Gas kilns/Firing results...

[preeta]

At our school we have 2 updraft gas (updraft small, updraft medium) kilns and a year old downdraft big kiln. 

We fire to cone 6 reduction but can get to a soft 7.  

School is out. My aging brain doesn’t remember details as I didn’t write it out so I can’t tell you make and size of kiln. 

School fires updraft small (small is relative here.  I could sit inside the kiln and stretch my legs) and down draft big regularly.  The medium one gets used once or twice a semester at the end.  

Both the professor and assistant are intuitive firers. They don’t pay so much attention to the temp reads as much to the cones and the colour of the flames, the smell and sound. Which is what I am trying to learn from them so alas I didn’t note down kiln details.  

So here are my questions. 

1. Is there a difference in the quality of reduction between adown draft and updraft kilns? That is what I am finding.  All the kilns get the same type of reduction - body around 018 and two reduction cycles. After the second reduction (both usually half hour each) there’s I think a half hour more of cleaning out the atmosphere and then the kilns are turned off.  

The glazes behave differently between the updraft and down draft. The down draft is not reduced that much.  For instance in some big bowls depending on placement the inside won’t be reduced but the outside would. Which is great. Exactly what I want. Even the outside is not as fully reduced outside as it would in the updraft kiln.   

I am trying to understand why there is a colour difference? Due to a new kiln vs  an older kiln? The teacher is still getting to know the down draft kiln mostly playing with the flue.

2. Heat work vs temperature. 

The big updraft Fired rarely, takes a long time to reach temperature. It stalls at temperatures like 1900 and then takes a much longer time to reach the next level.  So it definitely fires a few hours longer than the other two kilns. No matter how they play with gas and air it’s hard to get it to climb at the rate the staff wants.  

That means more shelves to clean as all glazes have melted quite a bit. The colours also come out darker. 

Its like Goldilocks.  One kiln has too little colour change, one has too much colour burnout and the other has just right.  

Which I find exciting because I want to use the use that difference to get 15 different surfaces out of my 5 favourite glazes. So it’s all testing next semester. 

Now about maturity. What is the maturity of glaze.  Is it the colour that tells you.  Obviously you can definitely tell underfire.  But what is overfire? Melts than usual and darker colour  results?

Does a ^6 have to hit^6? Let’s say the kiln doesn’t go beyond ^5 BUT it fires a few hours longer than a kiLn that does hit 6.  The prof.  Always continues firing till the cone is down. Cone 6 no matter what the temperature reading is.  

Can you stop when come 6 is soft but not fully down yet? To avoid the Melty and wash out of colour.  

In other words the stalling - kinda a forced hold - could it mature the glaze.  

My whole premise is based on this little fact I learnt.  Ken Matsuzaki woodfires mid range.  He says since he fires for 2 weeks he doesn’t try to reach high temperatures.  I have no idea about his glazes. But I notice some of his surfaces look like higher temperature woodfire results.  He probably doesn’t mix high fire glazes and then fire to mid range - which I assumed he did. Which must be wrong. 

3. Is there a subtle underfire result? Can you visually tell if the piece is slightly underfired? Will your eyes be able to tell or do you have to do the vinegar test or send a piece to the lab?

if you have made it this far - thank you. Now that the semester is out and I am not exhausted from frantically throwing, all those niggling queries have returned as tsunamis.  

I am so grateful you all are here to answer. I have a place to go.   

[Pres]

I have moved this strand to Equipment Use and Repair as it seems to be more relevant than being  In the Studio.

 

best, 

Pres

[neilestrick]

There is no difference in reduction depending on the type of kiln. It's caused by how it's fired. But downdrafts and updrafts have to be handled differently, so if you're seeing different results it's the teacher's fault, not the kilns. That said, not every kiln is designed well, and you can get areas where the reduction doesn't penetrate as well, or hot spots or cold spots. One thing that can add consistency is to reduce for about 45 minutes, with the kiln stalled out. I would also reduce at a higher temp, like around 012 if you're firing any shino glazes, or 08 if not. 

If one of the kilns if over-firing and the glazes are melting too much, that's a result of not shutting off the kiln soon enough, not the longer firing. If your teacher is watching the cones, then it doesn't matter how long it takes. Cone 10 is cone 10, whether it took 8 hours or 12 hours.

Cones measure heat work, not temperature. It's temperature over time. The longer it spends at temperature, the more heat work is developed. So yes, you can hold temp when cone 5 drops and in 20-30 minutes, cone 6 will drop, and so on.