Help with 3rd/4th Glaze test results

[shawnhar]

Feeling a bit lost on my test results so far and looking for feedback please.

My first glaze fire was fullish kiln load ^6 with 10 min hold and fired hot, 6 cone was drooped down on the shelf and I'm guessing was a soft 7. The glazes were pretty smooth but still kinda bumpy.

Second firing was fullish kiln load ^5 with 15 min hold Med speed and fired hot I think, 6 cone was bent all the way over tip touching the shelf. All glaze was riddled with acne, little bumps with a hole in the middle

Third firing was mostly empty kiln, used all the furniture I have and most of the shelves to add some mass but still a lot of empty space. Fired to ^5 with no hold Med speed. 5 cone bent over tip touching shelf and 6 cone bent 30 degrees. All glaze has the acne, not as bad as the second firing but still there.

Fourth firing kiln loaded exactly the same as the third, but fired ^5 FAST speed, no hold, 5 cone bent over tip touching, 6 cone bent over 90 degrees. I thought running a fast glaze would create less heatwork and the 6 cone would be bent less, but that's not what happened. The pot on the top shelf is riddled with horrible acne, BUT, the pot on the BOTTOM shelf is way better, the acne is still there, but almost completely healed over (at least compared to the top pot!).

I am hoping to understand WHY the acne is better/worse in each of these tests before I use a custom program for my next test. It "seems" like the more heatwork there is, the better the glazes are coming out, but that means I am firing more than a full cone over what they are firing at the studio just to get a smooth(ish) glaze, which is confusing.

 

[liambesaw]

Are you using their same bisque schedule?

Same clay body as the studio?

Sometimes those bubbles are from bisqued pieces that were fired too fast

[shawnhar]

I have ruled out the clay, bisque fire and glaze by mixing pots fired at the studio and home. It is def an issue with my fire at home. The last piece I picked up from the studio was bisqued at home and fired there, came out with no blisters using that same turq glaze.

Pretty sure the issue is the ramp up/cool down of the last 200 degrees, but the way my 6 cones are bending over is confusing and I'm not sure if I should be firing to a lower temp and a slow climb/cool around the peak, and what peak temp to use.

[neilestrick]

I think your glaze is over-fluxed. Add equal parts silica and EPK in 3% increments up to 12% and see if it stops pinholing.

[Babs]

Seems less prob. When single dipped   left h.s. mug middle photo. So thickness of glaze may be aggravating this.

I'd address that either by less hold in glaze bucket or dampen pots.

You are wiping your bisqueware with damp cloth,

and slow down rate of rise from  1100deg Celcius to end of firing and soak.

Cones in peep hole vision and note temp. when C 6 touches its toes and turn off. Can actually see glaze surface thro peep hole also

Keep good logs of firing and results. Change one thing at a time.

How does refiring affect your blisters.

Advice given to me was to grind blisters slap glaze on them and refire with slower top end 

[Min]

Cones in the "Fast speed, no hold" are at a perfect ^6. The tip of the ^6 is level with the top of the base, exactly where it should be for self supporting cones. I think the question is did you have cones on all shelves? What cone did the pot on the bottom shelf get to from that firing? Need that information. As to the fast firing cycle dropping the cones more than the med cycle it could be due to the amount of mass in the kiln, especially in the area around the thermocouple. Just to confirm, your kiln has 1 thermocouple, roughly in the middle of the kiln?

[Babs]

What length of time was each firing?

[Mark C.]

Its an age thing -kids always go thru an acne stage

[shawnhar]

Hahaha, I wasn't sure it was blisters or pinholes or pitting. Read up on blisters and assumed that is the issue, created a schedule that slows down around the peak temp and started the kiln on lunch yesterday, it's now at 400F and I took a quick peek, cones look perfect and I didn't see any blisters. Fingers crossed!

I have tried to look in the peepholes but I think my vision can't detect such a low contrast, once the kiln gets above dull cherry I can't see anything. I do only have one thermocouple, put cones in the bottom this time to see the difference, and a few of the blistered mugs to add mass. 

 

[liambesaw]

2 hours ago, shawnhar said:

Hahaha, I wasn't sure it was blisters or pinholes or pitting. Read up on blisters and assumed that is the issue, created a schedule that slows down around the peak temp and started the kiln on lunch yesterday, it's now at 400F and I took a quick peek, cones look perfect and I didn't see any blisters. Fingers crossed!

I have tried to look in the peepholes but I think my vision can't detect such a low contrast, once the kiln gets above dull cherry I can't see anything. I do only have one thermocouple, put cones in the bottom this time to see the difference, and a few of the blistered mugs to add mass. 

 

You have welding glasses? They help a bit with looking at cones.  I hear a Lazer pointer too

[shawnhar]

I will see about getting some welding glass, thanks!

Tried the following schedule:

Ramp	Temp/hr	Temp/hold
1	100	220/0
2	350	2000/0
3	120	2050/0
4	108	2150/10
5	-120	2050/10

 

Moving in the right direction but still pock marked, refired pots from previous firings were "better" but not healed. Pot on the bottom shelf is "almost" good, top shelf still riddled with holes but they are much smoother than previous. Not sure if I should increase the top temp or add more hold time. Even with a peak temp set well below ^5, it's still starting to bring down the ^6 cone.

[GEP]

The next thing I would try is to make and apply your glaze thinner. Maybe do a “line blend” of sorts where you have 5 small pots with increasingly thicker glaze application, ranging from very thin to very thick. See if there is a noticable result. Right now your time/temperature variations are yielding results that don’t have a noticeable pattern. 

[neilestrick]

Definitely looks like there's not enough stabilizers in the glaze. Add EPK and silica.

[Min]

Shawn, would the studio let you put some cones in their kilns along with test tiles of the same glazes that you both use? Your glazes plus theirs. I can't remember if they told you how long their firings took but I'ld get that info too. Not just the speed the firing is programmed for.

[shawnhar]

OK i have ruled out everything except the actual glaze firing of my kiln. I took my bisque pots and my glazes to the studio and everything came out perfect. I am sure adding stabilizers to the glaze would make a difference, but I would rather avoid that right now if possible, things have gotten complicated enough already.

 The studio fires using the cone program mode, no vents, all the peeps closed the whole time. Said they change the firing depending on the age of the elements. I didn't write it down but I think he said generally with new elements, cone 5 med no hold and when it starts taking longer to fire he moves to cone 5 fast with 8 minute hold, he stated a ^6 witness cone would be all the way down. He suggested I am going through the high end too fast for the glaze to heal over. They run 8.5 to 9 hours for a glaze fire.

I am drying my next set of test pots and thinking I might try the following schedule hoping to slow down the entire range that includes the bubbling.

Ramp	Temp/hr	Temp/hold
1	100	200
2	350	1976
3	108	2187/10
4	-108	1976

[Min]

Glad you got some more info to work with. For pinholes what works well for me is to do a drop and hold firing, similar to your schedule above but after #3 I drop by 100F and hold for 15-20. As this works for pinholes it should help with your blisters also, adds a little heatwork but not much. 

It's good you got confirmation that the studios thermocouple(s) are likely off, I really wouldn't fire with no venting and peeps in like they do.

[shawnhar]

Thanks Min, how fast is that drop before the hold?

I think the studio fires that way because they found that is what works and more better is the enemy of good enough.

[Min]

23 minutes ago, shawnhar said:

how fast is that drop before the hold?

9999