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aljmarr

Very New to Clay. Need Help with Firing Temps!

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HI all, 

I've referred to this forum many times in the last few months, so thank you all for sharing your knowledge with the world. I just touched clay for the first time in August. After a short class, I fell in love with slab building and geometric shapes. In my class, we only used cone 10 clay, SOOO, when I left class and started playing with clay at home, I got Cone 10 clay. Now that I have LOTS of pieces to fire, I'm calling around to studios and art centers to find they they all fire greenware to bisqueware at 04. Can my Laguna cone 10 clay greenware be used for potted plants, vases and general art if it's just fired to 04? 

I would glaze fire to cone 6.

Any help is appreciated and sorry for this and many future dumb questions.

Thanks

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Glazing cone 10 clay with cone 05-06 glaze and firing it to anything hotter than cone 05 could cause the glaze to run right of the pots and onto your shelves.

You need to match your clay, glaze and firing temperatures.  Not doing so is a waste of your time and cost of your raw materials.

It's almost like putting diesel in a petrol car, just because they are similar.

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48 minutes ago, Chilly said:

Glazing cone 10 clay with cone 05-06 glaze and firing it to anything hotter than cone 05 could cause the glaze to run right of the pots and onto your shelves.

Chilly, and others,

I have a colleague  that has been using a commercial cone 05-06 clear glaze as the base glaze for all of his cone 10 firings of porcelain for about 8 years.   He adds various ingredients (red iron oxide, cobalt carbonate, yellow iron oxide, copper carbonate, rutile, zircopax, … ) to create color and texture.  He has never had the glaze run off the pot.   His application process is either spraying or brushing.  

I have often use thin coats of a similar clear cone 05-06 clear glaze on my cone 10 stoneware work to produce a low gloss to otherwise raw clay texture.  No problems.

In both of these situations, the low fire glaze begins to melt at about the same temperature as the cone 10 stoneware  was bisqued , therefore the surfaces of the ware is essentially at the same state as ware the glaze was designed for.  As the temperature rises, the glaze becomes more fluid and flows into the pores of the ware rather than just running down the pot onto the kiln shelf.  Meanwhile the stoneware clay body matures with the glaze melt being absorbed into the surface of the ware instead of remaining on the surface.      Application thickness certainly is important;  and testing on both vertical and horizontal representative surfaces of the final forms being glazed is a necessary step toward confidence in using this technique.   

LT

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True lowfire glazes mature at lowfire. They likely contain up to 0.5 boron to get them to melt this early. All glazes really are cone ten and simply lowered in some fashion to get them to melt at the lower temperatures. Cone ten clay will not vitrify at cone 04 just sinter so it seems to me a bit confusing as to what is trying to be achieved.

  • for claybodies fire to their rated cone to fully vitrify, gain strength and limit absorption. (Cone 10 clay, fire to cone 10)
  • for glazes, fire to their cone to form a fully fused glaze. Overfiring past a glaze rating can make it run or dry up or both

 

 

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On 12/20/2018 at 10:42 PM, Magnolia Mud Research said:

Chilly, and others,

I have a colleague  that has been using a commercial cone 05-06 clear glaze as the base glaze for all of his cone 10 firings of porcelain for about 8 years.   He adds various ingredients (red iron oxide, cobalt carbonate, yellow iron oxide, copper carbonate, rutile, zircopax, … ) to create color and texture.  He has never had the glaze run off the pot.   His application process is either spraying or brushing.  

I have often use thin coats of a similar clear cone 05-06 clear glaze on my cone 10 stoneware work to produce a low gloss to otherwise raw clay texture.  No problems.

In both of these situations, the low fire glaze begins to melt at about the same temperature as the cone 10 stoneware  was bisqued , therefore the surfaces of the ware is essentially at the same state as ware the glaze was designed for.  As the temperature rises, the glaze becomes more fluid and flows into the pores of the ware rather than just running down the pot onto the kiln shelf.  Meanwhile the stoneware clay body matures with the glaze melt being absorbed into the surface of the ware instead of remaining on the surface.      Application thickness certainly is important;  and testing on both vertical and horizontal representative surfaces of the final forms being glazed is a necessary step toward confidence in using this technique.   

LT

I do the same with lots of commercial glazes, but always test them with cookies under.  I have  some that work, some that definitely don't work, and some that kinda work, but change colour.

But I still think one should start with matching stuff to stuff rather than diving all over the place.

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liambesaw:

The answer to your question requires a consensus on the definition of stable.    

The glazes developed by my colleague used a low-fire clear glaze as a base; colorant was added to that base.  None of his glazes ran, boiled, crawled, or fell of the ware.  His work is mostly sculptural.  Glazing  is 99%  application.  I routinely use a suite of cone 5-6 glaze recipes when firing to cone 10 R without having problems, but I have 10+ years  trials and errors guiding my application techniques.   When the results were no as expected, I began to focus on what caused the difference between expectations and reality; sometimes expectations were required to be changed.

Edited by Magnolia Mud Research

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@Magnolia Mud Research

Agreed! Hate the word stable almost as much as food safe. Durable is probably a better qualifier. As far as anyone’s preference to do this, I assume it is for artistic reasons and not to necessarily enhance durability of the complete “thing”. It may improve the overall durability or overfire itself into oblivion and increase porosity, hard to say. 

Experience counts, if it fulfills his needs and fits his way of working then lots of testing until he is satisfied. Myself, unless this an exotic look I would just try and dial that look in with a cone ten glaze with a durable flux ratio. But that’s just me.

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Sorry, by stable I meant without defect and colorant leeching below drinking water levels.  Not sure what else stable would mean in this context or how durable is any better of an adjective.  The books I've read use the term stable so I'm sorry for upsetting you, they're my only frame of reference.

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Not your fault. The entire industry has a brain freeze around this which is forever confusing. I am aware of some colorant leaching tests copper as I recall, and potential towards personal permissible limits. Stabile to me is too broad and yet sounds specific by design I assume.  Leaching below some level assumes consumption, timeframe, solvency as well as other factors.

at this point if a glaze is durable compared to other glazes that is most relevant to me with respect to: is this the best functional glaze I can make for its intended use. Food safe is great in that it  excludes elements known harmful but that is about it.

I can simply mix a non durable “garbage glaze” and declare it food safe based on what is not in it. It might dissolve in  your mouth piece by piece or your dishwasher and since levels of dissolve silca are likely not at issue I can represent it as food safe. 

Add to that  every show I go to  where folks are screaming food safe, microwave safe, dishwasher safe (what The heck is that?) ........ it all gives me a headache.

again, not your fault, just my pet peeve. I choose to construct my stuff as best I can which at this point means durable glazes when functional. I know most others will not and even will tell me their stuff is  food safe.    Maybe it is, have some silica!

just joking about the last part but there is really little direction for most potters

Edited by Bill Kielb

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The difference in terminology from yesterday to today  rests in 20 years of the internet and self published books. This forum for instance adheres to some level of professional standards: even though there is often diversity of opinion. Those opinions are at least given within the confines of accepted theorem. Over the course of time, because anybody and everybody can create websites: standards become diminished and "pottery speak" replaces terminology.

i have suggested to fellow clay dogs that published recipes be assigned cone value of original formula and if the formula was intended for functional or non functional use. EX. C6 Redstone NF or cone 6 Redstone non functional. I can hope!

T

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I mean one of the definitions of stable in the dictionary is: not liable to undergo chemical decomposition, radioactive decay, or other physical change.  And to me that sounds about right.  Guess it has taken on another meaning, I had no idea.  I only this year read Roy & hesselberth, it's pretty old I suppose.  But I read a lot of Tony hansens stuff too and he's pretty deep on chemistry and whatnot.  Paint me out of the loop haha

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I think the salient point in MMR’s post is he has 10+ years of trial and error to sort out what works and what doesn’t and the appropriate application of his glazes.  For functional work make durable glazes with as much silica and alumina as you can get into them and keep the colouring oxides to accepted levels then fire the glaze to a fully melted state. If in doubt spend the $30- to get a glaze tested for suspected leaching if that's an issue.

Another term that gets misused is “vitrify”. A clay fired to maturity doesn't necessarily mean it's also vitrified. 

 

Edited by Min

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Unfortunately 30.00 gets a test for the “food safe” no go chems. Last test for copper leaching I read certainly showed copper leached but one would not be likely to ingest a dangerous level over a lifetime given known permissible limits.  A pretty exhaustive test which assumed some potential solvents but not nearly all.

not bad, just impossible to do for every glaze and colorant combination. I like all the ideas above but my idea of functional is likely  different than someone else’s.

we all have  sipped from the crazed factory made mug and not thought a bunch about it until we finished our coffee and looked in the bottom. How many folks are happy to drink from soda  fired vessels? All of these are testable, but likely way too complicated to test or too expensive.

30.00 will get a test for lead and cadmium, I am  mixing my glaze and there is no lead or cadmium. I don’t even keep it in my store of chems. As a matter of fact no vanadium, no manganese, .............. oh chromium? I like the colors I get!

I think the standard for durable chemistry is a good one,  We all know fluxes and we all know the range that produces durable glazes. We also know that far outside these levels usually means non durable glazes. Sometimes it is obvious,  the silicon carbide volcano glaze certainly does not appear durable. But what of those that take cone 10 glazes fire to cone 6 because now they have an interesting flat matte look. Is that durable?

lots of issues to ponder that most potters have no basis to even think about. I am not sure of the answer and actually folks have been using all sorts of questionable pottery for generations. Maybe it’s not as dangerous as I thought.

then again it’s always fun to see folks tell me it’s, food safe, microwave safe, and dishwasher safe! My dishwasher sleeps much better at night while eroding away the glaze a ten thousandth at a time.

fun discussion!

Edited by Bill Kielb

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3 hours ago, liambesaw said:

I mean one of the definitions of stable in the dictionary is: not liable to undergo chemical decomposition, radioactive decay, or other physical change.  And to me that sounds about right.  Guess it has taken on another meaning, I had no idea.  I only this year read Roy & hesselberth, it's pretty old I suppose.  But I read a lot of Tony hansens stuff too and he's pretty deep on chemistry and whatnot.  Paint me out of the loop haha

I understand your point but all things especially glazes undergo chemical decomposition and physical changes from the first day they are put in use. As far as the qualifier not liable, well I am not sure what that means but ok it sort of works. Stabile does not mean durable as durable is comparative with like things within the class. So my preference is durable but I get what you are saying with respect to stabile.

unfortunately (in my view) potters are really not given enough information and so this Jello like concept has evolved where most decent  folks are looking conscientiously for credible information but it’s availability is limited.

Further  complicating things are commercial glazes. Anyone have any idea what their recipes are? If it’s a flat metallic look it probably contains manganese. If it’s an interesting glaze that breaks and floats it’s probably got a non durable flux ratio.

what is a potter to do? The word Stabile is fine, I just think that as a whole the average potter is not equipped to have a reasonable perspective  and folks that study this are truly conflicted and often try for best known practice during their tenure as potters.

in the end, maybe most of this stuff is safe for the human animal - thousands of years of testing and consumption.

Edited by Bill Kielb

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12 hours ago, Min said:

I think the salient point in MMR’s post is he has 10+ years of trial and error to sort out what works and what doesn’t and the appropriate application of his glazes.  For functional work make durable glazes with as much silica and alumina as you can get into them and keep the colouring oxides to accepted levels then fire the glaze to a fully melted state. If in doubt spend the $30- to get a glaze tested for suspected leaching if that's an issue.

Another term that gets misused is “vitrify”. A clay fired to maturity doesn't necessarily mean it's also vitrified. 

 

So just to ask, your last sentence points to the fact that vitrification is the formation of glass. Earthenware, low fire claybodies can be fired to maturity but not fully vitrified (ever) and claybodies fired below their rating are likely not fully matured or fully vitrified.

overfiring results in devitrification if you will and the claybody begins to become less dense likely two, three or more  cones over its rated temperature.

just asking, this is my understanding which I think agrees with your statement.

Edited by Bill Kielb

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Just now, glazenerd said:

The difference in terminology from yesterday to today  rests in 20 years of the internet and self published books. This forum for instance adheres to some level of professional standards: even though there is often diversity of opinion. Those opinions are at least given within the confines of accepted theorem. Over the course of time, because anybody and everybody can create websites: standards become diminished and "pottery speak" replaces terminology.

i have suggested to fellow clay dogs that published recipes be assigned cone value of original formula and if the formula was intended for functional or non functional use. EX. C6 Redstone NF or cone 6 Redstone non functional. I can hope!

T

Well that might be true Tom, but terminology was written when cone 10  gas kilns ruled this trade. In the 80's cone 6 came on the scene, then later low fire: then finally polymers. Industry has developed clay and glaze that fully vitrifies at 800C using nano-particle technology. So this trade has struggled trying to classify new techniques into some digestible and applicable terms. The great dividing line has been and always be: functional or non-functional. If functional, then testing is always required regardless of established norms. If non-functional then freedom of expression is the order of the day. 

Yet my sense has been this trade has been going through yet another period of historic change. The large commercial potteries such as Rookwood gave way to the studio pottery movement of the 50's. Materials and firing ranges from the 80,s until now. The studio pottery movement is slowly given way to part time craft and hobbyist. Making a living in pottery is becoming tougher with each passing day: I personally would not even attempt it. Actually I am waiting for the day when I hear: " Alexa, throw me a bowl." 

......always wanted to argue with myself 

Tom

 

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11 hours ago, Bill Kielb said:

So just to ask, your last sentence points to the fact that vitrification is the formation of glass. Earthenware, low fire claybodies can be fired to maturity but not fully vitrified (ever) and claybodies fired below their rating are likely not fully matured or fully vitrified.

overfiring results in devitrification if you will and the claybody begins to become less dense likely two, three or more  cones over its rated temperature.

just asking, this is my understanding which I think agrees with your statement.

Yes, we are on the same page.

I think this is another term that in ceramics means something different than in other industries. We consider a clay vitrified if it does't leak, but this doesn't necessarily mean it has zero porosity. We are getting into the area of open or closed cell porosity and absorption versus porosity.

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When a clay is overfired, I would say it becomes “overvitrified”, meaning a greater percentage of the claybody ingredients have been pulled in to the glaze melt (ie, turned in to glass) than originally intended by the claybody designer.  

Under these conditions the clay body becomes more dense, not less.  Porosity goes down even further, approaching zero.  

Some clay body ingredients which are well-behaved at the designated firing temperature may begin to off-gas at overly high temperatures.  With the now very glassy body melt in full swing, filling every available pore, these new bubbles cannot escape from inside the body, hence small bloats begin the occur, which can be seen on the surface of the clay body.

Since the glassy part of the clay body has low viscosity (is very fluid) at those higher-than-intended temperatures, and as more and more glass is being created inside the body as heat work continues, the the whole clay body becomes “structurally compromised”.  The ratio of glassy phase to solid phase (ie, remaining unmelted identifiable particles of original clay body ingredients, such as grains of silica) in the clay body becomes so great that the pot begins to succumb to the forces of gravity.  

The shape of the pot begins to change on its own.  Making problems you thought you had masked or repaired re-appear (, thick/thin throwing problems).  Rims of wide bowls lose the perfect roundness they had off right the wheel.  Carefully applied handles on thinly thrown mugs begin to sag down, changing the shape of the mug rim.  Slight imperfections in the shape of the kiln shelf underneath the pot begin to change its shape as the pot melts down to conform to the kiln shelf unevenness.

Keep going like this and eventually it is all a molten mass stuck to the kiln shelf like those cone 10 earthenware disasters we always see pictures of.

Devitrification (ie, “de-glassing”) is actually glaze ingredients re-crystallising (rather than remaining in an amorphous glassy state) after having been melted at some point.  Often this happens from slow cooling (deliberate or accidental), but is a fairly complex topic on its own.  What kinds of crystals form is highly dependent on the nature and amounts of the original clay body ingredients and what intermediate molecular forms they take.  

Usually we think of glazes devtrifying (eg, clear glazes getting cloudy) rather than clay bodys, which routinely have lots of crystal phase within and amongst the glassy phase.  Eg, Mullite and Cristobalite are both commonly found crystal structures in clay bodies.

 

Edited by curt
Clarifications

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Interesting, I have heard that explanation although when I look at curves for claybodys they seem to become most dense at or near design and then maybe two or three cones beyond, actually decrease in  density and appear to expand. Tony Hansen has some nice examples of these mysterious expanding overfired bars. Formation of significant amounts of crystobalite seems to require among other things firing in the range of cone 14 so hopefully for ordinary potters cristobalite formation is minimal and rare at the temperatures they are working, else I expect lots and lots of cracked pots on cool down and total disaster for any refires.

Clay chemistry is fascinating to me but definitely not something I know the nuances of. When I look at @Min response it is the typical view I believe potters have held  of  the meaning of fully vitrified and often matches curves presented to explain the process. Sorry about the devitrified label I suspect sometimes a convenient way to say it becomes less waterproof.

Technically what Is the predominate mechanism that makes clay begin to increase in size and decrease in density if overfired beyond design or does this rarely occur?

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Yes, we’ve probably all seen clay bodies that have been grossly over fired to the incorrect cone and resulted in a puddle of glassy mess on a kiln shelf. But I don’t think that all clays when overfired go from their minimum porosity at their specific maturity and necessarily stay in that minimum porosity state when taken over their optimum firing cone. It’s my understanding that after maturity there can be a range where the clay increases in porosity. I’ve actually had this happen with 2 clay bodies, Glacier Porcelain (Tacoma Clay Art Center) and Plainsman 450. The former is a cone 4-6 body that I had a mishap with and it went to 8. Plainsman 450 is a cone 10 stoneware, I’ve had several pots go about 3 cones hotter and in both cases the pots thunked when tapped rather than ring and they leaked. If graphed the absorption would likely start high, go to the optimal level and plateau there for X number of degrees then increase then probably decrease. 

We are going beyond the scope of the OP's post, if we are going to continue this conversation a new thread needs to be started so we can find this discussion in the future, thanks.

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Sounds good. This thread started with what to fire to and it seems the conscientious answer is to the design temperature and not much over for structural and absorption reasons.. I too have seeen many pots overfired exhibit the thunk sound and the leaky pot as a whole symptom.

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