Clay composition and crazing

[Hulk]

Unloaded second glaze firing Sunday last - overall, I'm pleased (will post some pics under status later)! ...and there's soo much room for improvement.

I'm testing three clear glazes. All three seem to behave fairly well with Aardvark srf, srfg, Sedona (red clays), however, all three craze on Aardvark bmix - the patterns are fine enough that a much lower expansion recipe seems to be in order (over tweak downs/adjustments). Next glaze firing will include some tests, tbc...

 

What I'm curious about - and haven't found any info on (yet) - looks like the bmix influences the glaze behaviour, iow, the crazing is not due only to the differences in clay body and glaze coe; is that possible?

In the collage below, starting upper left corner and moving clockwise

  a bmix bowl with srf slip applied inside, no crazing at all! All the other bmix pieces in this load had some crazing in the clear,   every    single    one*!

  a red clay bowl, the outside looks ok

  inside of same bowl, bmix slip had been applied, clear is crazed!

  another red clay bowl, outside looks ok

  inside, bmix slip had been applied, clear is crazed!

  the other two pics just to round out; I like the bmix and plan to continue using it once glazing is sorted (I bought six clays to test out - will likely stick with three or four)

Back to the question that's buggin' me, does something in the bmix cause the glazes to craze and vice versa for the red clay? My guess is that the coatings of slip don't change the underlying clay body coe (much); I'd believed that crazing was due to mismatch in coe, and elasticity perhaps also a factor. Now I'm thinking that glaze is also influenced by the clay (dare I say) chemistry - not the same glaze on different clays. Hmm...

Cone 5 clays, mid-fire glazes, two shelves hit about 6, one about 6.5, the top shelf not considered for this question, underfired. The recipes aren' secret, can post'm if anyone's interested. 

 

*My bad! First glaze fire proved that the three clears work ok with the red clays, should have tested bmix before loading up so many pieces. Every one has some crazing on the inside, many on the outside as well. Good news is a ceramic drill bit, some water, time, and drill press turns crazed pots into little planters quickly! ...btw, the bmix seems much tougher than the sanded/grogged red clays, takes longer to drill'm.

 

 

[Min]

If I'm understanding your question you are asking if there is anything besides a mismatch between the thermal expansion characteristics of the body and glaze that can cause crazing. One simple possibility would be if the glaze wasn't fired to maturity but I seem to remember you do use cones so that can probably be ruled out. Another possibility, though it is a very long shot, is the crazing is actually what's called "crows feet". It can happen with a very stiff glaze and a strong surface tension, usually over groggy clays. I have a friend who was sure her crazing, on Cone 5 B-Mix with grog, was from this. It does look different that regular crazing, explanation of it here on page 85. I doubt this is what's happening but in theory it could happen. Rub some ink on the crazing and see if the pattern matches the image below.

The far more likely possibility is that it's crazing. Could you post your clear recipes?

From the Aardvark website the Sedona Red has a posted absorption @ ^5 of 3.5%. The SRF @ ^5 is 6.5%. The Bee Mix @ ^5 is 2.5%.  I couldn't see a SRFB body but there was a SRFG clay with a posted absorption of 7% @ ^5. Clay would more than likely leak with the red claybodies and maybe leak with the Bee Mix @ ^5, but would be a good idea to do your own absorption tests firing with your schedule.

 

[Hulk]

Thanks Min!

srfg, aye.

Had cones on each shelf this time - there was clear over bmix crazing on every shelf, all three clears.

Looks like the typical crazing; I'm using the unsanded no grog bmix.

The third clear is Aardvark's Artisan series cone 5 clear.

Looks like a much lower expansion clear is in order for the bmix.

I'm just curious why the thin layer of red slip appears to stop crazing on the bmix, and thin layer of white slip on the red clay causes crazing! Next firing I'll have some test tiles in thar and look to reproduce.

 

[liambesaw]

I believe the reason is because glaze only "penetrates" very little into a clay body, so the areas of slip are still the areas affecting the coe.  I am willing to bet that the uncrazed area with the red slip is either also crazed (but less) or will eventually though.

Obviously the slip made of b-mix is still b-mix interfacing with the glaze, so it will still craze.  

I also had issue with b-mix crazing my Hansen 20x5 clear, I'm guessing most porcelaineous(?) Style clays would be similar.

[Min]

10 minutes ago, Hulk said:

I'm just curious why the thin layer of red slip appears to stop crazing on the bmix, and thin layer of white slip on the red clay causes crazing!

Doesn't really work this way. The glaze is on the red slip and is fitting that, take away the slip and put the glaze on your white clay and it will craze. Reverse that for what's happening on your red clay with the white clay slip.

I've started using the crystal clear glaze below with good results on my low expansion clay. It started out as a Tony Hansen glaze that I altered, it contains an expensive frit, Ferro 3249 but theres not an excessive amount of it in the recipe. If you try it and it doesn't craze on your body then I would strongly suggest you do a dunting / shivering test to make sure the COE isn't too low.

 

 

[glazenerd]

B-mix (white stoneware) has 10-25% kaolin content- hence the "white."  B-mix usually blends 50-60 mesh fire clay in lower %, which still allows for some body but without feeling any tooth when throwing. In addition, higher % of fine particle ball clay, with higher % flux= lower absorption values. But that also means higher COE values. (stoneware only application) you can use absorption data to help pinpoint COE values if none are given. To lower absorption: the body requires less large particle, more fine particle, with higher flux% in order to lower absorption: which means higher COE. conversely  larger particle with less flux = higher absorption! with lower COE. ( generalization).

Tom

[Bill Kielb]

On 4/7/2019 at 6:34 PM, Hulk said:

Thanks Min!

srfg, aye.

Had cones on each shelf this time - there was clear over bmix crazing on every shelf, all three clears.

Looks like the typical crazing; I'm using the unsanded no grog bmix.

The third clear is Aardvark's Artisan series cone 5 clear.

Looks like a much lower expansion clear is in order for the bmix.

I'm just curious why the thin layer of red slip appears to stop crazing on the bmix, and thin layer of white slip on the red clay causes crazing! Next firing I'll have some test tiles in thar and look to reproduce.

 

Here is a glossy clear that you can try that we routinely use over Bmix and porcelain with no issues thus far. It is extremely glossy, has a durable flux ratio and  a touch more Boron to help it melt over thick refractory underglazes.. we have been using this for several months now throughout the studio and so far everyone loves its high gloss, clarity and melt. Almost forgot - Cone six glaze 

oops, forgot to post the 100% recipe. Corrected below

 

 

 

 

 

[Bill Kielb]

On 4/7/2019 at 6:53 PM, Min said:

Doesn't really work this way. The glaze is on the red slip and is fitting that, take away the slip and put the glaze on your white clay and it will craze. Reverse that for what's happening on your red clay with the white clay slip.

I've started using the crystal clear glaze below with good results on my low expansion clay. It started out as a Tony Hansen glaze that I altered, it contains an expensive frit, Ferro 3249 but theres not an excessive amount of it in the recipe. If you try it and it doesn't craze on your body then I would strongly suggest you do a dunting / shivering test to make sure the COE isn't too low.

 

 

Just curious, why so much silica in the recipe above?

[Hulk]

Thanks for the feedback!

I'm looking at several low expansion recipes, including Bill's, Min's (both above), three of Hansen's, and Neil's (from another thread), and a few others. Hansen's point re more MgO and less KNaO sounds good (as in Min's recipe, above), however, likely I'll hold off on buying new material for now.

Earlier tests indicate "Functional clear" (local JC recipe, below) fits Aardvark bmix, however, I haven't decided for/against gerstley borate for the long haul ...yet (didn't buy a big bag o't yet); this recipe clouds on the red clays. On t'other hand, the bmix will need its own clear. hmm.

Looks "clear" enough Aardvark cone 5 bmix requires lower expansion glaze than the other clays I'm trying out.

My understanding is (still) that thermal expansion mismatch between the piece and its glaze causes crazing. The aforementioned red slip is a very thin layer; whilst its own coe may be different than the bmix it's on, is not the bmix in the driver's seat, expansion wise? Would not any coating be subject to same forces, whether layered or not?

I don't get how clear glaze over bmix is subjected to different force than clear glaze over red slip over bmix - the bmix should be moving the same in both cases, forcing the coating/coatings to move right along with.

Perhaps not the best analogy: latex paint on a brick wall cracks, as it doesn't stretch enough, so we choose elastomeric paint, problem solved. From there, we like the latex better, so latex over the elastomeric - it still cracks, for it's being forced to move/stretch same as before.

 

 

 

 

 

[Bill Kielb]

7 minutes ago, Hulk said:

Thanks for the feedback!

I'm looking at several low expansion recipes, including Bill's, Min's (both above), three of Hansen's, and Neil's (from another thread), and a few others. Hansen's point re more MgO and less KNaO sounds good (as in Min's recipe, above), however, likely I'll hold off on buying new material for now.

Earlier tests indicate "Functional clear" (local JC recipe, below) fits Aardvark bmix, however, I haven't decided for/against gerstley borate for the long haul ...yet (didn't buy a big bag o't yet); this recipe clouds on the red clays. On t'other hand, the bmix will need its own clear. hmm.

Looks "clear" enough Aardvark cone 5 bmix requires lower expansion glaze than the other clays I'm trying out.

My understanding is (still) that thermal expansion mismatch between the piece and its glaze causes crazing. The aforementioned red slip is a very thin layer; whilst its own coe may be different than the bmix it's on, is not the bmix in the driver's seat, expansion wise? Would not any coating be subject to same forces, whether layered or not?

I don't get how clear glaze over bmix is subjected to different force than clear glaze over red slip over bmix - the bmix should be moving the same in both cases, forcing the coating/coatings to move right along with.

Perhaps not the best analogy: latex paint on a brick wall cracks, as it doesn't stretch enough, so we choose elastomeric paint, problem solved. From there, we like the latex better, so latex over the elastomeric - it still cracks, for it's being forced to move/stretch same as before.

 

 

 

 

 

Interesting Boron amount. I try not to exceed 0.5 for a cone 04 (lowfire glaze) have you tested this?

[Min]

40 minutes ago, Bill Kielb said:

Just curious, why so much silica in the recipe above?

 I know the silica is above the limits and the silica could probably be reduced but the glaze is perfect (for me) with this amount. There is no crazing from an oversupply of silica not being taken into the melt.  I've noticed a few good clear glaze recipes that do oversupply the silica like this one does, as long as the high silica doesn't create an unmelted glaze and/or crazing from it I think it makes for a really durable glaze. Alumina is high too, just within the limits.

[Bill Kielb]

1 hour ago, Bill Kielb said:

Perhaps not the best analogy: latex paint on a brick wall cracks, as it doesn't stretch enough, so we choose elastomeric paint, problem solved. From there, we like the latex better, so latex over the elastomeric - it still cracks, for it's being forced to move/stretch same as before.

Food for thought,

If we viewed the crazing issue  from a speed perspective and we theorized that the thin layer of glaze shrinks much quicker than the underlying structure at some point the tensile stress in the glaze makes it craze or crack.  So the failure is affected by time and  is also very dependent on the thickness or section of the glaze or slip?

maybe from that perspective things will make more sense. Just a thought though.

[Hulk]

The "functional clear" sees extensive (cone 5) use at local JC ceramic lab*; I've used it on all "my" clays as well, including the Aardvark bmix (just one piece tho'). High boron, aye, it gonna melt and stretch, low expansion (per GlazeMaster) and cheap to make. The drawbacks I'm seeing are a) clouding, especially over the red clays, b)requires sieving before each use (at the JC lab, any road - haven't made up a batch at home). I'm curious what the forum regulars think o'that recipe.

Hansen makes a case for more silica, when the recipe will tolerate; sounds good t'me.

 

*very much appreciate the instructor copying some of their recipes for me!  ...the only experience with glazes I had at the time were with their theirs - even if I didn't use them at home, having the recipes gave me a starting point in terms of analysis, behaviors, characteristics, etc.

[liambesaw]

Looks like a pain in the butt to me!  3 ingredients relying heavily on GB which gels, hence the sieving.  

Maybe try something like G2926B which is tony Hansen's updated clear that replaces his 20x5 recipe?  He always aims for a good starting point that can be adjusted to suit a specific clay body.

[Bill Kielb]

26 minutes ago, Hulk said:

The "functional clear" sees extensive (cone 5) use at local JC ceramic lab*; I've used it on all "my" clays as well, including the Aardvark bmix (just one piece tho'). High boron, aye, it gonna melt and stretch, low expansion (per GlazeMaster) and cheap to make. The drawbacks I'm seeing are a) clouding, especially over the red clays, b)requires sieving before each use (at the JC lab, any road - haven't made up a batch at home). I'm curious what the forum regulars think o'that recipe.

Hansen makes a case for more silica, when the recipe will tolerate; sounds good t'me.

 

*very much appreciate the instructor copying some of their recipes for me!  ...the only experience with glazes I had at the time were with their theirs - even if I didn't use them at home, having the recipes gave me a starting point in terms of analysis, behaviors, characteristics, etc.

In my opinion

It’s chemistry is not super aweful except degradation test have shown at about 0.55 Boron things go south quickly. It has a less than exciting flux ratio of  about 0.15:0.85 so not a pillar of dishwasher durability  either.  For functional clears I like them to be as clear as practical and have a very durable flux ratio. 

[Bill Kielb]

6 minutes ago, liambesaw said:

Looks like a pain in the butt to me!  3 ingredients relying heavily on GB which gels, hence the sieving.  

Maybe try something like G2926B which is tony Hansen's updated clear that replaces his 20x5 recipe?  He always aims for a good starting point that can be adjusted to suit a specific clay body.

Personally definitely not a fan of that recipe. I don’t get the chemistry.

R2O :RO. - 0.41:0.59

B2O3 - 0.37 ( Probably melts cone 02 or less)

NA2O- 0.36

sio2/al2o3 - 10.85:1 

I seriously don’t get it but to each their own I guess.

[Hulk]

5 minutes ago, liambesaw said:

G2926B

Aye, top o' my list, that one ...oops, second, G1216M is - don't have all the ingredients for either one just now, MgO frit, nor Minspar, and low on talc.

 

Thanks for the feedback Bill, I'd been reluctant to spend time on the Functional Clear, so much gerstly (ghastly to spell too). Was sieving it at the JC lab on account o' the little bits, like a grain of sand, where a cloudy spot will form.

[Bill Kielb]

@Hulk

If you ever mix up the hard candy we suggested I would be very curious of your results and thoughts

[Min]

1 hour ago, Hulk said:

The "functional clear" sees extensive (cone 5) use at local JC ceramic lab*; I've used it on all "my" clays as well, including the Aardvark bmix (just one piece tho'). High boron, aye, it gonna melt and stretch, low expansion (per GlazeMaster) and cheap to make. The drawbacks I'm seeing are a) clouding, especially over the red clays, b)requires sieving before each use (at the JC lab, any road - haven't made up a batch at home). I'm curious what the forum regulars think o'that recipe.

 

Like Bill said the boron level for that glaze is super high. When you look at the glaze calc with Insight the COE comes out the same as my recipe above. The problem is that when boron is way oversupplied it actually induces crazing. There is a limit to the amount you can add to help keep the COE figure down but there is a tipping point after which in creates crazing, in addition to making a "soft" glaze that isn't durable. 

[Bill Kielb]

On 4/9/2019 at 10:39 AM, Min said:

as the high silica doesn't create an unmelted glaze and/or crazing from it I think it makes for a really durable glaze. Alumina is high too, just within the limits.

Your high Alumina mention brings to mind something that stumped me for a bit about a year ago.

For true matte glazes in the 3:1 - 5:1 si/al range that fire matte regardless of cool down what is one to do if the matte crazes?

Adding clay / silica at 1.25 :1 is difficult because it drives the glaze towards gloss.

Easy solution to try : (have not seen it fail yet)  - set flux ratio to 0.2:0.8, no higher than 0.23:0.77 and set  alumina at 0.56. All in about a .15 Boron composition for cone six.

The general result: crazing goes away, melt is fantastic and true matte fires time after time, no need for special cooling and a durable RO.   If we take some Alumina out because it seems too high the glaze begins to craze as we approach 0.50 Alumina.  

I have Attached a recipe we have been using for porcelain and bmix (studio use) for about a year now.  Added a touch more Boron to melt over heavy underglaze. It’s been extremely dependable even for student use.

Talk about high alumina!

 

 

[Hulk]

Still curious about this topic, found this Ron Roy statement on clayart:

"The reason for that is - glazes are influenced by the clay they are fired on. They pick up some of the material from the clay and - the clay, being short of fluxes (compared to the glaze) will help lower the expansion of the glaze."

Hence, perhaps my observations may be more that the red and buff clays are lowering the expansion of the crazing clears, whilst the bmix and Sedona red do not - that could explain why the thin layers of slip exert their influence as described. I was focused on the opposite - why the bmix and Sedona red cause crazing.

I don't buy that the glazes "fit" or don't fit the thin layers of slip. The underlying body should move (expand/contract) the same, hence force the thin layer of slip along for the ride - I don't see the slip layers being elastic enough to give the give the glaze a break, expansion wise.

Any road, I don't have a proven clear for either problem body, yet. A lower expansion clear (last firing) crazes on the bmix and Sedona red - not as badly as the previous trials - looks like something 6.0 or below is in order, hence I'm looking to invest in some FF3249 or review the substitutions for that frit ...again! 

Meanwhile, next few loads will be all buff and red clay!

[liambesaw]

It's funny you mention that, I just had a few slip experiments come out of the kiln and one glaze that normally crazes on my clay body (a matte black) pulled the bisqued slip off in places and incorporated it into the melt.  At first it looked like it just shivered off, but I couldn't find the pieces in the kiln, and I looked closer and saw that the slip had pulled off and melted into the glaze.  So I think the slip does affect the fit of the glaze by acting as a buffer between the body and the glaze.  It IS elastic enough to incorporate itself to both the underlying body and the glaze.  Id bet if you smash a piece and look at the cross section under magnification you'll see 5 layers instead of 3.  You'll see clay body->fluxed slip mixed with body->slip->slip fluxed with glaze->glaze and maybe that pure slip in the middle is like a solder connecting two dissimilar metals.

[Callie Beller Diesel]

@Hulk, out of curiosity, did you do a freeze/thaw test to make sure your slipped pieces weren’t doing any delayed crazing?

[Min]

@Hulk, I'm out of town right now and can't access my glaze book but I've got a low expansion glaze that is based on the G1215U from Hansen that I've redone without the FF 3249. I'll pm it to you when I get back home if you want the recipe. It works on my low expansion clay with no crazing, in theory it should be fine for B-mix.

[Hulk]

The red (Aardvark srf blunged and sieved) slip doesn't appear diminished after bisque, however, it does diminish in glaze firing - very thin layer disappears, thin layer becomes translucent; looks like it is being absorbed into the glaze.

I still don't buy that the slip is elastic enough to accommodate the body and glaze coe differences, all due respect Liam (much respect!!)

I'm not applying red slip to bmix to prevent crazing (I like how it looks, experimenting, etc.), just curious about the what I'm seeing in respect to crazing. Some of the red slipped bmix pieces do have some crazing over the slip Callie, however, all exhibit more crazing where there is no slip. Since they all have crazing where the clear is over bmix, they all are regulated to home only use and/or repurposing to succulent pots - I have a drill press with a ceramic bit!

Min, I have one of your low COE recipes from another thread; if you have a lower coe mid fire clear, I'm curious!

I don't have minspar, dolomite, or FF3249 - will have to decide which way to go.

From a Min posting:

Wollastonite 5.70

EP Kaolin 23.70

Talc 3.30

Silica 32.80

Minspar 12.60

Gerstley Borate 19.50

Dolomite 2.40

100.00

 

These are Sedona red clay pieces repurposed due to crazed clear liner