Resources on formulating a clay body

[baetheus]

Hello All,

My name is Brandon and I'm starting a studio pottery in Davis, CA. I've collected a good amount of old and often broken equipment that I've repaired, the latest of which is a Soldner Studio mixer. In honor of this equipment I've decided to start making my own clay from local sources (Exciting for me!). To that end I visited Donny over at the Gladding McBean pit in Lincoln, CA today and he gave me spec sheets as well as a bit of their Lincoln 60 and Lincoln 8 clays. Now I'd like to turn these raw clays into something useful for throwing and firing in oxidation at cone 6, but my normal resource for these things (Clay and Glazes for the Potter) is rather slim and advises experimentation but nothing really specific as far as clay composition is concerned. Of course I'll be experimenting, but it's nice to have a starting point! It's also nice to have a clear goal. Mine is:

* A good plastic throwing body for functional ware.
* Capable of being slipped, raw glazed, and single fired to cone 6.
* Final absorption of 2%-.

Here are some of the things I think I know:

Lincoln 60 Fireclay Technical Data (Dated 07/09/02):

Silicon Dioxide: 54.31%
Aluminum Oxide: 24.49%
Ferric Oxide: 1.92%
Calcium Oxide: (Less than detection limit)
Magnesium Oxide: 0.06%
Sodium Oxide: 0.33%
Potassium Oxide: 2.02%
Titanium Dioxide: 2.23%
Manganese Dioxide: 0.006%

LOI @ 1750F: 9.50%
Moisture @ 105C: 2.48% (Assuming this means chemical water)
Undetermined: 2.65%

Lincoln 8 Fireclay Technical Data (Dated 07/09/02):

Silicon Dioxide: 55.91%
Aluminum Oxide: 23.92%
Ferric Oxide: 4.00%
Calcium Oxide: (Less than detection limit)
Magnesium Oxide: 0.06%
Sodium Oxide: 0.34%
Potassium Oxide: 1.96%
Titanium Dioxide: 1.90%
Manganese Dioxide: 0.012%

LOI @ 1750F: 9.10%
Moisture @ 105C: 2.21% (Assuming this means chemical water)
Undetermined: 0.59%

Where possible, I'd like to use only local clays.
If I need it there is a company mining Kaolin down in Ione, CA that I can potentially get Kaolin from if they ever return my calls.
I haven't found a local ball clay, and I think I might need one if I'm to raw glaze, but that's a problem for future me :D.

Anyway, thank you for reading this far and I look forward to the wealth of knowledge this community has to offer!

[baetheus]

Also, general/practical (read: not included in academic books) information about formulating and testing clay bodies would be useful for me, as I'm still very new and inexperienced. I'm sure some of the tests are common sense. I could see testing absorption being something like weighing a fired piece, soaking it in water, and weighing it again. And since I'll be starting with a clear base glaze for this body I believe any glaze fit issues will be glaringly visual. I'm sure there are other methods and techniques that you've all developed or learned that aren't in the literature I have access to and I'd love to learn them too! In short, I am in your care.

[Hulk]

Hi Brandon,

Welcome to the Forum!

IMCO Clay in Sacramento is fairly close.
I have tried several of their clays* and bought materials from them as well.
My understanding is they mine local deposits.
I've found them to be very helpful and friendly.
They have a line of potters' supply/material/tools, however, they also do custom milling and blending, and supply bulk industrial clays and crushed rock.

There are some regular contributors who may respond to your queries soon; check back!

*I use their Red Velvet Classic and DC 3-5

 

Edited June 21, 2023 by Hulk
check back after while...

[baetheus]

Hello Hulk! Thanks for the info. The red velvet classic looks pretty close to what I'd try to make if I could find the right ingredients nearby. I've been to IMCO a few times and have used their Great White a fair amount in the past. They are a great company and I have a few friends who get their work fired there. I haven't looked at their dry mixes but that seems like a great idea! For reference, Gladding McBean quoted $118/1000# for both the 60 and 8. Assuming 30% wet to dry that's $118/1300# wet, or $0.09/lb or $2.25/bag, so that's a decent starting target way below most suppliers if it throws and fires good as is. I'll be mixing some up tomorrow and should get a cone 6 oxidation soon and a cone 10 reduction firing in the next month or so.

I read somewhere (can't find the link now of course) that the IMCO 800 clay is actually Lincoln 8 milled to 325 mesh, which is pretty cool too. They also list their Burgundy raw clay as a local CA clay. I wonder where they mine it.. maybe they will tell me if I ask nicely. Anyway, thanks for the response!

[Kelly in AK]

Sounds like you’re in a good place to get some clay someone else dug up, cleaned out, and processed for you. I wouldn’t mind that one bit!

Lincoln 60 is a winner in my book. I used it as part of my clay recipe for a few years until shipping costs became prohibitive. (Mine’s very different from yours, cone 03).

My first thought was to suggest blending a local red/common earthenware with Lincoln until you get the mix that vitrifies at cone 6. Tony Hansen suggests it ought to be possible by just adding feldspar (link below).

https://digitalfire.com/material/lincoln+60+fireclay

[glazenerd]

When reviewing clay specs: the alumina content will predict plasticity. The lower the alumina content; the more plastic the clay will be. Example: kaolin is 37% alumina, and nearly non-plastic. Larger grain (micron) ball clays have 27-31% alumina, and typically rated as medium plasticity. Fine particle (sub-micron) ball clays with high plasticity run 24-27% alumina. As alumina levels drop, plasticity increases: bentonite (very high plastiity) has 20% alumina. BentoneMA (even more plastic than bentonite) is highly processed hectorite with less than 2% alumina. Particle size also effects plasticity: fire clays have a percentage of large (20-80) mesh particles that lower plasticity; even though it has lower alumina. High plasticity ball clays are under 1 micron particle size; or sub-micron particles. Example: Kaolin run 2-20 microns typically; which also plays into its non-plastic rating. OM4 ball clay runs 0.67 microns and is medium plasticity. CMC ball clay is just below 0.50 microns and is rated high plasticity. I use Taylor ball clay (not available commercially) that is 0.27 microns and extremely plastic.

When formulating: plasticity ratings matter. For example: a common formulation basis is 25% kaolin, 25% ball clay, 25% silica, and 25% feldspar (cone 10)  25% OM4 ball clay will create workable plasticity. If you changed that 25% to CMC ball clay: the body would absorb water rapidly and collapse quickly on the wheel because CMC is much more plastic than OM4. Let me express this another way: 8% of my Taylor ball clay will produce more plasticity than 25% of OM4. So you have to understand that parameter when formulating. Remember: high plasticity equals high water absorption. Randomly switching ball clays in equal additions (25% OM4 verses 25% CMC) will turn a plastic clay into a “fat” clay quickly. 

Alumina will predict plasticity in most all cases. Exceptions: as mentioned, larger particle sizes will lower plasticity even when alumina is lower. 2. Higher calcium content will increase plasticity when alumina content is higher because calcium creates isomorphic substitution (don’t ask) at a higher rate than sodium or potassium. 

Fireclays have higher inorganic sulfide levels which equate to higher LOI at 1750F. Inorganic sulfides = lignite coal particles.

Tom

[Callie Beller Diesel]

It’s less common now to learn about formulating your own clay body because 1) there’s a lot of really good commercial clays readily available, and 2) mixing your own clay on the regular is a lot of backbreaking work. Soldner mixers are the right height ergonomically for no one. Word to the wise: have a look into how to mitigate some of the bending over and lifting of heavy wet clay out of the mixer when you’re setting it up. 

Lincoln 60 was one of the ingredients in the beginner cone 10 stoneware we all mixed up in college. The Lincoln was included because it gave a wider particle size variety to the epk and ball clay. This gave the clay a really good green strength and made it very forgiving shrinkage-wise, on the wheel and when making attachments. It will add quite a bit of speckle to the end result, especially if you’re firing in reduction. That speckle will bleed through all glazes, so using it exclusively will give you a very heavy 1970’s Peter Volkous look. By itself it’s suitable for sculpture, but not for functional ware. 

If you haven’t found Digitalfire.com yet, it’s a really good free clay and glaze chemistry resource, and there’s a few articles on formulating clay bodies that are good jumping off points. Definitely be sure to check the links to expanded info at the bottom of the page.  I like glazy.org as a calculator, but I still prefer Digitalfire’s materials database. I find it contains more material provenance. 

 

 

[neilestrick]

An easy place to start is an equal parts body- koalin, fireclay, ball clay, silica, feldspar- which makes a good cone 10 body. From there you can increase the feldspar to get it down to cone 6.

One thing to thing about is where you really want to spend your time as a potter- making clay or making pots. Since you have a resource near you (IMCO) that can mix clay bodies quickly and efficiently while still using local materials, you've got the best of both worlds and it seems like you don't really need to mix clay yourself. Most places that make clay will mix custom bodies, so consider using your Soldner mixing for testing and then having IMCO mix larger volumes for you. Clay mixing and pugging is really miserable work that's really hard on the your body and potentially dangerous to your lungs, so the less of it you can do the better.

[Min]

21 hours ago, baetheus said:

Lincoln 60 Fireclay Technical Data (Dated 07/09/02):

21 hours ago, baetheus said:

Lincoln 8 Fireclay Technical Data (Dated 07/09/02):

This bothers me. 21 year old data.

For what it's worth, I tried out Oregon Red with and without sand from Tacoma ClayArt and had issues with the absorption being much higher when I tested them compared to the posted theoretical absorption. Spoke with the clay tech at Tacoma ClayArt and she said they have been seeing issues with all their bodies that use Lincoln 60 (including the two bodies I tried). I also found fairly large pieces of stone in it. Might consider running it through a screen before mixing it.  I asked Gladding McBean for a more recent analysis and just heard crickets.

Edited June 21, 2023 by Min

[baetheus]

Woo! This looks like a good amount to start with. Thank you all for your replies! I will answer them each below.

@Kelly in AK I've heard from a few people that Lincoln 60 and Lincoln 8 are pretty solid as a starter for a formulation so I'm glad to hear a bit of an echo for that from you. I picked up some Nepheline Syenite, Bentonite, and grogs at 100 and 200 from IMCO today and am starting with bars and cups at cone 6 oxidation next week as well as melt tests for the non-grogs. The Digitalfire site is new to me, but I'm a little concerned about the drastic mismatch between their chemical composition and the sheet I have. Either way, I'll be finding out what's what. There is also a question of how drasticly different the recent extractions are from historic Lincoln clays.

@glazenerd If I'm reading this correctly alumina and particle size are the primary indicators of plasticity. This is important because plasticity is an indicator of water absorption, which in turn affects how well the clay throws (strength under deformation), how it dries (shrinkage), and likely other properties (how it takes to raw glazing when leatherhard or bone dry). In my specific case I am beginning with two clays with 24% and 24.5% alumina (or 29% and 32.5% according to Tom Hansen). If I'm to start with the simple test of adding nepheline syenite a270 into each clay starting at 5%, my instinct is that plasticity should increase. This is because wikipedia lists the alumina content of nepheline syenite at 20.96% and a270 indicates a somewhat fine particle size. In the case that the clay bodies I have are closer to Tom Hansens numbers this should be fine, since the higher alumina there indicate medium plasticity, and I might need to add in 1-2% bentonite to achieve a "fatness" that feels good to me. In the case that the sheets from Gladding McBean are correct then I expect that the body will be much too plastic, and I suppose I'll look into sourcing a kaolin from ione mineral company to compensate. Am I moving in the right direction here?

@Callie Beller Diesel I agree with the backbreaking work, taking apart the soldner was its own adventure in strength vs ingenuity. I have the whole mixer up about 11" total on a platform with fat casters so I can scoop out the peels without stooping at all. I also plan to make my rolling clay bins taller but not deeper for this same reason. Hoping my body thanks me for it, since I'm also subjecting it to an old lockerbie kick wheel. The reason for my choosing the lincoln clays as a starting point is that they are dirt cheap and only a 45 minute drive away. I did some napkin math yesterday and it's looking like I can keep this clay down around $0.12/lb including additional materials. To me this is the difference between breaking even at 140 mugs sold a month and 230 for the cheapest dry commercial blends in my area, so it's at least worth exploring the option. This digitalfire site is awesome! I wish I had joined this forum earlier and found out about it! Thanks for the link.

@neilestrick "One thing to thing about is where you really want to spend your time as a potter- making clay or making pots." If I'm being honest, I want to do it all. If I were ten years younger starting out I would likely be hunting clay veins on blm land to stake claim, and I still might do so in a year or two. For me making the pot starts well before the wheel. That said, it's entirely possible that I'll get six months into making my own clay and decide to bail out of it. I don't think that'll happen but if it does you can be sure I'll be taking your advice. As far as the easy clay to start with, the truth is that the Lincoln mine gifted me a good amount of dry material and I'm simply excited to see what I can do with it.

@Min I agree, that and the big difference between my spec sheets and Tom Hansen's numbers (and glazy's https://glazy.org/materials/15480)  made me leery at first. However, after reading all of this good advice, digging into digitalfire, and coming up with a plan for either case, I'm more excited than concerned.


Again, thank you all for your replies. If anyone is interested I'll post up my first test plan and the results when I have them.

[Kelly in AK]

Looking forward to seeing how it all develops. I too am afflicted by this disorder, “good pots begin with digging good clay,” though I don’t believe it’s officially listed in the DSM (yet). 

I noticed the same thing: Tony Hansen and Glazy’s specs on Lincoln are different than what you’ve got. This is some special clay though, you’re fortunate to have it nearby. The proof is in the pudding, physical testing will show you more faster than any chemical analysis can, in real time, in real life. In a sense, materials analysis is just physical testing taken to an atomic level. Evaluating plasticity, shrinkage, firing deformation, and absorption are things you can do in house. Data you get from those tests are often validated by the spec sheet, but can’t be always discerned by just looking at a table of percentages unless you are a wizard. When you know enough, you could probably estimate the chemical composition of a clay by tests you can do yourself (“If this, then that, if not, then this, this, or some other thing…”). 

Anything Tom (@glazenerd) says about clay formulation and testing you can take to the bank. Continually thankful for his input wherever it occurs. Wizard. And Tony Hansen (Digitalfire)  is some kind of saint. You will find more information, more documentation of tedious testing, more valuable analysis of clay there, freely given, than anywhere else on the internet. Glazy runs a close second. 

I stand by my suggestion to try a bit of “common clay,” local red earthenware, as a means to flux Lincoln to maturity at cone 6. My impression is that it won’t take much, since it already seems to vitrify around cone 8. I’m no wizard, just a fan of keeping it local.

Glad to hear you raised the mixer. That move will save you a lot on physical therapy later in life! 

[glazenerd]

Baetheus:

All clay are aluminosilicates (alumina/silica) in various percentages; which gives indication to plasticity. Feldspars and silica are inert; and are non plastic. 

Clay formulation basics: Porcelain is 50% kaolin, 25% silica, 25% feldspar (Cone 10), and 2% BentoneMA creates a premium body. 35% kaolin, 15% ball clay (plasticizer), 25% silica, and 25%feldspar (Cone 10) is a basic high white porcelain. 50% kaolin, 20% silica, 30% feldpsar, and 2% BentoneMA is a premium cone 6 body. Less feldspar is needed at cone 10 because of the extra heat work created firing to that temp. More feldspar is added at cone 6, because less heat is done. Porcelain relies on glass development to create vitreous wares. Silica + flux = glass. Silica + flux + alumina = stronger glass. Porcelain will always have lower absorption rates due to its high glass content.

Stoneware relies on density more than glass to lower absorption. Feldspar additions typically run 10% at cone 10, and up to 15% at cone 6. Unlike porcelain, flux additions in stoneware are to prevent cristobalite formation, more so than glass development. Yes, some glass does develop when fluxes absorb silica; but its primary function is to lower cristobalite formation.Cristobalite forms when excess silica is present: that fact is what limits silica additions in stoneware to a maximum of 10%. By nature, ball clays have much more silica in them compared to kaolin; so silica additions are much lower than porcelain. Cristobalite in stoneware is a primary cause of dunting in the low end of the cooling cycle, when pieces are microwaved or hot fluids are poured in. So stoneware body formulation 101: limit silica additions. 

Stoneware formulation body basics: 80% total clay, 10% silica, 10% feldspar (cone 10) 75% total clay, 10% silica, 15% feldspar at cone 6. Just like porcelain: more heat work at cone 10, and less heat work at cone 6. The “80% total clay” means any combination of clays cannot exceed 80% of the total recipe at cone 10, and cannot exceed 75% at cone 6.  25% Hawthorne 35, 30% Imco 400, and25% OM4 ball clay = 80% of the recipe. You can have 2-3-4-5 different clays; just do not exceed 80/75% of formula. The other formulation criteria is PSD (particle size distribution). The greater the % of large particle clay, the greater the medium and fine particle clay additions required to fill the voids creates by large particles. (porosity) In my testing; a maximum of 17% Hawthorne 35 (large particle) can be used, and then blended with medium and fine particles to keep absorption around 2%. Once I passed that 17% mark, absorption started climbing incrementally. If doing non-functional; wares; not a factor per se. Doing functional ware; big factor. 

Years back I loaded 100+ stoneware/porcelain recipes into Glazemaster software. I did so to determine a basis of “typical” values for both. To get you started: Porcelain SiAL ratio 4:1. Stoneware: SiAL ratio 5:1. Remember, these are target values, not etched in stone. You will be under or over a bit every time. If you get way over or under; rethink your formula. 

Tom

[glazenerd]

Pic below is unglazed “iron bering” clay fired to cone 6. (unglazed) Im= Imco burgundy. New = Newman Red. Hem= hematite bearing wild clay. RA= Red Art. Mag = magnetite bearing wild clay. 

[baetheus]

@glazenerd Sorry for the late reply. Thank you for this information. I'm still waiting on an electrician, the city of Davis, and our electrical company to sort out whether I actually have the necessary 48Amps available to drive my kiln before they run the copper. Lots of fun, that is. Anyway, I've come back around to thinking through clay recipes and I am hoping that you'll offer more wisdom. Unfortunately, I'm reliant on someone else to fire my test bars right now so I don't have any solid data to interpret yet, thus everything below is theoretical. To make things easier, I've put the final versions of my chicken scratch notes for clay and glaze starting points on glazy.org here: https://glazy.org/u/baetheus/recipes

First, let's start with clays.  https://glazy.org/recipes/358868 and https://glazy.org/recipes/358852

1. Based on this part of your comment, "Higher calcium content will increase plasticity when alumina content is higher because calcium creates isomorphic substitution (don’t ask) at a higher rate than sodium or potassium." I started looking at wollastonite as an ingredient for my clay body instead of straight sil-co-sil. According to Tom over at digitalfire the physical properties of wollastonite reduce shrinkage and from your comment the calcium could potentially aid in plasticity (although I sense potential nuance here).

2. When mixing up test batches of lincoln 60 and lincoln 8 I found the fresh mixed (+30% water across all blends was sufficient) wet clay to be fairly plastic but not immediately throwable. I did not age any of the blends yet, but I am confident that I will need to increase plasticity--thus the addition of 2% bentonite as a start.

3. Using your basic notes of 75% Clay, 15% Feldspar, 10% Silica as a starting point I adjusted ingredients, keeping total clay below 75%, until glazy listed the Si:Al ratio as somewhere around 5:1. Does this seem sensible?

 

Second, your namesake, glazes. Originally, I had planned to start my glaze journey by simply using the "cheapest" clear glaze recipe I could find in the John Britt midfire glaze book. This looked like it was going to be the Val Cushing Transparent 3 glaze. However, after spending the entire morning reading about the end of Gerstley/Gellespie Borate and then finding the physical sources of all of the other ingredients in the clear glaze table of Britt's book, I decided that it might be better to formulate my base glaze in the same spirit as my first clay body. This is to say, start from scratch with the same constraints. To this end I used the Val Cushing Transparent 3 glaze as a starting point. https://glazy.org/materials/1577. I have two starting points, one using Kaolin from a mine here in Ione, CA and another using the Lincoln 60 or Lincoln 8 clays. With the exception of silica added to the glaze using kaolin, my approach to both formulations was that same. Again, please correct me where my approach is incorrect.

1. I started with a similar amount of feldspar, choosing minspar only because of the drift I've read about in custer feldspar. After reading through digital fire I convinced myself that the feldspars (neph sye, custer, minspar) could not work as the only melters in a glaze, since they don't melt on their own at cone 6.

2. From there I added 10% kaolin to suspend the glaze.

3. Then I added 1% zinc oxide as a primary flux instead of one of the borates. Although gellespie borate comes in at less than half the cost of zinc oxide, it requires 13x as much material than this formulation. My hope is that 1% zinc is enough. My concern here is that it's likely my first colorants will be the iron oxides, which I've read are dulled by zinc oxide. I'm also concerned that 1% will not be enough to achieve a full melt.

4. Lastly, I shored up the glaze with wollastonite and silica, believing that the calcium would strengthen the glaze and using the silica to push the Si:Al ratio as close to Val Cushings 8.2 as I could.

From there I pretty much played with materials until the UMF formula was as close to the V.C. Transparent 3 glaze as I could get it.

In conclusion, I hope that this all makes sense and any input you'd have on how I might change my planning process would be greatly appreciated. Additionally, if you have any advice for which materials to start with as far as line blends are concerned that would really help!

P.S. I didn't mean to single out glazenerd here. I would love to hear thoughts from everyone on any of this process. So please, help me learn!

Edited July 23, 2023 by baetheus

[glazenerd]

12 hours ago, baetheus said:

1. Based on this part of your comment, "Higher calcium content will increase plasticity when alumina content is higher because calcium creates isomorphic substitution (don’t ask) at a higher rate than sodium or potassium." I started looking at wollastonite as an ingredient for my clay body instead of straight sil-co-sil. According to Tom over at digitalfire the physical properties of wollastonite reduce shrinkage and from your comment the calcium could potentially aid in plasticity (although I sense potential nuance here).

2. When mixing up test batches of lincoln 60 and lincoln 8 I found the fresh mixed (+30% water across all blends was sufficient) wet clay to be fairly plastic but not immediately throwable. I did not age any of the blends yet, but I am confident that I will need to increase plasticity--thus the addition of 2% bentonite as a start.

3. Using your basic notes of 75% Clay, 15% Feldspar, 10% Silica as a starting point I adjusted ingredients, keeping total clay below 75%, until glazy listed the Si:Al ratio as somewhere around 5:1. Does this seem sensible?

1. Water hull and stretched membrane were both theorems introduced by F.H. Norton, Phd. The water hull in simple terms meaning a single molecule of water is nearly equal to sub-micron clay particles, and a single calcium particle. Without paragraphs of science; the theorem states that sub-micron clay particles, and calcium particles are more effective at creating plasticity because they are of equal size. Sodium and magnesium are much larger; and the water molecule has to “stretch” ( stretched membrane) to encapsulate either. That gelantious effect created when you put bentonite in clay or glaze is in part a result of “stretching” the water hull. To further illustrate is Darvan. Powerful suspension agent, but few know that Darvan works in part by neutralizing sodium and magnesium ions. This allows remaining calcium to work more potently, in addition to the strong negative ionic charge created by high alkalinity. In discussing wollanite: then you get into alternate silica structures. Polymorphs I believe is the correct term. Pure silica has a high COE 12-14, while melted silica has a low COE 4.50 or so. So silica that has undergone thermal changes due to volcanic, or heat/pressure form a different crystalline lattice- thereby lowering it natural COE. For that reason, wollastonite reduces shrinkage, and adds to plasticity due to the calcium content.  Personally, I add 2% whiting. 

2. Plasticity develops over a 5-7day period unless it is ran through a de-airing pugger. Plasticity levels climb over the next several weeks. Plasticity is never an immediate reaction. A big mistake potters make is increasing ball clay/plasticizers until it is plastic upon mixing. However, in 5-7 days they have a bag of silly putty. Your projected plasticity in a week or so govens ball clay additions; not what it feels like when you mix it. Also note that not all ball clays impart the same level of plasticity.  Kentucky 5 for example is on the lower end of plasticity; yet it is still deemed plastic. OM4 is a medium plasticity ball clay, and FHC (Foundry Hills Creme) is high plastic. You have to calculate those levels when adapting a clay recipe. A common clay recipe might call for 25% OM4, and if you replace it with 25% FHC: you will have silly putty. If it calls for 25% OM4, and you replace it with 25% Kentucky 5; it will mature to the short side of plasticity.  Ball clay specs will often give CEC (cation exchange values. OM4 has a CEC around 5.8 (last I looked), and FHC is above 9. (from memory). The higher the CEC climbs, the more plastic it is. A recipe calling for 25% OM4, can be replaced with 15% FHC to achieve the same plasticity level.  Formulation rule #2; the higher the plasticity, the more water the clay will absorb. That is also translated; the higher the CEC value, the more water it will absorb. You do not want a tile body to absorb or hold any more water than necessary to form: because excess water = higher shrinkage. Throwing bodies need enough water absorption to make it malleable, but absorbing higher amounts of water will cause it to slump or fold on the wheel. The exception is hand forming, moreso pieces that require several days, or multiple steps to create. In this case, the additional water absorption will delay drying, and make the clay more suitable for carving or detailing. 

3. In your earlier post, you listed the mineral composition of the clays you like. Did you notice the alumina hovering around 25%?  24-30% is common for ball clay and fire clays, although fire clays often drop below that. Kaolin typically runs around 37%. If you formulate using clay(s) with lower alumina and higher silica; then you will have higher SiAL ratios; 5:1 is normal for stoneware. Porcelain uses kaolin, which is higher in alumina, and lower in silica: so the SiAL for porcelain bodies hover around 4:1.  The additional formulation criteria also comes from these natural SiAl levels. Stoneware bodies naturally have lower alumina and higher silica: so the silica addtions are limited to 10% for that reason. Porcelain bodies (kaolin) have higher alumina and lower silica: so silica addtions run 20-25% for that same reason. 

Bonus note: Porcelian relies heavily upon glass/mullite development to create nearly zero absorption. This is the primary reason flux additions run in the 25-30% range. Stoneware bodies focus on PSD (particle size distribution) to keep absorption under 3% (functional ware).Stoneware recipes can have up to 5 different individual clays; which plays a role in particle distribution, but also in working properties, and final fired color. Stoneware being more dependent upon PSD, also means flux additions ae kept to 10-15% total. Flux in stoneware does develop some glass, and does lower absorption some. However, flux addtions serve  a more central target of preventing cristobalite formation.

Tom

[akilpots]

On 7/22/2023 at 5:39 PM, baetheus said:

@glazenerd Sorry for the late reply. Thank you for this information. I'm still waiting on an electrician, the city of Davis, and our electrical company to sort out whether I actually have the necessary 48Amps available to drive my kiln before they run the copper. Lots of fun, that is. Anyway, I've come back around to thinking through clay recipes and I am hoping that you'll offer more wisdom. Unfortunately, I'm reliant on someone else to fire my test bars right now so I don't have any solid data to interpret yet, thus everything below is theoretical. To make things easier, I've put the final versions of my chicken scratch notes for clay and glaze starting points on glazy.org here: https://glazy.org/u/baetheus/recipes

First, let's start with clays.  https://glazy.org/recipes/358868 and https://glazy.org/recipes/358852

1. Based on this part of your comment, "Higher calcium content will increase plasticity when alumina content is higher because calcium creates isomorphic substitution (don’t ask) at a higher rate than sodium or potassium." I started looking at wollastonite as an ingredient for my clay body instead of straight sil-co-sil. According to Tom over at digitalfire the physical properties of wollastonite reduce shrinkage and from your comment the calcium could potentially aid in plasticity (although I sense potential nuance here).

2. When mixing up test batches of lincoln 60 and lincoln 8 I found the fresh mixed (+30% water across all blends was sufficient) wet clay to be fairly plastic but not immediately throwable. I did not age any of the blends yet, but I am confident that I will need to increase plasticity--thus the addition of 2% bentonite as a start.

3. Using your basic notes of 75% Clay, 15% Feldspar, 10% Silica as a starting point I adjusted ingredients, keeping total clay below 75%, until glazy listed the Si:Al ratio as somewhere around 5:1. Does this seem sensible?

 

Second, your namesake, glazes. Originally, I had planned to start my glaze journey by simply using the "cheapest" clear glaze recipe I could find in the John Britt midfire glaze book. This looked like it was going to be the Val Cushing Transparent 3 glaze. However, after spending the entire morning reading about the end of Gerstley/Gellespie Borate and then finding the physical sources of all of the other ingredients in the clear glaze table of Britt's book, I decided that it might be better to formulate my base glaze in the same spirit as my first clay body. This is to say, start from scratch with the same constraints. To this end I used the Val Cushing Transparent 3 glaze as a starting point. https://glazy.org/materials/1577. I have two starting points, one using Kaolin from a mine here in Ione, CA and another using the Lincoln 60 or Lincoln 8 clays. With the exception of silica added to the glaze using kaolin, my approach to both formulations was that same. Again, please correct me where my approach is incorrect.

1. I started with a similar amount of feldspar, choosing minspar only because of the drift I've read about in custer feldspar. After reading through digital fire I convinced myself that the feldspars (neph sye, custer, minspar) could not work as the only melters in a glaze, since they don't melt on their own at cone 6.

2. From there I added 10% kaolin to suspend the glaze.

3. Then I added 1% zinc oxide as a primary flux instead of one of the borates. Although gellespie borate comes in at less than half the cost of zinc oxide, it requires 13x as much material than this formulation. My hope is that 1% zinc is enough. My concern here is that it's likely my first colorants will be the iron oxides, which I've read are dulled by zinc oxide. I'm also concerned that 1% will not be enough to achieve a full melt.

4. Lastly, I shored up the glaze with wollastonite and silica, believing that the calcium would strengthen the glaze and using the silica to push the Si:Al ratio as close to Val Cushings 8.2 as I could.

From there I pretty much played with materials until the UMF formula was as close to the V.C. Transparent 3 glaze as I could get it.

In conclusion, I hope that this all makes sense and any input you'd have on how I might change my planning process would be greatly appreciated. Additionally, if you have any advice for which materials to start with as far as line blends are concerned that would really help!

P.S. I didn't mean to single out glazenerd here. I would love to hear thoughts from everyone on any of this process. So please, help me learn!

I don't know if this glaze is going to work at cone 6.  You need some fairly specific zinc levels to get a bristol glaze reaction which will get the temp of the glaze to melt at cone 6 without using boron. the val cushing recipe you link has 0.15 B203 from the 13% gerstley borate and this is what is melting the glaze at cone 6. you need something like 0.2-0.5 Zn and the rest of your RO flux getting you around that 0.7 sweet spot for a durable glaze. (ideally you'd have a R2O:RO ratio of 0.3:0.7) you'll be getting some Ca from the wollastonite so it's just about dialing in the proper amounts for the type of glaze you want and the materials you are using. you will for sure have muted colors from the zinc in the glaze. personally i think it's easier to use a boron source to bring the temp down to get a good melt and durable glaze at cone 6.

 

 

Edited July 26, 2023 by akilpots

[Callie Beller Diesel]

 

+1 for what akilpots says about boron and flux ratios. Your current 0.16:0.84 is likely not going to be durable, although that amount of calcium can skew that usual rule. I'd personally want it closer to 0.20:0.80 to start, and plan to adjust.

I have 2 thoughts regarding your glaze materials choice and the logic:

1) I think you’re being penny wise and pound (dollar?) foolish by focusing only on material costs. If you have reasons like “it’s a challenge to see if I can do this” or “this material gives a specific quality to the end work I find valuable” then carry on by using the borax instead of a frit or other less soluble material. At that point, it’s a logical material cost. But consider that borax is cheap and plentiful, yet folks are loosing it because Gerstley Borate is going away again. There’s a reason for that: GB provides other materials to the mix, sure, but it’s also less soluble and easier to work with than straight borax. This brings me to…

2)  That much borax (10% plus that much clay (25%) of any kind is going to significantly gel your glaze. At a glance, and just from a materials perspective, this glaze is going to be a pain to mix (wollastonite agglomerates) and will in all likelihood try and crawl off the pot while wet. This is going to lead to more testing to get your application down, and you’ll have to know how to change how the glaze flows without changing it’s specific gravity, or the amount of water in it. You may also find you have to calcine part of the clay, adding time and energy costs, plus testing time and probably a bottle of Darvan to your shopping list. And maybe some sucralose to help break up the wollastonite so you’re not spending hours with a sieve. (Here’s some work we did as a group to explain the sucralose thing.)

 

[baetheus]

@Callie Beller Diesel Hey, thanks for the response! I must apologize because I've made a ton of changes to the glaze recipes I've been mucking with on Glazy. At one point I had somewhere around 30 different iterations that I was basing on various books, other glazes, and conversations with local potters. I realized that I wouldn't be able to test any of them until I nailed down the clay body so I deleted all but one of them. The one I kept (and that you probably looked at) is not fully cooked! It is a quick copy of a cone 6 clear from here: https://ro.uow.edu.au/cgi/viewcontent.cgi?article=2753&context=theses that I dropped the bentonite from and subbed wollastonite for whiting/silica, and upped the kaolin to bring it out of the crazing zone on the stull chart. I'm really appreciative that you even looked at it! That R2O:RO ratio isn't mine though, and whenever I pull in a glaze and keep the "chemistry" while modifying ingredients I feel like I'm butting against my own serious lack of knowledge about materials.

Ah, yeah, borax.. I suppose the ultimate reason for me wanting to start with borax is that it sounds exciting! I'm a fresh potato when it comes to glaze formulation and mixing and using borax will force me to practice since I'll only be mixing up as much glaze as I think I'll use in each glazing session and won't be keeping it around. For me personally this kind of thing is invaluable, right up there with using a kick wheel and making my own clay. It leads to all kinds of unexpected outcomes. It also helps that I have around 85sqft of studio space so keeping buckets of glaze around is almost a non-starter.

However, you are right that I focus perhaps a bit too much on optimizing costs, I definitely have the spreadsheets to prove it. So far making my own clay is a substantial increase in my hourly pay. For mugs alone it's a $14/hr pay increase. The best I've been able to get out of my glaze cost calculations is about $1/hr and that's within a margin of error, but I don't mind playing the numbers game while I wait on the electrician.

As for an actual glaze recipe I don't have one yet! But in general I 100% agree with everything you've outlined. When playing with glaze calculations I have been targeting 0.22:0.78 R2O:RO ratio, 7.5-9.5 Si:Al ratio with Al around 0.35, and 0.13-0.17 B2O3 as the primary flux. I've probably put together two dozen recipes that are in this ballpark but until I can get my clay body dialed in it's all been theory work. It is good to know about sucrose helping with wollastonite agglomeration, I'm sure I'll run into all kinds of problems like this as it's in my plan to raw glaze and single fire, both of which offer their own sets of chemistry and material complications.

However, I am moving towards some real data. I mixed up a biaxial test of clays today and should have time to hydrate them this week.

[Kelly in AK]

Woohoo! Refreshing update. Thank you! You know you’re lucky to live so close to the clay mine, right? Wish I could pay someone to to dig my clay up for me.

Something about borax, in addition to challenges it makes with glaze formulation and application, consider that it will flux your clay body (it soaks in). It can change the fit of glazes, perhaps in an inconsistent, unpredictable way. I’m talking about having half a kiln load shatter on cooling, or having a mug crack clean in half when filled with hot water. Not conjecture, I know someone personally who did work with borax washes and these things happen. There is something known as calcined borax available that’s less soluble (apologies if it’s been addressed earlier in the thread), relatively less expensive than frits, and still has a sort of “purity of material” I sense you value, which could be worth exploring. 

Just saying stay loose and be ready. You’re doing great work, I love it! 

[PeterH]

1 hour ago, Kelly in AK said:

There is something known as calcined borax available

Interesting to know. Probably not an easily DIY-able process (unlike calcined alumina, kaolin, etc.) as seems to be melted & reground not simply pre-heated.

Anhydrous Borax
Alternate Names: Calcined Borax, Dehydrated Borax, Pyrobor
https://digitalfire.com/material/anhydrous+borax
Anhydrous borax is made by fusing hydrated borax into a glass and regrinding it.

[Callie Beller Diesel]

Wow! That article brings back the college memories! I do NOT miss the days of nonexistent glaze software access. So much fumbling! Thankfully, better starting points for cone 6 glazes have definitely come on the scene since. Tony Hanson’s stuff on digitalfire are great, but some Americans find those recipes need to be adjusted for crazing. John Britt and Joe Thompson (Old Forge Creations) are also good resources. Both Tony and Joe make a LOT of info available for free, John has a bit of a paywall for concise info. 

Re: wollastonite agglomeration. If you plan on using your glazes the same day they’re hydrated, you could get away easily with using plain sugar if you don’t happen to have sucralose lying around. I had concerns about possible bacteria growth, thus the suggestion for the artificial sweetener. It’s not a common practice. It was something we were noodling with at the time. 

15 hours ago, baetheus said:

I feel like I'm butting against my own serious lack of knowledge about materials

We got you! You found a bunch of folks who do glaze chem and research for fun. 

[baetheus]

@Kelly in AK I do feel very lucky, and not just with clay! Almost all of my equipment was free or cheap! Free kick wheel, free clay mixer, free wareshelving, an old skutt KM1227 for $1000 with furniture, free space in our shared garage, etc etc. Most of it needed a lot of love but ultimately it was much cheaper than buying new/used. I've made a few trips now to IMCO to talk clays and ingredients and they've been very generous with their time and information about material sources and compositions (even compositions of their "proprietary" clay mixes).

@PeterH I did read about anhydrous borax and ended up on several welding and knife making forums! Turns out borax can be used for surface prep in those fields. There was questionable information about cooking borax decahydrate into your own anhydrous borax but that would be a last resort for me. I'm not yet interested in making my own frits :D. That said, there is a little hope on the Borax Decahydrate page on digitalfire. It seems that the granular borax (which is what my source is) might not be very soluble. Either way, I've got 50lbs of it and I intend to find out what's what.

@Callie Beller Diesel Interestingly, I've share a few emails with John Britt  with questions on information (or, as you say, concise information) from his midfire book. He's been quite patient with me but I don't really want to push it. Besides, I think I'm a little bit outside the standard glaze path and that necessitates testing on my end anyway. That said, good to know table sugar will probably work. I was headed in that direction already :). I'm glad I decided to sign up here, you've all been incredibly welcoming and helpful!

@akilpots You're spot on. Originally, I had pulled the 4% zinc oxide in the recipe you likely read from Val Cushing's Transparent 3. I read all about Zinc and it's interesting effects on color changes when used with coloring oxides and didn't really want to use it. I've since been on a Borax kick, which is in the direction of the advice you've given if not exactly the standard choice. Thanks for your reply!

@Kelly in AK and @Callie Beller Diesel I've attached a photo of my space in it's almost finished state. Callie, the rolling stand for my mixer brings it to where I barely have to bend over to reach the bottom of the mixing bar, and my clay bins with the red stripes on the left are even with the top of the mixer so I'm not breaking my back scooping out clay. Also, you can see that aside from a little space to the left of the window there isn't much room for buckets of glaze to hang out without perpetually being in the way.. I've been thinking on building another rollie cart for buckets but I'm going to wait until I experience that problem before I put together a solution. Kelly, aside from the bench on the left everything else was pretty much donated by friends and strangers alike. I am extremely lucky and it's helped me want to keep working hard. I'll be waiting a week after I hydrate those clay tests before I roll them into shab bars and test cups so I expect to have an update for you all in 1.5 to 2 weeks. Thanks again for all the awesome feedback.

[Kelly in AK]

Holy wow! On your way, looks like.

I found out about anhydrous borax by accident. I had a bag labeled “borax” among a bunch of stuff I bought from a retired potter. I went to put some in solution and the dang stuff didn’t dissolve! 

[Callie Beller Diesel]

The raised clay bins are really smart, and the light is really nice in there!

As a fellow small space user, any time you can build wall shelves, do it. 

If you ever decide to go for making up some larger buckets of glaze, I keep mine in 5 gallon buckets and stack them on each other behind my studio door. I also made a conscious decision to keep the number of “big bucket” glazes to five or less.