Plates Breaking

[ronfire]

I have to figure out why i am now having an issue with plates breaking. I purchased a used olympic kiln 1827 and have fired 17 plates in it with only 1 or 2 plates breaking now the last fire ½ of the plates where broken. I stack them 3 high on a shelf with spacers between each plate. I started now to use clay rollers from my extruder as they are more consistent in size and easier to come by than the pieces of stilts I used to use.   Wondering if it could be caused by the elements getting old, I know the top of the kiln is not good at getting to temp so I have been lowering the setting on the infinite switches for the mid and bottom sections to allow the top to get to temp. You would think that the slower firing would work better. The broken plates are the same amount top to bottom in the kiln. They are not the top plate in a 3 stack always the bottom or middle plates. 

Might have to try firing them on edge but I worry that if one plate breaks it will tip the other plates over.

I might go back to firing  plates in my Skutt 1027 next time instead but they don't fit as well without wasting a lot of space. If it works better in the larger kiln at least I get the plates made.

Trying to decide if I should spend the money and put new elements in the smaller kiln or just sell it. I mostly use it for the plates because they fit so well in it.  All the elements work in it but I am sure they do not heat equally.

 

[Mark C.]

I assume you mean in the bisque fire-you do not state this but if you are stacking them its a yes.

Elements old or new I think is a red herring meaning its does not matter.

Your plates are breaking for one of these things

A-to wet-that means they are not BONE dry enough for your firing cycle

B-not supported well-that means the foot to foot concept carrying the weight is not set right-whats with the spacers" thats sound like uneven pressure points.I would not use any spacers ?

I stack mine lip to lip but more often on top of one another-usually no more than 3 high for a bisque on 10 inch dinner plates

C-firing to fast or cooling to fast

D -shelves need to be FLAT

You can decide which one of the above is your issue

 

I would avoid the very bottom of kiln as it heats up last-just as the top heats up fast-put them in the center of electric for even heating.

I use a gas kiln for most of my bisquing but have never had this issue when I use my electric on plates

one photo would help here of the stack

[Babs]

Do you put the spacer between the shelf and the first plate? Sounds like a too fast firing, too fast cooling thingie., though Mark says it all above.

Are you placing an edge of the plates close to the elements? Better in the middle of the shelf in a small kiln I think. No uneven pressure on the lips?

[terrim8]

Good to read as i am working on plates too. No problems so far & I use the lip to lip & one on top for 3. I tried stacking more and it worked too but maybe I had a lucky day.

[ronfire]

Sorry if I did not mention, yes this is on a bisque fire cone 04. I place ¼ " round extruded rods/roller under the plates in a star pattern and the same between the plates  with the weight bearing on the plate foot and fire them flat. I do not trim much for a foot, just enough so I can glaze the bottom with a 2" centre as well for a support. The plates are in the centre of the kiln on 1 piece shelves. The kiln is 18" so there is not room for anything else. The Olympic kiln has progressive switches so I start with the bottom on 2 for 1 hour then add the centre to 2 for an hour then then add the top to 2 for an hour. Repeat the sequence with a 4.5 setting with 1 hr intervals. I will then set all the switches to 8 for an hour then all to high until the sitter drops or the cones show they are done. I have not changed the way I trow the plates.

Tested all the elements and the measure the right resistance with 2 of them within 10% of specified resistance so the elements should be fine.

Letting the Kiln cool  usually takes more than 24 hours with the 3" bricks.

 

Noticed that the indicator light was wired wrong so I decided to  placed the wire to what was in the wiring diagram . Well I mixed up a couple and now require a new switch after I turned on the kiln to paper test the elements. Guess I was not paying enough attention to the diagram. I am usually good at fixing this sort of thing but S**t happens.   The hassles of getting a used kiln.

[Mark C.]

I think you think about the weight of each plate and the bottoms should bear the weight in a stack so the contact point is straight up if that makes sense-In most situations thats foot to foot in a cross section if you cut them in 1/2 for the view.

To help you further I would need to see a good photo of plate bottoms,stacked and not stacked

If they are indeed flat they should stack well if they do not make them so they do not need with spacers but in the throwing stage make them so they stack.

I perceive this not as a kiln issue at all but in a stacking spacer foot issue.

Maybe in this small kiln it cannot go slow enough at the start? Never had an 18 inch kiln except a raku one and speed was king as fast as the clay could take.

I may have missed one point but someone else needs to bring it up?

[ronfire]

I wil try a slower firing schedule than I have ben using for my skutt 1027.  The plates do stack well but without the spacer they can nest tightly and rim lock and then they will crack.

Found it odd that the top plate of the 3 stack did not crack only the lower ones.

[Callie Beller Diesel]

Ron, it might be a weight thing, especially if M370 behaves like H570. I had high losses when I stacked my plates on top of each other, and a big +1 for Mark's point about making sure the foot rims are directly in line with each other. That clay will tolerate exactly no weight on the rims.

Something else you might try that sounds super counterintuitive, but works like a hot d@mn is to stack them on their sides like you might tiles. They heat more evenly this way, there's no drag against the shelf, and it can be a more space-efficient way to stack.

[Mark C.]

They cannot rest on they rims-thats a no-no-try the bisquing on side -if it where me I would change the way they are made so they nest better

This is the easiest solution

[ronfire]

Might try firing on the edge, never done that with plates yet. I always have the weight bearing through the foot rims. It just have something to do with the weight on the M340 clay because the top plate is always fine so it must not be my throwing with spiral cracks. I will try a set of 3 or 4 plates on edge but have been worried they will deform with the weight on the rim. Have heard of it being done but never tried it, guess now will be the time.

[Mark C.]

Since the form does not stack well I would fire them on edge.

[Callie Beller Diesel]

This lady does it. This is Brenda Danbrook, out of Opal, Alberta. That big one is a 22" platter.

[ronfire]

Those plates are leaning right against the kiln wall, surprised they fire well. Also see that there are a lot of chips out of the kiln bricks.

[Mark C.]

I have leaned plattesr against the wall-it works-You could lean them against  a 1/2 shelve if that works better for you?

[Callie Beller Diesel]

She says that this method actually heats things more evenly: the flat, problematic part faces the elements uniformly, instead of edge-on when the plates sit flat. She figures that the more unbroken plates she can sell, the easier it is to replace kilns that have worn out. She factors a certain amount of wear and tear into her price.

[Babs]

Those chips look more like damage on removing and replacing elements. I stack birdfeeders and bird waterers this way. Get heaps more in and no cracks from uneven weighting.