Jump to content
saltcreekstudio

Mystery firing problem. Not for the faint of heart.

Recommended Posts

45 minutes ago, glazenerd said:

Vashion Red w/ grog is a cone 04/06 product. Your original post stated ^5 fast fire?

red bodied clays use Newman red, Red Art, or Imco Burgundy. Given this is a West coast supplier: Newman or Imco: Red Art is mined in Ohio as I recall. Regardless, red bodied clays have 5%+ iron content! and Imco Burgundy has more. Iron is part of the problem , but fast firing is also part of the problem. In addition, iron reduces even in oxidized kilns if fired too quickly when sulfides are present. Both Imco and Newman have higher sulfide ( carbon) content. When iron reduces , it becomes an active flux: which in turn can.react with both magnesium and titanium: which both are also present. Magnesium can cause color shifts, which I believe is part of your problem. You do not have one specific problem, but a chemical chain reaction that starts with "fast firing."

Read at the bottom of page 2. 

gallery_73441_1183_279308.jpgthese are porcelain test bars with varying degrees of iron, magnesium, and titanium. As you move from left to right, magnesium levels increase. Point being, varying levels of magnesium and titanium create color shifts. Tile 9 is actually porcelain with enough iron to produce the classic red color.

T

Hey there. No, Vashon Red w/ Grog is a ^04-^6 clay. I've achieved perfect, bright and vibrant glaze results (see pic of pine cone tile above) with a ^5 medium speed firing when my kiln will was purchased, but terrible results (see sea turtle tile next to pine cone tile) not long after. Your info is really interesting and I need to digest it a bit more. I find that I keep thinking, "aha! that's it!"  but then remember that I've proven the theory incorrect over this past year with trial and error. I plan to do another test firing, a really tedious one, to share and maybe whittle down some of the mystery. Thank you so much!  **edit... I think the longer, medium speed firing is giving me darker, worse glaze results.

 

Edited by saltcreekstudio

Share this post


Link to post
Share on other sites
39 minutes ago, glazenerd said:

Vashion Red w/ grog is a cone 04/06 product. Your original post stated ^5 fast fire?

red bodied clays use Newman red, Red Art, or Imco Burgundy. Given this is a West coast supplier: Newman or Imco: Red Art is mined in Ohio as I recall. Regardless, red bodied clays have 5%+ iron content! and Imco Burgundy has more. Iron is part of the problem , but fast firing is also part of the problem. In addition, iron reduces even in oxidized kilns if fired too quickly when sulfides are present. Both Imco and Newman have higher sulfide ( carbon) content. When iron reduces , it becomes an active flux: which in turn can.react with both magnesium and titanium: which both are also present. Magnesium can cause color shifts, which I believe is part of your problem. You do not have one specific problem, but a chemical chain reaction that starts with "fast firing." 

Read at the bottom of page 2. 

gallery_73441_1183_279308.jpgthese are porcelain test bars with varying degrees of iron, magnesium, and titanium. As you move from left to right, magnesium levels increase. Point being, varying levels of magnesium and titanium create color shifts. Tile 9 is actually porcelain with enough iron to produce the classic red color.

T

Vashon red is a cone 6 clay.  http://www.seattlepotterysupply.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=sps_ecat&Category_Code=SCD

Share this post


Link to post
Share on other sites

Are you putting cone packs all over your kiln?

C 04- c6 is a very wide firing range.

I for same red clay atC03 and again atC5.

The colour difference between the 2 firings is marked! As is the vitrification.

C03 typical red brick colour.

C5-6 dark brown.

Share this post


Link to post
Share on other sites

Misread it indeed. Cone 04- 6. 

Which means Newman Red would top the list: Red Art and Imco  Burgundy would be brown at cone 5. Newman is now a blended clay,the original is no longer mined.  If the premise of being a bad batch or impurities applied: then logically  all the tiles from that batch would be the same color, but different from previous batches. 

Have you broke any of these tiles open to check for carbon coring? That would certainly account for color shifts.

T

Share this post


Link to post
Share on other sites
11 hours ago, Babs said:

On the same shelf? Same distance from elements

?

Uniform upper and lower clay discoloration? Same density of pack?

Yes, on the same shelf. I think the darkest clay outcome is from those that that are fired at the hottest parts of the kiln. I will do a test firing and load the kiln in such a way that addresses all the responses/questions. I stopped loading wares on the top an bottom parts of the kiln b/c it seemed to be underfiring more than a cone, but will load test tiles in those areas for the test fire. 

Share this post


Link to post
Share on other sites
11 hours ago, Babs said:

Are you putting cone packs all over your kiln?

C 04- c6 is a very wide firing range.

I for same red clay atC03 and again atC5.

The colour difference between the 2 firings is marked! As is the vitrification.

C03 typical red brick colour.

C5-6 dark brown.

Yes, I use the self supporting witness cones throughout the kiln. I've always bisque fired to ^04, then glaze fire to ^5. In a thread above, I added pics of the way my tiles used to turn out (see the pine cone tile) and the way they often turn out (the sea turtle tile). And many come out of the kiln a variety of shades in between. 

Share this post


Link to post
Share on other sites
3 hours ago, glazenerd said:

Misread it indeed. Cone 04- 6. 

Which means Newman Red would top the list: Red Art and Imco  Burgundy would be brown at cone 5. Newman is now a blended clay,the original is no longer mined.  If the premise of being a bad batch or impurities applied: then logically  all the tiles from that batch would be the same color, but different from previous batches. 

Have you broke any of these tiles open to check for carbon coring? That would certainly account for color shifts.

T

Ha, I wish the answer was as easy as "Oh, I've just been firing low-fire clay as a mid-fire!" I have not broken up the tiles to look inside. Carbon coring is not something I know about. 

Share this post


Link to post
Share on other sites


I have been following this discussion from an Root Cause Analysis perspective.  My comments:

Have you carefully checked to verify that you have a uniform oxidizing atmosphere throughout the kiln?  The surface darkening of an iron containing clay body is a consequence of a lower oxygen level (specifically oxygen fugacity) in the surrounding atmosphere than in the lighter colored surfaces.  The initial post indicated that the kiln has a downdraft vent system.  Is the air flow specified for the vent actually being met throughout the firing - including the cool down periods for the unglazed tests?  Is the location of the color patterns related to location in the kiln?  If so, would the pattern be consistent with all the induced air going down the walls of the kiln and missing the kiln shelf area with the disliked color pattern? 

Given that your clay supplier has confirmed that the clay body composition has not significantly changed over time, I would focus my efforts on something associated with the firing steps.  Also ask a neighboring potter to fire some test pieces or raw clay to see if the results are consistent with yours. 

Pull up your production history and carefully list EVERYTHING that has changed from the 'good ole days' to the 'now' production.  Are you stacking the kiln the same way, what is different between 'then' and 'now'.  Make the complete exhaustive list before evaluating the consequents of the change.  After the list is complete take a break.  After the break, evaluate the changes and make note of why or why not that change is a cause or not a cause; be specific of the data you are using and how reliable that data is.  If you are making assumptions, clearly highlite those assumptions so that you can come back to verify that the assumptions are true. (as Jessie Stone said: "If you don't like the answers, check your assumptions")  

An alternative business approach is to make a definitive change in your product mix; add the dark colored tiles from your product list and closeout the light colored tiles.  (add a Smiley here)

LT


 

Share this post


Link to post
Share on other sites

One last point since your production is using a high iron bearing clay it will always be on the edge of problem issues.

All heavy iron clays can carbon core pit or wreak havoc with glazes.Your clay in photo is just full of colorants mostly iron-the sample on the left looks cored to me for sure. When I use high iron clays in my salt kiln I expect some issues-although the results can also be spectacular .

For you to base a line of tiles on these clays I would expect issues to pop up and I would pay special attentions to Magnolia MR post above to try to remedy  them. With a high colorant content clay issues are always just a fire away. You could change out to a buff clay and suffer less issues or be prepared to really keep a tight ship on all your firing stacking and loading and glazing and you still will have the occasional issue.

I used a high iron clay for about 15 years in reduction fires-filially dropped it do to body issues I got tired of fighting. 

Another body could cure all this.

Testing is in order now

Share this post


Link to post
Share on other sites

I think I said this before, but just because the clay manufacturer isn't aware of any changes to their raw materials doesn't mean that there haven't been changes. We've seen many materials changes that the manufacturers deny, or that they simply aren't aware of because they haven't done an analysis recently, or because they simply don't care because it doesn't matter to their primary buyer. This happened recently with Custer Feldspar. Remember that potters don't use enough of any raw material to matter. It could be there's a change in the amount of iron, or the amount of organic material in the clay.

If your vent is working, reduction shouldn't be an issue because it will be pulling oxygen into the kiln. I'd try firing slower and see if that helps.

Share this post


Link to post
Share on other sites
3 hours ago, saltcreekstudio said:

I will do a test firing and load the kiln in such a way that addresses all the responses/questions. I stopped loading wares on the top an bottom parts of the kiln b/c it seemed to be underfiring more than a cone, but will load test tiles in those areas for the test fire. 

Need to start with the bisque firing, for this test I wouldn't use test tiles. If you've already spent nearly a year with this problem start from scratch to rule out all changes. Load the bisque loosely, don't stack anything, could even put some thin coils under the tiles for the bisque firing to raise them up off the shelf so the clay can off gas from all sides. As others have said, check the venting. Could be there is more to burn off now than in previous batches of the clay. Yes, that is black / carbon coring in your photo above. Slow down the bisque between 1300 and 1650F, if there is still black coring then add an hour soak at 1650F.

Share this post


Link to post
Share on other sites
1 hour ago, Magnolia Mud Research said:


I have been following this discussion from an Root Cause Analysis perspective.  My comments:

Have you carefully checked to verify that you have a uniform oxidizing atmosphere throughout the kiln?  The surface darkening of an iron containing clay body is a consequence of a lower oxygen level (specifically oxygen fugacity) in the surrounding atmosphere than in the lighter colored surfaces.  The initial post indicated that the kiln has a downdraft vent system.  Is the air flow specified for the vent actually being met throughout the firing - including the cool down periods for the unglazed tests?  Is the location of the color patterns related to location in the kiln?  If so, would the pattern be consistent with all the induced air going down the walls of the kiln and missing the kiln shelf area with the disliked color pattern? 

Given that your clay supplier has confirmed that the clay body composition has not significantly changed over time, I would focus my efforts on something associated with the firing steps.  Also ask a neighboring potter to fire some test pieces or raw clay to see if the results are consistent with yours. 

Pull up your production history and carefully list EVERYTHING that has changed from the 'good ole days' to the 'now' production.  Are you stacking the kiln the same way, what is different between 'then' and 'now'.  Make the complete exhaustive list before evaluating the consequents of the change.  After the list is complete take a break.  After the break, evaluate the changes and make note of why or why not that change is a cause or not a cause; be specific of the data you are using and how reliable that data is.  If you are making assumptions, clearly highlite those assumptions so that you can come back to verify that the assumptions are true. (as Jessie Stone said: "If you don't like the answers, check your assumptions")  

An alternative business approach is to make a definitive change in your product mix; add the dark colored tiles from your product list and closeout the light colored tiles.  (add a Smiley here)

LT


 

When you ask if I've carefully checked to verify uniform oxidizing atmosphere throughout the kiln... you mean by testing samples? Not yet, but I'm going to. I assumed my vent was working fine because I checked it with a lighter flame (it was being pulled into the holes in the lid after I turned the vent on). Maybe something with the vent is amiss despite that.  Thanks for suggesting the detailed list idea... I will do that before I do my test firing. I appreciate the "root cause" perspective. Thanks.

Share this post


Link to post
Share on other sites
56 minutes ago, Mark C. said:

One last point since your production is using a high iron bearing clay it will always be on the edge of problem issues.

All heavy iron clays can carbon core pit or wreak havoc with glazes.Your clay in photo is just full of colorants mostly iron-the sample on the left looks cored to me for sure. When I use high iron clays in my salt kiln I expect some issues-although the results can also be spectacular .

For you to base a line of tiles on these clays I would expect issues to pop up and I would pay special attentions to Magnolia MR post above to try to remedy  them. With a high colorant content clay issues are always just a fire away. You could change out to a buff clay and suffer less issues or be prepared to really keep a tight ship on all your firing stacking and loading and glazing and you still will have the occasional issue.

I used a high iron clay for about 15 years in reduction fires-filially dropped it do to body issues I got tired of fighting. 

Another body could cure all this.

Testing is in order now

Thank you for your suggestions and info...I'm learning a lot here and have a long way to go. I'm beginning to think harder on switching to a lighter colored body. Having achieved nearly perfect results with the Vashon Red w/ grog (VRG), then to have it all go pear shaped is beyond frustrating. It sounds like it's just the nature of this finicky clay and it's components.  I suppose I'll try a few tests to see if I can get more carbon burn-out in the ^04 bisque firing, and see what I can learn in the ^5 glaze firing tests too. Thanks again. 

Share this post


Link to post
Share on other sites
14 hours ago, Babs said:

 

I followed glazenerd's iron body schedule for burning out sulphur and carbon, I smashed a jar this morning and it is uniform, no carbon coring.  I am hoping I get less bubbles too.  I always bisqued slower but not nearly as slow as his schedule called for.  From 1200-1750 at 100f per hour.  Started firing at 1pm and didn't get done til 1am, woof, all that for a bisque!  I am firing in a gas kiln though so it's pretty hard to nail the 100f/hr ramp.  Fingers crossed, I'll be doing glaze next weekend

Edited by liambesaw

Share this post


Link to post
Share on other sites
15 minutes ago, Min said:

Need to start with the bisque firing, for this test I wouldn't use test tiles. If you've already spent nearly a year with this problem start from scratch to rule out all changes. Load the bisque loosely, don't stack anything, could even put some thin coils under the tiles for the bisque firing to raise them up off the shelf so the clay can off gas from all sides. As others have said, check the venting. Could be there is more to burn off now than in previous batches of the clay. Yes, that is black / carbon coring in your photo above. Slow down the bisque between 1300 and 1650F, if there is still black coring then add an hour soak at 1650F.

Ok, I will. By "test tiles" I meant starting from scratch, making new tiles to test theories. I'll be sure not to stack and will double check my venting. I'll program the kiln to run a slow firing schedule for the bisque and see what happens.  I like the idea of the soak at 1650 if need be. Thanks for that. 

Share this post


Link to post
Share on other sites
2 minutes ago, liambesaw said:

I followed glazenerd's iron body schedule for burning out sulphur and carbon, I smashed a jar this morning and it is uniform, no carbon coring.  I am hoping I get less bubbles too.  I always bisqued slower but not nearly as slow as his schedule called for.  From 1200-1750 at 100f per hour.  Started firing at 1pm and didn't get done til 1am, woof, all that for a bisque!  I am firing in a gas kiln though so it's pretty hard to nail the 100f/hr ramp.  Fingers crossed, I'll be doing glaze next weekend

A 12 hour bisque?! Yowza. 

Share this post


Link to post
Share on other sites

Yep, first run, my old 1027 took that long (longer), with fuddling, to get enough time at the temps indicated by Nerd's post on "Cone 6-10 Firing Schedule for dark or red bodied clay bodies." - as most of the pieces were red, with a few black in thar too. Will see how it turned out when kiln is cool enough after yesterday's glaze firing!

Share this post


Link to post
Share on other sites

Salt:

congrats, you have a serious case of carbon coring. I wrote an article in Ceramics Monthly named " Coring and Bloating" thinking it was May 2018 Techno File. It will give you the back ground. Your bisq temp is roughly 60-70 degrees to high in the 1250-1750F range. Organic carbons burn out below. 1000F, and inorganic from 1250-1750F. 

Yes, as, my schedule adds hours to the firing, but it cures many problems including pinhole and blisters from trapped spars. In addition, it will extend element life. No need to ditch your clay at this point, just need to work within its limits.

T

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×

Important Information

By using this site, you agree to our Terms of Use.