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Lustre / Reduction Chemistry Question

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Nerd,

 

yep!

 

What is the link to the whole MIT course? and the Name of the course?

 

Ellingham diagrams are useful, but they do not tell the whole story.  The most useful indicator for redox is the oxygen fugacity.  The most relevant source of information for potters would be how oxygen fugacity ​is used in igneous geochemistry textbooks or website devoted to that igneous geochemistry.  Other than the weird nomenclature, the concept is really simple.  To apply it correctly to the firing of pottery one needs to understand reaction equilibria and some physical chemistry. 

 

LT

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Guest JBaymore

We sometimes incorrectly refer to an electric kiln firing as an oxidation atmosphere, but technically it is a neutral atmosphere, the same as the natural air we breathe.

 

My turn to disagree...........

 

Since most electric kilns that studio artists use are not hermetically sealed... or are being pumped full of inert gases like sometimes used in industry...... they leak ambient air into the chamber both during the firing and afterward.  If a local pickup kiln vent is attached,... they are for SURE having a supply of air flowing thru them. 

 

That air contains roughly 20% oxygen.  That oxygen is available to anything that is at a reactive state that can be oxidized.  So electric kilns in most cases are actually oxidation firing.  In some rare cases, sections of such kilns (mainly those without local vents) can even be in a reducing potentail state if there are organics in the body enough to deplete the available oxygen and leave some CO. 

 

But in 99.9999999999% of studio artists cases, the outer surface of the work is always in an oxidized state because the instant the kiln is shut down, whether that is gas, oil, wood or whatever fired,....... the  chamber is (once again) not sealed to the leakage of air and hence the presence of oxygen.   The exception to this is the few folks that deliberately fire down to below reactive temperatures in a fuel-rich state.  Then the surfaces can maintain the reduced state and any color rendition produced by that being on the surface of the work.  Some American Raku fits this bill.  Traditional lustre firing also (not liquid lusters).

 

best,

 

...............john

 

Edit (fixed O2 percentage)

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We sometimes incorrectly refer to an electric kiln firing as an oxidation atmosphere, but technically it is a neutral atmosphere, the same as the natural air we breathe.

 

My turn to disagree...........

 

Since most electric kilns that studio artists use are not hermetically sealed... or are being pumped full of inert gases like sometimes used in industry...... they leak ambient air into the chamber both during the firing and afterward.  If a local pickup kiln vent is attached,... they are for SURE having a supply of air flowing thru them. 

 

That air contains roughly 30% oxygen.  That oxygen is available to anything that is at a reactive state that can be oxidized.  So electric kilns in most cases are actually oxidation firing.  In some rare cases, sections of such kilns (mainly those without local vents) can even be in a reducing potentail state if there are organics in the body enough to deplete the available oxygen and leave some CO. 

 

 

 

John,   Dry air is ~20.9 volume percent oxygen according to NIST.  so, are you using weight % when you say ~30%?

 

LT

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Guest JBaymore

 

......... there is a raku technique involving fuming with stannous chloride. These pieces are multi-fired, sometimes with precious metal luster glazes added, and the stannous chloride is poured onto hot bricks placed in the final firing, which then vaporizes and fumes the substrate glaze.

 

The same tin chloride fuming process is also sometimes used at stoneware firings.  Particularly for salt or soda firings.  It is CRAZY hazardous to do in a large gas kiln.  Done that wearing full head to toe fire resistant suits and face shield and respirator.  Great way to get killed...ripping a kiln door down and fuming with stannous.  ;)

 

best,

 

................john

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Guest JBaymore

 

 

We sometimes incorrectly refer to an electric kiln firing as an oxidation atmosphere, but technically it is a neutral atmosphere, the same as the natural air we breathe.

 

My turn to disagree...........

 

Since most electric kilns that studio artists use are not hermetically sealed... or are being pumped full of inert gases like sometimes used in industry...... they leak ambient air into the chamber both during the firing and afterward.  If a local pickup kiln vent is attached,... they are for SURE having a supply of air flowing thru them. 

 

That air contains roughly 30% oxygen.  That oxygen is available to anything that is at a reactive state that can be oxidized.  So electric kilns in most cases are actually oxidation firing.  In some rare cases, sections of such kilns (mainly those without local vents) can even be in a reducing potentail state if there are organics in the body enough to deplete the available oxygen and leave some CO. 

 

 

 

John,   Dry air is ~20.9 volume percent oxygen according to NIST.  so, are you using weight % when you say ~30%?

 

LT

 

Oops... typo.  GOing back and fix.  Thanks.

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Spot on, John!

I did an 8 year UV test in the out doors/sunlight using an acrylic sealant. The copper luster did not turn green where the sealant was applied.It did where there was no sealant.

Also, if you can keep the copper oxide under 5%, it will maintain the color ad not re-oxidize so quickly.

 

Marcia

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Ok, now it's my turn to disagree... oh, wait, I already did that once today, didn't I.... :D So that's a double negative, which makes an affirmative? My concern, Nerd, with your line of reasoning was with the notion (and I may have misunderstood you) that color changes and other reduction effects, whether in standard gas kilns or raku firings, are derived from CCVD. I have no information that supports that general methodology, other than, e.g., the occasional flashing of chrome greens by stray volatized tin flying around off other nearby work. Copper reds in a conventional reduction firing are so because we choked the flame somewhat and that sent carbon monox through the kiln, which stole an oxygen from the copper oxide in the glaze in order to complete itself back into stable CO2 on its way out to the flue, not because we CCVD deposited something new on it. Same for copper glazes in raku. Even with salt (and soda) firings, I'm not convinced we are actually depositing the decomposed free sodium on the pot and sending only the leftover chlorine up the chimney. My current belief is that the sodium vapor fluxes the clay on its way past causing it to create its own glassy surface, but then keeps going and recombines with the chlorine as it cools on the way up the chimney, leaving just a cloud of salty vapor, not the oft-stated awful cloud of toxic hydrochloric acid.

 

And yes, I was a bit sloppy with the disassociate/decompose terminology. My intent was that at kiln temperatures it's a big square dance. There is a lot of rearranging of partners, you probably won't go home with the same partner you came with, and some of the dancers, unable to take the heat, will just bail out up the chimney. But in the end, unless the sneaky carbon monox let in the back door by the dance caller takes away some of the chairs during the musical chairs game, everybody left at the end of the dance goes home happy. (And just because I'm an argumentative SOB tonight, in the crystalline glaze - I was under the impression that the operative transformation of the ZnO and SiO2 into the crystalline lattice of zinc orthosilicate was something like 2(ZnO) + SiO2 => O4SiZn2? Again, no oxidation or reduction, just a reorganization of partners into a big group hug at the end of the dance.)

 

And John, my notion of neutral atmosphere in the ordinary studio artist's electric kiln was in relation to ordinary atmospheric air, which as you note has ~20% oxygen. Our typical studio clay and glaze materials are already stable, non-reactive in ambient conditions, and firing them to kiln temperatures in the electric kiln generally doesn't result in the materials becoming any more reactive that they would automagically start losing or gaining oxygen atoms just because the air around them has some available. And making even more O2 than usual available from a tank won't start or stop anything either, nor would sealing it and filling with inert gas as in the industrial setting. Unlike reduction in a fuel kiln where the strategic production of carbon monox intentionally starts some specific reactivity. And yes, the very outer surfaces may re-oxidize, but the trick is to start reduction at a time when the core of the body/glaze is still open to the reactivity and keep it in reduction until the glaze matures and seals the core in its reduced state.

 

And that Ellingham biz... gonna have to spend a few more minutes to twist my head around that. Good stuff.

 

And final disagreement for the night - Nerd, you are smarter than many, most? But humble enough to share it, which is why I come to visit and learn from you, and all the others too.

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LT---sorry no, I do not have the information for the MIT course. Had that chart for years.

 

I use a band gap chart, in conjunction with the reduction potential chart shown above.

 

 

NERD;s Band Gap Chart

SiO2       0.033            Silica

Li2O       0.024            Lithium

Na2O      1.790           Sodium

K2O        0.830           Potassium

CaO        0.469            Calcium

TiO2       0.184            Titanium

Al2O3     0.054           Aluminum

ZnO        0.283            Zinc

Mo          2.160            Molybdenum

Although band gap gets into the Ev (electron volts) and ionization energies: the more glaze applicable approach is to just look at it as magnets. Molecules have electronegative/positive values (Paulings), so they can attract or repel just like any other magnet. The stronger band gap values will influence the lesser band gap values: when molecules begin to reform after being disassociated.

 

Nerd

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are derived from CCVD.

 

Yes, many misunderstood; but then again I may have phrased it wrong: giving that impression. I used CCVD, only in the sense to describe the process of depositing thin films on substrate: in no way was I implying that it was the actual chemical reaction. Actually, it is not totally accurate either: but the only defined process that remotely resembles a Raku firing.

 

Nerd

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DW said: I was under the impression that the operative transformation of the ZnO and SiO2 into the crystalline lattice of zinc orthosilicate was something like 2(ZnO) + SiO2 => O4SiZn2? Again, no oxidation or reduction, just a reorganization of partners into a big group hug at the end of the dance.)

Sorry, forgot to address this last time.

 

When writing chemical chains:the change in molecular structure comes after the element ZnO (twinned) 2 and SiO2 (twinned) 4  

Zinc silicate (crystalline is an ionic bond, so you are correct: an electrostatic, instead of a chemical bond. ZnO2SiO4 describes the actual HCP stacking pattern of

AbbAbbAbbAbb........ 

 

350px-Willemite.png  << even looks like a crystal.

Silica, zinc, and lithium all have hexagonal crystal structures: which reforms at the soak temps. The other reason sodium and potassium should be avoided in crystalline recipes: first because they have cubic crystal structures; which deform the hexagonal pattern: so you end up with spikes and cluster needles instead of round florets. Secondly, the have much stronger band gap values, which also compete with the lower values of silica and zinc: which also interferes with ionic bonding.

 

Nerd

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Ok, Nerd, now we are off in our own little corner talking about something completely different from the rest of the room, so y'all out there, pay us no mind...

 

I dabble (though some would say more than dabble) in the crystalline world, so I am very curious about your statement, "The other reason Na and K should be avoided..." I've got a zillion collected macro crystalline recipes from all over, some published in books, others are private favorites of fellow crystalliers, and some of my own invention, both "cone 6" and "cone 10" (my personal work is in the 6 range, though as you know in our corner of the world, the "cone" attribute is pretty loose). But nearly all of them involve a significant proportion (half or more) of assorted frits that are heavy in sodium - 3110, F413, F644, F493. What else is there? How can I avoid the sodium if it is that bad?

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Dick: lets blame Mousey for asking a technical question to begin with: it will release us of all guilt :)

 

Frit 3110 is just slightly over 7% (weight) sodium @ 50% of recipe, FF644 I believe is  the old soda glass standard of 70silica/30 sodium? Frits have already undergone even higher heat to melt the raw materials; so the direct effects of sodium is somewhat diminished due to decomposition: although Na cations still exist. That said, those levels of sodium we have to deal with; no choice.

 

The first thread I started a year ago was the Calclium Borate" thread, in which I talked about anisotropic etching to modify the properties of existing elements. Not received very well; but then again new things rarely are. I started doing this because I noticed silica, alumina, and yellow zinc all had certain levels of sodium. The silica I was using initially had nearly 10%, and the alumina nearly 8%. I was etching silica primarily to impair its crystal lattice; so it would disassociate earlier in the melt (cone 6). I still convert alumina hydrate into pure alumina oxide however. After more research, I switched to French process zinc, which is 99.7% pure, Imsil A25 silica, which is 99.9% pure, and use alumina oxide which is also pure. I noticed the change in outcome immediately; so my quest for purer elements was solely based at removing impurities that would directly effect crystal formation. I now use Imsil and XL500 zinc in all my glazes, as well as NZ kaolin: you will see the difference; especially in clears.

 

After correcting those glaze problems, it had already occurred to me that the clay was adding to the problem as well. Nep SY is the flux of choice in most porcelain bodies; and adds a significant amount of sodium directly to the glaze from leaching. 3-4 years ago I started studying and experimenting with clay bodies; which culminated in meeting Ron Roy last March in KC @ NCECA. I ambushed the poor guy with pages of notes and questions: to his credit he answered them. He recently sent me an email apologizing for getting me into "clay this deep", after he read my SAS thread. I now make a sodium free porcelain called Coma Clay (little memory) that produce much better crystals, and I rarely see spikes or clusters. In my Come Clay thread you will see some results. 

 

Coma Clay 01

 
The brown tile on top is cone 10 with 2.5 hours of soak, the green tile is cone 6 with 2.5 hours of soak: cone 6 is now larger than cone 10.
and now my liners are much cleaner, and I have more control (laughs) over the crystals

Base Line

 
Can you eliminate all sodium? NO... but you can keep it under 10% total including clay-which changes everything. I am currently formulating the clay to provide a no run glaze; making good progress.. I have ran 2-10" cylinders without catchers :).
 
Nerd

 

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NERD;s Band Gap Chart

SiO2       0.033            Silica

Li2O       0.024            Lithium

Na2O      1.790           Sodium

K2O        0.830           Potassium

CaO        0.469            Calcium

TiO2       0.184            Titanium

Al2O3     0.054           Aluminum

ZnO        0.283            Zinc

Mo          2.160            Molybdenum

Silica 0.033, zinc 0.283, and lithium 0.024 are the primary ingredients for crystals. I leave frit out of the equation because we cannot alter that decision. Band gap is relating Ev (electron volts) of these elements: which gets into all kinds of techno talk. Leave everything else out and look at only this: the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band ---- molecular magnets is very simple terms.

 

Zno is the strongest magnet at 0.283, and will attract the weakest magnet (silica) at 0.033: which is why lithium is used because its 0.024 band gap is not strong enough to interfere with the primary elements required to make crystals. The test books on crystals, including Insight state: "lithium has an affinity for zinc."  Actually that is wrong: lithium does not have a strong enough band gap to attract anything.; which is why it has the reputation of being the "safe" flux. However, sodium has a band gap of 1.790; and does have the electron affinity to attract weaker elements. It has a cubic lattice instead of a hexagonal lattice: which lithium, silica, and zinc all have. So you have this molecular magnet battle going on when you get sodium too high. That help? ..

 

Nerd

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Guest JBaymore

 Our typical studio clay and glaze materials are already stable, non-reactive in ambient conditions, and firing them to kiln temperatures in the electric kiln generally doesn't result in the materials becoming any more reactive that they would automagically start losing or gaining oxygen atoms just because the air around them has some available.

 

 

 

That oxygen is available to anything that is at a reactive state that can be oxidized.  So electric kilns in most cases are actually oxidation firing. 

 

Note that I did not say that anything WAS necessarily being oxidized, only that anything that CAN be would be in that kiln environment.  And there are things in bisque firings that need to be oxidized from the state they are supplied in.... such as carbonaceous compounds that are impurities in clay body materials.  And in some cases materials that go into glazes are in a form that will oxidize...such as say, black iron oxide then oxidizing to the red form (before maybe getting reduced in some cases back to the black form ;)) .  There is also the oxidation possibility for resinous luster carrier.  The need to burn off glaze additives like CMC and gums.  Oh... and wax from resists.  And so on.

 

best,

 

........john

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Nerd, damn, you da man, as they say in the 'hood. 10" test cylinders with no drips! There was a guy active in the crystalline discussion groups a few years ago, William Melstrom from Texas, who had it figured out so that he didn't need catchers at all, just a nice fat roll at the bottom edge. He never revealed his secret, and seems to have dropped out of the crystalline scene for the last few years (his other love was his motorcycle - serious competition...). The 10% limit on Na is useful info. Just checked a few of my recipes and most of them are within range except for the 644 ones; they are in the 12% range. And yes, that frit is roughly 70/30 soda glass, actually ~60 Si/10 Al/30 Na but who's counting... And no, I'll probably not go so far as to mix my own porcelain. My main interest right now is improving the repeatability and consistency, even if at a lower level of perfection. Having played with it over a few years, the local arts center director thinks I am sufficiently expert to teach a class - haha, does he ever have rocks in his head... At this level for the students, it is more about learning the practical aspects - form of the vessel, prep of the catchers and risers, and then the big bugaboo of glaze application - than technical development of the glaze chemistry. Especially since I have to work within the limits of their conventional clay and glaze materials, i.e., white stoneware body, only useful frit is 3110, standard crap Zn, standard crap Si, etc. Fortunately I have my own private stock at home for my work.

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John, you are absolutely correct in that the ambient air provides the necessary O2 for burnout of organics and other contaminants. I was trying to stay within what I perceived as the original question of raku, which veered off a bit into more general discussion of glaze reduction. With the exception of your apt example of black iron, the common basic ceramic materials we use are stable and nonreactive in the electric kiln. We don't do anything special during the electric firing to force an "oxidation" change to occur in the same manner as in the gas kiln where we tweak the damper and cause an unnatural "reduction" that otherwise would not have occurred. I had to chuckle at the black iron, as I can't remember how many times students have whined about how the black iron wash turned red... And you forgot the oxidation reaction in the electric kiln that is most troubling - the damn elements... :P

 

cheers

dw

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Guest JBaymore

 And you forgot the oxidation reaction in the electric kiln that is most troubling - the damn elements... :P

 

That's why god made globars  ;) .

 

I love all the folks that have to order more black iron oxide for a glaze..... when they have 50 pounds of red sitting there in a bag.

 

best,

 

................john

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john:

 

I know several crystalliers firing to cone 10 who use element wash (forgot the  name of it) who declare it extends life greatly. Maybe Neil has an opinion, but the premise is to prevent oxidation of the elements.

 

Dick:

 

I have talked to Bill Melstrom in email a few times over years, Bill Campbell, Avery, and a few other well known. My current favorite is from France: Jose to those who know him:  ceramicasjosemariscal.blogspot.com.es/

Although Peter Frolich from Austria grows some mean crystals.

 

William is a big fan of talc.. for the record.

 

Nerd

 

I will convert your red iron oxide to black iron oxide for just a small fee: contact me @ Thatwasajoke.com

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John, where can I order some globars for my L&L Jupiter?

 

John and Nerd, back to serious for a moment - the element wash is ITC 213. I have a jar of it right here, just waiting for my current elements to go. I haven't used it before, but I understand it is tricky to get on just right. My kiln brick interior is coated with ITC 100, as is my raku kiln fiber. Good stuff. Some crystalliers swear by APM elements, but you need to do your cost benefit analysis before springing that kind of cash.

 

Nerd, yup, know some of those names. My mentor was Bill Schran, worked with him as his college studio monkey for 10 years before he bailed the educational system and just moved to NC. We had a big gallery show and confab at the college back in '07 and many of them came to town for the opening. Had them all for dinner at my house. Blew my youngest son's mind to see two pottery magazine back covers, one with Jamie Koz hawking Skutt and the other with Bill Campbell swooning over L&L, both of them sitting side by side on the couch like chatting old friends. Jose is from Spain, and not only does extraordinary crystalline, his throwing is out of this world. The group had another confab at his studio in Spain a few years ago, Bill went but I couldn't get away. We were planning on another gathering this spring to coincide with NCECA in Portland, but Avi had some health issues so we cancelled the idea for now.

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Mousey:

 

2 things before this thread goes into forum archives.

 

1. Everyone that responded are NERDS!!!!!.. but I am the only one who admits it.

 

2, I would like to disagree with what everyone said, except me of course. I did not get the chance to disagree with any one, I feel left out, rejected, unappreciated, unheard.. in fact the sudden rush of emotions is making my left eye twitch. My palms are getting sweaty because I really disagreed with Dick, but did not say anything. Geesh, now I am getting facial tics.. someone book me on Dr. Phil...  

 

Its clay... enjoy it, make something perty.

 

Nerd

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Guest JBaymore

1. Everyone that responded are NERDS!!!!!..

 

 

Sssssshhhhhhhhhhhhhh.......................................................   ;)

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Guest JBaymore

John and Nerd, back to serious for a moment - the element wash is ITC 213. I have a jar of it right here, just waiting for my current elements to go. I haven't used it before, but I understand it is tricky to get on just right. My kiln brick interior is coated with ITC 100, as is my raku kiln fiber. Good stuff.

 

I've used ITC100HT for some clients on soda kilns built of Thermal Ceramics 2500 IFBs in the past.  It does extend the hot face brick life in that environment.  But not hugely.  I've tested it in wood.  Nothing really special I see there. 

 

The 213 on the metal elements is basically forming a "ceramic shell" casing around the elements.  Keeping the oxidizing atmosphere  out.  If there is a pinhole in the coating on an element....... right there is where it will fail.  So the coating process is key to success with that stuff.

 

And Dick,,....... just get a brick saw and cut a silicon carbide shelf up for your very own Globars.  You'll have to up the supply voltage and amperage a bit though.  ;)

 

best,

 

....................john

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Hi - I just wanted everyone to know that I have a board on Pinterest - 'Reduced Paste & Fumed Lustre'  an interest of mine.  There are visuals showing different technique examples; Paste, Fumed, Eosin, Kosai plus books & video references all of which I'm gradually sorting into categories.  Hope this may be helpful & if anyone knows of ceramists working in these fields please let me know so I can add their work...thanks Val

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