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Throwing Small Narrow Vessels In Porcelain


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#1 oly

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Posted 25 April 2016 - 07:40 AM

Having started using porcelain, and trying to keep walls as thin and even as possible has highlighted a problem in my throwing...

 

The vessels I'm throwing are using only about 200g (7oz) of porcelain and wet end up about 2" across and 4 or 5" high. I want to get the walls as thin as possible (to gain more height) but the torque resulting from pulling up the porcelain can start buckling the walls. And I can't reach all the way down to bring up the clay bit by bit.

 

Any tips please?



#2 Dick White

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Posted 25 April 2016 - 08:25 AM

When pulling taller forms, the diagonal buckling you describe (which we have all seen and experienced, nothing unsual) is the result of the friction of your fingers slightly restricting the speed of rotation at that point while the wheelhead continues at full speed, combined with the wall being insufficiently strong to withstand the slowing of the upper area. The two variables are 1) the declining strength of the wall, and 2) the increasing distance between the point of good strength (usually where the vessel is attached to the wheelhead) and the point of friction restricting rotation. In other words, thinness and height. Thinner, and the piece will wrinkle. Taller, and the piece will wrinkle. To resolve the discrepancy you need to disconnect thinness and height. But that sounds absurd when the objective is throwing thinner and taller.

 

What I teach is that you can, and must, disconnect the two concepts by working the vessel in sections. Throw the cylinder to a moderate thickness (and I use the word thickness purposely here, we are not yet in the realm of thin) as tall as you can. Try to reach the point just before you know the dreaded wrinkle will begin to occur. Stop pulling up and consolidate/strengthen/dry the wall of the cylinder with a good ribbing. Now begin a new style of pulling up the cylinder wall by starting halfway up. In stage one, only raise the top half of the cylinder, making it taller and thinner. Complete that section with another good ribbing. Because you left the bottom half thick and strong and are working only the top half which is (obviously) shorter than the whole cylinder, you have now defeated variable #2 above. The distance between the upper point of friction and the lower point of strength (the middle of the cylinder which is still thick and strong) is short enough that you can achieve thinness without wrinkling. Now shift to stage two, the bottom half of the cylinder. Reach down through the now completed top half of the cylinder all the way to the base, and begin to pull it up. As you reach the midpoint where you earlier began the top half work, ease up and transition out. Do NOT continue up through the already completed top half. Again, the distance between the friction of your throwing fingers and the lower point of strength (now the wheelhead again) is short enough that it won't wrinkle. The moment you continue up into the thin top half (which I told you not to do), you are in wrinkle territory again. From this point on, only gentle shaping is allowed, no more vigorous pulling up.

 

One final observation. You must always pull up the cylinder wall as quickly and efficiently as you can. The more times you pull it and the longer you take to do it will cause the clay to be overworked and weakened. There is nothing you can do to pull overworked weak clay any taller. It may seem to pull taller, but the moment you let go, it will slump back down. Strive for three pulls maximum before you shift to the top half/bottom half routine.

 

As I tell my students, now it's your turn to do it.



#3 glazenerd

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Posted 25 April 2016 - 07:29 PM

Dick:

I do not throw and I understood that: excellent teaching skills.

Nerd



#4 oly

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Posted 28 April 2016 - 09:24 AM

When pulling taller forms, the diagonal buckling you describe (which we have all seen and experienced, nothing unsual) is the result of the friction of your fingers slightly restricting the speed of rotation at that point while the wheelhead continues at full speed, combined with the wall being insufficiently strong to withstand the slowing of the upper area. The two variables are 1) the declining strength of the wall, and 2) the increasing distance between the point of good strength (usually where the vessel is attached to the wheelhead) and the point of friction restricting rotation. In other words, thinness and height. Thinner, and the piece will wrinkle. Taller, and the piece will wrinkle. To resolve the discrepancy you need to disconnect thinness and height. But that sounds absurd when the objective is throwing thinner and taller.

 

What I teach is that you can, and must, disconnect the two concepts by working the vessel in sections. Throw the cylinder to a moderate thickness (and I use the word thickness purposely here, we are not yet in the realm of thin) as tall as you can. Try to reach the point just before you know the dreaded wrinkle will begin to occur. Stop pulling up and consolidate/strengthen/dry the wall of the cylinder with a good ribbing. Now begin a new style of pulling up the cylinder wall by starting halfway up. In stage one, only raise the top half of the cylinder, making it taller and thinner. Complete that section with another good ribbing. Because you left the bottom half thick and strong and are working only the top half which is (obviously) shorter than the whole cylinder, you have now defeated variable #2 above. The distance between the upper point of friction and the lower point of strength (the middle of the cylinder which is still thick and strong) is short enough that you can achieve thinness without wrinkling. Now shift to stage two, the bottom half of the cylinder. Reach down through the now completed top half of the cylinder all the way to the base, and begin to pull it up. As you reach the midpoint where you earlier began the top half work, ease up and transition out. Do NOT continue up through the already completed top half. Again, the distance between the friction of your throwing fingers and the lower point of strength (now the wheelhead again) is short enough that it won't wrinkle. The moment you continue up into the thin top half (which I told you not to do), you are in wrinkle territory again. From this point on, only gentle shaping is allowed, no more vigorous pulling up.

 

One final observation. You must always pull up the cylinder wall as quickly and efficiently as you can. The more times you pull it and the longer you take to do it will cause the clay to be overworked and weakened. There is nothing you can do to pull overworked weak clay any taller. It may seem to pull taller, but the moment you let go, it will slump back down. Strive for three pulls maximum before you shift to the top half/bottom half routine.

 

As I tell my students, now it's your turn to do it.

 

That is really wonderful explanation and it will help with all my throwing I am certain (not just the little porcelain forms), it makes perfect sense. Thank you!



#5 MatthewV

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Posted 28 April 2016 - 12:59 PM

Normally I use one or two fingertips to pull the cylinder. On a thinner form, I will spread out the pressure using all four finger tips. This motion is much more difficult to do correctly.


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