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Cracking Platters


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I got some serious cracking on my last firing. I could use some advice.

 


LRW 11/11/16: Firing schedule

 

Ramp 1: 100ºF/hour to 190ºF Hold 3 hours

Ramp 2: 500ºF/hour to 1968ºF  5 min hold

Ramp 3: 150ºF to 2190ºF 5 min hold

Ramp 4: natural cooling to 1900ºF No hold 

Ramp 5: 100ºF/hour to 1400ºF  No hold, kiln off.

 

 

Kiln Skutt km1227 with computer-controller It is located in a large closed building where there are no drafts.

 

Clay G mix 6 with grog from Georgie's

 

Bisque to 04 

Glaze fire to 6

 

Plugs pulled at 1000°

 

Lid cracked 1 inch at 380°

 

ugly pictures attached

 

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Doc 

I cannot see how long it took to get to cone6? whats your guess?in hours

Do not pull your plugs at any temps.Let cool without drafts.

Keep kiln closed up until lower than 300 Degrees -keep closed longer with platters

Where was this platter in kiln

Middle ?

top?Covered with a shelve?

bottom?

 

One last note but its my bias -I'm not a Georgies clay body fan but as I said its just my experience which their bodes (high fire only)

they may make a mean cone 6  body

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Cracked before you even  glazed fired them. Fissures in the clay, and the mechanical stress of the glaze finished the job. Need to do the ping test: hold the piece with two fingers and gently tap it with something metal (softly!) If it pings it is solid, if it pongs it has a fissure. I use a metal glass cutter, with the metal ball on one end. $3

Nerd.

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The thickness looks uneven too. As both Mark and Bciske mention. Looks like the crack may have started where you indented the lip but the bottom looks very heavy. I have a chuck for trimming such shapes to avoid stressing lips. Highly recommend using something to support the pot when trimming more delicate shapes.

Marcia

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Babs:

 

Seen that crack pattern too many times: all I need to see. It is the classic bow/hook pattern: an arc towards one end that straightens out to the other. Where the hook/bow crack is: is also where the piece was making the most contact with the shelf. The body had stress points in it from making (will let others address that cure) and as the glaze began to harden upon cooling: it finally snapped it apart. Notice how open the gap is on the one end? If Doc would have been standing next to the kiln: he would have heard that one go.

 

Nerd

 

by the way Doc: get some silica under those pieces next time: reduce shelf drag.

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wow, thanks for all the good advice.I can see where I made a lot of mistakes. I'm just a raw beginner at this stuff

 

Actually there were a couple of platters that broke in a similar pattern. The bottom was probably a little thicker than the sides on both of them.

 

the whole firing process takes about 16 hours. I would guess it takes about six hours to go from 190  to 2190.

 

The platters were on the top two shelves. 

 

the top was not covered with a shelf.

 

it cooled pretty fast from 2190 to 1900 then it was slow after that.

 

I assume the ping test is done on the bisque or do you mean  after glaze firing. I do have one of those glass cutters like you mentioned.

 

I can see where I need to change a lot of stuff. 

 

My welding is so much easier than glazing.

 

tthanks all

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Babs:

 

Seen that crack pattern too many times: all I need to see. It is the classic bow/hook pattern: an arc towards one end that straightens out to the other. Where the hook/bow crack is: is also where the piece was making the most contact with the shelf. The body had stress points in it from making (will let others address that cure) and as the glaze began to harden upon cooling: it finally snapped it apart. Notice how open the gap is on the one end? If Doc would have been standing next to the kiln: he would have heard that one go.

 

Nerd

 

by the way Doc: get some silica under those pieces next time: reduce shelf drag.

Yes, dunting, I was referring to your conclusion  that it was already cracked prior to glazing..... It does look a very tortured piece as MarkC writes.

It's just that I have had pieces do this which have been intact till the glaze firing and the crack occurs when the kiln drops quickly, fibre kiln the culprit.

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I have an extension of my original question

 

This was just a test firing of a bunch of little   12 inch bowls. I have a bunch of similarly shaped 23+ inch .bowls. What additional advice would you folks have for firing these monsters without getting cracks in them, given that I implement all of the great suggestions I've gotten.

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Put either a layer of grog or silica sand beneath the platters or put them on clay coils or slats (bisque fired) so they are sitting above the kiln shelf; that will help allow them to heat and cool more evenly. Sitting on the shelf, they bottoms of the platters stay hotter as they absorb heat from the shelf while cooling.

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Go slower with fire and make sure they are covered by a top shelve.(I never heard how long your fire was?)Make sure they can move as Bruce said in above post.

Make sure they are BONE DRY

Let them cool SLOWER and do not open anything including peeps until cool.

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Doc:

 

FF02

 

Obviously you need to scale this principle according to size, weight, etc..... but you get the idea.

 

QUARTZ INVERSION  573C    Program 100F from 1000 to 1100F to avoid cracking a piece during this critical bisq temp.

 

The larger the piece, the more surface/shelf contact it makes: the more critical quartz inversion becomes.

 

Nerd

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