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mrcasey

Mathematical Definition of "Centered"

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What is a good definition of centered clay? (I'm especially curious about Tyler's and Baymore's  opinions)

(1)  What is the mathematical/physical definition of a centered piece of clay? and
(2)  Why does coning up and down center a piece of clay?  

 

 

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I've no answer to your question, but it's interesting to discover that people 'out there' (i.e. non-practitioners) do find this sort of thing interesting...

Regulating rotation speed in wheel throwing: Effects of mass and shape

...to one side, but relevant, from the Journal of Archaeological Science:

A proxy of potters’ throwing skill: ceramic vessels considered in terms of mechanical stress - (PDF)

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For me, centering is to coax the clay into a shape that is equidistant from the rotational center of the wheel head.

As for coning, it is a wheel throwing wedging technique that helps to align platelets and remove air bubbles from the clay. The technique of raising the clay then pushing it down achieves this, and at the same time the gradual movement of the form outward from the center helps to center the clay. Many texts have called this coning "Mastering" as a preliminary step to wedging. For the beginner, being able to cone up and down, is a skill process as it helps to understand the amount of pressure, water lubrication and coordination needed to get the clay to move for the potter.

All of this is in my understanding, and I am sure others may elucidate much better than I.

best,

Pres

Marcia Selsor and Benzine like this

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Old lady, up until a few days ago, I did not have a thumbs up/like button. Something to do with the account software on my name. So Lee and I had reason to be exasperated. However, as you can see, all is good.

 

best,

Pres

Min likes this

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I always thought centering distributed the clay evenly so that when the walls were raised, they were even in height, but this Morning I watched a video of Ben Owens beating a BIG lumpy ball of clay into an even shape and then pounding his fist into it with the wheel turning slowly to open it .  So, was that clay centered?  Or is he strong enough with large enough hands that he centered the pounded open ring and went from there?

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Clay is centered when it no longer moves your hand while the hand is resting on the rotating clay. But, from a technical stand point clay (or any object) can be considered as centered  when at that moment any point on the clay body is equidistant from the center of the rotating mass as the point 180 degrees from that point.  note that this does not require that a rotating body be round or circular, just that one point and the point on the other side be the same distance from the middle.   so you can center a rectangle, but on a wheel it would be really hard to throw a rectangle.  

Edited by Viking Potter

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Centered and on center are two different things. On center is the mathematical definition of being perfectly in the middle of the wheel. But for throwing, that is not adequate. I can take any piece of clay and get it on center, but I wouldn't necessarily call it centered. Being on center is one part of centering. Clay being centered is also a state of being, a condition. In addition to being on center, the clay must be smooth, consistent, homogenous, etc. I think that is more important than being on center. I can deal with a little wobble as long as it's consistent and homogenous and smooth. The wobble will even out as I pull. But inconsistency doesn't go away as you pull, it just makes the wobble worse.

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My opinion aligns with Neil's (bad pun, ducking and running). To put things in more physical terms, IMO, on-center is a perfectly round outer surface around the center line of the wheel's axis of  rotation. Fully centered is when the mass of clay is both on-center (round, as previously defined) and the vertical axis of the center of gravity of the clay mass aligns with the on-center axis of rotation. Clay as a material can have, and usually has in its natural condition, variable densities within the mass. Areas within the ball of clay where the platelets are more aligned are more dense, and consequently have more actual clay per cubic whatever than areas where the platelets are not well aligned. Thus, the center of gravity/mass is not in the center of the volume. If one were to open an on-center but not fully centered ball of clay, the side that has the aligned dense area will have more physical clay and thus will create a thicker or taller (usually both) wall compared to the other side that started with less clay. Once the ball of clay has been opened, clay does not move around much. Squeezing it to raise the wall causes the wall to get thinner at that location by displacing some of that spot of clay slightly upward (thus generating height in that location) or tangentially (thus generating circumference (which yields diameter) at that location). Otherwise, the clay does not move. In exaggerated stark terms, despite what it might look like, raising a cylinder wall does not entail moving clay from the base to the top and building up from there. Likewise, once the ball has been opened and raising the walls has commenced, clay does not move around the circle. If there is are any more dense and/or less dense sections, they will remain where they are and the resultant wall will be uneven at best. Think of it like tires on a car. Unbalanced wheels, despite being round, will be a bumpy ride.

The clay can, and does, move around inside the ball when it is still a solid mass. Coning up and down achieves this. Proper coning up produces a spiral jet up through the center of the cone as it rises. At the same time, the friction of your hands against the side of the rotating cone causes the platelets to collapse into alignment, to a depth of about a half inch. The aforementioned spiral jet brings additional disorganized clay up from the base for alignment as the cone rises. Taking the cone back down properly, redistributes the aligned clay evenly around the outer layer of the mass. Because it is not possible to get all the the clay aligned in a single pass of the coning, you must run it up and down several times. Each time the spiral jet brings more up from the base, until it is all aligned. You can tell when you have more alignment to do by watching the very top of the cone as you complete it. If the tip is lumpy, there is probably more in the lower center of the mass that hasn't yet been worked. Smooth the lumpy tip with your thumbs, take the cone back down properly, and squeeze some more disorganized clay up from the bottom center of the mass. When the cone tip finally comes up smooth and clean, you're done centering. Take the cone back down and carry on.

 

It is true that an expert potter with very strong and steady hands can pinch the rotating clay into some semblance of centered without all the coning, but the rest of us need to cone. JMO, YMMV.

 

dw

hantremmer likes this

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On 10/25/2017 at 3:50 PM, clay lover said:

I always thought centering distributed the clay evenly so that when the walls were raised, they were even in height, but this Morning I watched a video of Ben Owens beating a BIG lumpy ball of clay into an even shape and then pounding his fist into it with the wheel turning slowly to open it .  So, was that clay centered?  Or is he strong enough with large enough hands that he centered the pounded open ring and went from there?

The technique of pounding/slapping the clay to center is an old technique used in Asia. It usually is done in concert with the fist pounding of the clay while slow wheel rotation. Yes this is fully centered, and if done properly there are very few lumps. The technique is usually followed up with a very stable pull that really does not move a lot of clay, but smooths the walls of the knuckle bumps on the inside or outside or even the finger marks o the outside. It is an ambitious technique to learn, but I have used it often when throwing 20# jars or vases. It looks like it takes a lot of energy, but really does not take as much as you might think.

 

best,

Pres

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