Gas Kiln Firing Schedule Help

[tbaa]

My ignorance is probably getting old, but here is another question from me:  What increments in temperature are best when starting to fire.  I am dubious about the expertise I've received, and it is as below:

 

My test kiln has been fired once by a technician who came up from the company, and heres what he did:

Fired the kiln at 10:13 am.

 

 At 10:35 (22 minutes) the temp was up from mid 70s to 335C.

11:15 - 850C

12:45 - 991

1:05 - 1043

1:55 - 1152C

2:20 - 1192

2:25 - 1200C

2:40 - 1223C

2:49 - 1225C

2:50- 3:03 Kept temp stable : 1225-1222.  Roughly 14 min.

 

Turned off at 3:03

 

Temperature at  5:55 was down to 683C

 

From whatever I have read, this was not correct, but the tech was adamant.  But, I'd like to correct it, and understand the speed at which the temperature should rise. Some approximation at least.  

 

Test Kiln results will differ from the main one. Understood.  But the above process didnt seem to start as a 'gentle' one.  Even though I understand that the small kiln will reach cone 6 quicker, any suggestions on how to gauge this? 

 

Marcia Selsor's advice and John Britt's Charts help a lot, but it would be great if there was some reference for how much time it should take to go from whatever temp the loaded kiln is - 70s, 80s - to getting to the first stage of firing.  Just that basic start.  

 

When I used the big kiln, we took an hour to simply let the interior 'dry', and then one hour to get to 240C.  Then in we lit the second burner, and in twenty minutes we were up from 240 to 463 degrees.  Too fast?  or OK? That was a bisque firing, but even then, the same tech didnt follow any logical temperature increments.   

 

I hope the above makes sense.....?

 

 

 

[Marcia Selsor]

There are so many varying conditions to firing gas kilns from altitude, type of insulation, density of load, size, etc. you just have to wing it til you get to know the kiln. ONce there, you should be good for a very long time. IT may take 6-7 firings to get to know the kiln, maybe more. COnditions may not always be stable. I always paid attention to barametric pressure when I fired at 3000 altitude. YOu just need to get acquainted.

MArcia

[Marcia Selsor]

THis seemed to get buried. WIll bring it back to the foreground for you to find. YOu just have to get use to your conditions and the properties of your kilns. Marcia

[tbaa]

Found this site, which is very clear and articulate - and may be helpful for others who need the basics as I do: 

Linda Mosley. lmosley.com.  The firing chart below.  I will explore further, as I'm not sure if I understand the 12-18 hour part...

 

Temperature Range

Rate of °F. per Hour

Number of Hours

70 – 400°F

50 – 80

4 – 6

400 – 1200 °F

150

5 1/2

1200 – 1900°F

200

4

 

 

 

Total Heating Time

 

12 – 18 hours

Total Cooling Time

 

12 – 18 hours

[tbaa]

Thanks Marcia.  I will indeed keep trying until I get familiar with the kilns.  Linda Mosley's chart is a good start for me to understand how many degrees per hour - which is the part I wasnt able to find anywhere.  Especially with regard to the first Hour or two. Still not sure though, as The time for the initial temperature stage seems long to me, so I'm hoping to get more pointers, as clearly, I have an awful lot to learn.....  

 

Oh, also, not sure why the chart came out the way it did, and here is clearer version for anyone who was wondering:

TEMP RANGE                     RATE of F per HOUR                       # OF HOURS

70-400                                  50-80                                                4-6   (? surely not??)

400-1200                              150                                                    5 1/2

1200-1900                             200                                                   4

 

Total heating time 12-18 hours 

Total cooling time 12-18 hours

[Guest JBaymore]

Firing cycles are based totally on the ware IN the kiln and ITS needs.  So there is no such thing as a totally "one size fits all" firing cycle.  You have to adapt your firings to your clays and glazes to get the absolute best results.

 

Published cycles from others are really just starting points.

 

There are certain physical and chemical things that happen in firings.  THOSE points dictate a lot about firing cycles.  Those points are combined with the way a given kiln performs (even-ness of temperature and atmosphere distribution, for example) to develop an optimum cycle for YOU.  Best to spend time to understand what the demands of the clay and glazes are...rather than follow a "cookbook".

 

As a very BASIC example........... for bisque firing...... water changes state at precisely 212F (100C).  A  tiny bit of liquid water becomes a pretty large amount of steam (water vapor).  If that water is within a clay wall...... it is possible that it will not be able to escape thru the pores of a clay body fast enough... and will blow the wall section. 

 

The amount of water content that can "escape" thru the pores of a clay body wall will depend on the porosity (particle size and packing density) of the clay body itself to water vapor.  Different clay bodies react differently.  What is fine for MY clay body may not be for yours.  There's the start of difference in firing cycles. 

 

Then there is the even-ness of the temperature distribution at the roughly 212 F / 100C point in MY kiln versus yours.  The pyrometer only measures the temperature at the point the probe is located.  How hot is it elsewhere?  If the kiln is very even... then that is a good representative reading.  So I can use caution about getting just under 212F / 100C on that meter..... and be sure the water is totally dry before going above 212F/100C. 

 

But what if the hottest part of MY kiln is 25 degrees hotter than that pyrometer reading?   I'd better start being careful of rate of climb at a reading of more like 190F.  And what if the coldest part of MY kiln is lagging 50F below that reading.  Then if I use the pyrometer as a guage to increase the rate of climb when it reads 215F......... I might have issues in the COLD part of the kiln with steam.

 

And how does MY kiln compare with yours?

 

So......... take existing firing charts with a "grain of salt".  Good for general concepts at best.

 

best,

 

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

[Marcia Selsor]

Thanks Marcia.  I will indeed keep trying until I get familiar with the kilns.  Linda Mosley's chart is a good start for me to understand how many degrees per hour - which is the part I wasnt able to find anywhere.  Especially with regard to the first Hour or two. Still not sure though, as The time for the initial temperature stage seems long to me, so I'm hoping to get more pointers, as clearly, I have an awful lot to learn.....  

 

Oh, also, not sure why the chart came out the way it did, and here is clearer version for anyone who was wondering:

TEMP RANGE                     RATE of F per HOUR                       # OF HOURS

70-400                                  50-80                                                4-6   (? surely not??)

400-1200                              150                                                    5 1/2

1200-1900                             200                                                   4

 

Total heating time 12-18 hours 

Total cooling time 12-18 hours

 

As John says, all kilns have different requirements. When you say surely not, surely not on you test kiln. That is a micro kiln. You can't use Linda's schedule for it.

Marcia

[tbaa]

O.K.!  Well, enough wondering what to do I guess. will start with firing first thing tomorrow morning.  Thank you all for your help, AND patience.  

[Dick White]

In addition to the very good advice from John about being very careful about the water phase change at 212F/100F, there are several other physical inflection points that you must manage. (And by the way, the water-to-steam issue is not only for exploding greenware in a bisque firing, but if a coating of glaze is not completely dry, the interior water turning to steam may cause spots of the glaze to erupt and spatter off the piece.) In the bisque firing there is a second phase of water release at around 900F, when the hydroxyls in the clay molecule are released and reform into water. While this newly formed water is already steam, it needs to get out or it will shear off the surface of the piece. Then there is the silica inversion at around 1100F, where the ceramic goes through a sudden 1-2% size change. You need to slow the rate of temperature change through this phase (and it applies to both bisque and glaze firings). And next in a glaze firing you have reduction starting in the 1500F range. The reducing flame is, intentionally, less efficient and so the rate of temperature increase will naturally slow. Reduction is managed through both damper changes and gas pressure adjustment, and mismanagement of either can cause the kiln to completely stall. It is counter-intuitive, but you might need to turn down the gas to make the temperature rise. It all depends on how YOUR kiln design reacts. Finally, at the very end you need to slow the temperature rate of rise to around 150F/hour for proper penetration of the heat into the ceramic to achieve your intended cone.

[Guest JBaymore]

 Then there is the silica inversion at around 1100F, where the ceramic goes through a sudden 1-2% size change.

 

That's at 1063 F (  572.8 C ) to be precise (called alpha/beta quartz inversion).  It's a geometric rearrangement of all of the crystalline silica molecules in the clay, glaze, posts, shelves, and kiln structure.  Note that it does not happen to silica that is in the glass state. 

 

So you protect your equipment also by knowing about this particular item.

 

As Dick says... the are a number of other points to be aware about.  Reasons to "study up". 

 

best,

 

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

[Babs]

AND keep a log of all you do, others more learned have spelled it out, note the temp rise, gas input, color in kiln, temp range, hours into the firing schedule, and you'll work it out.

Go for a gentle start, you may find you can shorten your schedule as you gain experience with the kiln. I fired my little gas kiln with the pilot light only, it was adjustable until way into the firing as the burner whooped the temp up before the crucial changes mentioned above..

[tbaa]

two firings, and its not as difficult as I thought it would be!  Still, a LONG way to go, but the above advice helped.  Thanks!