Jump to content
docweathers

Cracking in 20 inch greenware platters

Recommended Posts

Recently I got into making large (20 inch) platters for wall hangings. To dry them I put them upside down in large plastic bags and let them set for a couple months. I took some out today and several have cracks about 1/3 of the way across the center. These are not S cracks but relatively straight cracks slicing the platter in half. They do not follow any design feature.

I'm using , G-mix 6 with grog.

They are just beginning to turn white. The humidity is low and the average room temperature is about 45°F.

From my thinking, what I'm doing should work but obviously I'm missing something..

Share this post


Link to post
Share on other sites

at risk of being tossed off the forums, may i ask WHY everyone seems to think that slow,  s l o w,    s  l   o   w      drying is good for preventing cracks  in every case.

i can see some advantage in putting a mug with a handle that has been attached into a plastic bag for maybe one day to even out the moisture content BECAUSE there are two parts but not something made all at once of one piece of clay.

    

 

Share this post


Link to post
Share on other sites
6 minutes ago, oldlady said:

at risk of being tossed off the forums, may i ask WHY everyone seems to think that slow,  s l o w,    s  l   o   w      drying is good for preventing cracks  in every case.

i can see some advantage in putting a mug with a handle that has been attached into a plastic bag for maybe one day to even out the moisture content BECAUSE there are two parts but not something made all at once of one piece of clay.

    

 

I only box my mugs for a day before setting them out, I didn't see any difference between 1 day and 2 days to equalize moisture in mugs and handles.  I only ever box or bag if it's something I'm putting handles on or decorating with slip, I haven't heard of slow drying to prevent cracks, it makes sense if it's over a period of days, but months seems excessive!  Either way, I don't think the long drying period caused the cracks, I'm thinking friction plus gravity.

Share this post


Link to post
Share on other sites

Not enough info to diagnose ?

Did they have feet and if thats a yes then why upside down?

Did it ever freeze in those months? 

next time no need to wait that long . I throw huge platters-flip and trim them next day and dry them usually in matter of days.I only flip them to dry the backside to trim then after trimming they sit on the foot.And thats wiyh porcelain. I assume you are using stoneware thast takes a beating. Or is it a talc low fire body? If so thats another issue

Keep the trash bags for trash 

whats the clay body and are they thrown or handbuilt

I'm not a doctor but have breathed to much clay dust in my life for sure.

Share this post


Link to post
Share on other sites

How frustrating doc.

How are you making them? 

Thrown or slab?

Are the rims even or irregular?

I support some slab platters by placing them on a hump oftowelling or newspaper so weight is not resting fully on the rim.

How they are constructed will give a window into what is happening.

 

Share this post


Link to post
Share on other sites

I don't dry my plates or platters upside down.  Nor do I dry that slowly.  Either with porcelain or stoneware.  If I think the plates might hump up in the middle I put a bag o rice on them and loosely cover them with plastic. They dry as long as it takes me to fill the kiln.  I wonder if the upside down drying might have something to do with your cracking?  

Share this post


Link to post
Share on other sites

If the plastic bags were fairly sealed up any condensation that might have formed within the bags would be trapped and then drip back down onto the platter. I think this could cause water splitting the clay. Silty clay seems more prone to this. Since clay cracks are a result of a stress put on the pot I would be looking at even drying, be it fast or slow. 

Share this post


Link to post
Share on other sites

My guess is you are letting them dry on plastic or some other smooth surface. The rim might have adhered to the surface (lots of moisture trapped under plastic could make a small amount of slip where the rim touched the surface). When the foot and insides of the platter shrank, the rim stayed put and caused the cracking. 

One fix for this is to use paper or foam under the pot while it dries so the clay can easily move on the surface as it shrinks. 

Share this post


Link to post
Share on other sites

all of my shelves are made of drywall.    everything gets slapped down onto it  (if a slab) or placed if a trimmed foot ring.  air is always moving in either studio and my clay will dry evenly.  except now that the clay has changed it takes longer to dry even a small coaster, spoonrest or small item.    with the new clay, i sometimes have to move it once to a dry shelf after a day.  wish they would fix my clay back to perfect!

Edited by oldlady
correction

Share this post


Link to post
Share on other sites

The root of cracks such as these is found in the opening process. The clay and the metal wheel head have a terrific attraction for each other. If you opened the clay by starting at the center and pulling out to the edge, clay directly in contact with the wheel head the clay does not move uniformly from the center. The clay particles in the base will not be lined up as they are in the wall at the completion of throwing. The shrinkage will be uneven from the sides/edges to the center resulting in the cracks such as yours. To insure that the clay particles are lined up in the base as they are in the walls, extra attention needs to be given to the base by moving your hands from the edge back towards the center so that the base is subjected to similar pressure to the walls.  I also am careful not to open  by pulling the mound out from the center. I gradually enlarge the opening by pressing straight down on the edge so that the clay moves outward little by little. The base is also compressed with each move. I finish by going back and forth on the base several times, moving toward the center more than outward. 

Alternatively, you can leave a goodly amount of clay under the base to be wired off, removing the unaligned clay before making the foot. I prefer the first method.

Hamer and Hamer has a great discussion of cracking in their Potter's Dictionary.  

Let me know if my explanation needs more explaining. 

Ruth

PS. Any reasonable even drying will not cause these cracks.

Edited by RuthB

Share this post


Link to post
Share on other sites

I am a compression person so my first inclination is to always make sure the clay is compressed to the same density especiallly in the center. I am not an alignment guy though. To me being as dense throughout means less space or holes in between the clay particles and strength in the areas that are compressed correctly will be greatest while those that are not will be least. Partical  size in claybodies varies considerably so less holes between particle means more strength and lower shrinkage. Alignment to me is a bit too unrealistic. To me, simply fill in all the open spaces with the variety of partical sizes available in the body. This will  actually displace the water occupying those holes.  

Anecdotally, I have seen spiral cracks in bodies not wedged and right out of the bag where after thrown they begin to delaminate in a spiral direction while drying. Polarizing or aligning things would appear  to have some real downside potential.

Simply pressing down on things is not necessarily compressing it as where unconstrained the clay will squirt out. Attachments similarly can benefit greatly with a joint that is outlined and compressed with a suitable smooth scribe. Super slip becomes better as a glueing agent as we seek to make it thicker in clay content and as dense as practical all for the same reason, more uniform shrinkage and greater strength. I always felt that scoring joints and then compressing the connection actually provided space for the joint to  fill with as much clay as possible, just like applying tile glue with a proper V notch spreader.

slow drying and evening out water content are two things to me so having something in a bag for a day or two to even things out makes sense. Three months, seems counter productive for something that is uniformly flat. For sculpture I understand the need for slow to maintain even drying  between differing thicknesses of clay.

I think your inadvertent keep them in a bag test proved there was a weak area in your finished clay (assuming reasonable storage and freedom to move during drying) so now the question would be what construction method was responsible for the loss of strength in those areas?

Edited by Bill Kielb

Share this post


Link to post
Share on other sites

I got the dry slowly thing from Tony Hansen. The plastic bags were an attempt to slow the drying is much as possible. The dry upside down thing came from a South Korean MFA  who does a lot of videos on YouTube, but I don't remember his Korean name. I vented the bags at least once a week or more often to make sure there is no condensation building up inside them. The bottoms are highly compressed over and over and about quarter inch thick and quite even. None of the crack seem to have anything to do with any decorative features. The platters do not have feet or a foot... They were for wall hangings.  The cracks were relatively straight crossing the center with no spiral or S crack pattern.  They were thrown on Masonite bats and removed as soon as they were stiff enough to handle.

They were made from new G-mix 6  w/ grog  with some wedging

Share this post


Link to post
Share on other sites
2 minutes ago, docweathers said:

None of the crack seem to have anything to do with any decorative features. The platters do not have feet or a foot... They were for wall hangings.  The cracks were relatively straight crossing the center with no spiral or S crack pattern.  They were thrown on Masonite bats and removed as soon as they were stiff enough to handle.

Interesting, and you believe when throwing you compressed the shapes from out to in as much as practical so the clay in the center was truly as dense as that which had been dragged away from the center while opening this to its final size?

Share this post


Link to post
Share on other sites
4 minutes ago, Bill Kielb said:

Interesting, and you believe when throwing you compressed the shapes from out to in as much as practical so the clay in the center was truly as dense as that which had been dragged away from the center while opening this to its final size?

Since I throw completely dry with some roller tools that I built, I can put a huge amount of pressure on the clay to compress it. If it's not compressed after what I do in it is not going to be compressed with anything.

 

What is this super slip you mentioned?

Share this post


Link to post
Share on other sites

Downward pressure is not always compression. Press down at the edge of a slab and it squirts out, definitely not compresssed more.  When I see platters beginning to crack I often ask to watch them being made. Usually I find folks pressing down hard enough but not really evening out the clay structure. I can’t remeber when this hasn’t worked actually but usually when we ask them to work their clay out to in so the center is dense these problems go  away. We usually make them work the clay with their fingers from outside to inside almost like a pop up lid so they can feel the wave of clay they are moving back into the center. We also ask them to finish out to in with their rib in the same manner.

My thought is the crack in the center verified it was too weak and fractured under the forces developed during drying even though this was dried at a snails pace. My only idea is compression (not necessarily  the pushing down hard type) ....... or the claybody cannot sustain the force of drying. Sorrry, my only thought at this point.

Share this post


Link to post
Share on other sites

Bill
I use my roller tools to push the clay out to in and out to in a bit before I move to get a final shape.

If what I'm doing doesn't fit your definition of compression then we need to find some other terms such as mixing or destroying particle memory. Both of which I think I'm doing a pretty good job with my roller tools.

 

Min

I think I've got my cracks patched so it would be very hard to get a decent picture of them. Breaking my platters and half would be a heartbreak not just the platter break.

They are just generally straight very fine hairline cracks that jiggle back and forth just a little bit  across about the center third. of the platter.

Share this post


Link to post
Share on other sites
21 minutes ago, docweathers said:

What is this super slip you mentioned?

Super slip is just clay glue where it is  created with lots and lots of clay supersaturating it by using Darvan, clay, a blender and minimal water.  when you use it you add some fully saturated epsom salt to flocculate it. It’s super sticky, super dense and has less water so shrinkage is less. It is sort of pottery glue that allows folks  to stick their handles on quickly and move on to the next.

Share this post


Link to post
Share on other sites
Just now, docweathers said:

I will try some super slip. It sounds like it could have a lot of uses. 

Magic slip is pretty much the same but with an additional flux.  It's magic water (sodium silicate, soda ash, water) super saturated with clay.  I use it to attach handles if my mugs got a little too dry before I could get to them.

Share this post


Link to post
Share on other sites
5 minutes ago, docweathers said:

Bill
I use my roller tools to push the clay out to in and out to in a bit before I move to get a final shape.

If what I'm doing doesn't fit your definition of compression then we need to find some other terms such as mixing or destroying particle memory. Both of which I think I'm doing a pretty good job with my roller tools.

Sorry, compression is my only thought at this point. I think we confirmed that for whatever reason your clay cannot take the forces of drying so I am at a loss really. You could not dry this any slower or more even it appears.

Share this post


Link to post
Share on other sites
3 minutes ago, docweathers said:

I've used magic water. That seems like good stuff. Why do you folcculate before application.

I'm gonna guess to make it sticky.  Flocculating reverses polarity like on a magnet, the particles go from repelling each other to attracting each other.  Might help to cause it to stick to clay, but seems like it would turn it really thick

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.