this? Disconnected as well?!? I have no idea how the couplers became disconnected as they are spring loaded and the spring is fairly hefty.

 

So, this means both glaze and bisque have been fired unvented! What are the implications of unvented firing? How might an unvented firing manifest its self in glaze faults? We've been having a series of faults, and now I'm wondering if I might have found the problem...

 

Blistering?

Running?

Color Distortion?

Orange Peel?

 

Your thoughts, other than... Go post this in the thread on "It's OK to make mistakes!" :-)

 

 

Merry Christmas!

 

Mike

[clay lover]

I would like to know more about what you are seeing as to glaze differences, and evenness of firings. Cones are always hotter in center at thermocouple. Do you have any idea how long the vent has been disconnected? When the glaze issues started? I do not have a vent, my kiln is in an outdoor shed, but am considering one to even out firings and maybe improve glaze surface.

[MikeFaul]

Our temps are even and consistent by location (vertical & horizontal), it's not a matter of temperature. We consistently achieve a cone 6 firing with a consistent bend on the 6 cone regardless of placement. 

 

I have no idea how long the venting was disconnected, we first started seeing serious faults about 4 weeks ago, and they have gotten progressively worse. We suspected it was the clay body, which we introduced about the same time as the faults. But, now I'm not as sure of that as a I was yesterday. The coupling may have been disconnected during a clean up, but no one is fessing up.

 

Often, I am finding these issues are multifaceted, and have solutions which may include clay / glaze combination changes, firing schedule changes, loading methodology, and venting / circulation during firing. The issues we've had include those in the original post. And, I'm inclined to replace the clay body rather than go on a wild goose chase trying to make a marginal clay body work. I'm mostly interested in gauging the venting issue's impact on the faults as it will help in developing procedures and test plans for our new clay bodies when they arrive. I may want to fire unvented and vented and do a comparison when the test kiln arrives too. 

[clay lover]

I fire unvented, always have, with a Skutt 1027, some variation in cones, but none of the faults you mention. I would be looking at the clay.

One question, what do you bisque to? Has that changed? Before you change clays, what about trying a higher bisque temp?

[timbo_heff]

Vent can make a difference in bloating (maybe what you mean by blister). Lots of air during bisque fire can prevent that fault.

Some colors can be dull when not vented during glaze fire: red/orange colors especially.

[Guest JBaymore]

So, I'm loading our glaze kiln (Skutt 1231 PK) 31 inches tall, 28 wide, 11.5 cubic foot firing chamber. And the kiln slides on the floor. Now it's on casters, but the casters are suppose to be locked down, and even if they aren't the Envirovent coupler is spring loaded and should drag on the floor... Weird!?!

 

So, I get down on the floor to inspect this oddity, and find... what do I find? The Envirovent coupler is disconnected, not engaged, laying on it's side far away from the vent holes in the floor of the kiln. Meaning we have been firing our kilns unvented. How long? Unknown. Did I say Kilns? Well, me rolls over to inspect Meshach (Our glaze kiln is called Shadrach). Meshach, who does most of our bisque firings is... What's this? Disconnected as well?!? I have no idea how the couplers became disconnected as they are spring loaded and the spring is fairly hefty.

 

So, this means both glaze and bisque have been fired unvented! What are the implications of unvented firing? How might an unvented firing manifest its self in glaze faults? We've been having a series of faults, and now I'm wondering if I might have found the problem...

 

Blistering?

Running?

Color Distortion?

Orange Peel?

 

Your thoughts, other than... Go post this in the thread on "It's OK to make mistakes!" :-)

 

 

Merry Christmas!

 

Mike

 

Mike,

 

So to track this down defect wise .... you have to think what "no venting" means.   Then take the ceramic physics and chemistry in firing and match the two up to see what "pops".  Like being a doctor.  Take the patient history, look at the symptoms, maybe runn a few tests, and see if you can track it down.

 

No active venting.  So the kiln chamber had a far more static atmosphere than it would have if the vent wa operating.  So what does that mean?

 

There was nothing helping get things like gases and fumes evolving out of the the clay body out of the kiln.  There was no good supply of fresh air (and hence oxygen) into the kiln.  There was reduced convective movment of gasses withing the chamber.

 

SO.............. I'll list one potential example here.......

 

If there was not a lot of fresh air circulating... there was not a lot of oxygen available for reactions that needed oxygen to go to completion.  So for example, any carbonaceous contaminants in the clay body MAY not have been oxidized fully by the time the kiln had dropped below the reactive temperature.  This fact might or might not be visible in any way on the fired bisque wares.

 

So it is possible that in the body, there remained stuff that could still be oxidized later in the up cycle in the following glaze firing.  That action will cause more outgassing IF the surface of the glaze layer has not yet started to sinter or melt and become gas impermeable. Outgassing thru melting glaze can cause pinholes, blisters, and craters.

 

Another potential aspect of this is that the compounds that are looking to be oxidized, in a fast climbing glaze firing, may find that they can steal an oxygen atom/molecule from another compound in the clay body before any oxygen from the kiln atmosphere reaches it thru from the clay surface.  So it can sometimes cause reactions like reducing the iron oxide in the clay body to the FeO state.  This causes a color change in the iron... but more importantly FeO is a powerful low temperature acting flux on silica.  So this can change the way that the body is melting. Which can change the color, maturity, and reaction with overlying glaze.

 

You can see many differnt potential scenarios depending on the exact clay bodys, the exact firing cycle, and the exact fiollowing glaze usage.

 

Another faster example...... sulphur concentrations in the clay body can readily cause color changes with certain materials. If there is not enough O2 present in the bisque, you don;t get all the Su changed to sulphur di and tri oxide. It then comes out on the glaze firing... to casue color changes.

 

Blistering, orangepeel surface, and color changes are all very possible from this lack of venting.  I could even see the running scenario in certain situations... but less likely..... OTHER than the lack of convective heat transfer possibly causing som "hot spots" in the kiln that you have no idea exist... because you typically do not have cones all over the place.

 

best,

 

.........................john

 

PS:  Want a real eye opener........  Take about 30 -40 cone packs and scatter them throughout your glaze firing kiln.  Fire the load.  Check out the cones.  LOTS of assumption often go out the window there. 

 

Want an even bigger eye opener?  Make some cone packs up that have a lower end point cones in them.  So maybe if you are firing to cone 9-10.... a set of packs with only 012, 010, and 08.  Then another pack that has 02, 1, 3.  Then another that has 4, 6, 7.  Then anotehr that has 8, 9, 10.  Make many multiples of these.  Plan a firing load.  Mark the exact locations that you will again scatter many, many cone packs in.  The difference this time is that while you are intending to eventually go to cone 9....... you will use your main single sitter or controller to fire first to cone 010... and stop.  The let it cool and open the kiln and check the cones.  Replace with the next  cone pack up the sale...... and fire to cone 1 and stop.  repeat this all the way up.  You might be amazed at how uneven the kiln is at intermediary times in the firing...... that evens out only at the end.... and your finish fire cone packs all look nice and even..... and you wonder why some results are different. 

[neilestrick]

This is the major downfall of the vent-cup-wedged-under-the-kiln venting systems. It takes very little to know them out of place. I would say that about half the kilns I work on at schools don't have the cup properly aligned. L&L figured this out a long time ago and screws their cup to the stand or kiln so it can't move. So you should check that vent cup every time you fire.

[MikeFaul]

This is the major downfall of the vent-cup-wedged-under-the-kiln venting systems. It takes very little to know them out of place. I would say that about half the kilns I work on at schools don't have the cup properly aligned. L&L figured this out a long time ago and screws their cup to the stand or kiln so it can't move. So you should check that vent cup every time you fire.

Yup already thought of this, modified our firing protocol to check the cups, and if multiple potters are assisting with the firing each is responsible for checking and verifying. I think it's even easier to dislodge them because we have caster mounts, basically a big steal box with cross members that support the kiln, the box has steel casters, if they are not locked down the temptation to push a kiln out of the way is great. So, now we're checking the caster locks with each loading and unloading of the kilns too.

[MikeFaul]

 

So, I'm loading our glaze kiln (Skutt 1231 PK) 31 inches tall, 28 wide, 11.5 cubic foot firing chamber. And the kiln slides on the floor. Now it's on casters, but the casters are suppose to be locked down, and even if they aren't the Envirovent coupler is spring loaded and should drag on the floor... Weird!?!

 

So, I get down on the floor to inspect this oddity, and find... what do I find? The Envirovent coupler is disconnected, not engaged, laying on it's side far away from the vent holes in the floor of the kiln. Meaning we have been firing our kilns unvented. How long? Unknown. Did I say Kilns? Well, me rolls over to inspect Meshach (Our glaze kiln is called Shadrach). Meshach, who does most of our bisque firings is... What's this? Disconnected as well?!? I have no idea how the couplers became disconnected as they are spring loaded and the spring is fairly hefty.

 

So, this means both glaze and bisque have been

 

 

Merry Christmas!

 

Mike

 

 

Mike,

 

So to track this down defect wise .... you have to think what "no venting" means.   Then take the ceramic physics and chemistry in firing and match the two up to see what "pops".  Like being a doctor.  Take the patient history, look at the symptoms, maybe runn a few tests, and see if you can track it down

Another potential aspect of this is that the compounds that are looking to be oxidized, in a fast climbing glaze firing, may find that they can steal an oxygen atom/molecule from another compound in the clay body before any oxygen from the kiln atmosphere reaches it thru from the clay surface.  So it can sometimes cause reactions like reducing the iron oxide in the clay body to the FeO state.  This causes a color change in the iron... but more importantly FeO is a powerful low temperature acting flux on silica.  So this can change the way that the body is melting. Which can change the color, maturity, and reaction with overlying glaze.

 

You can see many differnt potential scenarios depending on the exact clay bodys, the exact firing cycle, and the exact fiollowing glaze usage.

 

Another faster example...... sulphur concentrations in the clay body can readily cause color changes with certain materials. If there is not enough O2 present in the bisque, you don;t get all the Su changed to sulphur di and tri oxide. It then comes out on the glaze firing... to casue color changes.

 

Blistering, orangepeel surface, and color changes are all very possible from this lack of venting.  I could even see the running scenario in certain situations... but less likely..... OTHER than the lack of convective heat transfer possibly causing som "hot spots" in the kiln that you have no idea exist... because you typically do not have cones all over the place.

 

best,

 

.........................

No active venting.  So the kiln chamber had a far more static atmosphere than it would have if the vent wa operating.  So what does that mean?

 

There was nothing helping get things like gases and

 

PS:  Want a real eye opener........  Take about 30 -40 cone packs and scatter them throughout your glaze firing kiln.  Fire the load.  Check out the cones.  LOTS of assumption often go out the window there. 

 

Want an even bigger eye opener?  Make some cone packs up that have a lower end point cones in them.  So maybe if you are firing to cone 9-10.... a set of packs with only 012, 010, and 08.  Then another pack that has 02, 1, 3.  Then another that has 4, 6, 7.  Then anotehr that has 8, 9, 10.  Make many multiples of these.  Plan a firing load.  Mark the exact locations that you will again scatter many, many cone packs in.  The difference this time is that while you are intending to eventually go to cone 9....... you will use your main single sitter or controller to fire first to cone 010... and stop.  The let it cool and open the kiln and check the cones.  Replace with the next  cone pack up the sale...... and fire to cone 1 and stop.  repeat this all the way up.  You might be amazed at how uneven the kiln is at intermediary times in the firing...... that evens out only at the end.... and your finish fire cone packs all look nice and even..... and you wonder why some results are different.

 

I just ordered several boxes of cones..., I was hoping the venting issue would solve the problem, but we unloaded a glaze kiln tonight. It was a mixed load of Standard 181 and 240G. All were bisque together to Cone 05 on slow. The kiln was unvented. All wares were platters except one bowl, the only piece made from 240g. Only the 240g cratered. None of the 181cratered. So, I'm back to organizing a clay / glaze test matrix for when the new test kiln arrives. Well just have to do this old fashioned way, one clay / glaze combo at a time.

[neilestrick]

You could invest in one of THESE, and mount it to the side of the kiln. Here it is mounted to the side of my big Davinci:

 

 

The other great thing about these is that they are adjustable, so you're not drawing more hot air out of the kiln than you need to.

[Mark C.]

Mike

Electric Kiln vents are a somewaht new thing- for 40+ years they did not have a vent-other than leaving the lid cracked.

I know they can improve results just saying.

Mark

[MikeFaul]

You could invest in one of THESE, and mount it to the side of the kiln. Here it is mounted to the side of my big Davinci:

 

Side Vent.jpg

 

The other great thing about these is that they are adjustable, so you're not drawing more hot air out of the kiln than you need to.

I have the Skutt version of one of those... That's what was disengaged, I suppose this one screws onto the kiln and can't be disengaged, which is a plus...

[Guest JBaymore]

Mike

Electric Kiln vents are a somewaht new thing- for 40+ years they did not have a vent-other than leaving the lid cracked.

I know they can improve results just saying.

Mark

 

Mark,

 

But for a long time people also had "firing issues" that they had to deal with. 40+ , 30+, 20+ years ago we had less knowledge about the technical side than we do now. Yes, you CAN fire just fine in an unvented electric kiln. NO QUESTION of that. But it makes the potential of problems for the ceramist higher. So the ceramist has to understand exactly how they need to approach that firing in many ways to have high success rates. The vents help to prevent problems, and make things easier.

 

Not to mention make a healthier studio environment.

 

For example... a little too much ware crammed in there in the wrong ways... and then you DO get some issues that will only show up in the glaze firings later. If you have the technical knowledge to KNOW that the glaze defect you are seeing is coming from the bisque firing........ then you know where you screwed up. But if you don't..... you waste a lot of time trying to figure out what your glaze changed, your glaze firing schedule is screwing up, your clay supplier changes something, and so on.

 

And the idea of doing so much bisquing in an electric kiln is a somewhat new phenomoma also. It really has taken off only in the past 25+ years or so. It is one of the BEST accomplishments of the computerized controllers. Instead of firing your gas or wood kiln to bisque and pay lots of attention to it.... you can sort of "load and forget". Before the controllers, people who had them tended to bisque in their gas, oil, or wood kilns.

 

There actually is an argument to do so... because of the potential of very good airflow in active draft kilns. Not to mention environmentally it (usually) is far more efficient to fire a single 100 cubic foot kiln once than a 10 cubic foot kiln 10 times. And with the losses involved in burning fossil fuels and then converting the steam to electricity via mechanical means (generators) and then the transmission losses, and then the typically underinsulated electric kiln walls...... that makes electric firing a little less environmentally attractive also.

 

I bisque here at my home studio in my gas kiln. It works best for me. At the college we typically bisque in the electric kilns.... becasue it is EASY to manage the huge workflow. But we also make it clear to students that bisquing is not somehow ONLY to be done in electric kilns.

 

best,

 

...............john

[Mark C.]

John 

I to bisque in my gas kiln and only use my electric to bisque in when the gas kiln is in use. Its unvented  and outside and only ever (rarely) gets used for bisque.

For me its way easier to load a car kiln than bending over an electric. As I am not firing any glazes in that electric and its outside venting makes very little sense for my application. If my whole operation was firing in electrics I'd be venting away I'm sure .

I'm still a cone 10 gas man-not giving this up-maybe in the next life.

Mark