Glaze Bilsters...

[Gokul]

I have this heavy blisters and pinholes issue and I m doing the Bisque at cone 08 and glaze temperature is at cone 6 + 40 mins soak.,

Whats the reason behind this blisters and pinholes ? Is it a clay issue or its something to do with a glaze chemistry ?

 

[glazenerd]

Gokul:

unfortunately this is the down side of laterite clay. 8% hematite ( Feo3).  All your 40 minute hold is doing in this case is causing glaze run off. Laterite also has other specific properties that make burning off sulfides more difficult than typical Western clays. Western clays have iron disulfide (pyrite) and you have hematite.  Hematite is molten by 900 C, so your schedule needs to burn off carbons by that temperature: because  the iron starts vitrify ing above that temperature. Early vitrification creates a denser body which causes escaping spars to escape under more pressure: causing craters in lieu of pinholes.

http://community.ceramicartsdaily.org/topic/15676-cone-6-firing-schedule-nerds/?tab=comments#comment-120634

Firing schedule for dark and red bodies on page 2.

t

[glazenerd]

By the way, break those examples open ( smash actually into small pieces). Look for darker, to black blotches, discoloration, etc. All early signs of carbon coring.

t

[Gokul]

50 minutes ago, glazenerd said:

Gokul:

unfortunately this is the down side of laterite clay. 8% hematite ( Feo3).  All your 40 minute hold is doing in this case is causing glaze run off. Laterite also has other specific properties that make burning off sulfides more difficult than typical Western clays. Western clays have iron disulfide (pyrite) and you have hematite.  Hematite is molten by 900 C, so your schedule needs to burn off carbons by that temperature: because  the iron starts vitrify ing above that temperature. Early vitrification creates a denser body which causes escaping spars to escape under more pressure: causing craters in lieu of pinholes.

http://community.ceramicartsdaily.org/topic/15676-cone-6-firing-schedule-nerds/?tab=comments#comment-120634

Firing schedule for dark and red bodies on page 2.

t

But Tom,

  We are not adding any red clay (Laterite) to our receipe, but still its pinholing and blistering...

[GEP]

@Gokul You might simply be firing your kiln too hot. Running glazes and blisters are a common outcome of overfiring. Are you using witness cones to measure that you are actually firing to cone 6? If you already know you are hitting cone 6, you can try firing without the soak, or lowering target temperature. 

I refer to my glazes as “cone 6” glazes, but at one point I switched claybodies and started getting pinholes in one of my glazes. Through trial and error I solved the problem by changing my firing program to cone 5.75 instead of 6. 

Edit to add: I also think you have applied your glazes too thickly. This can also contribute to both running and blistering. 

[Pres]

I believe Mea has really hit the nail on the head, too thick a glaze process, and possibly an incorrect firing schedule for you body/glaze.

I would recommend a series of test tiles placed at different areas in the kiln with a series of cone packs to measure temp/results. When doing the test tiles make certain to mix your glaze to a chocolate milk consistency or just slightly thicker, then double or even triple dip your test tiles in stages so that you have areas with 1, 2, and three dipped coats. Firing a load in this manner should not take long to set up, but will tell you a lot about your kiln, your glazes and your process.

 

best,

Pres

 

[Min]

5 hours ago, Gokul said:

Bisque at cone 08 and glaze temperature is at cone 6 + 40 mins soak.,

 

What cone are you aiming for with your glaze firing? A 40 minute soak is going to take you way past cone 6 if that is your target. For the bisque I would increase your cone to 04, vent the kiln as much as you can and bisque fire slowly.

[Callie Beller Diesel]

I agree with the overfiring hypothesis. Pinholes from improper carbon burnout are usually smaller, and not clustered the way yours are. Your glazes look more like boiled glass. 

[neilestrick]

I agree. The glaze is over-fired. Cone 6 with a 40 minute hold gets you to cone 8. I used to fire cone 8 that way. What was your reason for the hold?

[glazenerd]

Gokul.:

the Than clay and ball clay also have 2-3 times the iron compared to Western clay. You also have higher carbon and sulfide contents. The joys of sub- tropical  alluvial soil. However, as Mea pointed out, glaze application is on the heavy side.

easy enough to test: cut the 40 minute hold to five minutes. Cut the application back 20% or so. Fire the same glaze for accurate comparison.

the foot ring on the second picture is telling as well. See the color shift from the bottom to just under the glaze line?  The clay is certainly mature.

[Min]

@Gokul, may I ask about the ball clay(s) you are using? When I look up suppliers to the Bengaluru area of India is seems like there is ball clay available with fairly low iron content. A quick search brought up this one, Fe2O3 @ 0.58%  for example.

[Gokul]

6 hours ago, Min said:

@Gokul, may I ask about the ball clay(s) you are using? When I look up suppliers to the Bengaluru area of India is seems like there is ball clay available with fairly low iron content. A quick search brought up this one, Fe2O3 @ 0.58%  for example.

Min,

  Here is the chemical of the ball clay which we are using. 

[Gokul]

10 hours ago, neilestrick said:

I agree. The glaze is over-fired. Cone 6 with a 40 minute hold gets you to cone 8. I used to fire cone 8 that way. What was your reason for the hold?

We are using the hold for the glaze to even out and the body to mature well.

[Gokul]

17 hours ago, GEP said:

@Gokul You might simply be firing your kiln too hot. Running glazes and blisters are a common outcome of overfiring. Are you using witness cones to measure that you are actually firing to cone 6? If you already know you are hitting cone 6, you can try firing without the soak, or lowering target temperature. 

I refer to my glazes as “cone 6” glazes, but at one point I switched claybodies and started getting pinholes in one of my glazes. Through trial and error I solved the problem by changing my firing program to cone 5.75 instead of 6. 

Edit to add: I also think you have applied your glazes too thickly. This can also contribute to both running and blistering. 

Yes, I ll cut down the soak to 15mins and see hows its coming., Yes the glaze application is thick as well and does the thick application cause these kind of craters and bilsters ? 

[Gokul]

7 hours ago, glazenerd said:

Gokul.:

the Than clay and ball clay also have 2-3 times the iron compared to Western clay. You also have higher carbon and sulfide contents. The joys of sub- tropical  alluvial soil. However, as Mea pointed out, glaze application is on the heavy side.

easy enough to test: cut the 40 minute hold to five minutes. Cut the application back 20% or so. Fire the same glaze for accurate comparison.

the foot ring on the second picture is telling as well. See the color shift from the bottom to just under the glaze line?  The clay is certainly mature.

Tom,

The colour in the foot ring is because of the carbon coring or its because of the sulphides which causes the colour ? Also we are using the same ball clay for our glaze as well but i dont know this particular glaze has ball clay in it., And even if its because of the sulphides and carbon wi is it not happening with all the glazes happening with only the particular glaze ?

[glazenerd]

Gokul:

the color of the foot ring attests to the iron content of your than and ball clay. You sent me the spec sheets a month or so ago. You have unusual circumstances in India: it either takes months to get testing samples, order by multiple tons, or just wait. I think we discussed the clay Min suggested a couple of months ago as well: availability issue as I recall?

to give you a point of reference:

Western clay with 1.50-1.80% iron with 500-600 PPM sulfates/ sulfides run 9.50 LOI typically.

the clay Min suggested is well below 1% iron, yet the LOI is nearly 13%. Even though it is a large particle, much cleaner ball clay: it still has much higher sulfate/ sulfide content. What makes clay formulation in India so tough. If the calcium bentonite would ever show up; that i had you order two months ago: could reformulate a whiter stoneware body.

hematitie is FeO3, and has ferro- magnetic properties: even on a molecular level. Unlike iron pyrite, hematite binds sulfides to it, making it much harder to burn them off. Now how that effects glazes with these clays included, I am uncertain. Few years back I ran several glaze tests on clean porcelain : testing sulfides in the glaze.  Below is a pic of 4% moly sulfide on porcelain at cone six:

classic glaze defect blister. However, when iron/ sulfur in the clay body causes early vitrification: the escaping spars escape under pressure: causing a raised area around the blister: like a volcanic cone. 

And when off gassing spars are not under pressure, but simply have not been given time to escape: you get small pinholes and bubbles.

follow the bubble pattern and it will tell you the causes.

tom

[GEP]

5 hours ago, Gokul said:

Yes, I ll cut down the soak to 15mins and see hows its coming., Yes the glaze application is thick as well and does the thick application cause these kind of craters and bilsters ? 

Yes, a too-thick glaze application can contribute to the problem. If gasses are escaping from the clay and glaze (which is normal), a too-thick glaze is producing more gasses, which need to penetrate out through a thicker glaze layer. Some of those bubbles don’t make it all the way out in time for the holes to heal over. This is just one possibility. Callie mentioned “boiling” earlier and I also think your blisters look like boiled glaze. This is more related to overfiring. Just like all things in ceramics, there are many factors that need to be put in balance. 

Just to emphasize, the larger problem with applying glazes too thick is the runnyness. Kiln shelves are expensive. 

If I were in your shoes, I would take Pres’s advice to make test tiles and fire this clay/glaze combo at varying temps, with varying thicknesses, with witness cones next to each tile. That’s the fastest way to test all the factors that need to be put in balance. 

[Rae Reich]

Agree that it looks like the overthick glaze has "boiled" and I think maybe (with thinner application) any remaining bubbles from that firing schedule might have smoothed out more if the kiln had cooled more slowly. Like when boiling lava cools so quickly you can still see the bubbles.  

The glaze color and iridescence are beautiful, Gokul might not want to risk losing that. 

[Mark C.]

Application looks to thick and its fired to hot as noted in above posts..

[Min]

9 hours ago, Gokul said:

We are using the hold for the glaze to even out and the body to mature well.

So your clay isn't mature enough at cone 6? And you said your other glazes work on this claybody without craters etc when fired to cone 6 with a 40 minute hold right? If you come to the conclusion that what you have is a cone 8 body (approx) with the blistering glaze a cone 6 glaze then it seems the easiest thing to do would be to raise the firing temperature of the glaze rather than messing around trying to change the clay. If the clay works with your other glazes, brilliant, don't mess with it. If the clay is good at cone 8 then fine, go with that. If the blistering green glaze is the only problem then raise the firing temperature of the glaze itself by altering the recipe so it doesn't run off the pot and boil, change it into a cone 8 glaze. By the way, I think the iridescence shown in some of your pictures is another indicator of an overfired glaze.

[glazenerd]

Just now, Gokul said:

Tom,

The colour in the foot ring is because of the carbon coring or its because of the sulphides which causes the colour ? Also we are using the same ball clay for our glaze as well but i dont know this particular glaze has ball clay in it., And even if its because of the sulphides and carbon wi is it not happening with all the glazes happening with only the particular glaze ?

Gokul:

one thing about clay and glaze testing: you have to keep detailed notes. The recipe, application rate, firing schedule, etc. When you retest this issue: same clay, same glaze: but only change the hold time and application rate. The usual protocol is changing one thing at a time. If multiple things are changed at one time; then figuring out the source of the issue gets complicated. By changing the hold time only, you will  know if the glaze boiled. Then change the application rate in a second test. If problems persist: then look at the recipe.

t

[neilestrick]

If you need to get to cone 8 for the clay body to mature, then add some kaolin and silica to the glaze in equal amounts, in 3% increments, until it works at cone 8.