Cone 5 plus hold = Cone 6 ?

[Rockhopper 163]

I think I've read somewhere in the forum that firing to cone 5 and adding a hold would take it to cone 6.  I think I also read that this can help reduce pin-holing with some glazes.

I've tried to search for that thread - but I either get every thread that as 'cone 5' in it - or I get nothing that has 'cone 5 plus hold' or 'cone 5 with hold' , etc.

Would like to know 1) How long would I need to hold for ^5 to become ^6 ?  and 2) What are the pro's & con's of doing so ? 

Is this something that's only done to address issues with specific glazes, or are there potential benefits in doing this as a general practice ?   [I'm working with stoneware - mostly Standard 112, and Amaco PC glazes - in an L&L E23S kiln.]

[liambesaw 2,810]

Cone 5 to cone 6 is a pretty big gap, but if you take a look here at the orton cone chart, cone 5 at 108⁰f/hr is the same as cone 6 at 27⁰f/hr.  This gives you a sort of hint as to how heatwork works.

 

[Min 3,529]

You can get to cone 6 by firing to 5 and doing a soak, but if it's to reduce pinholes I'ld suggest trying going hotter than ^5 then doing a soak then dropping 100F and doing another soak. With my kiln what gets me a perfect cone 6 is going to 2180F then holding for 15 minutes then dropping at 9999 to 2080F and holding for another 15 minutes. There are a lot of reasons for pinholes but if it's from the glaze not smoothing out this schedule works really well. If you can monitor the cone(s) at the end of the first soak you are looking for the cone 6 to be where you want it, the second soak doesn't add much heatwork. If you aren't comfortable monitoring the cone(s) toward the end of the firing then it will probably take a couple firings to get your kiln and schedule dialled in for how long a soak you need at the top temperature. (kiln goggles necessary for checking cones)

I use this soak then drop and soak schedule for just about all my firings. It's part of my slow cool schedule. If I'm firing an entire load of  either of my transparent gloss glazes I don't need to do the soak / drop and soak schedule but it doesn't hurt these glazes if I do include them with that firing.

Edited December 20, 2020 by Min

[Bill Kielb 1,179]

Just to add, common is cone 5 plus a 15-20 minute hold gets most folks to cone six. It’s heatwork stuff so the flux silica and alumina need enough time to melt. This is usually predictable to about a cone or so, so dropping to cone 4 plus forty minutes likely will not work.

Everything said above is super pertinent as pinholes can be tough and the intuitive idea that soaking at higher temp so everything smoothes out  often makes things worse.

One other reason I can think folks fire one cone less with a hold and that is to preserve color. Some underglazes lose or change their color as the temp goes higher so cone 5 + 15 min hold can be a strategy to preserve color and still fire effectively to cone 6 without having to hit that top temp.

Edited December 21, 2020 by Bill Kielb

[neilestrick 4,671]

1 hour ago, Bill Kielb said:

This is usually predictable to about a cone or so, so dropping to cone 4 plus forty minutes likely will not work.

I used to fire to cone 8 by going to 6 with a 40 minute hold, and it worked well. It increased my element life by about 25%, too. I tested soaking cone 4 to get to 6 and it took about an hour. Some of my cone 6 glazes worked fine, but some did not like it at all. They needed more heat, not heatwork. The glazes that worked with the long soak tended to be higher in frits, which melt early. Those that were higher in feldspars and low in frit didn't do well at all.

[Bill Kielb 1,179]

1 hour ago, neilestrick said:

Those that were higher in feldspars and low in frit didn't do well at all.

Yep, super hard to predict the amount of energy needed for the reaction. Not to mention how well had the clay vitrified in this whole process. Usually a cone is doable, outside that it gets sketchy and with respect to the claybody, even harder to know.

Seger only wanted to verify glaze melting points  with his cones so he set his flux at  0.3:0.7  K2O: CAO and varied alumina and silica

• cone 4.  - 0.5 AL: 4 SI
• cone  5 -  0.5 AL: 5 SI
• cone  6 -  0.6 AL: 6 SI
• cone  7 -  0.7 AL: 7 SI
• cone  8 -  0.8 AL : 8SI

Even by composition there are a couple jumps in there.

 

[liambesaw 2,810]

Speaking of all this, I am currently performing an experiment and am hoping an hour at cone 6 doesn't freak my clay out too much.  Will post results though I'm expecting quite a bit of bloat on the stoneware and some slumping with the porcelain.  You never know until you try, right guys?

[Bill Kielb 1,179]

5 minutes ago, liambesaw said:

You never know until you try, right guys?

Some clays have very sharp firing curves on the order of one to two cones higher. If you had the curves, you might be able to be more predictive.

[Rockhopper 163]

3 hours ago, Bill Kielb said:

Some underglazes lose or change their color as the temp goes higher so cone 5 + 15 min hold can be a strategy to preserve color and still fire effectively to cone 6 without having to hit that top temp

Ahh...  This sounds familiar - may be the context of the previous thread I saw.    I think someone also told me that the sample pieces on Amaco's website are fired to ^5 + 15minutes

I may just need to slow things down a bit.  My old manual kiln took about 10hrs for ^04, and around 14 hrs to hit somewhere around ^5-1/2 - ^6.  (Both fired at 2hrs all switches on low, 2hrs med, then high 'til the sitter drops.)  No Pyrometer to check actual temp's reached - so may have been doing more of a ^4 or ^5 + extended hold without realizing it.

 

[Bill Kielb 1,179]

9 minutes ago, Rockhopper said:

I may just need to slow things down a bit.  My old manual kiln took about 10hrs for ^04, and around 14 hrs to hit somewhere around ^5-1/2 - ^6.

Seems a bit slow for glaze actually. Once a kiln can’t make 25-50 degrees per hour in the last 200 degrees of the firing things seem to get funky as the glaze just simmmers.

[neilestrick 4,671]

57 minutes ago, liambesaw said:

Speaking of all this, I am currently performing an experiment and am hoping an hour at cone 6 doesn't freak my clay out too much.  Will post results though I'm expecting quite a bit of bloat on the stoneware and some slumping with the porcelain.  You never know until you try, right guys?

In my experience, stoneware clay bodies hold up to soaking better than you'd think. If the normal absorption is, say, 1.5%, there's some room there before bad things happen. Porcelain is more likely to have problems because it's already much closer to its melting point at the recommended cone. But there's also something with the fact that clay bodies don't go into melt like a glaze and therefore don't respond the same. Again, the lack of early melting ingredients like frits.

[glazenerd 1,641]

The 2190F peak comes from an U of I study done on feldspar reactions (KnaO) At this temp, potassium and sodium are spent; having achieved their mxiumum potential- past this temp it is all heat work. Ougland & Brindley (British Ceramic Society) measure the effects of heat work on a triaxel porcelain blend: 61% glass at cone 6 and 66% glass at cone 10- measured effects of heatwork. The 15-30 minute hold came from a study in Rio (Brazil) using a gradient kiln which found it takes heat an additional 30 minutes to penetrate an 1/2" clay wall. Edward Orton, Jr. Came up with 108F an hour climb when doing studies on iron bearing clays to ensure inorganic burn-out; which also proved effective in melting KnaO in the clay body: not just the glaze on the surface. I have long advocated that pinholes come from off gassing spars in the clay body which has not matured: in lieu of glaze that is directly exposed to the ambient kiln temp. The key is to either slow down st 2050F (key clay reaction temp) and climb 108/125F to peak with a hold, or hit peak with a hold so the spars in the clay stop out gassing. The hold temperature will vary pend ing kiln chamber size, brick thickness, and load density.

Tom

[Hulk 821]

Repeatability for results, what matters, eh?

That said, I'm seeing that a a full cone 6 is too much for some of the clays I'm using (or have used) vs cone five, slow drop to, then extended hold at -100F.

Any road, by the time small ^6 in the sitter allows the drop, it's too hot - having a pyrometer, notes, and cones on each shelf is helping.

[Rockhopper 163]

1 hour ago, Bill Kielb said:

Seems a bit slow for glaze actually. Once a kiln can’t make 25-50 degrees per hour in the last 200 degrees of the firing things seem to get funky as the glaze just simmmers.

Yeah the old one was really slow...  My first glaze fire with my new L&L was about 8:20, and almost every piece in it (mostly coffee mugs, with various PC glazes) had pin holes - so I'm thinking either the bisque or the glaze went (or both) may have been a little too fast.  Was considering doing the ^5 + soak, instead of a straight ^6 on the next one.

[Min 3,529]

Go by what your clay is mature at, if it can take a ^5 plus X amount of time in a soak and the glazes look okay then you're set. Since you are making functional ware test the claybody at the cone you fire to, be it 5 or 6 or somewhere in between.

Glazes melt along a range, there is a spectrum of both time and temperature upon which they both melt on the way up and solidify on the way down. The reason the drop and soak works on a matured claybody is that some glazes have a surface tension that is too great to allow glaze bubbles / pinholes / craters to heal over; lowering the temperature by between 100 - 200F and doing a soak gives time for the glaze to heal over without over firing the claybody. Some glazes are fine with just going to peak temperature/cone and shutting off the kiln, others are fine with going to near the peak and doing a soak, for others the drop and hold works really well. For all my glaze firings I program the last 200F degrees at 108F/hr regardless of whether I'm doing a drop and hold or not. I don't get pins/blisters/craters with my transparent glazes when I go to peak, soak and shut off the kiln. Same claybody with my other glazes and I get pinholes if I don't do the second hold at the lower temperature. Both fired to cone 6, glaze dictates which schedule to use.

Edited December 21, 2020 by Min

[liambesaw 2,810]

Well here's my test results.  The porcelain was fine for the most part, some tiny bloats on the bottom, it self glazed making the bottom fairly shiny.  

The stoneware did not like it, it started to melt and as it did, pulled itself apart.

Good test, reached cone 8 with a 45 minute hold at cone 6.  I'm not too surprised at the results because the red stoneware has no absorption at cone 6, and the porcelain is fairly clean so not really much in it to cause bloat.

 

[glazenerd 1,641]

Liam: looked in your original post but did not see: what cone was the clay rated to? 

[liambesaw 2,810]

1 minute ago, glazenerd said:

Liam: looked in your original post but did not see: what cone was the clay rated to? 

Both are cone 6 clays

[Bill Kielb 1,179]

6 hours ago, liambesaw said:

Good test, reached cone 8 with a 45 minute hold at cone 6.  I'm not too surprised at the results because the red stoneware has no absorption at cone 6, and the porcelain is fairly clean so not really much in it to cause bloat.

Ouch! Thought you were going an hour. This is consistent with some firing curves I have seen, some appear fairly abrupt actually. My thought at that time best to fire to maturity, over looks less desirable curve wise .

Edited December 22, 2020 by Bill Kielb

[liambesaw 2,810]

2 minutes ago, Bill Kielb said:

Ouch! Thought you were going an hour. This is consistent with some firing curves I have seen, some appear fairly abrupt actually. My thought at that time best to fire to maturity, over looks less desirable curve wise .

Well at 45 minutes I took a peek and saw the large bowl beginning to collapse, so I ended the experiment early, was a 24 inch bowl and could have slumped into the brand new elements.

I knew it would overfire, I just wanted to see what it would look like overfired, and especially what this glaze looked like overfired

[Bill Kielb 1,179]

6 minutes ago, liambesaw said:

what this glaze looked like overfired

Makes me realize the test kiln is a really handy thing to have actually. Never gave it much thought when looking for an effective glaze range until now.

[liambesaw 2,810]

Just now, Bill Kielb said:

Makes me realize the test kiln is a really handy thing to have actually. Never gave it much thought when looking for an effective glaze range until now.

That would be nice. But I also wanted to test out the new elements... Their first trip to cone 6, actually kept up with the firing schedule!

[Bill Kielb 1,179]

4 hours ago, liambesaw said:

But I also wanted to test out the new elements..

Holds at peak temperature  are reported to be hard on elements but I suspect a hold at six to get to eight is still better than straight firing to eight.

Edited December 22, 2020 by Bill Kielb

[liambesaw 2,810]

1 hour ago, Bill Kielb said:

Holds at peak temperature  are reported to be hard on elements but I suspect a hold at six to get to eight is still better than straight firing to eight.

Yeah I suppose it's all the same as far as the elements are concerned.  Doubtful heatwork only applies to ceramics!

Needless to say, the new elements are working cherry.  Now I just need a few non-rainy days so I can glaze the rest of this stuff!

[Chilly 908]

On 12/20/2020 at 8:52 PM, Min said:

but if it's to reduce pinholes I'ld suggest trying going hotter than ^5 then doing a soak then dropping 100F and doing another soak

Thanks @MinThat sounds like just what I need.  I've printed that bit out and will try it when (if) I ever get back to the community studio.  (My little kiln won't go to ^6.