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#1 Natania

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Posted 18 April 2012 - 10:59 AM

I don't usually get S cracks, but lately I've been throwing little cups off the hump, and I get the cracks on the bottoms of the cups. I do compress the clay when I open up, but I feel like the clay isn't getting compressed on the other side since it isn't the wheel head there, but rather another lump of squishy clay. Any suggestions?
Thanks!,
Natania

#2 neilestrick

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Posted 18 April 2012 - 12:48 PM

This is very common when throwing off the hump, especially with porcelain or smooth clay bodies. Try compressing the bottoms after they have set up a bit, before they get to leather hard.
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#3 Mark C.

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Posted 18 April 2012 - 06:56 PM

I beat on them when almost set up with a tool on the bottom of pot.
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#4 Pres

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Posted 18 April 2012 - 07:12 PM

I don't usually get S cracks, but lately I've been throwing little cups off the hump, and I get the cracks on the bottoms of the cups. I do compress the clay when I open up, but I feel like the clay isn't getting compressed on the other side since it isn't the wheel head there, but rather another lump of squishy clay. Any suggestions?
Thanks!,
Natania


I had this problem quite a bit years ago and then did a commission of 2000 small vessels-throwing off the hump was the only way. I found an old video of a Japanese potter throwing off the hump-he would make a flat pancake, compressing the pancake on the hump, then draw up the sides of the pancake to form the walls of the pot then pull once or twice more. My percentage of cracking went from 25% to maybe 2%, in a week, the total of pieces was about .2% Worked out well, and I use it now all of the time.

Simply retired teacher, not dead, living the dream. on and on and. . . . on. . . .                                                                                 http://picworkspottery.blogspot.com/


#5 StefanAndersson

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Posted 18 April 2012 - 09:37 PM


I don't usually get S cracks, but lately I've been throwing little cups off the hump, and I get the cracks on the bottoms of the cups. I do compress the clay when I open up, but I feel like the clay isn't getting compressed on the other side since it isn't the wheel head there, but rather another lump of squishy clay. Any suggestions?
Thanks!,
Natania


I had this problem quite a bit years ago and then did a commission of 2000 small vessels-throwing off the hump was the only way. I found an old video of a Japanese potter throwing off the hump-he would make a flat pancake, compressing the pancake on the hump, then draw up the sides of the pancake to form the walls of the pot then pull once or twice more. My percentage of cracking went from 25% to maybe 2%, in a week, the total of pieces was about .2% Worked out well, and I use it now all of the time.


Yes.. Another way is to pull out the bottom a bit extra and after you lift the walls the first time you pinch the bottom from the outside to the width it shold have. This usually makes the bottom cone shaped and in need of some final shaping before the second pull of the walls. Much the same idea as the pancake method.

Best of luck.

#6 ThisIsMelissa

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Posted 18 April 2012 - 09:51 PM

One guy I watch on youtube throws a lot off the hump.
He insists that the reason he gets no s-cracking is that he trims the bottoms very thin... like 3-4mm.

#7 JBaymore

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Posted 19 April 2012 - 07:46 AM


I don't usually get S cracks, but lately I've been throwing little cups off the hump, and I get the cracks on the bottoms of the cups. I do compress the clay when I open up, but I feel like the clay isn't getting compressed on the other side since it isn't the wheel head there, but rather another lump of squishy clay. Any suggestions?
Thanks!,
Natania


I had this problem quite a bit years ago and then did a commission of 2000 small vessels-throwing off the hump was the only way. I found an old video of a Japanese potter throwing off the hump-he would make a flat pancake, compressing the pancake on the hump, then draw up the sides of the pancake to form the walls of the pot then pull once or twice more. My percentage of cracking went from 25% to maybe 2%, in a week, the total of pieces was about .2% Worked out well, and I use it now all of the time.


I was about to share this technique when I read the first posting in the thread ... but as I scolled down... I can see that Pres beat me to it. (Good stuff Pres) This "open to a flat disk" approach is a pretty common technique in Japan..... I've seen it all over when I've been working there. I;ve been using it for many many years. It helps greatly.

Different clays react differently to the invariable less compaction and the torque stress set up in the foot area by this technique. Compared to most commercial western clay bodies, the Japanese bodies are verry large particle sized and way less dense..... and tolerate this technique far more readily. Sometimes changing clays helps..........some bodies are really prone to issues with it.

Sometimes the clay manufacturers change the commercial clay body recipes a bit without telling you... and this can cause sudden changes in clay body behavior.

best,

...............john
John Baymore
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Professor of Ceramics; New Hampshire Insitute of Art

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#8 Natania

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Posted 19 April 2012 - 08:01 AM



I don't usually get S cracks, but lately I've been throwing little cups off the hump, and I get the cracks on the bottoms of the cups. I do compress the clay when I open up, but I feel like the clay isn't getting compressed on the other side since it isn't the wheel head there, but rather another lump of squishy clay. Any suggestions?
Thanks!,
Natania


I had this problem quite a bit years ago and then did a commission of 2000 small vessels-throwing off the hump was the only way. I found an old video of a Japanese potter throwing off the hump-he would make a flat pancake, compressing the pancake on the hump, then draw up the sides of the pancake to form the walls of the pot then pull once or twice more. My percentage of cracking went from 25% to maybe 2%, in a week, the total of pieces was about .2% Worked out well, and I use it now all of the time.


I was about to share this technique when I read the first posting in the thread ... but as I scolled down... I can see that Pres beat me to it. (Good stuff Pres) This "open to a flat disk" approach is a pretty common technique in Japan..... I've seen it all over when I've been working there. I;ve been using it for many many years. It helps greatly.

Different clays react differently to the invariable less compaction and the torque stress set up in the foot area by this technique. Compared to most commercial western clay bodies, the Japanese bodies are verry large particle sized and way less dense..... and tolerate this technique far more readily. Sometimes changing clays helps..........some bodies are really prone to issues with it.

Sometimes the clay manufacturers change the commercial clay body recipes a bit without telling you... and this can cause sudden changes in clay body behavior.

best,

...............john


Great suggestions. I tried compressing the bottoms with a metal rib today, before the became leather hard. I've been looking for a video of the "pancake" method, but haven't found one. If anyone does find one, I'd love to know about it... it's a little hard to envision without a demo.
Thanks!

#9 Pres

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Posted 19 April 2012 - 08:11 AM



I don't usually get S cracks, but lately I've been throwing little cups off the hump, and I get the cracks on the bottoms of the cups. I do compress the clay when I open up, but I feel like the clay isn't getting compressed on the other side since it isn't the wheel head there, but rather another lump of squishy clay. Any suggestions?
Thanks!,
Natania


I had this problem quite a bit years ago and then did a commission of 2000 small vessels-throwing off the hump was the only way. I found an old video of a Japanese potter throwing off the hump-he would make a flat pancake, compressing the pancake on the hump, then draw up the sides of the pancake to form the walls of the pot then pull once or twice more. My percentage of cracking went from 25% to maybe 2%, in a week, the total of pieces was about .2% Worked out well, and I use it now all of the time.


I was about to share this technique when I read the first posting in the thread ... but as I scolled down... I can see that Pres beat me to it. (Good stuff Pres) This "open to a flat disk" approach is a pretty common technique in Japan..... I've seen it all over when I've been working there. I;ve been using it for many many years. It helps greatly.

Different clays react differently to the invariable less compaction and the torque stress set up in the foot area by this technique. Compared to most commercial western clay bodies, the Japanese bodies are verry large particle sized and way less dense..... and tolerate this technique far more readily. Sometimes changing clays helps..........some bodies are really prone to issues with it.

Sometimes the clay manufacturers change the commercial clay body recipes a bit without telling you... and this can cause sudden changes in clay body behavior.

best,

...............john


From what I understand in reading analysis of the technique, in a standard thrown piece the flat particles of the bottom are aligned differently than the side walls putting more stress on the floor. This technique starts with the particles all aligned the same in the pancake, and then as it is lifted to the walls the particles curve around the form giving a more homogenous structure to the form.

Simply retired teacher, not dead, living the dream. on and on and. . . . on. . . .                                                                                 http://picworkspottery.blogspot.com/


#10 JBaymore

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Posted 19 April 2012 - 07:09 PM

I'll try this in words.......

Center the top section of clay into a ball of the appropriate size for the piece you are making, make sure to define the "bottom" point that you will eventuually cut it off at with the string.

In Japan they usually initially open with the thumbs of both hands, but I often use only the thumb of the left hand. Using the pad area of the thumb (or thumbs).....not the "tip" of the thumb...., open the ball of clay into a curved-bottom very shallow bowl form.

Bring the walls of the shallow bowl downward and outward thinning them VERY slightly so that the form that then results looks like a smoothly curved thick shallow plate. Do a quick "compression" pass outward towear the edges, back inward toward the center point, and then back outward toward the edges.

Then using both hands cupping the outside of the plate from underneath, collar the thick plate form back upward into a more cylindrical thick form.... like the starting point for the usual cylinder pulling business.

When you get proficient at this ........ it all blurs into almost one fluid movement.

And also........ don't let water sit in the bottom of the form much at all. get it out of there. Working fast is important when throwing off the hump also.

best,

................john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

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#11 StefanAndersson

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Posted 20 April 2012 - 01:51 AM

I'll try this in words.......

Center the top section of clay into a ball of the appropriate size for the piece you are making, make sure to define the "bottom" point that you will eventuually cut it off at with the string.

In Japan they usually initially open with the thumbs of both hands, but I often use only the thumb of the left hand. Using the pad area of the thumb (or thumbs).....not the "tip" of the thumb...., open the ball of clay into a curved-bottom very shallow bowl form.

Bring the walls of the shallow bowl downward and outward thinning them VERY slightly so that the form that then results looks like a smoothly curved thick shallow plate. Do a quick "compression" pass outward towear the edges, back inward toward the center point, and then back outward toward the edges.

Then using both hands cupping the outside of the plate from underneath, collar the thick plate form back upward into a more cylindrical thick form.... like the starting point for the usual cylinder pulling business.

When you get proficient at this ........ it all blurs into almost one fluid movement.

And also........ don't let water sit in the bottom of the form much at all. get it out of there. Working fast is important when throwing off the hump also.

best,

................john


This vod is in swedish but notice the pinch of the bottom to compress and the scissor grip to turn the shallow bowl into a cylinder. This man has another technique (in Japanese) to compress the bottom via making it during centering (It takes some practice to get the right amount of clay).

I am not sure why you say that it´s important to work fast off the hump. Can you explain why you feel that´s necessary?

#12 JBaymore

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Posted 20 April 2012 - 07:38 AM

I am not sure why you say that it´s important to work fast off the hump. Can you explain why you feel that´s necessary?


Stefan,

Having worked with a lot of "less skilled" Posted Image throwers...... as in students..... there can sometimes be a real tendency to do a lot of "pulling" and "fiddling around" with a form that really does not directly contribute to the final form. (They usually do this also when throwing directly on the wheelhead also.... but it is not as much of an issue for S cracks in that case.)

All of that unnecessary frictional contact between the fingers/hands and the upper parts of the off-the-hump piece cause a lot of excess "drag" on that part of the plastic wet clay form as it is driven at the botom by the force of the wheel. The torque induced by this sheering action is not distributed evenly in the lower part of the piece ... the base area... where it is "attached" to the hump of clay and the clay particles are less well aligned (which is inherent in the process). So that tends to make the uneven torque stresses that develop between the upper walls of the piece and the bottom area greater.... also a factor contributing to the problem of S cracking ... even in clays that tolerate off-the-hump work pretty well for a more skilled thrower.

The Japanese video you just posted is an excellent example of what we have been discussing in general. That should really help the OP.

best,

..................john
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Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

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#13 Pres

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Posted 20 April 2012 - 07:47 AM

I am not sure why you say that it´s important to work fast off the hump. Can you explain why you feel that´s necessary?


Stefan,

Having worked with a lot of "less skilled" Posted Image throwers...... as in students..... there can sometimes be a real tendency to do a lot of "pulling" and "fiddling around" with a form that really does not directly contribute to the final form. (They usually do this also when throwing directly on the wheelhead also.... but it is not as much of an issue for S cracks in that case.)

All of that unnecessary frictional contact between the fingers/hands and the upper parts of the off-the-hump piece cause a lot of excess "drag" on that part of the plastic wet clay form as it is driven at the botom by the force of the wheel. The torque induced by this sheering action is not distributed evenly in the lower part of the piece ... the base area... where it is "attached" to the hump of clay and the clay particles are less well aligned (which is inherent in the process). So that tends to make the uneven torque stresses that develop between the upper walls of the piece and the bottom area greater.... also a factor contributing to the problem of S cracking ... even in clays that tolerate off-the-hump work pretty well for a more skilled thrower.

The Japanese video you just posted is an excellent example of what we have been discussing in general. That should really help the OP.

best,

..................john


I think anyone trying this technique should realize because of its nature it will not be mastered in the first hump of clay, but over time with practice. It is well worth mastering in that it will allow for quicker throwing of a variety of cylinder forms with much more ease. I have found that well wedged clay is a prerequisite for being able to use the entire hump without fail.

Simply retired teacher, not dead, living the dream. on and on and. . . . on. . . .                                                                                 http://picworkspottery.blogspot.com/


#14 Natania

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Posted 20 April 2012 - 07:50 AM

Both those videos were extremely helpful, as was the description in words as well. I think I may be using a bit too much water as well. I'll try this technique and see how it goes. Thanks!

#15 SusanM

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Posted 07 May 2012 - 10:04 PM

Thank you for the information - I just ran into this problem throwing a series of small shallow bowls (wasabi/sauce dishes). If using the technique shown in the video from Japan - would I just flatten out the form with my thumbs, compressing the clay on the hump and make that my shallow bowl/sauce dish?


Thanks.

#16 koreyej

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Posted 08 May 2012 - 10:23 AM

Thank you for all the excellent tips! The class I'm teaching right now is throwing cups for their final. I have lots of ideas to share with them!

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#17 StefanAndersson

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Posted 09 May 2012 - 05:40 AM

Thank you for the information - I just ran into this problem throwing a series of small shallow bowls (wasabi/sauce dishes). If using the technique shown in the video from Japan - would I just flatten out the form with my thumbs, compressing the clay on the hump and make that my shallow bowl/sauce dish?


Thanks.


I think that if you look at the video ha has a mall dish in his hands when he has pushed the cone down after centering. I would grab it between thumb on the inside and the index finger on the outside (is the index finger the right word? The one next to the thumb) and pull it out.
But that is for speed. If you can take it slower I would make a cylinder and push it out into the dish with a thick rib tool (with the dish profile). This will make the dish stroger at the point where it usually get weakest and you can therefor make the wall thinner (and skip some trimming)




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