Question on Formulating Cone 5 Porcelain Clay Body

[kraythe]

I have been purchasing my porcelain from a local provider but the problem is they won't tell me what is in it and sometimes it doesn't perform the way I want to. Now disclaimer, I am a hobbyist, not a professional so please keep that in mind. The thing is I have been Formulating a Porcelain by  Digital Fire and trying to figure out where to start.  Digital Fire doesn't tell me the vitrification temperature of the clay or how to calculate what cone the clay body is. So how can one calculate the temperature for a clay. Another question is where to start, I am trying to find recipies for this kind of porcelain to start with so I know kind of a direction to go. I want to create small batches and try them on the wheel and so on and then when I get one I like, my supplier will make 1800# for me. 

[neilestrick]

Don't let @glazenerd see this thread or you're in for it!

What exactly is the problem with the porcelain you're currently using, and what properties are you hoping to get out of your own formula?

The most basic cone 10 porcelain formula is 25% silica, 25% feldspar, 50% kaolin. Going from there, for a cone 6 porcelain you'll have to increase the feldspar to get it to fuse at a lower temperature. You'll also need to add plasticizers to make it more workable. I'm a big fan of VeeGum T, but there are other (cheaper) white bentonites and such that work well, too. The best porcelains use English Grolleg for their koalin. If you plan to have your supplier mix it for you, then you'll first need to find out what materials they keep in their inventory that are available for you to use in your formula. Convincing them to get in certain materials just for your mix will be difficult and expensive. Bear in mind, too, that this will take a long time, like maybe years to get just right. It may be easier to simply try out other porcelain bodies from other suppliers.

[kraythe]

48 minutes ago, neilestrick said:

Don't let @glazenerd see this thread or you're in for it!

What exactly is the problem with the porcelain you're currently using, and what properties are you hoping to get out of your own formula?

The most basic cone 10 porcelain formula is 25% silica, 25% feldspar, 50% kaolin. Going from there, for a cone 6 porcelain you'll have to increase the feldspar to get it to fuse at a lower temperature. You'll also need to add plasticizers to make it more workable. I'm a big fan of VeeGum T, but there are other (cheaper) white bentonites and such that work well, too. The best porcelains use English Grolleg for their koalin. If you plan to have your supplier mix it for you, then you'll first need to find out what materials they keep in their inventory that are available for you to use in your formula. Convincing them to get in certain materials just for your mix will be difficult and expensive. Bear in mind, too, that this will take a long time, like maybe years to get just right. It may be easier to simply try out other porcelain bodies from other suppliers.

Well more certainty in the knowledge of the clay body. I don't have a clue what is in it or what percentages and nearly every glaze crazes  and since I am a hobbyist I don't have a ton of stuff to fire together with good glazes and one trial glaze so I have a ton of unglazed pieces that I would like to finish and NOT ruin with a shivering or crazing glaze and I am having problems getting the right fit. My supplier offered "we sympathize" which is nice but of course useless. They wont tell me what is in it, I don't know how to fit a glaze and I am wasting time and propane in my gas kiln trying to figure it out to the point that I walk away from it fore weeks at a time in frustration. 

Furthermore, they say the CoTE is 6.0 but almost none of the glazes on Digital Fire match that super small thermal expansion in cone 5 and so I attempt to adjust the glaze using this article to try and match by adding stupid amounts of silica to try to bring down the expansion but then it does other stupid thing because I am not altering a recipe by 2% but by like 50% and that is obviously going to lead to problems. For example a V.C. 71 glaze I found on Page 87 "The Complete Guide to Mid Range Glazes" By John Britt has a CoTE of like 7.4 and crazes hard. Not surprising if the clay CoTE is really 6.0 but to adjust that glaze down to 6.0 I have turned it into something completely different that I don't know will even work, will it crawl, craze, core, dunt, run off the piece. I am just guessing here and its frustrating.  I have almost removed all Custer Feldspar and replaced with massive amounts of Silica to get the fit and even that is off possibly by 0.1. That seems insane to me. 

Hence, I would like to create a clay body that I can throw with and also manage to glaze so I dont have tons of pieces of useless bisque on my shelves. 

 

 

[glazenerd]

Correct Neil:

basic cone 10 is 50% kaolin, 25% feldspar, and 25% silica.

basic cone 6 is 50% kaolin, 30% feldspar, and 20% silica. ( which generally averages 6.00 COE) by the way.

cone 5  48% kaolin, 32% feldspar, and 20% silica should work just fine. 6.10 cOE by guesstamation. 

*** assuming vitrification is the goal.

 

[kraythe]

22 minutes ago, glazenerd said:

Correct Neil:

basic cone 10 is 50% kaolin, 25% feldspar, and 25% silica.

basic cone 6 is 50% kaolin, 30% feldspar, and 20% silica. ( which generally averages 6.00 COE) by the way.

cone 5  48% kaolin, 32% feldspar, and 20% silica should work just fine. 6.10 cOE by guesstamation. 

*** assuming vitrification is the goal.

 

How do you calculate vitrification temperature and Coefficient of Thermal Expansion based on a particular clay?  I guess digital fire will do the CoTE calculation but not vitrification. Basically I am wondering if you want to bring the CoTE up to make it easier to fit more glazes you can add some materials to do that but how do you know you havent adversely affected vitrification temp and turned it into a cone 4 or cone 10 body? 

[glazenerd]

You determine vitrification by "total alkali" molar. ( not total fluxes, total alkali.)

cone 5. 4.00 to 4.10 total alkali molar

Porcelain is dependent upon forming a "glassy matrix" within the body. As the glass content increases, absorption decreases. Glass formation in a porcelain body is pottery chemistry basics: silica + flux + heat = glass. How to manipulate that is where clay chemistry comes in.

[kraythe]

2 minutes ago, glazenerd said:

You determine vitrification by "total alkali" molar. ( not total fluxes, total alkali.)

cone 10.   Total alkali. 3.25 to 3.50

cone 6 total alkali. 3.75 to 4.00.   3.75 will result in 1-2% absorption.  4.00 will produce 0 absorption.

as the cone value decreases, particle sizes increase.  Ex. Cone 10 - 200 mesh.   Cone 6. 325 mesh. 

Porcelain is dependent upon forming a "glassy matrix" within the body. As the glass content increases, absorption decreases. Glass formation in a porcelain body is pottery chemistry basics: silica + flux + heat = glass. How to manipulate that is where clay chemistry comes in.

Wow you are an encyclopedia of knowledge, thanks. Just one question, how do I determine the  "total alkali" molar in a recipe perhaps using digital fire. I can see the chemistry, do I just have to add up the mole% for certain materials? 

[kraythe]

@glazenerd While i have your expertise handy, do you see the alterations I made to the tea dust glaze above and do you see any problems? 

[Min]

pssst look at your silica to alumina ratio

[kraythe]

6 minutes ago, Min said:

pssst look at your silica to alumina ratio

@MinUhh for which question? The glaze itself ? I had to radically alter it to fit toe CoTE, What do you think will happen with it? 

[glazenerd]

Sodium and potassium = total alkali (Knao)  there will be very low % of lithium and calcium that occurs naturally. ( do not  add to alkali). Formulate the body, set your molar ratio before adding the plasticizer/s.  Plasticizer will spike the magnesium (MGO) % exponentially.

i do not use, nor am I familiar with Insight. I would suspect there is a setting to present the info in molar %, but others will have to answer that. I program my own calculator with clay formula limits that I wrote. 

I will let Min answer your glaze questions, she is better at explaining them than I am.  Except to say there is not enough flux to produce a cone 5 melt. 

[Min]

32 minutes ago, kraythe said:

@MinUhh for which question? The glaze itself ? I had to radically alter it to fit toe CoTE, What do you think will happen with it? 

sorry, I was vague. Look at the pictures of the two recipes you posted for VC's Teadust. Have a look at the Si:Al ratio that is on the right side of each picture. Original has 8 point4 : 1, now look at your altered glaze, the ratio is 16 point 5 :1  What this is telling you is a couple things, how the glaze will melt and therefore the gloss of it. Have a look at what Custer has in it, the main fluxes are sodium and potassium When you removed those you lowered the COE, good start. But you need to replace those fluxes with lower COE ones and yet not change the quality of the glaze. So, to replace high COE fluxes (or just part of them) you need to bring in low COE ones. Magnesium and lithia are good places to start. You already have some magnesium in the glaze (from the talc), that would be one place to start increasing it. I would also add some spodumene but don't go crazy with how much you add. Could also play with the calcium level, probably could lower that a bit also. 

Sometimes it's easier to think in terms of cooking. If you have a brownie recipe with lots of fat and you want to make a lower fat recipe what would you do? Take out some of the butter right? But if you do that alone you will have nasty dry brownies so you need to replace it with something. Applesauce instead of all or some of the butter will reduce the calories and yet the brownies will still be brownies. make sense?

To look at mole percentage instead of a unity formula with Insight look in the lower left area of the screen and under "Calculation Type" click the button and scroll through to Mole% then on the right side of the screen click the box with the red X and it will take you to a list of choices of Targets and click "Eppler Cone 6" or whatever cone you are firing to.

[curt]

This looks like a classic case of why it is sometimes easier to adjust your clay body than your glaze, and it seems that after a long journey that is about where you are at in your thinking. 

But...a few comments on your glaze:

Your adjusted glaze as it stands does not look viable to me, especially not at cone 5.  Your recipe now has a much larger than usual proportion of very refractory ingredients (silica and alumina), and very little in the way of "power fluxes" such as potassium and sodium to melt those refractory ingredients.   I know you have dumped most of those fluxes for COE reasons, but you can only take that process so far before, as you suspect, the glaze is totally out of wack and I suspect it will not melt.  I think the trick with these fluxes is to lower them some, but not too much.  Only testing will tell how much.

You have lots of calcium, the "bread and butter" flux of stoneware glaze temperatures, but at midfire temps (cone 6 and below), your calcium is not going to do much melting work.   Calcium has a eutectic with silica and alumina (eutectic = the lowest temperature all three will melt at together if they are in just the right proportions to each other) at somewhere around 1200 Celsius, but this will barely be getting going just as you are hitting your top temp for a cone 5 firing.  So calcium is kind of sitting on the sidelines in this glaze.

Magnesium (which is even more refractory as a flux than calcium), is also not going to be doing much to help melt things at these temperatures I suspect.  

So even though your Silica:Alumina ratio appears that it would give you a super glossy glaze, due to the very small amount of low temperature fluxes I think you would find this glaze dry and unsatisfying due to all that silica, and probably not even properly melted.  Might want to try a line blend with ever increasing silica in this glaze to see first. 

The one thing that might save this glaze if you are firing in reduction is all that iron.  It is a very active flux in reduction, and it might just come to the rescue in the glaze melt.   But that is a long shot in my view, never a good idea to depend to much on reducing iron as a flux driver, not least of all because reduction can be uneven about the kiln.

All this is one reason glazes start to lean heavily on frits, and particularly the hefty amount of boron they contain, once one leaves the realm of stoneware temperatures.   That is something I see you have not tried (?), ie, upping the amount of (relatively lower expansion) frit instead of depending on increasing silica to crazy levels.     Frit 3124 is billed as a low melter, but I think you might want to sub in 3134 because it is an even better melter, mostly because it has no alumina.  Anyway, just a thought about another way to go with adjusting your glaze.

[kraythe]

@curt That is pretty much what I thought. The glaze is so radically altered that it cant be used and now I cant use a low expansion glossy I found because my supplier doesn't carry ferro frit 3249 so I am a little screwed with a glaze I found that supposedly would fir the clay (below) So I am quite frustrated. 

 

[curt]

I would not get too hung up on one particular frit.  If you don't like 3134, there are hundreds of different frits out there and probably a few others with not dissimilar chemistry would do the job for you if you wanted to play around with it.  All it really means is that you will need to tweak the chemistry of the whole glaze to accommodate the particular frit you CAN get your hands on.   You could even go upstream to boron itself (rather than boron  packaged up in a frit) if you can get that (borax? boric acid?, etc.).

But like everything boron has its trade-offs.  It is all a game of compromise....

I think you are right to look at the claybody.  I am not quite the purist regarding only potassium and sodium as some others - they are not the only fluxes that can aid in glass formation, Limoge being a perfect example.  But there can be little doubt that they will beef up the COE of a clay body.  Particle size aside, the high shrinkage of most commercial porcelains is directly related to these two fluxes, potassium and sodium. 

[Marcia Selsor]

On 10/20/2017 at 1:21 PM, kraythe said:

I have been purchasing my porcelain from a local provider but the problem is they won't tell me what is in it and sometimes it doesn't perform the way I want to. Now disclaimer, I am a hobbyist, not a professional so please keep that in mind. The thing is I have been Formulating a Porcelain by  Digital Fire and trying to figure out where to start.  Digital Fire doesn't tell me the vitrification temperature of the clay or how to calculate what cone the clay body is. So how can one calculate the temperature for a clay. Another question is where to start, I am trying to find recipies for this kind of porcelain to start with so I know kind of a direction to go. I want to create small batches and try them on the wheel and so on and then when I get one I like, my supplier will make 1800# for me. 

here are some recipes from Linda Arbuckle's handouts. BTW Dale Neese is in San Antonio. Join the San Antonio Potters Guild. Find out what the members are using for ^6 porcelain and talk to them.

Marcia

 

 

I'd recommend finding a clay that works for you. I like Frost ^6 and also Archie Bray's ^6. I adjust my glazes to fit the clay body. Don't get a clay that says "cones 6-10" as there really is no such duck. Where are you located besides 95 miles from San Antonio? Austin and Houston have good suppliers.

Marcia