Laguna 15 cracking

[SabClay]

I am an experienced potter trying to find the right porcelain for my work. As of late I have been using cone 6 Laguna 15 because it is super white and super vitreous. Both of these things are important to me because I leave much of the exterior of my pots unglazed, so the porcelain needs to look very white and not stain from absorption over time. 
However this clay cracks like crazy!!! About 50% of the mugs I make have cracks around the handles. Also, lots of cracks along the bottoms. I have tried everything I can think of to prevent cracking- compressing like CRAZY, adding more clay to attachments, different glaze/clay combos, slower firings with holds... at this point I’m wondering if it’s me or if it’s just the clay. 
Has anyone else had this problem with Laguna 15? Has anyone found alternative super white, vitreous clays that don’t have cracking problems? This clay is so beautiful I wish I could figure this problem out, but a 50% loss rate is too high!

 

[Bill Kielb]

We use Frost for its whiteness but it is finicky. Compressing was key to solving these issues but how it was compressed became critical, especially around handle joints. Here is an old bad video about compressing https://youtu.be/jVNJELUpclk. Once we started being religious about our compression the cracks disappeared. Same for joints, which often we clear glaze, so any crack shows up. Best idea, re-evaluate how things are compressed and whether they are truly denser when you are done.

The bowl below is frost, the mug Is frost, just warm lighting,  it reveals the type of handle joint which deserves lots of compression at attachment time when it is clear glazed.

Hope that helps!

Threw  in a bisqued version below before spraying the clear glaze. Frost is very white.

 

Edited June 6, 2021 by Bill Kielb

[Morgan]

I love frost when it turns out but it’s finicky is not worth the effort imo…it is hard to match but I found standard 365 to be better in almost every regard. I can throw it paper thin if needed, it’s super white translucent etc. months ago I tested every porcelain I could pretty much get my hands on and 365 won for me as it checked every box.

[Roberta12]

@SabClay  I have been using Laguna #16 for a few years.  Quite happy with it.    Have you tried that one? 

[Candace bell]

Yes! And I’ve used Laguna B mix for years and only had this problem in the last 12 months. Having to baby all handles like they were porcelain. Very slow drying and using a paper/vinegar mix in my slip. 

[Morgan]

Reviving this old thread since I was somewhat forced financially into using frost, which I love when it works, but oh the cracks. I never really had an issue before because I wasn’t making a ton with it but after making a hundred mugs or so I noticed quite a bit of cracking in the bottoms. Not S cracks but almost perfect circular cracks (almost never go all the way through). Is this still just a compress the heck out of it issue? I use a sponge on a stick to remove water at the end and it oddly seems like the circular cracks follow the final brush bristle lines if that makes sense. I have never had any attachment issues as I always attach and trim pretty darn wet still as to not ever have to worry about that but I am super annoyed right now and have a literal ton of this sitting here.

[Bill Kielb]

1 hour ago, Morgan said:

Not S cracks but almost perfect circular cracks (almost never go all the way through). Is this still just a compress the heck out of it issue? I use a sponge on a stick to remove water at the end and it oddly seems like the circular cracks follow the final brush bristle lines if that makes sense

We have found frost to be a bit picky so we teach what we consider effective compression or for bottoms of cups / mugs …….. some pattern of ribbing from the outside perimeter to the very center as in the video. Folks simply pushing down hard definitely stand a chance of the clay growing outward or less compressed on the whole. For Frost though, I spend a lot more time on handle joins after having knocked several off after a bisque run just to see how well they were attached. If when fractured, the parent material broke off with the join, I considered that well made. If the join simply broke at the slipped connection, then I considered it an ok but weaker than the parent material join and for Frost subject to hairline cracks. (Sort of a welders view of good and average joins)

For frost I definitely make sure to compress as properly as practical and for handle joins - compress and often infill to be sure it’s all similar strength and density.  Overall, I rarely use a sponge or overwork even greenware with a rib and sponge while trimming as well as burnish things perfectly. I find overworking can lead to delaminating and really strange surface cracks (maybe more like you describe) presumably due to local shrinkage and density differences of areas I overworked.

Having said all that I think it has made me better. I still like frost, and the extra attention really does not seem to slow me down a bunch.

Edited June 6, 2023 by Bill Kielb

[Morgan]

thanks, ya I believe I am so used to "compressing" by just pushing down back and forth over the bottom it is probably more of an outside inward (or lack of) compression. 

 

Edit: are you saying you do not do much burnishing or you do? I burnish the bottoms of all of my pots with a rib...wondering if that is what prevents the cracks from going all the way through but causing on the inside bottom, just a random thought.

Edited June 6, 2023 by Morgan

[Bill Kielb]

3 hours ago, Morgan said:

Edit: are you saying you do not do much burnishing or you do? I burnish the bottoms of all of my pots with a rib...wondering if that is what prevents the cracks from going all the way through but causing on the inside bottom, just a random thought.

I try not to trim much and just throw nearly finished. (Lazy, but good practice throwing) I stopped excessively burnishing especially if rewetting with a sponge. Definitely have caused cracking issues with excessive wetting and burnishing. The bottoms are trimmed very smooth and only very lightly burnished. After firing they all take a few turns on a diamond grinding disk anyway so butter smooth. I do glaze inside the foot though so another reason burnishing is not really important to me.

[neilestrick]

IMO, burnishing the bottom after trimming likely has very little effect on preventing cracking, because the clay is already pretty well set up to crack or not at that point. Sanding after firing is going to be necessary regardless of how well you burnish.

[Mark C.]

One point that is not covered above is that Frost scraps and slurry, trimmings do not play well with other clays. Many art centers do not want it in thier slurry due to that fact.For single users this is not an issue but a thought to consider

[Morgan]

2 hours ago, neilestrick said:

IMO, burnishing the bottom after trimming likely has very little effect on preventing cracking, because the clay is already pretty well set up to crack or not at that point. Sanding after firing is going to be necessary regardless of how well you burnish.

ya I did not think so, then again I have never in my life seen perfectly circle cracks, S cracks sure, but not these. I too do very very minimal trimming. Rib quickly and angle cut corner, done.

another random thought...these came as a large bulk order so they were pretty rock hard blocks that I end up slamming on concrete to loosen up, then they are fine...is it possibly this clay needs to be wedged way way more? And just to reiterate the responses, the main fix here is more and more compression outward in? Certainly not a thickness issue, my walls and base are identical and have only had this issue with Frost and maybe decades ago in high school

Edited June 7, 2023 by Morgan

[Min]

After you wedge the clay how are you prepping the mug weights of clay before throwing? Are you placing the spiral of the wedged clay face down on the wheelhead or turning the lump of clay on it's side? Or wedging a large mass then cutting pieces off and patting them into shape before throwing?

[Morgan]

5 hours ago, Min said:

After you wedge the clay how are you prepping the mug weights of clay before throwing? Are you placing the spiral of the wedged clay face down on the wheelhead or turning the lump of clay on it's side? Or wedging a large mass then cutting pieces off and patting them into shape before throwing?

Good question and the latter.  When I make mugs, or any amount of smaller weighted things, I usually weigh out and wedge about 8-10lbs at a time, or a third of a 25lb block, spiral wedge. Then I slap stretch that into longer narrow pieces I can quickly wire off and weigh a 1lb ball for my standard mug. Slap round and add to pile.

when I say the clay is hard I actually thought it was bad when delivered at first, as some of the blocks showed a visible crack in the clay block bag itself. Sure enough some concrete slamming wakes it up and it seems perfect consistency…in fact I like it better a bit stiff.

the only reason I thought about this is because I have thrown at least 50lbs of frost before, all mugs, and I don’t recall any cracks at all. The only difference is that was what I consider a “fresh” small direct order to test, much more soft than what I have now. Some of you may recall my laguna frost glaze issue thread too, ha!

Edited June 7, 2023 by Morgan

[Hulk]

Any chance the clay composition/recipe had changed?

Curious, that the cracks are appearing on the insides, in a circle.

While I don't believe that clay actually compresses (forced to occupy less volume than before), nor do I believe that the potter can wring water out of clay with their fingers (or a tool), I absolutely believe that how the clay is worked makes a vital difference.
The clay swirls opposite the direction of the wheel; I prefer the term "flow" - that's what it looks and feels like to me.
The walls get a lot more work than the base - around and around. Working the base so the clay has been swirled more seems to make a big difference, particularly close to the center; working the base to relieve the stretch imparted by opening, seems to make a big difference.

My guess, the cracks are along a seam imparted in the clay prep, and/or a shear imparted in the opening.

Much as I'd like to conduct some serious and somewhat thorough testing, base cracks aren't at the top of my list!

I'd like to think that wetter clay would be more prone to cracking, as it has further to shrink down that drier clay.
On the other hand, wetter clay moves more easily, and drier clay is (err, may be) more prone to shearing*.

I haven't done thorough testing to try to prove that clay prep makes a difference - my clay prep, that is - I do believe it makes a difference though**.

I'm prepping each clay ball. It's a habit now.
Contact with the wedging surface dries the clay, so I rotate the ball ninety degrees, several times, hence the outside is folded in, and in, and in.
I believe that continuing the spiral imparted on the wedging board is helpful, so I tip the clay ball up on its right side (I throw clockwise) and roll it around some to seal the bottom afore stashing (I keep the prepped clay balls in a plastic bin so they don't dry out as much).
Inconsistency in the clay will reveal itself; this is almost certainly true.
 

*Once the clay has been pushed to the point where there's some tearing, the tear won't heal - it will "be back" again.
Dryer clay can be worked longer afore it gets too wet and floppy, however, it also takes more time and force to move it.

**The testing I have done: I prepared clay balls for classmates who were struggling to center and throw. The majority (by far) had a better experience!
Perhaps due to two factors, a) more consistent, for any dry bits, bubbles, chunks, et cetera disrupt the flow of the clay, which leads to wonky wobbling frustration, and
b) maybe, maybe the swirl imparted via wedging gets the clay moving in the "right" (or left) direction?
However, maybe my classmates were just more focused, idk.

Edited June 7, 2023 by Hulk
and

[Min]

Did you  solve the massive crazing issue you were having?

As a test I would suggest prepping the clay by doing a spiral wedge on a dozen or so chunks of your standard 1 lb weight then turn them on their side (so the spiral is not face down) when placing on the wheel head. See if that makes a difference. 

Edited June 8, 2023 by Min
Typo

[Morgan]

That is a good idea min thanks, I will try that. I also wonder if stretching the wedged clay out on the table might be messing it up. Just in some form of common sense it seems as if I properly wedge them then elongate/stretch them by sideways slamming  it would add stress to the clay? Either way will try that.

I did fix most of the crazing issues but not all. Some I just had to adjust my application and others I need to just get around to either tweaking the glaze or saying goodbye to some of them with frost. Most of the crazing should be fixable (based on the size and distance of cracks) through hopefully just some silica/epk additions. if not, such is life. Such a drastic change from 365…

Edited June 8, 2023 by Morgan

[Morgan]

Just an update and I do not wanna jinx anything but I did about 50 more mugs wedging each ball individually per @Min’s suggestion and another chunk wedging larger amounts but then doing my normal elongating wedged clay to wire off smaller balls. I then truly spent an insane amount of time compressing bottoms. No visible cracks on any of the individual wedged ones and a handful on the ones using my normal method but with intense amounts of compressing. Looks like this is just going to be part of the process with frost. Thanks for all the suggestions and hope that is the end of that.

I do want to add some compliments to frost as well. It is hands down the best looking glazed porcelain I have ever seen. I also love how even when fairly wet it holds it shape very well for handling. This is especially important for me as I do all attachments much sooner than leather hard so I never have to really worry about joint cracks. So I may have to spend more time on the wheel but handles etc are much more efficient for me now.

off to do some glaze fit adjustments now…

Edited June 17, 2023 by Morgan

[Min]

@Morgan, glad it's sorted out!

[Morgan]

Sorry, but yet another update that I hope helps anyone else that goes through the headaches I have gone through with this clay body.

So, all of the suggestions in this thread did help, the wedging, the compression direction (slow drying vs not) etc but I still found I was getting a 5-10% crack rate. Very minor, never through, but nonetheless present.

That said I have gotten this to 100% success rate adding a few more steps into my routine so I hope this helps.

First off, as many have mentioned, lets assume you have consistent thickness in your bottoms and forms, wedged well. From there two things seem to be the game changers for me. Instead of just compressing out > in, it seems the clay really loves to be forcefully spread in > out, then forcefully out > in. Take your time with this, I start my forms with a rather thick bottom and by the time I am done really compressing in both directions it is the right thickness. The last thing I do is when the pots are leather hard, if not a bit before, is use a tool of some sort, a spoon will work, a sponge stick, your finger, whatever....and compress manually outward in with said tool. With these two additions I have fired over 200 mugs with zero cracks.

When I say forcefully above I mean really forcefully. The way I came across this was by making some plates, which I laughed at while making, thinking of coarse a plate would crack. When I make plates I use a ton of force with my fist getting it wide, then compress in to even the surface…wouldn’t you know it, no cracks.

Hope this helps to those dealing with this in the future!

Edited September 14, 2023 by Morgan