A Couple Of Odd Things That Occured In My Bisque Firing

[Guest]

For the most part I'm pleased with how the bisque firing went. That said two things happens and I'm more curious and not upset that it happen.

 

This one I fired upside down since there will be glaze on the base. I don't know how well you can see but the inside on the clay is a different color than the outside. This also happen to a mug that was fired right side up but only the rim was a different color. Based on the cones it looks like it was properly fired to ^04 and this was in the middle shelf. (rim is waxed)

 

 

This one is weird, the pot chipped right where I had a notch for a spoon (sugar bowl). What's weird is the fragment was found inside the pot. This wasn't chipped before I placed it in the kiln

[Benzine]

Hmmm...

 

I have had the same clay body turn out different colors, depending on its shape, how I stacked everything, etc.  I am not an expert, but I believe this is due to gases coming off the clay body.  It should not affect anything during the glaze firing.

 

In regards to the phantom chip, I would guess that the chip was started, before loading and just not visible.  Then when it was fired stress built up, and was released at that point, launching the chip towards the inside.

 

Ceramics is awesome.  So much science going on.  And if none of that can explain it, just blame the Kiln Gods...

[Pres]

Inside outside thing, I usually attributed to some sort of reduction going on with the trapped gasses, especially so with some clays.  The other problem on the sugar jar looks like a bit of spawling, It may be that the rim area was not quite chamoised enough, and that left that area weak.

 

Just my personal thoughts Ben, but then again others will chime in to help you out.

 

 

best,

Pres

[Babs]

Think chip inside came from notch still hanging loosely from lid,

Other chip swhere un your kiln still😉

[neilestrick]

Different colors due to different amounts of oxygen. Totally normal, nothing to worry about. The chip could have been due to an impurity popping out during the firing. It just happened to pop into the hole in the lid.

[Guest]

Thanks for all the feed back

 

Good to know the color isn't a big deal.

 

The chip is from the sugar bowl since it fits in divot nicely.

 

Neil, that is the most logical assumption as to what happen. What are the chances, I should have played the Lotto that day.

[High Bridge Pottery]

Why is the rim shiny?

[glazenerd]

Joel:

The rim is shiny because there was enough off-gassing of the potassium to self glaze it.

 

 

In this 3/4" test bar you can see the mason/dixon line where the white area had completely off gassed and matured, while the tan'ish area still shows the discoloration of the off-gassing potassium. Potassium and sodium, both become gas as they melt: potassium much more so than sodium. Potassium is the primary flux in stoneware: the reason stoneware has a disproportionate amount of pin holing issue over porcelain. So both the discoloration and the shiny rim are caused by potassium off-gassing.

 

This piece will be prone to pin-holing: I am seeing white dots in the bisq body. Those white dots are feldspar minerals, that are greater than 40 mesh. 40 mesh and lower are visible to the human eye. Might be camera flash, but they look like white specks to me.

 

 

I am seeing more and more large particle feldspar minerals showing up in fire clays: so I fully expect to see more and more issues with pinholing and craters in glaze due to their presence in the clay body.

 

Nerd

[Guest]

Nerd

 

On one pot the exposed rim was lighter, a second pot (the one pictured above) the rim was sitting on the shelf and the interior was lighter. A third item was also sitting on the rim and the interior is the same color as the exterior. Both second and third were in the middle shelf near the center. To be clear the open end of the later two was sitting on the shelf and the insides where not "exposed" to the outside..

 

If the clay was off gassing it would need to be on the exterior of the pots, not the interior. Since only one pot has a different color interior I respectfully suggest that you may be incorrect. Yes I know you are way better informed about this then I am so I apologize if I'm wrong

 

May I suggest another reason why the rim is shiny.  The rim was waxed and the flash bounced off the rim.

 

HBP, sorry I didn't reply to you sooner.

[glazenerd]

Ron:

It would take me ten paragraphs to explain sublimation, and I have not had enough coffee yet.

Nerd

[Guest]

Thanks, I may learn something. I do have a sublimation printer so I'm guessing it might be similar.

[Sheryl Leigh]

Inside outside thing, I usually attributed to some sort of reduction going on with the trapped gasses, especially so with some clays. 

 

 I often notice glaze coloration differences between the outside and the inside of pots and had attributed it to heat differences, but this makes more sense.