big pots -how to dry. Please help me.

Hi potters.
I try to make(I am trying about 10 years) biger pots with many kinds clay with 20-50% grog (0-3mm). And every time the same surprise during drying time.
I used "creaton-stone ware clay", "WBB-clays",etc. I mixed own clays with grogs, wood...and still craks.
I roll mixed clay on slab roller and connect for slip clay or press into mold. Wall thickness: 8-20 mm depends on the pots.
After cover plastic bag for 2-4 days-pots stay on feets and additional middle feets.
Next I cover pots fat cotton-stay on feets. After 7-15 days crack bottom or walls.
Please tell me what is wrong. How long to cover and what kind of cover? It kills my passion...
I read many articles and so...nothing.
Thank You.
Regards. Martin from Poland

Martin. it may be that you are over-working the wet clay. In an effort to achieve a fine finish or to refine the shape it is easy to work the clay past it's plastic point. This would vary widely from clay type to clay type. It is important to cleanly and efficiently join the plastic slabs, working to maintain an even moisture content throughout. Overworking, or working past the plastic point can result in micro-cracks that of course open as the pot dries. If this is the case no amount of careful drying will avoid cracking. Of course, you want all the slabs to be the same moisture content before you start working, and if you can equalize the thickness of the clay walls you will also minimize cracks. Once you roughly join the slabs cover the pot to slowly stiffen the clay. If you need to further refine the pot after the clay is leather-hard do not re-wet the clay or re-wet individual areas as this will also promote cracking. As for support while drying, I dry my pots upside down resting on temporary foam-block supports. Depending on the form and size, I will also often bisque the pot upside down (on it's rim). I hope this helps. Warm Regards and Good Luck - Tim

Thank You for reply.
I now about all rules what You talk: join,the same moisture,the same thickness of the clay walls, do not re-wet, etc.
I will try dry pots upside down resting on temporary foam-block supports- sometimes I turned back...maybe this is my mistake.
Please tell me how long to cover and what kind cover to use?
Thanx- Martin

Tim,

How does potting clay cure? Is it plainly by drying or some sort of chemical reaction? I am curious because concrete is through chemical reaction and you can prevent/reduce cracking by submerging it in water (or spraying with curing compound or evaporation retardant). Just wondering if there are other options you can employ.

Another thing, I used to collect a lot of green (wet/fresh) wood for my other hobby and the biggest problem is cracking. I usually paint the ends to slow it down. Later I found that I can dry wood super fast without cracking by vacuum. The secret is uniform drying...cracks happen when the exterior dries and the interior is still wet. The vacuum solved that (in wood). Anyone want to give this a try and experiment (even small scale first)?

Thanks!

Martin - I am sorry that you are experiencing cracking even though it sounds like you are experienced with clay. I set wet pots on plywood shelves and cover with plastic sheeting. Of course the key is even drying throughout the clay walls to equalize shrinkage. Are you using very fine grained/plastic body for your large pots?

Dario - clay does not "cure" in the same way that concrete does. First it dries (non-molecularly attached water is driven off) and then it is heated. The heating/firing drives off molecularly attached water and then at some point components of the clay body begin to melt together. At a given temp. the clay will no longer slake in water and is considered "fired". I am somewhat familiar with concrete. It is not possible to submerge an unfired pot in water to dry it as it is possible with concrete to promote "curing". I do not know about drying pottery in a vacuum. It may be advantageous but sounds cumbersome to me, especially depending on the size of the green pot. Certainly potters use wax resist/liquid latex to protect thin/thinner sections from drying too quickly. Also used are ad-hoc plastic sheets, wet newspapers etc. The point is to equalize wet shrinkage as much as is possible to prevent green cracking. I hope this is informative. Regards to all.

The approach used commercially to dry larger clay items rapidly (think toilets and such) is to put them in an oven at 80-90C (below the boiling point of water) and high humidity to match the moisture content of the clay body. The humidity level is then reduced until the pot is dry. The advantage of this process is that the unbound water is much more mobile at the higher temperature so the clay body stays at the same moisture content throughout as it is dried. Same idea that has been expressed before, but sped up.

Thanks. That helps. In that case even drying is the only way.

As with my analogy with wood, you can slow it down by coating/covering or speed it up with steam (kiln dried) or vacuum. Steam kiln drying is almost what Marty described done with toilet seats.

Vacuum (when low enough) causes water to "boil" out sucking the water from inside out so there is even drying/shrinkage. It won't work for big projects but I also used microwave to dry small wood. Same concept, expanding the inner water to drive it out faster...maintaining even moisture throughout the piece. I do this in short (20 secs) bursts while the piece is in a plastic bag. Let it out to acclimate for a few seconds and repeat until no more "sweat" forms in the plastic bag.

Just ideas/options that may work and worth looking into.

Makes me laugh what we do for our hobbies.

Thanks. I use white clay with grog(1-3mm biger grog, about 30-50% in clays)...is thin, not plastic like red clays etc.
How long do You cover with plastic sheeting and what next?
Martin.

timatkinson wrote: ...Once you roughly join the slabs cover the pot to slowly stiffen the clay. If you need to further refine the pot after the clay is leather-hard do not re-wet the clay or re-wet individual areas as this will also promote cracking. As for support while drying, I dry my pots upside down resting on temporary foam-block supports. snip

The cracks you have are classic stress when the rim drys faster then the body. Look no farther than that super good advice from Tim Atkinson above. I really like your style and know the effort you put into that pot and understand that sad feeling of it not even going into the fire.

My pottery motto

Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
Samuel Beckett

good luck
big Dave

My advice is simple, let it stay covered until it's dry. My one meter long pot had to stay 4 months on the shelf.

Same advise as Tom. If I make a big pot then it is covered completely and left covered until bone dry. As Tom says this can take a long time but the slow drying is well worth the wait. You can speed up the drying process for smaller pots made from one piece of clay with nothing added, but for large pots take your time in drying them out

Regards

Andy

I'll chime in and agree with Tom and Andy, slow drying is the key. Two to four days covered isn't nearly long enough. For a large pot it can take weeks to dry properly, although you can push that a bit if you are careful. Also, in the early stages after joining walls and or feet, etc. I will cover tightly for a few days to let the moisture levels equalize in the pot before loosening the covering to allow slow drying.

Roger

Hello Martin

May I put in my 2 cents.
I've got the same opinion as Tom, Andy and Roger when it comes to drying! You have to do it slowly!

BUT there's another thing I have to mention.....and that is a proper clay-preparation before making a pot!
How do you do that Martin? Do you have a pugmill or do you work by hand?

Kind regards
Martin

For really large pots I wrap them with a bed sheet then wrap with plastic. If you try to do a really slow drying you probably notice condensation on the inside of the plastic. When beads of water drip on the clay it can cause uneven drying, even though its under plastic. The bed sheet distributes the moisture evenly. Also you can make a sacrifice slab. Sometimes, especially with straight sided pots, if you dry on the rim it will warp or crack in the firing. And if you dry on the feet they will crack. Make a slab the same thickness as the feet and cut it to fit under the pot. Lay down a thin layer of sand on a super flat ware board or kiln shelf. Put the slab on the board and make sure it's perfectly flat. Next lay a single sheet of newspaper on the slab, cut it to fit, and smooth it down on the slab with a rib. Make sure the sacrifice slab has the same wetness as your pot. Set your pot on the slab, wrap with a bed sheet and plastic, and it will shrink evenly without cracking the feet. If it was done on a kiln shelf, it can go right in the kiln after its dry.
-Preston

-Also you could try nylon fiber in the clay. It makes it difficult to cut, but it WILL NOT crack like your photo showed.

Thank You for help!
I make new pots and try to dry slower under "roof plastic cover"-one way moisture.

Klaudia and Martin....Yes I have pugmill and try new clays mix. I think they are so "thin"...to much grog etc.

Prestontolbert...thank You for idea with slab and sand under pot.

I am still trying and don't give up.
What do You think about firing ramp? Electric furnace-2192 F (about 1200 C)ramp time: 212 F(100 C) per hour, about 10 hours to end and half hour hold in boilng (100 C) time.
Thank You and Regards.
Martin

I personally bisque big pots much slower. Hold 8 hours at 100C then 75C per hour to cone 08 orton. It might not be necessary to go that slow, but what are a few bucks in electricity compared to the work involved in your pots?

Hello Martin

I second Preston's words! We are not doing the same but heating up slowly by 70° C per hour to 650° C ... and than straight through to 1200.

I have taken another look at the photo of your pot. It looks like you are working with very wet clay .... am I right?
This could also be ONE reason for cracking while drying ..... or better to say it "promotes/encourages" cracking.

Kind regards
Martin

My bisque firing if it helps

15 degrees an hour for 4 hours hold at 60 degrees c for 4 hours then rise at 60 degrees an hour till 650 then full power to 980 degrees

I used to have alot of cracking problems and by doing the pre bisque drying, 15 degrees an hour for 4 hours, drastically reduced my losses

Regards

Andy

Thank You all for reply and help.
1. ramp time... I will slow down to 60 C/hour to 650 degrees.
2. I don't now... my clay is wet or dry . I can't make "hand made" small, about 1 cm fat, 10 cm long "stick" and turn him without crack. I think there is too many grogs etc. but I afraid about drying and cracks... have You good and tested clay? I mix and preparation many kinds but I am not sure that's good way. Once time I used "creaton stoneware" (very expensive in Poland) and I was happy-that is it! but... first pot from whell fired at 1200 degrees deformed like pot with clasik "fat" red clay
Martin.
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