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SunsetBay

Throwing And Trimming Off The Hump

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The chuck does not need to be totally "symmetrical"'..... it just has to hold the PIECE in that condition )or close to it.... depending on how "loose" you are working).  Often I just hand pat the chuck clay to a rough centered shape and then trim on that.  No water used at all..... so there is nothing that tends to "stick".  The chuck surface is just patted clay ...so is not that "sticky".  Does not easily stick to leatherhard clay.

 

Another approach ........... let the "chuck" get to a leatherhard stage.  Then KEEP the chuck for later use.  Wrap it tightly in plastic and it will keep that way for a long time.

 

In Japan years ago I learned to also often trim on bone dry chucks.  They work great also if they are designed well for the forms you are trimming. 

 

And then you can also bisque fire chucks and keep them.

 

Experiment.  Best teacher.

 

best,

 

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

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For a temporary chuck with wet clay, cover it with plastic wrap. It'll keep it from deforming and from sticking to your pots. I keep an assortment of bisque fired chucks in my studio. Some I've had for 5+ years, others get remade whenever the students break them.

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Any suggestions for a "tried and true method" to compress the bottom of the pot to prevent S cracks when throwing off the hump? 

 

Compress as much as you can, then try to compress more when it's firm enough to handle it- compressing/burnishing with a wood tool after trimming can sometimes help.

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I think I've mentioned this before on here when this subject comes up.... but can't remember clearly (getting old).

 

There is a technique to the off the hump process I was taught in Japan that deals with this potential issue.  And even knowing this trick that helps to even out the shrinkage torque stresses in the base area of forms.... different clays trend to respond to this process in different ways.  Some clays just do not like to be formed in this manner.  Most folk processes of forming come from what the particular clay body in the area needed to be used to work well.  (In Jindezhen a couple of weeks ago, I just watched them joining porcelain parts bone dry.  It works with their clay.)

 

The whole use of the term "compression" by potters is a bit of a misnomer, and I think sets up a thought process that is not all that accurate a picture.  By channeling the malleable plastic material thru some sort of 'narrowed channel' (between fingers, or tools, or fingers or tools and wheelhead, etc.) you realty are simply aligning the clay particles surfaces with one another a bit.  There is nothing to really "compress out" of the clay.  It is not like there are major voids or airspaces to get rid of.  This alignment of particles helps with keeping the rate of shrinkage when the clay starts to dry out the same in all parts of the wall sections.

 

S cracking in off the hump stuff happens when the directions that the clay particles are aligned does not "match up" well in all parts of the form.  So different parts shrink at slightly different rates... and it sets up stresses that must be relieved.  The impacts of throwing torque in off the hump stuff is the main culprit in this issue.

 

So on to the stuff I was taught.........

 

The top part of the mound is coned up and then back down aggressively as the very last move before opening.

 

The ball of clay at the top of the hump is centered off from the rest of the mound as quickly as possible.  It is basically part of the coning downward move mentioned above.... it all happens sort of at once.  The longer you take to do this separating and centering part of the process with the ball at the top, the more torque effects are placed into the clay mass at the narrowed dividing point between the top (the piece you'll throw) and the bottom of the overall larger mound.  This sets up potential stress issues.

 

The clay is then opened and then is immediately formed out into a flat dish shape with a smooth slightly curving upward inside base.  (Looks like a small thick dish.)  Usually done with both thumbs on the inside at about 180 degrees to each other (9 o'clock and 3 o'clock).   This is done even if the end form is a Chawan, a bowl, a bottle, a teapot body, or whatever.  Then the base inside area is gone back over with the fingers from inside to outside at least once and back in to center to "compress" the area.  Sometimes a rib or tool is used to do this "compressing" also.  There is a definite slight "lowering" of the overall floor profile here at this point.

 

Then the initial dish shape is cupped upward quickly into a thick "ball" or "cylinder" type form to begin the real pulling actions.  Then the piece is formed.  Last step is a slight "compressing" of the interior floor...... either with fingers or something like a cow's tongue rib.

 

Speed is key in all of this....... long times of fussing and skin or tool contact with the top part of the clay compared to the mass at the bottom increase the torque effects that get concentrated at the "connection point" (where eventually you'll cut it off).  Another factor is not letting water (or lots of slurry) pool in the base of the inside of the form....increasing its water content... and hence shrinkage compared to the rest of the walls. 

 

Trimming is also important.  Thickness should be quite even compared to the walls.   In Japan the trimming direction is often opposite to the throwing direction.... helps even out torque stresses.  And sharp trimming tools also... so as to not add torque stresses......... cutting not scraping.

 

Seems to work for me.  I get very little S cracking in forms I make off the hump... and I do a lot of stuff that way.

 

But some clays just don't work for this process.  (Related....... I've used some clays in Japan that you simply CAN'T cover with plastic..... they crack 100% of the time.  You cover them with newspaper.  Or ones that can't be places in the sun to dry for even 1/2 hour.... crack 100% of the time.  And so on.)

 

best,

 

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

Min and OldUberGoober like this

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John, Another thought as I re-read your post...  Would more grog in the clay help or hinder?  Currently I'm using Standard 181 or 182.  BTW, I have 2 cows tongues and find them most useful.  My flat pieces stay flat.  Thanks again for sharing your knowledge. FW

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In MY experience the coarser and more large grained clays seem to handle off the hump better than the fine grained clays.

 

But there are a couple of very fine grained clays I've used in japan that seem to do just fine with off the hump....... so...... as the commercials say.... 'your mileage may vary'.

 

I've been in ceramics long enough to know that you can never say "always" about most stuff ;) .

 

best,

 

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

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I thought we had had much of this discussed early this year. 

 

http://community.ceramicartsdaily.org/topic/7880-throwing-off-the-hump/?hl=%2Bthrowing+%2Boff+%2Bthe+%2Bhump

 

Throwing off of the hump is as much a learned skill as learning to throw in the first place. The method that John describes is quite successful for me also, I learned it from researching the s crack problem and found an old video of an japanese gent throwing in this manner. If I remember correctly he was in the states on a tour and doing a pottery workshop on a campus. With practice I was able to use it to my advantage for throwing bowls for chalices.

 

 

best,

Pres

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