Vitrification

[Dave K]

I use Standard 563 stoneware and fire to cone 6. 

My firing schedule is

60  200   2.0

200   250   0

400   1978   0

100   2185   .10

My kiln fires hotter by about 35 degrees. The self standing  pryrometric cones are almost touching the kiln shelf or base they are on, so to me a little over fired.

Two firings ago I changed the last ramp rate to 150 from 100 . The vases all leaked water when I tested them. The pyrometric cones indicated a very slight under fire but only by a hair. The prior firing at 100 everything seem fine.

Last firing I changed it back to 100 and re-fire the ones that leaked along with 3 new ones. The re-fired ones are fine and don't leak now but the 3 new ones leak. They were all on the top shelf in no particular order. My kiln indicated a power failure on the bisque fire of the 3 new ones but the 04 pyrometric cones had just started to move so I didn't redo it. 

Any thoughts as to what my problem is?

Thanks, Dave

 

 

[neilestrick]

7 minutes ago, Dave K said:

I use Standard 563 stoneware and fire to cone 6. 

My firing schedule is

60  200   2.0

200   250   0

400   1978   0

100   2185   .10

First, this firing schedule is slower than it needs to be. The first hold is completely unnecessary unless you just glazed your pots and they're really wet. Even then, with a ramp of 60/hr, they'll dry out just fine by the time they get to 200. If they're dry, you could put the first ramp at 150/hr, without a hold. The second ramp is unnecessary, and I would put the final ramp back at 150/hr.

The bisque temp won't matter. Are these pots all from the same batch of clay? I'm thinking it's bad batch- one that wasn't weighed properly or wasn't mixed well enough. The refires tightened up due to being refired, but the new ones need a little more heat. The web site says the absorption rate at cone 6 should be 1%, plenty tight to prevent weeping, even at cone 5 it shouldn't weep. I would fire a test bar and do an absorption test to see how bad it is. If your glazes can handle it, fire a little hotter. I would also try firing hotter, but without a hold, to achieve cone 6. Sometimes heat work doesn't have the same effect as just going hotter. I've seen some glazes that are like that. Are your pots thick? it could be that they're not getting hot enough due to thickness. The cones are melting, but the heat hasn't penetrated the ware fully. Are the cones on the same shelf as the pots?

[Dave K]

Thanks for the reply Neil. I will change my firing schedule. With dropping the .10 hold ,going back to 150 and trying for a little hotter.  What temp would you recommend? 

The thought of the pieces being too thick is a real possibility. I once had a problem with the bottoms cracking because they were too thick. I make an effort to make sure the bottoms and lower part of the vases are no more than 1/2 inch thick and hopefully like 3/8 inch.  That being said I am pretty new to pottery so anything is possible.

As far as the clay being from the same batch, maybe. The dates on the boxes are the same but what be a batch number can vary. At my supplier that batch number are rarely the same on any given palate of clay. 

Would you explain what a test bar is and how to test absorption?

This has all been very helpful.

Thanks again, Dave

[neilestrick]

Make a bar of clay, about 6 inches long, 1 inch wide, 1/4 inch thick. Fire it with your pots. Weigh it. Let it soak in water for a day, then weigh it again. Math it out to see what % it has increased in weight, that's the absorption rate. You can also do a shrink test at the same time by marking a 10cm line on the bar when you make it, then measure it again after firing to see how much it shrank.

Shoot for 3/16 thick on your pots.

[Min]

@Dave K, if you are making pots with 3/8 to 1/2" bottoms I would do a couple test bars that thick as well as what Neil suggested. May as well test them all and see the difference a thick bottom makes. A bit more on absorption tests here, about 2/3 the way down, explains the math if you need it. I like going at 108F the last couple hundred degrees, works for my glazes and clay, we all have our preferences, trial and error. 

[Dave K]

What is an acceptable percentage on the absorption test especially on a test bar that is 1/2 inch thick? I know Standard rates the 563 at 1%, but I don't know how they arrive at that.

 

[neilestrick]

Acceptable at any thickness would be 2% or less.

[Mark C.]

Are cone 6 clay and glazes though to get a good match that vitrifes and is leak proof? There seems to always me someone with leak issues in this temp range.

This issue can happen at any temp range but there must be a post every few months on this issue at cone 6.

[Min]

^6 can be just as tight as ^10 , I think a lot of the issues that come up here are because people are using wide firing range clays and not firing to maturity. In Dave K's case having a really thick base is in effect doing just that, not mature for that thickness therefore either more heat/time to mature the base or needs a thinner base.

[Callie Beller Diesel]

Yeah, I don't have any issues getting my cone 6 not to leak, but it took a few tries when I was learning. But I think that might be the case any time you learn a particular clay. I found I needed a soak hold at the end of the glaze fire, and to push things a bit hotter than the manufacturer of the clay I use strictly recommends.

[glazenerd]

The research done in the 50-60's has been lost in modern pottery times. Lawrence, Ougland, Brindley and a host of other PHd's long ago studied, tested, and developed the chemistry and firing cycles of clay. For instance, standard feldspars reach their peak melt potentials at 2190F, yet it is common to fire under that temp. Ougland and Brindley also studied glass formation (including mullite) and found through X-ray diffraction that peak development occurs between 1200-1250C.  Once you cross the 1250C, cristobalite formation increases. Most kaolinitic bodies melt completely at cone 32. Doubt any potters will reach that temp, so pushing a clay body by a full cone is not an issue.

The reason you have to test each body as correctly recommended above is due to the variance in alkali molarity. Cone six bodies can run as low as 3.30% and upwards of 3.90% alkali molarity. 3.30 requires higher temps and longer holds, and 3.90 can be fired slightly higher with no holds. So the recommendations given above are correct. When switching to a new clay, the first thing on a potters list is to test fire several bars to dial in a firing schedule for THAT body. So now you have all the techno-blather to affirm the recommendations given.

For the record; on occasion you will find certain bodies that are so poorly formulated: that no amount of firing cycle corrections will cure.

Mark: this cone six body probably has more glass content than most cone 10's. Absorption: 0. Shrinkage: 10.5

[Chilly]

None of my ^6 mugs have leaked.  Until someone drops them and they crack.