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Newbie questions about firing and mildew


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#1 SueG56

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Posted 04 May 2013 - 03:12 PM

Glad to have found this forum and hope someone can help me with some basic questions!


I am making ornaments - I am using a cone 6 porcelain - i have rolled out the porcelain on a pasta machine to about 1/8" thick.
I have had success making my ornaments, but am having problems once I have glazed them and start painting them (china paints). Apparently I am not firing correctly, because after all this work, I am finding many pieces with mildew spots.

So, I am running a new batch. This time I am firing very slowly - turning up the heat half a notch every hour or so. I don't have any fancy controllers on my kiln - it is only a doll kiln with a kiln sitter.

Anyway, I plan on firing to cone 04 and then sanding down the rough edges and refiring to cone 6 and then glazing at cone 06. Here are my questions:

1. After I have fired to cone 04 and then sanded the edges down, when I refire these to cone 6, do I need to slowly ramp up the temperature again, or does all the water come out with the first firing? Or can I just fire them on high until it reaches cone 6?

2. Am I in the right ballpark in assuming that my mildew problem is from trying to fire too fast the first time and excess water is not getting out?



Thanks for any help you can provide!

#2 OffCenter

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Posted 04 May 2013 - 03:47 PM

1. After I have fired to cone 04 and then sanded the edges down, when I refire these to cone 6, do I need to slowly ramp up the temperature again, or does all the water come out with the first firing? Or can I just fire them on high until it reaches cone 6?

2. Am I in the right ballpark in assuming that my mildew problem is from trying to fire too fast the first time and excess water is not getting out?


1. Just fire them. No need to go slow.

2. No. The mildew has nothing to do with the firing and is probably caused by the china paint and/or whatever you put on them after they were fired.

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#3 Chris Campbell

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Posted 04 May 2013 - 05:31 PM

I will offer the following ....

If you are cutting them out with a cookie cutter, put dry cleaner plastic between the cutter and the clay and you will have far less clean up to do.

You could color them with underglazes at dry or bisque and save yourself a firing.

I also cannot see how the mildew could be from anything fired ... What temp are you firing the china paints to?

Chris Campbell
Contemporary Fine Colored Porcelain
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#4 Diane Puckett

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Posted 04 May 2013 - 05:34 PM

Anything that has been fired in a kiln should be sterile. No mildew, no bacteria, no virus, nothing. Even if there were water or organic material left, it would be sterile. If what you are seeing is really mildew, it is not coming from clay which has been fired. I am wondering if it is mildew or something else that looks similar. It is common for unfired, wet clay to get moldy, especially if it sits around in plastic for awhile, but I don't know how mildew would grow on your fired work. Do you have mildew on other things in your studio that might be contaminating your finished pieces? Is this only where they are painted? If so, maybe it is something in your paints.
Diane Puckett
Dry Ridge Pottery

#5 SueG56

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Posted 04 May 2013 - 06:23 PM

Anything that has been fired in a kiln should be sterile. No mildew, no bacteria, no virus, nothing. Even if there were water or organic material left, it would be sterile. If what you are seeing is really mildew, it is not coming from clay which has been fired. I am wondering if it is mildew or something else that looks similar. It is common for unfired, wet clay to get moldy, especially if it sits around in plastic for awhile, but I don't know how mildew would grow on your fired work. Do you have mildew on other things in your studio that might be contaminating your finished pieces? Is this only where they are painted? If so, maybe it is something in your paints.



There are black splotches on the fired pieces. I think I have thrown all of them out as they come up but I will see if I have any more and post a picture. The clay is new - no mildew on that. No mildew anywhere around either. The paints are freshly mixed with oil based medium. The first batch I fired to 015 and then the second batch I fired to 017. I will have to see what happens if I fire some blanks to 015 without adding china paints to them. It is very frustrating to get that far and have the splotches appear!

#6 SueG56

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Posted 04 May 2013 - 06:37 PM

Here are a few of the pieces. The one on the bottom left has had one firing with china paints. The other two have had none, but they still have the black spots, so it doesn't seem as though the black spotting is coming from the china paints.

Attached Files



#7 Diane Puckett

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Posted 04 May 2013 - 07:44 PM

Here are a few of the pieces. The one on the bottom left has had one firing with china paints. The other two have had none, but they still have the black spots, so it doesn't seem as though the black spotting is coming from the china paints.

Hmm..it does look like mildew, but there is no way mold would make it through a firing. I assume these have not gotten wet and been sitting around for awhile after firing. Or even just sitting under plastic. I would try mixing a solution of Clorox and water, pouring it on one of the pieces, and letting it sit a few minutes. If it is mildew, it should disappear. If it dos not, you will know it is not mildew.

This is baffling. What clay are you using? Assuming you are firing these directly on your kiln shelves, are they clean and not discolored from something that was fired previously, maybe something with black iron oxide?
Diane Puckett
Dry Ridge Pottery

#8 SueG56

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Posted 04 May 2013 - 08:53 PM

Nope. They have not gotten wet after firing, nor have they been sitting around. Tried the clorox and it did nothing. The black spots go all the way through the one piece that had the paint on it and stops just short of the bottom of the piece. If it was the china paints, it went through the cone 06 glaze and then into the porcelain. On the other piece I broke it was from the bottom up - i.e. the back of the piece has the spots on it and the top (where there is glaze) does not.

I am using Highwater Clay porcelain http://www.highwater...21&ParentCat=37



The kiln shelves are clean - nothing with black iron oxide (don't even know what that is - I fire just the porcelain, the glaze and then the china paints)

Sorry for the crummy picture. You can only get so close!









Attached Files

  • Attached File  m1.jpg   66.85KB   66 downloads
  • Attached File  m3.jpg   73.63KB   64 downloads


#9 bciskepottery

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Posted 04 May 2013 - 10:02 PM

Possibly black coring? Can happen when wares are fired too fast in bisque firing . . . may not show in bisque, but shows in glaze firing.

#10 Chris Campbell

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Posted 04 May 2013 - 10:38 PM

That is so strange ... Have you had a chance to call Highwater?
I've used fairly new P5 within the past two weeks and had no problems but I would definitely send them your images and ask them if it could be something in the clay.

Chris Campbell
Contemporary Fine Colored Porcelain
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#11 jo4550

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Posted 05 May 2013 - 01:51 AM

Hello Sue
What you are describing is quite prevalent in refiring glazed ware at at China paint temperatures. It is often called mildew but it is actually black spotting that occurs on the UNGLAZED surfaces when the clay body has NOT been fired to full maturity. Hence you will normally find it under the work (where it touches the shelf)and footrings. It is not trapped carbon from poor bisqeing. It is mostly found in older pieces that have been around a while. It is carbon being trapped in the open pores of the body and this can be generated from a variety of sources.

The carbon can become reabsorbed and trapped from the media being used so it is more prevalent to occur from oil based mediums than water based mediums. This coupled with poor circulation and ventilation in the kiln during firing allows the pores of the unglazed clay surfaces to reabsorb this new carbon. This carbon can be burnt out with repeated firings especially if the temperature is raised slightly higher. The easy answer to all this is that if you are making things from scratch make sure you fire to the full maturity of the clay or otherwise get a clay that vitrifies at the temperature you want to fire to. Then this problem shouldn't occur. Always make sure you have the lid or door of the kiln slightly ajar in the overglaze firing until at least 650.C This allows the gases generated from the mediums to be dispersed. Always allow plenty of space between the shelves in the kiln during an overglaze firing. Always try to fire to the highest temperature in your overglaze firing depending on your glaze type. eg if using porcelain fire to 800-820.C rather than the lower 760-780.C option. This allows more carbon to burn out. If at all possible fire your pieces lifted OFF the shelf by little wads or pins.

I have attached a small .pdf made from page 125 of "Ceramic Faults and their remedies" by Harry Fraser. This deals with this topic in greater detail and it saves me from having to type it all out.
Attached File  Black spot.pdf   492.53KB   27 downloads

Regards

Johanna

#12 OffCenter

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Posted 05 May 2013 - 08:12 AM

Looks like bciskepottery and jo4550 solved the mystery for you. Interesting.

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#13 SueG56

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Posted 05 May 2013 - 09:16 AM

Wow! Thanks!

Reading your post and the link you posted, I find multiple things that I could have done wrong... which brings up more questions (isn't that always the way?)

Since some of the pieces have not been fired at china painting temps and still have the spots, I'm thinking that the cause of my problems happend before I got to that stage. Here is where it is going to be painfully obvious that I have no idea what I'm doing:

What I've been doing up till now is keeping my little kiln on high, loading it and firing it to the temp I want it. (I know... I know) My story is that I didn't care about the translucency business, since I was going to paint over them anyway. Who knew it was more complicated than that? Not me! I'd bisque fire, sand the pieces down, refire to cone 6, and glaze with my 06 glaze. I have not been cracking the top of the kiln during the glazing stage (or at all for that matter). Are you all cringing yet? The pieces have been laid on kiln shelves and placed fairly closely together. I don't have any fancy equipment on my little doll kiln - only a kiln sitter that turns the kiln off when the pyrometric bar bends to let me know I've reached the proper temperature.

And here is what I think you are telling me to do: Please correct me if I am wrong! I have already bisqe fired the butterflies to 04. Am I to understand I can get a clear glaze that fires to cone 6 and apply it to the bisqued pieces now, thereby saving myself a firing instead of using the cone 06 glaze? When I do fire to cone 6, am I to do this slowly or does it not matter since the pieces are already bisqued? Whichever glaze I use, I will need to vent the top of the kiln to allow gasses to escape during the glazing stage. And now the really stupid question - how do I know when I have reached 650 degrees and can close the top of the kiln? How far open should the top of the kiln be during this stage? Would a 1" kiln post prop it open enough or too much? Does anyone know how much of a temperature increase each one of those little dots on the temperature knob represents? And last but not least, if I do decide to keep using the 06 glaze, can I set the emp on high and fire away, or does it need to be done slowly as well? Oh, wait... one more question - can I use cone plaques to prop my pieces on? If so, do they need to be coated in kiln wash first?


Sorry for all the questions - I really appreciate the opportunity to pick your brains -- and your patience with someone who obviously doesn't know much about ceramics and using this kiln!


Sue G

#14 OffCenter

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Posted 05 May 2013 - 10:05 AM

And here is what I think you are telling me to do: Please correct me if I am wrong! I have already bisqe fired the butterflies to 04. Am I to understand I can get a clear glaze that fires to cone 6 and apply it to the bisqued pieces now, thereby saving myself a firing instead of using the cone 06 glaze? When I do fire to cone 6, am I to do this slowly or does it not matter since the pieces are already bisqued? Whichever glaze I use, I will need to vent the top of the kiln to allow gasses to escape during the glazing stage. And now the really stupid question - how do I know when I have reached 650 degrees and can close the top of the kiln? How far open should the top of the kiln be during this stage? Would a 1" kiln post prop it open enough or too much? Does anyone know how much of a temperature increase each one of those little dots on the temperature knob represents? And last but not least, if I do decide to keep using the 06 glaze, can I set the emp on high and fire away, or does it need to be done slowly as well? Oh, wait... one more question - can I use cone plaques to prop my pieces on? If so, do they need to be coated in kiln wash first.


Yes. Use a cone 6 clear glaze instead of refiring to cone 06. If you use commercial glazes any will work but Opulence 125 clear is almost foolproof and will not muffle the sound of the chimes much. Be careful not to get whatever clear glaze you use too thick since you are used to applying glaze to vitrified clay. bisqued clay will absorb the glaze much better and faster.

You're making this a lot harder that it should be. Don't leave the lid or peeps open just fire it as fast as it will go until the setter turns it off.

"Does anyone know how much of a temperature increase each one of those little dots on the temperature knob represents?" Each dot represents 12.5 degrees except on Tuesdays when each dot represents 14.0 degrees.

A third firing to cone 06 would be completely counter-productive and probably soften the sound but regardless of what you decide to do about glaze firing, just fire the kiln without leaving anything open and as fast as it will fire.

Not clear on how you plan to use the cone plaques but if they touch glaze they're going to stick to the piece with or without kiln wash.

Jim
E pur si muove.

"But it does move," said Galileo under his breath.

#15 SueG56

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Posted 05 May 2013 - 10:45 AM

Oh... duh. I did not realize the china paints were the OVERGLAZE!




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