Iron

[porcelainbyAntoinette]

Hi Guys, I am looking for answers everywhere........Maybe I will find it here.........

I have mixed up a iron red glaze that I did not mix for a while. I used ingredients that is 10 years old and older. The end result was a flat brown instead of the rich red brown that it was before.

I repeated the recipe, thinking I made a mistake, ending up with the same results.

Then I thought maybe I used the wrong recipe and compared it with similar recipes getting to the conclusion that I did not make a mistake.

So someone said the problem is the iron that changed over time. This was the first time I heard that in all the 36+ years I am in clay. Is that true and if so, can I fix the iron, or do I trash it? I assume that if that is true, it has to do with the oxidation process.
Then someone  mentioned the bone ash - artificial versus real, which raised the question with me if the bone ash may "expire", since I used the real thing. 

 

As I said:  I used the exact same materials that I used on porcelain before. All these (except the silica and maybe the Custer) came from batches that I had in my studio for the best part of 15-18 years. (yes I have some valuable materials.....)

The recipe is no secret, it is similar to many others available online, but the reason I want to do it again is because over time some of the plates that I glazed with it, wore off, which told me that there is maybe not enough silica in this recipe. So I want to alter it some, but first need to make sure I have the color right. 
 

 Ralph’s Terracotta.

 

Custer F.                           41                   

Talc                                     9                    

Bone ash                          13                     

Lith Carb.                            2                       

Kentucky Ball clay            13                     

Silica                                 13                     

 

Add:

 

Iron ox                                 9    
  

Ideas please...........

 

PorcelainbyAntoinette

 

TeachinArt

[Min]

One thing that can make a big difference with iron reds is the firing and cooling schedule. Are you firing the same way in the same kiln? Any chance it's a faster cooling firing now?

Also, have you tried a strike firing of one of the brown pots to 04?

[neilestrick]

Custer has changed, that has been proven. There's a recent thread that has the new formula. Iron will not change, nor will the bone ash.

[douglas]

It might be worth doing some small batches with new iron oxide, firing alongside the glaze using the old, and see if you get different results.

 

I did not think iron oxide changes, but maybe it continues to oxidize over time, changing the color.

[Dick White]

The traditional iron red glazes have been troublesome in recent years, a lot of hair has been pulled out of many potters' heads. If you are using a stock of ancient materials you bought long ago, unlike fine wine I've not heard that the usual materials change just by aging in a bin under the bench.However, there have been some significant changes in the materials as produced in recent years. Specifically in your recipe, Custer has not been right for at least 7 years that I have been tracking it (much lower in potash than before) and the current gray talc from Texas responds differently from the white talc from the NY mine that closed in 2008. And, as you alluded, there is the difference between natural bone ash (roasted and ground bones left over from the meat processing industry) vs. synthetic bone ash (tri-calcium phosphate). Maybe the natural bone ash does go stale after a time? And there is an resolved debate over the results using the various types of red iron - high purity (with a higher price), "ordinary" (which is whatever your supplier has), crocus martis, Spanish (maybe the same as crocus?), rust scraped off a bridge overpass, etc. But again, I've not heard that the stuff goes bad just sitting for a decade.

 

And there has been some of research into cooling rates as mentioned by Min, together with some of the standard advice given through the ages that these glazes need a heavier application and firing on the hot side. And, as already mentioned by Min, refire to bisque temperature to restart and simulate the slow cooling to bring out the crystallization of the iron. I've put a few pieces in the next bisque firing, but it you have enough you could refire an entire load on a bisque cycle. However, even this may not exactly replicate the original plates.

[neilestrick]

According to DigitalFire, bone ash is chemically inert. I'd check the Custer.

[Dick White]

According to DigitalFire, bone ash is chemically inert. I'd check the Custer.

That's what I would have thought too, as any organic content in the bones should have been burned out in the calcining. But it's the only thing in the recipe that was not naturally already a million years old when dug out of the ground. What's another 10 years under the bench? Nerd once mentioned that lithium carb reacts slightly with plastic storage containers, is better stored in glass, but there is only 2% in this recipe.

[Dick White]

Antoinette - do you by chance have in your collection some old true G200 feldspar or slightly newer G200HP feldspar (both of which are unavailable now). The analysis of the old true G200 was close the old Custer, you could try that as a sub. Or if you have some of the G200HP when they switched mines to a higher-potash spar (hence the HP designation) you can mix it 70% G200HP/30% Minspar200 to attain the same content. The problem with trying to adjust around the current Custer is it has less potash than before (~7% now vs. ~10% before) but also less alumina (~15% now vs. ~17%) and more silica (~74% vs. ~69%) so just bumping up the amount of Custer in the recipe to regain the potassium will change other things too. Even if you recreate a recipe that is nominally exactly the same chemically, it may fire differently. Good luck with it.

[neilestrick]

 

According to DigitalFire, bone ash is chemically inert. I'd check the Custer.

That's what I would have thought too, as any organic content in the bones should have been burned out in the calcining. But it's the only thing in the recipe that was not naturally already a million years old when dug out of the ground. What's another 10 years under the bench? Nerd once mentioned that lithium carb reacts slightly with plastic storage containers, is better stored in glass, but there is only 2% in this recipe.

 

 

The calcining would get rid of any organic matter, and all that's left shouldn't be soluble. I get what you're saying, but I'd still check the Custer.

 

I'm wondering if the clay body is at all different? The iron red glaze I use looks completely different on each of the 4 clay bodies we use in my studio. It gives the least red color on porcelain, then gets progressively more red as you work towards the higher iron, lower silica bodies. The speckled brown gives the brightest red. That and firing schedule would have a huge impact. Are you firing any cooler?

[glazenerd]

There are some synthetics that I recall having issues. One in particular, was completely synthetic ( meaning it was not reduced from a natural iron source.) that I recall reading having some issues. Been awhile since I read that abstract, so I do not recall precisely the exact problems. A synthetic will have a fine particle/ dusty feel to it: almost like a red flour type feeling.

 

Nerd

[Fred Sweet]

Antoinette-

 

You might want to look at this from today's message from CeramicArtsDaily:

 

http://ceramicartsdaily.org/ceramic-glaze-recipes/glaze-chemistry-ceramic-glaze-recipes-2/techno-file-four-ways-to-red/?utm_source=WhatCountsEmail&utm_medium=CAN%20ListWarmUp-Day%2011_7-18-17_AM%20(21)&utm_campaign=Coming%20Soon%201

 

Don't know if the section on iron reds will help, but offered just the same.

 

Regards,

Fred

[bciskepottery]

don't discount that the composition of your porcelain has changed . . . in fact, if you are using the same glaze ingredients from 10 years ago, then the variable is your clay, which has likely undergone changes in materials.

[porcelainbyAntoinette]

bciskepottery, you nailed something that I thought of after I posted here. My clay changed from a grolleg based one to Southern Ice, which means I do not know which feldspar is in the clay! 

Thank you guys, I think this glaze may have to go back to basics from what I saw you say here. 

Dick I actually do have a batch of the old G 200. I use it very sparingly, but it is worth a test series, because I am working on a glaze workshop for my students. 

I've got info from other sources too that the Custer is probably the culprit; the last place where I would have looked. Since I do want to make the glaze somewhat harder, it may be worth it to add more Custer, but I think it will need a total re calculation. According to Ron Roy, all Custer as far back as 15 years ago, lacks about 3 % potash. 
 

I've also received information that the bone ash may be hygroscopic, changing the volume needed - Mississippi is extremely humid.... The suggestion was that I add more bone ash in increments to see if it may fix it. 

 

 

Thanks for the link Fred. I will check it. 
I wish I had my Matrix software program under my belt by now, but I am still learning to use it, because that may help me understand the glaze better........which is what I am really after...

Okay, I will work on it and let you know if I hit something worthy. 

[Min]

"I've also received information that the bone ash may be hygroscopic, changing the volume needed - Mississippi is extremely humid.... The suggestion was that I add more bone ash in increments to see if it may fix it." 

 

Might be an idea to take it up to red heat and re-calcine it to burn off any moisture/water then store it in a screw-top container. Wouldn't have to fiddle around trying to guess how much water it's absorbed.

 

John Post has done a lot of work on iron reds, his website is down until the end of the month but you can see one of his iron red recipes on the Glazy site. It uses NepSy for the spar instead of custer. There are a couple versions of it, one includes lithium carb. Don't know if this is of any help but perhaps a different direction to look in. 

[douglas]

Antoinette-

 

You might want to look at this from today's message from CeramicArtsDaily:

 

http://ceramicartsdaily.org/ceramic-glaze-recipes/glaze-chemistry-ceramic-glaze-recipes-2/techno-file-four-ways-to-red/?utm_source=WhatCountsEmail&utm_medium=CAN%20ListWarmUp-Day%2011_7-18-17_AM%20(21)&utm_campaign=Coming%20Soon%201

 

Don't know if the section on iron reds will help, but offered just the same.

 

Regards,

Fred

Is anyone else freaked out with the casual way they are recommending cadmium stains? They caveat that any glazes for food use should be tested, but many/most potters just use recipes and don't send glazes out for testing. Maybe I am overreacting, I am not a chemist, but since even trace amounts can cause cancer or be fatal, this seems like a poor ingredient to recommend to independent studios.

 

https://en.wikipedia.org/wiki/Cadmium_poisoning

[glazenerd]

Posted Yesterday, 10:31 PM.

" My clay changed from a grolleg based one to Southern Ice, which means I do not know which feldspar is in the clay! "

 

High translucency bodies can have up to 20% more spars than a standard body.

Grolleg ( Coleman's) runs 3.58% molar, and southern ice is probably above 4.25%. ( cone 10)

Not uncommon to find sodium in highly translucent bodies to produce higher glass content.

 

Nerd