Why do kiln shelves break?

[KarunaS]

Opened my kiln this morning after a slump firing to 1275 and found that the 21" diam. kiln shelf had a crack straight down the center.
What causes kiln shelves to crack? Old age? Shelf and mold were not freshly kiln-washed.
Best place to buy replacement? Best shelves? BE doesn't sell round ones otherwise I'd go there.
Thank you,
Karuna

[The Hobbyist]

My only guess is that the shelf suffered a physical trauma before going in the kiln like a drop on its edge. The heat then would exaserbate the injury and finish it off.

I bought my 21" shelf at a pottery supply store. They were even kind enough to let me pick through the inventory and search for one that was reasonably flat. As I remember it was about $60 around 8 years ago in Miami.

Jim "The Hobbyist"

[Bert Weiss]

They can be thermal shocked, or if supported with stress, that can crack them. For hard shelves, the big deal is to find a flat one. A depression in the surface can cause a lot of problems. I agree with Jim, Go to the pottery supply with a straight edge and check them out, yourself. You will do best hand picking one.

BTW, I once toured a factory where kiln shelves are made, and sitting in stacks along with the shelves and saggers were the pizza stones. If you are lucky, maybe you can find the right size pizza stone.

[KarunaS]

Thank you for the responses; don't recall physical trauma to the shelf or when it could have heat shocked.

Wish I could go to a pottery supply store to pick one out! None over here on Maui. Often order my odd supplies from Delphi but found their handling and packing somewhat sloppy at times (but then they are very helpful when something breaks in transit). So if anybody knows a better retailer who understands the importance of 'flat', I'd appreciate being pointed in that direction.

Thank you.

[Valerie Adams]

I've had a shelf break when I was fusing a thicker piece.

Bullseye's shelves are great; I know Kim Treloar could order you some.

[Kevin Midgley]

Near Vancouver Canada source.
Pdf is 2150kb
I believe they are made in England.
http://www.greenbarn.com/PDFs/G_KILNS%2 ... 023_27.pdf

[Arnold Howard]

Opened my kiln this morning after a slump firing to 1275 and found that the 21" diam. kiln shelf had a crack straight down the center.
What causes kiln shelves to crack? Old age? Shelf and mold were not freshly kiln-washed.
Best place to buy replacement? Best shelves? BE doesn't sell round ones otherwise I'd go there.
Thank you,
Karuna

Cooling the shelf rapidly or unevenly can crack it down the center, especially if a mold is on the shelf. Lift the mold off the shelf with half inch posts. This will help to prevent uneven cooling.

Sincerely,

Arnold Howard
Paragon Industries, L.P., Mesquite, Texas USA
ahoward@paragonweb.com / www.paragonweb.com

[Morganica]

The only times I've had a mullite shelf break are when I've had a casting mold sitting directly on the shelf instead of raised up so that air moved between the mold and the shelf, as Arnold mentioned. It usually doesn't happen the first time (unless it's a big, honkin' mold), so I suspect the shelf starts stressing or cracking internally on the first firing and doesn't actually come apart until the second or third firing.

The second time I did this, actually (you'd think I'd only need one expensive broken kilnshelf to learn this lesson, wouldn't you?), the shelf remained intact through three firings, prompting me to say "Well, that whole "don't put the mold on the shelf thing is a myth." Then I set a quarter-billet of Bullseye on the cold shelf, dead-center, and the shelf collapsed. It had obviously been waiting for just a little weight to finish breaking. After that, lesson learned, I took the kilnshelves entirely out of the kiln when using molds. Now I raise them up with kiln furniture, an inch or two above the kilnshelf floor. Haven't lost a shelf since.

[Lauri Levanto]

I secomd MOrganica's report.
Especially plaster molds warm up very very slowly.
As long as there is a tad of moisture in the plaster it does not get over boiling point. Even after that the water in the chrystalline lattice resist heating.

-lauri

[KarunaS]

That must have been it! I had the 12" BE square slumper mold sitting directly on the kiln shelf.
What a great idea to set them on kiln posts directly on the kiln floor...duh!
Thanks all.

Shipping of kiln shelf to Maui = as much as the kiln shelf (approx $80 + $80 UPS ground)...price of living here, along with groceries being about 3x the price.

[Arnold Howard]

That must have been it! I had the 12" BE square slumper mold sitting directly on the kiln shelf.
What a great idea to set them on kiln posts directly on the kiln floor...duh!
Thanks all.

Shipping of kiln shelf to Maui = as much as the kiln shelf (approx $80 + $80 UPS ground)...price of living here, along with groceries being about 3x the price.

I used to live on the Isle of Hawaii. One time I visited Texas and discovered that a papaya was more expensive in the Big Island grocery stores than it was in Texas!

Sincerely,

Arnold Howard
Paragon Industries, L.P., Mesquite, Texas USA
ahoward@paragonweb.com / www.paragonweb.com

[Barry Gitelson]

I have lived on the Big Island for over 40 years and rarely buy fruit that can be grown (Papaya being one of them) and they taste better than the store bought ones that were picked barely ripe. But it is hard to get a good peach or plum here. Safeway has some organic grapes that are delish but very pricey.

I do relate to the problem (cost) of shipping here. Glass costs drive me up the wall. I too will need a new shelf at some time and dread spending the money to get it here. I am using my Kaiser Lee board for a second shelf and it seems to work just fine.

[Morganica]

I have lived on the Big Island for over 40 years and rarely buy fruit that can be grown (Papaya being one of them)

Uhm...what other kind of fruit is there?

[KarunaS]

I have lived on the Big Island for over 40 years and rarely buy fruit that can be grown (Papaya being one of them)

Uhm...what other kind of fruit is there?

Mangoes, figs, tangerines, oranges, bananas, also apple bananas, lychees, sapote, egg fruit, oh yes - pineapple...

[Barry Gitelson]

Cynthia... you are very funny. Of course all fruit is "grown". But stay away from the GMO ones.

[Morganica]

I have lived on the Big Island for over 40 years and rarely buy fruit that can be grown (Papaya being one of them)

Uhm...what other kind of fruit is there?

Mangoes, figs, tangerines, oranges, bananas, also apple bananas, lychees, sapote, egg fruit, oh yes - pineapple...

The whole fresh-pineapple-off-the-bush thing was one of my favorite parts of Hawaii. I think the pineapple juice had just about worn a hole through my tongue by the time I left.

Hmmm. What are apple bananas and egg fruit?

[Barry Gitelson]

At one time I was the prez of our local rare fruit growers club and you would not believe the varieties of fruit that can be grown. Amazing stuff. Apple bananas are one of the many banana types that you can grow and seems to be one of the more popular. Egg fruit is a type of sapote.

[KarunaS]

Hmmm. What are apple bananas and egg fruit?[/quote]

There are apple and strawberry bananas...they are a lot smaller than regular bananas and taste a bit like apple or strawberries.
Egg fruit I knew from India as "chikus"; they are egg-shaped and little larger than an egg. They taste like nothing else I know...a bit caramelly, sweet, malty and a slightly grainy texture like a pear..one of my most favorite fruit: http://en.wikipedia.org/wiki/Manilkara_zapota