Mystery firing problem. Not for the faint of heart.

[saltcreekstudio]

I've been having problems with my once consistent tile making process. Is it the glaze, the clay, the kiln? Manufacturers  have not been able to solve the problem. My once bright and vibrant (Spectrum glaze) low-relief tiles on red-brown stoneware (^5 /fast firing 8-9 hours)  are muddied and dull and inconsistent.  Some look like they've been overfired, as evidenced by the post-firing unglazed clay surface, which isn't red-brown as is should be, but rather dark chocolatey brown. Some tiles are worse than others. The top and bottom of the kiln (Skutt 1227 KM...pretty new when this all started, and it has a downdraft vent) are underfiring by at least a cone +.  I don't put wares on those shelves now. I know there  is naturally a cooler top and bottom, but this is more than the norm I think. The elements, despite being fairly young, have all been replaced; thermocouple too.  Amps seem accurate. Witness cones are showing that the middle sections are reaching ^5, but how on earth is the clay getting so dark and therefore greatly darkening and muddying up my glaze?  I've had to stop my line of handmade tiles due to having ruined so many loads.  Any guidance or thoughts on what is going on is appreciated. 

Thanks,

Maria

[Arnold Howard]

Maria, make sure the lid is closed all the way and the peephole plugs are inserted since you are using a downdraft vent.

Have you changed batches of clay lately?

Are you using witness cones?

Could you post a sketch or photos of the way you are loading the kiln? It would be interesting to see the spacing, in inches, between shelves.

Arnold Howard

[neilestrick]

What is the voltage and phase of your kiln? Assuming you're doing a full load of tile, that is a very dense load for a KM1227 if it's not running on 3 phase service. Also, you're dealing with a kiln that only has one thermocouple, in the middle section, and they are notorious for running cold on the bottom and very top. In general, to get them to fire evenly you need to load the bottom and very top looser, the middle more dense. That will help to even things out.

As for the color difference, if the cone say you're firing to an accurate cone 5, then something has probably changed in the clay body, not the kiln.

[saltcreekstudio]

14 minutes ago, neilestrick said:

What is the voltage and phase of your kiln? Assuming you're doing a full load of tile, that is a very dense load for a KM1227 if it's not running on 3 phase service. Also, you're dealing with a kiln that only has one thermocouple, in the middle section, and they are notorious for running cold on the bottom and very top. In general, to get them to fire evenly you need to load the bottom and very top looser, the middle more dense. That will help to even things out.

As for the color difference, if the cone say you're firing to an accurate cone 5, then something has probably changed in the clay body, not the kiln.

Hi Neil. It's 240 volt, single phase.  I do try to load evenly, but I wasn't getting the temps I needed on the top and bottom, so I stopped loading those areas.  

I thought maybe it was in the clay too,  so yesterday I took samples and drove to Seattle and I asked if their clay changed or if the supplier of the ingredients changed. There were no changes. What makes me think it isn't the clay is that I get some tiles that are dark brown and some that are closer to the correct color, and they are all from the same batch of mixed clay and from the same firing. Here's a pic of the darkened tiles. The tile on top is what the outcome of the fired clay SHOULD be. Reddish brown.  The pine cone tile is how the glaze should look on the properly fired clay, something I can't achieve these days due to this problem. The dark sea turtle next to it is an example of what happens when the clay turns out dark. 

Also, I was recently told I ought to be using a production kiln for the type of work I'm doing; one that's hard wired, beefier and gives consistent results.  

[neilestrick]

Hard wired or not isn't the issue, but if you go more powerful it'll have to be hardwired, because once you get above 50 amps it's required. I would also say that zone control would help a lot with getting more consistent firings out of dense tile loads.

Do you get multiple colors of clay on each shelf, or are the color differences from shelf to shelf? Have you put cones in the middle of every shelf to see if there are differences in temperature related to each fired color?

How are you applying the glaze? If it didn't know anything that you told us so far, I would say the difference in the glazed tiles was due to glaze thickness.

 

[saltcreekstudio]

2 hours ago, Arnold Howard said:

Maria, make sure the lid is closed all the way and the peephole plugs are inserted since you are using a downdraft vent.

Have you changed batches of clay lately?

Are you using witness cones?

Could you post a sketch or photos of the way you are loading the kiln? It would be interesting to see the spacing, in inches, between shelve

2 hours ago, Arnold Howard said:

 

Arnold Howard

Hi there. Yes, lid is closed and plugs are in. I didn't change clays either. Yes, I use cones and they are consistent. I think that's why it is so frustrating...there are no big red flags waving so I don't really know where to go from here. I usually have two elements between shelves. Sometimes three. The bottom shelf is about an inch of the floor, so that element is right at the shelf. Sometimes there is more space around the thermocouple, in order to keep shelves an inch or so away from the thermo. See my response to Neil for a couple of example tiles. Do you have experience with this kiln model? Is it weird for it to take 8 hours and 45 minute to fire to ^5 on a fast fire program? I'm wondering if it is taking an extra say, 45 minutes to get up to ^5, therefore cooking everything too long, but getting to the correct temp eventually. I've stopped doing a medium speed program b/c it just takes too long 9+ hours. I used to always fire with the medium speed and I remember correctly, it'd take 8.5 hours. Still doesn't answer why some tiles turn out dark and others a little less dark...OR DOES IT? lol

 

[saltcreekstudio]

2 minutes ago, neilestrick said:

Hard wired or not isn't the issue, but if you go more powerful it'll have to be hardwired, because once you get above 50 amps it's required. I would also say that zone control would help a lot with getting more consistent firings out of dense tile loads.

Do you get multiple colors of clay on each shelf, or are the color differences from shelf to shelf? Have you put cones in the middle of every shelf to see if there are differences in temperature related to each fired color?

How are you applying the glaze? If it didn't know anything that you told us so far, I would say the difference in the glazed tiles was due to glaze thickness.

 

Admittedly, I'm not savvy with the electrical part. Maybe by the end of this I will be. Ha. I think I get most of that, but I was thinking of converting the kiln into a production kiln (as opposed to the "hobby" kiln I have now) which will draw more power  and is "designed specifically for volume production and fast turnaround of high mass loads. It has the power and industrial components to go straight to Cone 10 with a full load of tile or stoneware without struggling to reach the final temperature." 

Yes, I get multiple colors on each shelf. So weird. I recently double-stacked 4 x 8 field tiles in the kiln for a bisque firing to ^04, but I accidentally programmed a glaze firing. So I messed up a bunch of tile, but was able to see all of these differences clearly. I thought maybe the dark brown tiles were so dark b/c they were on the bottom of the stacks, but that wasn't the case: the results were mixed, with some of the top tiles dark and some of those that were touching the shelves were dark.  There doesn't seem to be a correlation between the witness cones and the coloration, although at the top and bottom of the kiln, where it's underfiring by a cone or so, the glaze is not AS muddied, but it's still not right, like the pine cone tile previously mentioned.

I brush three coats by hand on each tile. I've been doing it the same way for years and I'm confident that I'm brushing it the same way every time. I even tried to go very light on the coats several months ago, just to see, and it wasn't the problem. They looked like they didn't have enough glaze and they still were too dark. Aaack!

[Mark C.]

You have several issues-the kiln taking longer to fire from medium to fast-elements wearing out or loose connections or corrosion in the wiring-check out all your connections first.

clay getting darker-the kiln is oxidizing so I'm going out on a limb and say its the clay body. Darker or lighter is the iron in the clay not a firing issue.Now if its overtired thats another deal but you said the cones did not show that.

or as Neil has said glaze thickness issues. 

glaze thickness can change a lot.Your tile in photo on lefty looks like the glaze is just to thick

Leaving the top and bottom empty is not good as well-as Neil said loose fill in bottom and top. Full loads in center.

Is your floor(shelves) off the bottom of kiln floor?

The more info you give the better

I feel you have several issues not one.

[neilestrick]

29 minutes ago, saltcreekstudio said:

Admittedly, I'm not savvy with the electrical part. Maybe by the end of this I will be. Ha. I think I get most of that, but I was thinking of converting the kiln into a production kiln (as opposed to the "hobby" kiln I have now) which will draw more power  and is "designed specifically for volume production and fast turnaround of high mass loads. It has the power and industrial components to go straight to Cone 10 with a full load of tile or stoneware without struggling to reach the final temperature." 

Yes, I get multiple colors on each shelf. So weird. I recently double-stacked 4 x 8 field tiles in the kiln for a bisque firing to ^04, but I accidentally programmed a glaze firing. So I messed up a bunch of tile, but was able to see all of these differences clearly. I thought maybe the dark brown tiles were so dark b/c they were on the bottom of the stacks, but that wasn't the case: the results were mixed, with some of the top tiles dark and some of those that were touching the shelves were dark.  There doesn't seem to be a correlation between the witness cones and the coloration, although at the top and bottom of the kiln, where it's underfiring by a cone or so, the glaze is not AS muddied, but it's still not right, like the pine cone tile previously mentioned.

I brush three coats by hand on each tile. I've been doing it the same way for years and I'm confident that I'm brushing it the same way every time. I even tried to go very light on the coats several months ago, just to see, and it wasn't the problem. They looked like they didn't have enough glaze and they still were too dark. Aaack!

You can't cheaply/easily convert your existing kiln into a production kiln, although it may be cheaper than buying a whole new kiln. Talk to Skutt. You'd have to get a whole new control box, because the PK models have branch fusing, larger relays, wiring, etc. You'd also need new elements, feeder wires, bus bars, etc. Basically, everything but the brick parts. That's at least $1500, so cheaper than a new kiln but not cheap. You'd also have to upgrade your electrical service to 80 amps.

On glazed, unstacked tiles, are you seeing a difference from the middle of the shelf to the edges? Are you firing on shelves or tile setters?

[liambesaw]

What clay from SPS are you using?

[saltcreekstudio]

16 hours ago, neilestrick said:

You can't cheaply/easily convert your existing kiln into a production kiln, although it may be cheaper than buying a whole new kiln. Talk to Skutt. You'd have to get a whole new control box, because the PK models have branch fusing, larger relays, wiring, etc. You'd also need new elements, feeder wires, bus bars, etc. Basically, everything but the brick parts. That's at least $1500, so cheaper than a new kiln but not cheap. You'd also have to upgrade your electrical service to 80 amps.

On glazed, unstacked tiles, are you seeing a difference from the middle of the shelf to the edges? Are you firing on shelves or tile setters?

Ok. Thanks. On glazed tiles, I don't think I've noticed differences from middle shelf to the edges of shelf.  I don't use setters. 

[saltcreekstudio]

16 hours ago, liambesaw said:

What clay from SPS are you using?

Vashon Red w/ grog

[saltcreekstudio]

17 hours ago, Mark C. said:

You have several issues-the kiln taking longer to fire from medium to fast-elements wearing out or loose connections or corrosion in the wiring-check out all your connections first.

clay getting darker-the kiln is oxidizing so I'm going out on a limb and say its the clay body. Darker or lighter is the iron in the clay not a firing issue.Now if its overtired thats another deal but you said the cones did not show that.

or as Neil has said glaze thickness issues. 

glaze thickness can change a lot.Your tile in photo on lefty looks like the glaze is just to thick

Leaving the top and bottom empty is not good as well-as Neil said loose fill in bottom and top. Full loads in center.

Is your floor(shelves) off the bottom of kiln floor?

The more info you give the better

I feel you have several issues not one.

I hear you on the "more than one thing going on with the kiln" thing.

The elements are brand new and aren't worn out yet. I will definitely give my wiring connections a look. Thanks.

The clay body thing is what I want to believe too, but it's not adding up. I'm having dark and lighter color even from the same bag of clay,  in the same firing.  And I really don't think it's glaze thickness. The glaze seems the same consistency it always has been, and I'm brushing the same way I have for years. And, the clear difference is that darkened clay...

I should mention that occasionally I'll place a different kind of clay (white stoneware) on the higher shelves b/c that particular clay is tolerant. 

Floor is an inch from bottom. 

 

 

 

[Mark C.]

how about fire some small unglazed clay samples along with the glazed ones. That way you will see the body color and you can compare in various areas of kiln.

The key is to eliminate one thing at a time.

[liambesaw]

Did you always have the down draft venting?  I ask because I use SPS Klamath yellow and I'll get the different clay colors after bisque when I stack bowls or there's a lack of air movement in one area.  Sometimes a stack of bowls will be brown on the bottom bowl and top bowl and bright red to orange with the bowls in the middle, it seems really sensitive to localized atmosphere.  I'm sure you're not getting reduction atmosphere in your electric kiln, but all iron bearing clays will go darker with less oxygen in the kiln.

(Bottom draft venting creates a vacuum so maybe that is doing something?)

[neilestrick]

2 hours ago, liambesaw said:

Did you always have the down draft venting?  I ask because I use SPS Klamath yellow and I'll get the different clay colors after bisque when I stack bowls or there's a lack of air movement in one area.  Sometimes a stack of bowls will be brown on the bottom bowl and top bowl and bright red to orange with the bowls in the middle, it seems really sensitive to localized atmosphere.  I'm sure you're not getting reduction atmosphere in your electric kiln, but all iron bearing clays will go darker with less oxygen in the kiln.

(Bottom draft venting creates a vacuum so maybe that is doing something?)

The venting should be pulling more air into the kiln, making it more consistent.

Check to make sure your vent holes in the kiln bricks aren't clogged. I see that a lot.

[Mark C.]

One note is in ceramics 10,000 things can get you. Just when its all going well  it can all go to heck in a second. Thats the fun with clay right>?

Just keep working on a change at a time until you understand the process. I'm doubtful the clay in same bag is different colors-a simple bare clay tile test will prove this true or false.Also as melted in Liambesaws post stacking tight iron clays in bisque can yield different colors due to lack of burnt outs.

I assume you are stacking the snot out of tiles in a bisque(tight loads)

[saltcreekstudio]

3 hours ago, liambesaw said:

Did you always have the down draft venting?  I ask because I use SPS Klamath yellow and I'll get the different clay colors after bisque when I stack bowls or there's a lack of air movement in one area.  Sometimes a stack of bowls will be brown on the bottom bowl and top bowl and bright red to orange with the bowls in the middle, it seems really sensitive to localized atmosphere.  I'm sure you're not getting reduction atmosphere in your electric kiln, but all iron bearing clays will go darker with less oxygen in the kiln.

(Bottom draft venting creates a vacuum so maybe that is doing something?)

HMMMM.....   I like what you are saying here! So maybe the reason that clay color is varying  is due to the stacking at the bisque fire, and that the bottom tile is not getting as much airflow.  Oh, but I'm pretty sure I've doubled stacked and not had this issue. It's been long enough now that perhaps I'm not  remembering correctly and I was only putting one tile down. 

I will definitely test this theory. It seems like the only reason that some tiles are so much darker (the ones possibly not getting enough air on the bottom). I don't see a difference in the clay color after firing to 04 bisque, but all the bisqued tiles are jumbled up in preparation for the glaze firing, and I don't know what's what. I'll find out. 

I've held a lighter up to the downdraft holes in the kiln lid to make sure the vent is drawing air, and it is, but maybe I'll double check my ducting and the spot it connects to the kiln floor, too.

Thanks, 

Maria

***edit*** except when I fire the unglazed tiles there were very dark tiles both as the top tile and bottom tile in the stack. Ugh.

 

[saltcreekstudio]

25 minutes ago, Mark C. said:

One note is in ceramics 10,000 things can get you. Just when its all going well  it can all go to heck in a second. Thats the fun with clay right>?

Just keep working on a change at a time until you understand the process. I'm doubtful the clay in same bag is different colors-a simple bare clay tile test will prove this true or false.Also as melted in Liambesaws post stacking tight iron clays in bisque can yield different colors due to lack of burnt outs.

I assume you are stacking the snot out of tiles in a bisque(tight loads)

Hey Mark. That's the truth! Minus the "fun" part. Ha! This conundrum has been driving me nuts. I've been using the same glaze for years. I've had the same kiln model (only bought a brand new version a couple years ago, not long before this all started going wonky. The clay has changed from Highwater in NC to SPS, but I've achieved excellent results with the SPS clay initially. I can't remember that far back, but maybe I got good results because I didn't double stack that load for the bisque.

I only stack in twos. I'm guessing that still could be causing the problem (that the bottom tile in the stack is being deprived of air)? I'm going to test it. 

Thank you. ***edit*** except that I've had the top tile in a double-stack turn out that dark brown color. So confusing.

[neilestrick]

Any differences in color in the bisque should go away in the glaze firing.

[saltcreekstudio]

3 minutes ago, neilestrick said:

Any differences in color in the bisque should go away in the glaze firing.

Yeah, it's never been an issue for me before, but now he opposite is happening. The color of bisque doesn't vary, but the ^5 firing (glazed or not) is way too dark. 

[neilestrick]

Is all the clay from one mix batch? Could be you just got a batch that they messed up. And their could be a change in the mix even if they didn't change anything in their recipe, and haven't heard of any changes to their raw materials. The stuff is all out of the ground, so there can be some variation sometimes.

[Babs]

Well some iron coloured clay will darken when  fired higher........is than a possibility?

Do tiles placed on cool bottom or top change to a similar colour?

[glazenerd]

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. 

these 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

[saltcreekstudio]

On 10/13/2018 at 6:57 AM, neilestrick said:

Is all the clay from one mix batch? Could be you just got a batch that they messed up. And their could be a change in the mix even if they didn't change anything in their recipe, and haven't heard of any changes to their raw materials. The stuff is all out of the ground, so there can be some variation sometimes.

It's been happening for at least a year. I've wondered about whether or not something in the mining area was different. Maybe that's it. Still weird though that some tiles turn out really brown, and others less so, even from the same batch.