Jiggering Conundrum...help With Crack!?

[ernie]

We are jiggering plates (see attached) with an Axner Power Arm onto pure and simple hump molds.  We are experiencing a tremendous amount of cracking and need help!

 

We have done a very similar form without the jigger arm, by slabbing and attaching the foot and had no cracks at all.  We used a different clay body with grog but should have worked the same as 181 (has grog).  None of our other forms are experiencing cracking in the dry room, regardless of the clay body however none of the other forms are jiggered which leads us to believe it is related to jiggering.  

 

First guess is perhaps the pressure of the jigger arm is causing the clay to stick to the plaster.  When the clay dries too quickly it has no where to shrink and cracks internally on the plaster, then externally on the ware boards.  We don't know any way to remedy this other than just not jiggering....

 

Troubleshooting thus far:

-  Drying too quickly: Initially we thought it was only due to the forms drying too quickly and the humidity.  So we brought in a humidifier into the dry room.  Cracks still happening.  

-  Clay Body (grog vs no grog): We were previously using Standard 365 (no grog).  We switched to Standard 181 and...still the same amount of cracking.  

 

These are all just guesses as we are still experienced cracks galore...we'd certainly appreciate any thoughts!!   

 

Best,

Ernie

ernieniblack@gmail.com

[Mark C.]

I'm not familiar with either of those east coast clay bodys but the form you are jiggering in on a hump mold which as the clay dries it will crack unless removed as soon as it drys enough. Could be your plaster forms are not dry to start with and that will slow this down so forms stick more and dry to much. How long are the plates left on the hump mold after jiggering???How dry are the jigger molds?

[David Woodin]

I have done jiggering for years and never had the problem shown in your pictures.  I use Laguna  WC370 Amador clay which I also use for throwing.  My first thought to your problem is the molds themselves. Even when I leave my clay on the molds too long they just float off the mold, no warping, no cracking.  Was the plaster fresh and mixed to a ratio of 68 to 70?  I noticed that some of the mixtures for pure and simple don't use this consistency.  Perhaps  pictures of your mold surface would help.   My thesis was on Jiggering and done in May 2003.

David

[rayaldridge]

I think I would try a clay body with more (and perhaps coarser) grog.  There was a time when I used 181 and I did occasionally have cracking problems in large forms.  My perception at the time was that the body was too tight for larger or more stressed forms.

 

The only other thing I would wonder is: how thick is the plate in the bottom, inside the foot ring?  Just looking at the plates, they are cracking as they would were this area much thicker than the rims of the plates.

[Mark C.]

Even thickness should be one of the reasons you are jiggle ring?

I think Ray hit it on the head is this body good for larger forms

Also is this reclaimed clay or new stock?

More info is needed to help you

[Mug]

I think the outer edge of the plate  is where the trouble starts. It's thinner and uneven in roundness, causing the outer edge to dry quicker. Some people wrap the outer edge with plastic to allow the center to dry faster.

 

You could try making a new jig shape that will make a round plate that is even in thickness and goes all the way to the outer edge of the plate.

[High Bridge Pottery]

They certainly look similar to drying disks I have seen where you force dry the rim and hide the centre. A friend tried a bit of jiggering once and the fastest way to get them off was hanging upside down so the pot would drop 1cm once dry enough to come off without staying on the plaster too long.

 

Could make moulds of the inside food to add after jiggering to try and even out drying. or rim and foot. 

 

Could also be your clay prep. What do you do before putting the clay onto the mould?

[drmyrtle]

Could someone comment about wedging? Those cracks are all over the place (not consistent s cracks, for example), and made me wonder if the pre-existing planes from machine pugging are simply showing as they dry. I think if I simply sliced off of a block of clay and wrapped it on a form, I'd get the same effect. Those edges also give me the heebie-geebies; I'd at least wax them to slow down their drying. Not to mention cutting them clean before letting them dry and warp.

 

Anyone have an opinion?

[oldlady]

why jigger at all?  they look like almost flat slabs just slightly curved upward and the shapes are all odd. is that the intention or are they supposed to come out round at some point? if you don't need round,  just roll out slabs and put them into forms with cloth between the slab and the form.  the footring has to be in the form if you do this.

[ernie]

Hi All,

1000 THANKS FOR ALL THE HELP!!!!

 

Huge strides and really grateful for so much experience.  We always use new clay for jiggering, not wedged so the cracking would not come from that however, I did compile a list in order of what troubleshooting we are going to do to remedy which I think, combined should work.  Let me know what you think and we're going to keep moving forward, hopefully with no crack!

 

1.      Make new molds with 68/70 plaster/water ratio                                                                            

(We use the pure and simple suggested ratio.  We'll try this instead) 

2.      Use courser-grog clay body 

(no grog in the current clay body)

3.      Jigger an even thickness from center to rim. Perhaps make a new jig or adjust arm accordingly

(Our rims are much thinner than the center)

4.      Pop forms off as soon as possible                                                                                                

(Sometimes we let them sit O/N on the bats)

5.      Cover when drying.  Perhaps wax edges.                                                                                    

(We sometimes cover our pieces, depending on the season.  Hot and dry inside this time of year on the E. Coast)

6.      Jigger each mold 1x/day to avoid saturation.  Dry molds o/n without any forms attached

(We jigger each mold 2x/day leaving the molds saturated and not able to absorb properly)

 

Good times and thanks again!!!

 

Best, Ernie

[ernie]

Jiggering vs rolling it out takes is at least 3x faster.  This was our alternative which would of course result in no cracks but we'd really like to solve the problem for this commission as it becomes very daunting in high numbers, as well as avoid the problem in future commissions that require jiggering in the interest of time.  Thanks though, we will move to slabbing if we have to!

 

Ernie

[David Woodin]

Hi All,

1000 THANKS FOR ALL THE HELP!!!!

 

Huge strides and really grateful for so much experience.  We always use new clay for jiggering, not wedged so the cracking would not come from that however, I did compile a list in order of what troubleshooting we are going to do to remedy which I think, combined should work.  Let me know what you think and we're going to keep moving forward, hopefully with no crack!

 

1.      Make new molds with 68/70 plaster/water ratio                                                                            

(We use the pure and simple suggested ratio.  We'll try this instead) 

2.      Use courser-grog clay body 

(no grog in the current clay body)

3.      Jigger an even thickness from center to rim. Perhaps make a new jig or adjust arm accordingly

(Our rims are much thinner than the center)

4.      Pop forms off as soon as possible                                                                                                

(Sometimes we let them sit O/N on the bats)

5.      Cover when drying.  Perhaps wax edges.                                                                                    

(We sometimes cover our pieces, depending on the season.  Hot and dry inside this time of year on the E. Coast)

6.      Jigger each mold 1x/day to avoid saturation.  Dry molds o/n without any forms attached

(We jigger each mold 2x/day leaving the molds saturated and not able to absorb properly)

 

Good times and thanks again!!!

 

Best, Ernie

When cutting off the amount of clay needed for the jiggering, I take a wood mallet and pound the clay down, than finish the slab by running it though a slab roller for the final thickness.  Than place the slab on the jigger mold and starting from the center push it down on the mold with the wheel spinning.  Your molds should not be so wet that you can't get more than 2 a day, which means you must not be using the correct ratio.  I have 12 molds for plates and can get two a day easily from each mold.  The plates shouldn't be forced off the molds they will lift themselves off.  If needed you can pop them off with about 20 psi of air presssure.  Were the plates your design?

David

[ernie]

Hi David,

Great advice!  Looks like pounding and I think simply making the thickness of the plates consistent from foot to rim could help tremendously in the drying process.  The designs are my partner, Amber Kendrick's for a restaurant opening in Panama that we are delivering in the next few weeks.  We run the business together (Cloud Terre).  I lend a hand with production and the business end of things.  If it is beautiful, it is from her

[mregecko]

This has already been said I think, but looking at those plates with cracks on them, the outside is visibly considerably dryer than the center.

 

This kind of uneven drying will absolutely cause cracks. I'd recommend drying them upside down (on foam is a good option to prevent damage), and slowly. Wax on the rim isn't a bad idea either.

[Babs]

Well when my rims dry quickly THEY crack not the interiors...but then they are not jiggered.

Clay coming out of bag/pugmill maybe is being placed to be jiggered that encourages the cracking ie lengthwise along the pug..

[ernie]

I wanted to sincerely thank everyone for their feedback on this topic, especially David Woodin for encouraging us to stay the course but just adjust things slightly.  We look forward to digging up his thesis on jiggering from GW and reading through it with a fine toothed comb.  

 

We adjusted the jigger arm ever-so-slightly (quarter turn of the screw) to make the thickness of the rim uniform to the rest of the plate (change-1).  We then slab-rolled the clay (change-2) rather than taking it straight off the block from the manufacturers and cut them close to our form with a template (change-3).  After jiggering we covered them for ~12 hours with plastic (change-4).  After the 12 hours we uncovered them to dry and let them come off on their own (change-5), carefully monitoring them to make sure to pop them off as quickly as possible without prying them off (change-6).  We then set them face-side down rather than foot-side down (change-7) being that the face of the plate was cracking.  This I believe provided some additional covering to let the piece dry uniformly from the thickest part at the foot.  

 

And...voila!!  Not a one crack throughout the whole batch.  Thank you all for such kind help and advice!!!  We're delivering 200 of these to Panama in the next few weeks.  The restaurant will be happy that they have plates that are not cracked...bonus!  And...we have learned a ton in the process.  

 

Best,

Ernie