Cracking platters

[docweathers]

I'm having severe cracking problems on large 20 inch platters. I think it's from hairline cracks developing during drying. The platters were dried upside down for about four months inside large plastic bags. Every week or so I would open the end of the bag and flush some fresh air through to clear out some of the condensation on the inside of the bag.

Some of the platters were thrown in the traditional pull a cylinder and spread it out technique. Others were made from a slab laid on the bat on the wheel head. It doesn't seem to make any difference which way they were made. Attached are pictures of various styles of cracks.

 

Suggestions please

[dhPotter]

In your glaze firing, have you tried placing sand under the platters to reduce drag on the kiln shelf?

 

[docweathers]

No, I have not tried putting sand under the platters. I have been putting  five or six 5/16 inch ceramic rods  radiating out from the center of the platter.  Maybe sand would be better.

[neilestrick]

I'd try sand, and maybe even slow down the cooling from 1200F to 800F.

[Pres]

Doc

I just fired a bird bath top for a friend that let his freeze. I made mine by throwing upside down on a plaster domed bat with a 2 1/2" rise. Centered ball on top, flattened down around the edges. the center flange to hold the top on the base was thrown into the bottom from same piece, after drying to leather hard, added a thrown ring to top to deepen water area. This 22" platter was fired upside down in bisque, and upside down in glaze firing, leaving the rim unglazed. I was afraid it would collapse around the collar with such a cantilever. Thrown to glaze fire time was 1 1/2 weeks!

 

Buddy was very pleased with the replacement.

I believe as others that it is your glaze firing expansion/contraction causing the problem, not your making skills!

 

best,

Pres

[docweathers]

Thanks for the input.

Sand, firing upside down and slower cooling it will be.

[Min]

@docweathers, when I expand your photos and look at the cracks it looks to me like the glaze edges are rounded over the cracks. The left side of the crack on the second platter and the crack on the green platter both look like the glaze has rounded/smoothed off over the crack which means the cracks didn't happen on the cool down but they were there as the glaze was melting, perhaps from the bisque firing. Sharp edges on cracks with no glaze softening the fracture means the cracking happened on the cool down. 

[Roberta12]

Would the prolonged drying time in plastic have an impact on the structure of the clay??  Just wondering.  

 

[Babs]

Howcfrustrating Doc.

Flick your platters afterf you bisque them to hear how they sound. If have a ring to them then they are sound at this stage.

REALLY slow down your glaze firing to about 600deg C and even after that.

Would you have more success if the kiln was loaded with more than just platters?

Hard to see but edges of top platter very close to element and wall.

[Callie Beller Diesel]

I'm leaning towards a combination of Min and Roberta's ideas. If you're drying platters for four months, I think there's good odds that the platters are maybe re-wetting themselves in that closed humid environment, leading to weak points, leading to hairline cracks that show up in the glaze. I get some similar crack patterns on slip when i've applied too much of it to a pot that's a bit too dry.

Four months is too long to be drying a platter that's finished being worked on. I know everyone gets told to slow down their drying, but even is better than slow. It makes sense to do things like invert bowls and platters, or just cover the rims so they don't dry faster than the centres. If humidity is collecting inside your plastic, it's going to rain back down on your piece and it'll begin to slake.

[docweathers]

Min

The sharp edges are a good observation. Yes, they are very sharp.  During bisque firing, would sand, upside down and slow cooling be the answer... Or something else?  Are certain kinds of sand better for this?

Babs

My new standard operation will be flicking platters and pitching if they don't ring true.

What's the logic of not firing all platters?

What's the minimum distance you would have  between the edge of a platter in the elements?

Callie ... Pres

The ultra long drying was an attempt to solve this cracking problem. How slow should I dry these things? Pres talked about a week and 1/2 from throwing to glaze firing? 

[oldlady]

doc, how thick is the average area on your platters?   a lot of people think thick is better for strength.   it ain't necessarily so.  

alice   (the person who makes a lot of long or big platters less than 1/4 inch thick, dries them immediately on drywall and single fires on sand after glazing)

[docweathers]

Min

The sharp edges are a good observation. Yes, they are very sharp.  During bisque firing, would sand, upside down and slow cooling be the answer... Or something else?  Are certain kinds of sand better for this?

Babs

My new standard operation will be flicking platters and pitching if they don't ring true.

What's the logic of not firing all platters?

 

How slow should I dry these things?

 

 

[docweathers]

16 minutes ago, oldlady said:

doc, how thick is the average area on your platters?   a lot of people think thick is better for strength.   it ain't necessarily so.  

alice   (the person who makes a lot of long or big platters less than 1/4 inch thick, dries them immediately on drywall and single fires on sand after glazing)

They are about 3/8 thick. What would be optimal?

[preeta]

20 hours ago, Pres said:

plaster domed bat 

WOAH!!! <Mind Blown> didn’t even knew such a thing existed. Other than this platter what other reasons would you use a domed bat for?

Curious Pres why you threw it and didn’t make it out of a slab. Maybe too big a slab to handle? 

Does throwing a slab make the slab stronger?

@Alice - i bow down in awe. Even I haven’t been able to fire 1/4 inch thick plate without warping. Maybe need to use sand too.

Doc by Golly!!! I would be so super disappointed.

Also i am wondering. You are slow drying your platters. If there’s condensation then was there any drying happening? Is there such a thing as covering too much? Ive been taught at school not to allow condensation to happen. In fact from some of the sculptors in my class (past students using the studio) they dont completely cover their pieces. They leave a little airhole. We have a damp room at school. The only sculptors who completely cover their work are those who take 6 months to finish a piece. 

[oldlady]

that seems a lot.  i started feeling the china in the big department stores many years ago.  it seems to be less than a quarter inch.

my platters are mostly slab made.  i start with the 1/4 and compress the slab by rolling texture from the various leaves that i use deep into the clay.   it is a very smooth clay, no grog.  so i am really using 1/4 inch of material smashed flatter by a pony roller and a final 30 inch  long rolling pin.

the thrown platters are made with a cylinder opened with a broomstick handle inserted vertically and then folded quickly down to horizontal (as close as possible)  so i do not know the final thickness.     it is trimmed away because the stick forces the thin cylinder wall to compress downward as it is being shaped, leaving a thicker area near the footring.  

if you watch the video on making plates by  Bill VanGilder, he shows making the plate the way you do platters and also explains the rising of the rim while drying.

[oldlady]

preeta, your post overlapped my slow typing.

just once i wish someone would say that they took my suggestion and tried it and it worked.    UNLESS YOU ARE ATTACHING A HANDLE OR SOMETHING ELSE slow drying is not necessary.

what can it hurt to try drying something flat on drywall or something that will absorb water from the bottom as well as having the top air dry?   (you can slide it around to see how much water is coming off the bottom a few hours after putting it down.)

[Callie Beller Diesel]

I don't even think slow drying is necessary for handles. You just need to cover them for a few hours so the moisture levels even out, and then leave things uncovered. I'm not at Mark's level of production, but I can spend a week throwing and finishing Mon-Fri, bisque the following Monday, glaze day is Wednesday, and photos and packing on the Friday. So 11 days, start to finish. The biggest items, slipped pieces and lidded jars get thrown first, because they take a bit longer than 48 hours to become bone dry after I've finished working on them.

I can do this if all my creative decisions are made, and because I know the clay I work with, and my climate is semi arid. If I lived somewhere more humid, that would definitely mess with my system. If I'm playing around with new forms or methods of decorating, that also adds time.

[Pres]

When attaching handles, I wait until pots are cheese to leather hard trimming usually 20 per batch. cover with plastic.  Then wedge up clay for extruded handles, load extruder, and extrude first tube. I attach each handle with magic water, as I believe fits the pot, back fill the bottoms and finish curve of handle using regular water on a finger wiping the inside curve only. Set the pots aside on fresh ware board, and allow to dry. No covering, nothing no cracks.

 

best,

Pres

[Pres]

Preeta, years ago, I was throwing some large platters, and they wanted nicely finished foot rings with a shallow curve to the form. This was not possible with out a lot of trimming. so I threw them upside down, on this bat, made the double foot ring with large enough outer ring to support the curve at out edge, then the inner foot ring to support the center. Worked very well. Sometimes the obvious solution to a problem is not the best one.  For a bird bath that was not flat, but evenly round, and with a foot ring to fit the pedestal base, this was the only solution I could believe most efficient, especially since I had the plaster domed bat.

 

best.

Pres

[neilestrick]

3/8 is too thick. Get them down to 1/4 or less, and don't fuss too much with slow drying. Get them to leather hard as evenly as possible, but don't worry too much after that.

[dhPotter]

Doc, use Play Sand from the hardware store. Spread it out under the platter. I set the platter where I want it be on the kiln shelf then, with a pencil, draw an outline around the foot, then put sand within the outline. Vacuum or sweep the sand up after you have pulled the platter from the glaze firing. Like OldLady, I also single fire platters this way.

[Pres]

If I remember correctly, Marsha S. uses little coils under large flat pieces also.

 

best,

Pres

[Min]

I find its the evenness of the drying that is super important and not so much the speed of it. Get the thickness even across the base and the walls as best you can. Trim the bottom as soon as you can, don't wait until the rims of the platters are too dry. Flip the platter center portion onto an upholstery foam covered batt so the main part of the platter is supported by the foam and the rim is hanging, don't support them on the rim while trimming. Trim a small support ring towards the center of the bottom when trimming if the platters are large so it has some support while firing and doesn't slump.  Once it's all trimmed then you might try slowing down how fast the rim dries. I take thin plastic and wrap it around the rim, platter is right side up on a batt and I wrap both the under and topsides with plastic and leave the middle of the platter unwrapped. Let the middle dry out then loosen up the plastic on the rim, you're trying to get them both to dry at the same rate but the rims always will want to dry faster. If you make some slab built ones then put  bit of weight in the middle to stop it lifting up, not necessary for wheel thrown ones.

I used to use a body with sand, it did watersplit if I rewet it too much as it was drying, I'm wondering if this is part of your problem. There isn't much shrinkage from dry greenware to bisque, the main shrinkage with clay is from wet to dry then from bisque to glaze firing (excluding some lowfire). Your shelves look good, if they were my platters I'ld bisque them without sand (or coils) and glaze fire with them.

The platter on the top shelf in your images had a double stress put on it. The center cup part is going to have added to the thickness of the base where it's attached, plus it didn't have a shelf above it to act as a heat sink. The bigger the platter the more it needs the cooling really controlled. Next time either don't put a platter on the very top shelf or put another shelf above it to help hold the heat as the kiln is cooling. It wouldn't hurt to put some mass around the edges of the platters on the other shelves. Just lay some triangular or square extra kiln posts around the edges of the platters. A couple times a year I do a refire of pots that had glaze issues, including lazy susan's and pots like my avatar, both are prone to cracking on refiring so I use a super cautious firing schedule. Even though this schedule is for my refiring it should be fine for bisque to glaze firing. Heating up: start off with whatever preheating/candling you normally use then I go at 350 an hour to 380 with 0 hold. Then slow it down to 50 an hour to 480 with 0 hold. Then 400 an hour to 1000, 0 hold, 50 hour to 1100 0 hold, 375 an hour to whatever your cone 6 is plus your usual hold if you do one. Cooling down: 9999 to 1100, 0 hold. 50 an hour to 1000 with 0 hold. Kiln off and plugs not removed until 200F. (If you do a slow cool down or a drop and hold schedule then add those ramps in)

[Babs]

16 hours ago, docweathers said:

Min

The sharp edges are a good observation. Yes, they are very sharp.  During bisque firing, would sand, upside down and slow cooling be the answer... Or something else?  Are certain kinds of sand better for this?

Babs

My new standard operation will be flicking platters and pitching if they don't ring true.

What's the logic of not firing all platters?

 

How slow should I dry these things?

 

 

Well I may be way off course but if more mass in kiln would mean the temp would drop less dramatically rate wise.. 

If shelves can be positioned so the platters are not near the element row.

I try to leave an inch all round if poss.

Platters not placed top or bottom .  In my kiln the bottom is cooler so I guess when the kiln goes on full the ware on bottom shelf may be slammed up temp. wise. Poss more important in a bisque.

And top opposite but heat loss on cool down more extreme.

Small pits or as someone above mentioned kiln furniture to increase mass of load.

Only my observations