Clay Slabs Crack When Bent

[Victoria Ahmed]

So I'm in my last month of pregnancy with my third child and attempting to put all my muscles into wedging my clay (rams head wedging).  Everything looks solid when I wedge. I use my wire tool to cut the clay and don't see air bubbles. I make a conscious effort to wedge smaller portions of clay and not be lazy about this. It's also brand new clay from a nearby art store. However when I roll out my clay, any attempt to bend it results in many cracks forming. The bigger the bend, the deeper the cracks. I'm assuming it's too dry, but I also want other opinions. I see a lot of information online about cracking when drying, but nothing about clay instantly cracking when you bend it.

If you cannot tell, I'm absolutely new to this type of pottery.  I grew up with greenware, bisque, pouring slip into molds, and we own kilns. This is new territory.

How should I go about helping my clay not to crack? If it's a moisture issue, how can I bring more moisture to my clay?

[liambesaw]

Sounds like a fault with the clay, not with your methods.

You may be drying it out too much with your wedging.  If it's pugged clay from the manufacturer try just using it straight from the bag and see if that helps.  

Some clays are just short.

[Benzine]

Like @liambesaw said, the wedging may be the culprit. 

I find that most the time, the clay out of the bag from the company is about where you want it (If not slightly too dry).  So while wedging is generally good practice, it may be counter productive in this case.

If you are wedging on a porous surface, especially plaster or cement, that's really going to pull moisture out.  Wood will do so, but far less.

A couple options on top of what Liam suggested:

1.  When wedging, spray down the work surface with a bit of water, throughout the process, that way you are getting the clay homogenized, without drying it out.

2.  Wedge as you were, but after rolling out the slabs,  use a damp sponge on both sides of the slab to both smooth the surface and work some water back into the slab.  

3.  Add moisture to the bag of clay before wedging.  There are a couple methods for doing this.  One is to poke a bunch of holes in the block of clay, add a bit of water and seal the bag up.  Another, and the one I have found to be very effective is to add about a cup of water to the bag and seal it  Then submerge the bag in a five gallon bucket of water.  The pressure from the water outside of the bag, forces the water inside the bag into the clay.  I found out about this method on these forums, but don't recall who first shared it.  Like I said, I have found it to be effective.

Best of Luck,

Ben

 

[Hulk]

Aye, have also pulled the block from the bag, sliced like bread, water spray each slice, wedge, maybe repeat, wedge thoroughly, and go to work, returning any portion not for immediate use to the bag.

I'm liking when the clay is on the soft side, where wedging brings the moisture level down to just about good-oh.

[Victoria Ahmed]

Thank you all. I appreciate the thoughts and advice. I will likely try everything suggested until I find something that remedies the issue. I have a good amount of clay to experiment with. It does still crack if you roll it into a slab right out of the bag, but not as severely. Maybe adding moisture to the slab prior to working with it would be ideal. I also need to use the clay I already have so I will try to work with it rather than switch brands or types right away. 

If nothing seems to remedy the cracking, I'm debating turning it into paper clay for non-functional work and just to be able to build anything. There's nothing more frustrating than finding out that every time you have enough time to sit down, it does not cooperate. Though I am sure it will not be the last time something goes wrong..... 

[Chilly]

3 hours ago, Hulk said:

Aye, have also pulled the block from the bag, sliced like bread, water spray each slice, wedge......

I do this too. 

If not in a hurry, spray, stack and wrap in a damp towel, then in plastic.Wait 24 hours.

If in a hurry, I slam wedge - stack, drop onto floor, squat, pick up, cut in half (like a sandwich), stack, drop, squat.............  Do that 20 times and it really evens out the moisture.  If you drop onto a piece of plastic, you can grab the side edge of the plastic and roll the clay into the other hand.

[Benzine]

7 hours ago, Victoria Ahmed said:

I also need to use the clay I already have so I will try to work with it rather than switch brands or types right away. 

There's likely no need to do that.  It's probably  nothing with the specific clay time in itself, just its moisture content when you go to use.

But Liam mentioned that some clay is just "short", and naturally not have much plasticity/ flexibility.  Most, if not all suppliers will list recommended applications in the clay description i.e. "Good for Hand Building, Good for Wheel Throwing, Great for reducing facial puffiness and detoxifying the pores"... Well, maybe not that last one...

[JohnnyK]

One of the things I do when storing a bag of clay that has been opened with some of the clay removed is to put a wet piece of towel about the  size of a hand towel in the bag before closing it back up. That seems to keep the clay's moisture at an adequate level during storage...

[oldlady]

victoria, how are you rolling the slab?  on what surface?    do you shape the slab immediately after rolling it or do you let it sit?   do you use a spray bottle of water?

Edited March 15, 2021 by oldlady
add

[Victoria Ahmed]

 

Edited March 16, 2021 by Victoria Ahmed

[Victoria Ahmed]

16 hours ago, oldlady said:

victoria, how are you rolling the slab?  on what surface?    do you shape the slab immediately after rolling it or do you let it sit?   do you use a spray bottle of water?

Sorry for the late response

I am using a two by two piece of sheetrock right now and am just wedging and rolling out.  I have been attempting to shape right after rolling.

[oldlady]

all the advice you have been given is valuable.   you will learn how to treat the  clay so it is just right to work with.

sheetrock will dry out the clay.   it is great for drying clay after it has been shaped the way it is to remain but it will suck out the moisture as you work with it.   put it aside for later.

stop wedging brand new clay out of the bag.  you cannot get air out of that kind of clay, a huge machine has already done that.   just slice a piece that will come out thicker than you intend to use and flatten it as quickly as you can.  you might put a pillowcase over a regular piece of plywood, spray an amount of water that will dampen, not wet it, and roll on that.  you can use a spray bottle to get a slight sheen of water onto the clay as you roll it.  as quickly as the sheen  disappears, roll out from the center into each direction.   flip and repeat until the clay is larger and as thin as you want.

you might be trying to roll the clay to the exact size you want to use, try using a bigger piece and cutting off those cracked edges.  try lifting an edge as you roll and you will see when the clay is just right for working.    at some point you will just do all this automatically.

one other thing,  just my personal observation is that clay that comes in a cube is more difficult to use for slab work than clay that comes in a brick shape.   cutting thick pieces from a cube to make something long is harder than cutting a slice from a brick shape that is already part way there.

[Ujjwal Sethi]

Hi

 first of all thanks to Victoria for doing this question. I myself am experimenting with slab but facing the exact same issue as Victoria.

 

 While I see many valid inputs that I will use for myself, I would like to add here that I have tried 4 types of clays including earthenware, stoneware, porcelain wedged on plywood and rolled out on multiple media (plywood, canvas, table cloth, pillowcase etc). In each case, I would get my slab cracked the moment i bent it. And I am still trying to reach where i do not crack it. India being warm, could be additional reason that moisture dries out too quick, as soon as I roll. 
 

A couple of projects where I could get some decent results were done by adding water during wedging (not sure if this is wrong method, inputs are highly appreciated) and use the clay right after to roll out slab and shape it immediately. 
 

In my opinion, it’s all about the right moisture. The real question is, how much moisture and how to measure it? Does anyone know this?

 

 Any information I shared above is from an experience of merely 2 months in pottery overall. So please add to my knowledge as much as you can and excuse me if any of the information is incorrect.

 

 Regards

[Mark C.]

Ok so I have a bit more time in ceramics . Clay is not all equal . Some clays are better for bending than others. We produce some slab baking dishes (like 12x 16 rectangles). The clay rolled out with a slab roller . Then pushed into a plater mold-the sides are just under 3 inches deep (or about 7 centimeters). We use a blend of open fine groged porcealin mixed with my throwing porcelain about 50/50 mix. Most clays will crack with type of bending. The fine grog is the key to this-that clay is 1/2 porcealin 1/2 fine groged white clay and then I mix it with my other porcealin to get ot so slab bend and form wear most porcealin will not do any of this.. The grog also helps with thermo shock .

The wetness of the clay matters and does how it rolled out. My power slab roller is two rollers compressing each side as it goes thru.

There are many factors here

if your clay is to short meaning it will not throw tall or cracks at bending then slab bending will not work. Short clay comes from many factors but aging helps and the right clay helps. Most earthenwares are not great as they are pretty tight bodies (not loose say with sand or grog) Stonewares are usually better for slab work depending on makeup of ingredients

A bit more info on how you are rolling and what type of clay body would help

Since you are in India your additives will be different than say what have access to. I think of clay there as red  and white earthenware and maybe stoneware as well. but the closest I have been is Cambodia and it was just about all red clay.

Edited March 15, 2023 by Mark C.

[Callie Beller Diesel]

+1million for all clay not being created the same.

I think the best thing anyone can do when they’re working in clay is to make their own observations about the materials they’re using.  Supplier practices, material availability, recipes and user expectations do vary heavily from one location to another. The more specifics we get, the better help we can be. Links to product specs can be really helpful too.