Bisque ware has discolouration in the middle.

[Jeanetta]

Hi!

just opened my cone 06 bisqueload and I noticed that some of my plates have discolouration in the middle.

I stacked them about four on top of each other in the firing.

this is my first year firing my own Pottery (electric Kiln skutt 1027) and my first time ever firing plates. Other possible barriers are it’s also my first time using my new kiln vent and as well it’s quite cold :about -5 Celsius outside air

is my suspicion that stacking these on top of each other caused the discolouration?

does this white patch mean it’s under fired? Does anyone think this will cause troubles in the glazing stage or that or that these pieces won’t be functional in the end?

many thanks for any thoughts 

 

[Mark C.]

tightly packed STONEWARE will take more time to burn off impurites. Stoneware has more junk in it than say porcelain. I think you will be fine in a glaze fire but lets see what the cone 6 folks say. I'm a cone 10 porcealin potter.I used to fire huge amounts of stone ware in the 70s and 80s and had zero issues at cone 06 bisque temps but cone 6 seems to be more fickle in may ways.

[JohnnyK]

What color is the bone dry greenware? Did you use witness cones and how did they turn out? Did ALL the plates come out with the spot? I don't have any experience firing plates. Nor do I use a vent. I would guess that the tight packing of the plates and the cold vent air might keep the plates from outgassing properly, thus leaving the spots in the middle of the plates. If it was me, I would refire the plates without the vent operating and see what happens.

[Bill Kielb]

8 hours ago, Jeanetta said:

Other possible barriers are it’s also my first time using my new kiln vent and as well it’s quite cold :about -5 Celsius outside air

Your vent should only draw a tiny amount of air from your kiln. Most of the air exhausted should be room air, very very little kiln air so I would check it is configured correctly. I would re bisque at least a few, my guess is the discoloration will burn off in which case I would rebisque the rest even though this likely will not affect your glaze, why chance it. Stacking too densely can inhibit off gassing for sure.

Incidentally, the discharge pipe from your kiln vent probably should not exceed about 50c - 55c assuming it’s in normal room temperature ambient air. This is often checked when the kiln is near top temperature. If the discharge pipe is too hot, that would indicate too much kiln air is being withdrawn and an adjustment to the bypass manifold it was supplied with would be in order.

[Jeanetta]

8 hours ago, JohnnyK said:

What color is the bone dry greenware? Did you use witness cones and how did they turn out? Did ALL the plates come out with the spot? I don't have any experience firing plates. Nor do I use a vent. I would guess that the tight packing of the plates and the cold vent air might keep the plates from outgassing properly, thus leaving the spots in the middle of the plates. If it was me, I would refire the plates without the vent operating and see what happens.

Hi @JohnnyK thank you for your thoughts! The bone dry green ware is a pinkish tan colour….

I did use witness cones. The cones near the middle of the kiln turned out fine for 06. The cones at the back and bottom bent the 07 cone but the 06 cone barely at all. These plates were on the top shelf.

This effect didn’t happen to any other pieces, and not all the plates in the stack got this blotch. I believe more of the plates on the bottom part of the stack were the most discoloured . 

[Min]

Glaze firing them as is would be a crap shoot, might work, might not. We don't know how much less organics got burned out of the incomplete bisque nor do we know how prone your glazes are to pinholes etc. from incomplete gassing out of bisque.

In general the cleaner the clay is the lower the cone the bisque can be fired at without gassing issues, regardless of if it's a mid or high fire body. I know when I started making pots the go to bisque cone was 010 to 08, since then it has become common practice to bisque to 04 but it isn't necessary for all claybodies. If your pots/glazes are fine after glazing with an 06 bisque then all is good. If you are finding pinholes etc to be an issue then consider raising your bisque to 04 and allow for the slight decrease in porosity of the bisque while glazing.

[kswan]

I had that happen once and someone told me it might be localized reduction (lack of oxygen) when too many pieces were stacked together. They need air circulation so instead of stacking things like you would in your cabinet, fire them rim to rim and foot to foot so that the air can get in there. I can't remember what happened to the pieces after I fired them but I think they were okay. It was one of my first times firing my kiln also. 

[neilestrick]

It will probably be just fine. Discoloration like that is common when stacking brown stoneware pots. I do recommend bisque firing hotter for brown clays, though, like cone 04. You may have to adjust your glazing technique slightly if you're dipping to glaze, as your bisque will be just slightly less porous at 04.

[Pres]

Fired a lot of plates to cone 6 with a ^06 bisque, and when stacked in such a manner, invariably they will not be the same in the center. However, as your glaze firing will be stacked individually, the center coloring will come to match the rest of the plate. IMHO.

I do box plates now, having them rim to rim on a layer two to a layer, saves some space and they are fine. You might try a couple next bisque to see what they do that way.

 

best,

Pres

[Russ]

I see this in every bisque firing I do. Has never affected anything afterwards in the glaze firing. I do admit this if far more common in a fuel fed firing vs. electric but I can see how localized reduction can occur. Shouldn't  affect anything in your glaze firing.

[Jeanetta]

On 12/11/2021 at 12:35 PM, Rockhopper said:

What clay are you using ?  Assuming it's a ^6 stoneware, I'm thinking ^06 is a little low  - especially since you didn't fully reach that in all areas of the kiln

Whether it causes glaze issues will probably depend on what glaze you use, and your glaze-firing schedule - but I think I would re-bisque at ^05/^04, and maybe use a slower firing schedule to allow the heat to more evenly penetrate the stacked plates.

I’m using a clay called “mid cal 5” from Tuckers it’s a cone 6. Thanks for your thoughts!

[Jeanetta]

Thanks everyone- this is all helpful. Especially the tip on stacking plates rim to rim @kswan and @Pres- I will Definitely be trying this in my next bisque.

The glaze firing is currently underway I am excited to see if there’s any difference in these plates (I opted not to re-bisque them).

I will update what Has happened when I open the kiln!

 

[Jeanetta]

Just to update for learning & interest’s sakes :

In the end the glaze firing went great and There was absolutely no difference in the pieces with this discolouration and the ones without.

[Callie Beller Diesel]

@JeanettaThank you so much for letting us know! We love to hear how things went.