Pitoon
Imperial Masterpiece
Maybe try switching clay?
Diving into pottery
- Thread starter mwar15
- Start date
TinyArt
Chumono
I've heard it said that it isn't hoarding if it's books...
Hope this is of help!
Hope this is of help!
sorce
Nonsense Rascal
Truth truth, if something is going to sag or otherwise warp, in drying or firing, putting something there is merely transferring that energy elsewhere.
So sometimes it will work, and sometimes it won't. That's not enough certainty for a process so uncertain.
I think your largest problem in this scenario, spare all the other things we can solve, but are mostly being prevented by your inability to change the firing schedule (cuz this is still #1 priority), is...
Your feet seem to made of a lengthwise piece of clay or slab. The shrinkage of that is greater than the compressed direction. So if you are using one flat compressed width of clay as the support, your feet will shrink more than the support. Your feet seem 2 flats tall, which would mean you are increasing the shrink difference, because for every layer in this configuration, you are increasing the difference in shrink rate between the feet and the supports.
It is true that clay body plays a roll, and overfiring could mean more sagging due to pyroplasticity, or a clays characteristic to sag during firing.
However, Iron content is something that mustn't be overlooked, because if it's not properly "cleaned", or burnt out, the additional flux of reduced Iron can and will always cause it to sag, regardless of peak temp, maturation, and pyroplasticity.
The reason non bonsai potters don't need to consider this, and do not need to adjust standard "bisque" schedules, the usual burnout time, is because no pots span the unsupported distance that bonsai pots do. So though they aren't properly bisqueing, it is never evident, so no one cares.
Also consider round is the most stable shape. For the same reason portholes are round, round pots are more capable of giving equal "pressure" and movement during shrink.
A square pot is worse.
By the time you add the 2 different lengths of sides a rectangle has, the more evident it becomes, how difficult our situation truly is.
What physics are actually at play.
This is why it is so hard to find rectangle rectangles outside of Japan, and other places that fire wood kilns for 3 days or more. This slow of a burnout and overall firing, make it almost impossible to sag even the most iron rich clays, because the organics have plenty of time to burnout, leaving less to cause localized reduction, further reducing Iron, and sagging the pot.
This is the reason Western Potters use oxide washes to darken white clay bodies for "unglazed". Fake ass bullshit!
So....
Penumbra is right about the only sag happens during drying, short the reduced Iron part.
That said ...well...I recently decided the with the way I operate, drying pots fully, open, right side up, I have to be certain the bottom is dry enough to not sag before adding feet. This allows the slight iron reduce sag inevitable with my learning the right firing schedule to be just enough to make the pot drain good with a slight sag.
So I'm all for using no supports, rather, using drying and your method as the way to reduce sag.
For instance, a guy who dries upside down and gets a little sag, can reverse the sag firing right side up. And vice versa.
Allowing sag while drying right side up, and firing right side up, can double sag, making the pot less perfect.
Point is, all sag, minus iron reduction sag and clays with high pyroplasticity, can be controlled with the least amount of headache by paying attention to how dry it is when left unsupported.
I haven't found a clay that sags when only overfired. Even porcelain which is supposed to be most pyroplastic. Seam cracks are much more likely.
I don't think it has anything to do with what you are doing as far as build goes. Unless you are only compressing one side of the slab.
You can test clays for this by firing rectangle strips that have been compressed in one direction on one side. If you are not finding problems there, your problem lies elsewhere.
Sorce
So sometimes it will work, and sometimes it won't. That's not enough certainty for a process so uncertain.
I think your largest problem in this scenario, spare all the other things we can solve, but are mostly being prevented by your inability to change the firing schedule (cuz this is still #1 priority), is...
Your feet seem to made of a lengthwise piece of clay or slab. The shrinkage of that is greater than the compressed direction. So if you are using one flat compressed width of clay as the support, your feet will shrink more than the support. Your feet seem 2 flats tall, which would mean you are increasing the shrink difference, because for every layer in this configuration, you are increasing the difference in shrink rate between the feet and the supports.
It is true that clay body plays a roll, and overfiring could mean more sagging due to pyroplasticity, or a clays characteristic to sag during firing.
However, Iron content is something that mustn't be overlooked, because if it's not properly "cleaned", or burnt out, the additional flux of reduced Iron can and will always cause it to sag, regardless of peak temp, maturation, and pyroplasticity.
The reason non bonsai potters don't need to consider this, and do not need to adjust standard "bisque" schedules, the usual burnout time, is because no pots span the unsupported distance that bonsai pots do. So though they aren't properly bisqueing, it is never evident, so no one cares.
Also consider round is the most stable shape. For the same reason portholes are round, round pots are more capable of giving equal "pressure" and movement during shrink.
A square pot is worse.
By the time you add the 2 different lengths of sides a rectangle has, the more evident it becomes, how difficult our situation truly is.
What physics are actually at play.
This is why it is so hard to find rectangle rectangles outside of Japan, and other places that fire wood kilns for 3 days or more. This slow of a burnout and overall firing, make it almost impossible to sag even the most iron rich clays, because the organics have plenty of time to burnout, leaving less to cause localized reduction, further reducing Iron, and sagging the pot.
This is the reason Western Potters use oxide washes to darken white clay bodies for "unglazed". Fake ass bullshit!
So....
Penumbra is right about the only sag happens during drying, short the reduced Iron part.
That said ...well...I recently decided the with the way I operate, drying pots fully, open, right side up, I have to be certain the bottom is dry enough to not sag before adding feet. This allows the slight iron reduce sag inevitable with my learning the right firing schedule to be just enough to make the pot drain good with a slight sag.
So I'm all for using no supports, rather, using drying and your method as the way to reduce sag.
For instance, a guy who dries upside down and gets a little sag, can reverse the sag firing right side up. And vice versa.
Allowing sag while drying right side up, and firing right side up, can double sag, making the pot less perfect.
Point is, all sag, minus iron reduction sag and clays with high pyroplasticity, can be controlled with the least amount of headache by paying attention to how dry it is when left unsupported.
I haven't found a clay that sags when only overfired. Even porcelain which is supposed to be most pyroplastic. Seam cracks are much more likely.
I don't think it has anything to do with what you are doing as far as build goes. Unless you are only compressing one side of the slab.
You can test clays for this by firing rectangle strips that have been compressed in one direction on one side. If you are not finding problems there, your problem lies elsewhere.
Sorce
mwar15
Omono
I am using the trail mix dark chocolate from Georgies(cone6)
I roll it out using a large dowl(2.5”) I roll, flip, roll rotate, flip, roll, compressing .25-.5” at a time. I then cut out my pieces. Let them dry leather hard before assembly.
As I assemble the pot I roll out the rim and feet pieces. Those are leather hard before attaching to the body of the pot.
I cut out support at the same time or use high density foam as supports. The pot goes in a plastic bag to equalize and start drying
also I have zero control over the kiln
I roll it out using a large dowl(2.5”) I roll, flip, roll rotate, flip, roll, compressing .25-.5” at a time. I then cut out my pieces. Let them dry leather hard before assembly.
As I assemble the pot I roll out the rim and feet pieces. Those are leather hard before attaching to the body of the pot.
I cut out support at the same time or use high density foam as supports. The pot goes in a plastic bag to equalize and start drying
also I have zero control over the kiln
penumbra
Imperial Masterpiece
Cool. I like local clays too but for me that's NC or KY. Also use Standard, PA.
You are using the wrong clay, try the toast version for two reasons:
1. The additional grog is meant to terminate crack propagation
2. The shrinkage is significantly lower.
The young's modulus of clay is ~50GPa. So your 2% shrinkage difference equates to about 1GPa more stress potential anywhere there is a discontinuity. That's significantly more than the tensile strength of any clay.
Georgie's even warns you to fire to cone 5. It is fluxed to shit
1. The additional grog is meant to terminate crack propagation
2. The shrinkage is significantly lower.
The young's modulus of clay is ~50GPa. So your 2% shrinkage difference equates to about 1GPa more stress potential anywhere there is a discontinuity. That's significantly more than the tensile strength of any clay.
Georgie's even warns you to fire to cone 5. It is fluxed to shit
Pitoon
Imperial Masterpiece
Do you use the clay right out of the bag and start rolling it? I don't know if you do it or not but first thing you should do is wedge the clay. It does make a difference.
penumbra
Imperial Masterpiece
As to the toast mentioned, although the shrinkage is less, the absorption is quite high. For bonsai pots and other outdoor use I like an absorption rate as low as I can get. My pottery is generally 1% or less and my dark clay that I use on most of my stone art pots is 0.25%.
Pitoon has it easy with his tiny pots.
This is true but it is a pain in the ass, particularly with the large amounts of clay needed for larger pots like this.
Pitoon has it easy with his tiny pots.
Pitoon
Imperial Masterpiece
Yup, I wedge balls of clay from plum to orange sizeAs to the toast mentioned, although the shrinkage is less, the absorption is quite high. For bonsai pots and other outdoor use I like an absorption rate as low as I can get. My pottery is generally 1% or less and my dark clay that I use on most of my stone art pots is 0.25%.
This is true but it is a pain in the ass, particularly with the large amounts of clay needed for larger pots like this.
Pitoon has it easy with his tiny pots.
sorce
Nonsense Rascal
I don't think equalization is the most appropriate move for rectangle Bonsai Pots.
This need and teaching of equalization comes from attaching handles to mugs. Even there, there is a more efficient way to go about it, but eff mug makers! Except Iker. That's my guy!
Once your slab is a wee passed leather hard, it won't shrink anymore, and shrink is the only place at this stage we need to be concerned about when drying concerning cracks, or stresses that cause cracks in the firing. (See my seam cracks)
So you are essentially taking a bottom slab that won't sag or crack, and allowing it to again become wet enough to sag, and succumb to drying cracks that you already beat by having it a wee passed leather hard.
The Dreamcatcher Theory comes into play again.
S cracks (read, slab cracks too)happen ONLY due to the woven part that catches the dreams being wetter then the already shrunken frame. The frame being your attachments, and the woven part being your slabs. When the wetter slab still needs to shrink, but the frame is rigid. Crack.
There IS a tension created by the wetter seams, but (I think @NaoTK knows what this is, Gpa?) It is a much lesser tension than that which happens with a wet slab.
This should be easily, lol, easier, overcome by better controlling the amount of water applied to attach walls.
When we consider the distance the long side of a rectangle needs to shrink, as compared to the short side, we see how equalization is actually working against our mission.
In a perfect world, we would have these 2 sides exposed to air in percentages that match the distance of the shrink of each side. Allowing the long side to shrink a little slower than the short side. Impossible. So there is a better way.
Sorce
sorce
Nonsense Rascal
As far as wedging goes......
The reason for the process has 2 purposes.
One. To remove air and homogenize what may be yet unmixed ingredients.
Two. To align the clay particles in a swirled pattern which they will continue to swirl into on a potters wheel.
So for slab building, 2 works against us.
One depends on the producer.
I noticed straight away the difference between Amaco clay and standard Clay, as far as air pockets go.
When I spoke with Jim of Standard, he informed me they use the most powerful deairing vaccums available on the Market. It shows. Amaco clay is airfilled like the head of a .....
.
So wether or not to wedge has hidden truths too.
In general, rolling out an air free slab is equal to wedging for the wheel when it comes to particle alignment.
Sorce
The reason for the process has 2 purposes.
One. To remove air and homogenize what may be yet unmixed ingredients.
Two. To align the clay particles in a swirled pattern which they will continue to swirl into on a potters wheel.
So for slab building, 2 works against us.
One depends on the producer.
I noticed straight away the difference between Amaco clay and standard Clay, as far as air pockets go.
When I spoke with Jim of Standard, he informed me they use the most powerful deairing vaccums available on the Market. It shows. Amaco clay is airfilled like the head of a .....
.So wether or not to wedge has hidden truths too.
In general, rolling out an air free slab is equal to wedging for the wheel when it comes to particle alignment.
Sorce
penumbra
Imperial Masterpiece
If that works for you in the Pacific Northwest, that is great for you.
It doesn't work here for me. I can get a few winters out of some bonsai pots here that later fail because of freeze & thaw. Four years ago we had two severe weeks where several days were single digits and nightime was minus 8 to minus 14 degrees.
I will not sell any pot with greater than 2% absorption and as stated most of mine are 1% or less. I find this appropriate for the mountains of the Mid-Atlantic.
You are right in that testing is a good idea. And I am sure there are many more considerations other than absorption rates.
sorce
Nonsense Rascal
Mas double truth?
This has to do with clay body as well.
In regards to this, there are 3 categories of clay.
One that always must be vitreous Enough to be frost proof.
One that will always be vitreous due to closed porosity.
And the bitch one....that, depending on composition and firing schedule, can be all over the board.
That's the one that will get us into trouble.
The orange clay I mix myself has tested vitreous in oxidation between 6 and 10.
In reduction, I believe due to the iron pyrites in Hawthorne Bond Fireclay, it tests frigging anywhere.
The slightest change in reduction schedule has a great and unpredictable effect on the outcome.
So far so good, but I won't sell one of these until it has wintered successfully.
Sorce
penumbra
Imperial Masterpiece
I have talked with Jim several times at the Clay Conference; good guy and all business. I have never had the problem of even a single small bubble in Standard clay. For slab rolling I cut it right off the block and pound it downs to about 1.5 inches and run it through the slab roller no more than .25 inch at a time (frequently less), turning it and flipping it each time. My slabs are pretty thick for my stone age pots, about 3/4 inches before I press them with stone. Some of my other projects call for as little as 1/4 inch. But on the wheel it is a different animal and I always wedge the clay to play it safe.
Freeze and thaw are the key words here, I agree completely that the east coast gets more freeze thaw. But colder temperatures than freezing do not matter; the main event is the ice transformation, then any colder actually reduces that volume change. Anyways, given the track record on the hobby pottery landscape I'm more inclined to believe pot failures are from construction and firing faults than the material itself.
penumbra
Imperial Masterpiece
No argument from me there. That is ice cubes 101. But the colder the zone the more likely a freeze & thaw cycle.
.... except when they are not. Many of the Chinese bonsai pots, though constructed to near mathematical perfection, and fired to exactitude will not hold up to our cycle based solely on the clay body itself. I have used a few for target practice. Also true for many hand made planters built by some our finest local professional potters who's usual fair is not planters but platters, and who treat the absorption factors with little regard.
There is really not a certifiable point of comparison here, our climates are apples and oranges. My pottery is made to withstand my climate and harsher climates, and absorption rate is an important factor. It doesn't matter that many of my pots have ended up in California, as some of them also end up in New England.
Good discussion.
sorce
Nonsense Rascal
Of slabs....
I been throwing them out like the Chinese. Some good Korean Onggi videos with it too.
After seeing my guy throw a slab near perfect with just throwing and a rib, I was convinced.
Heard a well trained dude here talk about having less cracks when throwing out slabs instead of rolling them, and the more I look, the more I see pro pro's doing it like that. Not guys who suck at pottery but fancy all the ladies that they think think they are God because they run a studio, real pros.
I had a hard time overcoming the thought of how much waste I may create if I mess it up, but after I started having 400lbs on hand and no more reclaim worries, I gave it a go.
I am still rolling out the last quarter to half inch, but it cuts the slab prep time by half or more, much more for larger pots.
Sorce
I been throwing them out like the Chinese. Some good Korean Onggi videos with it too.
After seeing my guy throw a slab near perfect with just throwing and a rib, I was convinced.
Heard a well trained dude here talk about having less cracks when throwing out slabs instead of rolling them, and the more I look, the more I see pro pro's doing it like that. Not guys who suck at pottery but fancy all the ladies that they think think they are God because they run a studio, real pros.
I had a hard time overcoming the thought of how much waste I may create if I mess it up, but after I started having 400lbs on hand and no more reclaim worries, I gave it a go.
I am still rolling out the last quarter to half inch, but it cuts the slab prep time by half or more, much more for larger pots.
Sorce
