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Ginny C

Large Platter Broke In Half In Bisque Firing. Anything To Do With It?

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Lots of work went into this large (15" diameter) agate ware platter, but it broke clean in half during the bisque firing! I had it sitting on sheets of Bullseye kiln liner paper to allow it to slide on the kiln shelf. 

 

I'm sure it is now headed for the driveway, unless someone can suggest another use, or even a way to repair it. (Ha!)

 

 

 

Just to show what it would look like with a clear glaze, here's another piece I made earlier:

 

 

FYI, the patterned clay strips are placed on a thin slab of the same clay (Laguna B-Mix for cone 5) and rolled hard until they are basically one unit. Then this platter was cut to shape and placed on a canvas sling to form the gentle curve while drying to leather hard.  I placed a ware board under it (under the sling) at the right distance to give it a flat bottom.  In case someone thinks it wasn't dry...no chance! I'm sure it was thoroughly dry.   The process seemed to work, until...I opened the kiln!

 

Ideas?? 

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post-1066-0-28128300-1461980568_thumb.jpgpost-1066-0-16530100-1461980801_thumb.jpg

 

Chris...I guess I do not understand how to post a photo. Thought I followed the directions to attach files, but evidently I need to do something different!  Or, can you see them now?

ginny

post-1066-0-28128300-1461980568_thumb.jpg

post-1066-0-16530100-1461980801_thumb.jpg

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It appears you could finish two 1/2 platters with a little smoothing of the edges.

 

 

So cracking... it is a problem that every beginner faces. Here is a list of things that promote cracks (things not to do)

-- over working the shape

-- bending or carelessly handling the leather hard piece

-- joining pieces with lots of slurry

 

Here are some good practices:

++ making a solid, smooth lip

++ leaving pieces to dry (no touching!) after the leather hard stage has passed.

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Hi Ginny:

Some questions-- How much of this platter made contact with the shelf? (foot ring)

Did you slow down the ramp during the quartz inversion temp?

Can you take a picture of the edge/s where it cracked and post please?

 

Think this is a mechanical issue, not a clay issue: but need more info.

The smaller the foot ring (shelf contact) the more forgiving the quartz inversion temp is. The larger the foot ring (shelf contact), the more unforgiving quartz inversion temp is. I fire large format tile on a regular basis: and I have blown them completely in half due to inversion stress. The crack pattern I see on your platter very much reminds me of this problem for several reasons. If it had been a forming technique issue; the break would have followed the pattern made by the clay. The break is nearly in half perfectly, and also crosses the ribbons in several places. The near equal split would equate to near equal stress in two directions. The jagged split on each end is where the platter was not in contact with the shelf: hence an unequal expansion co-efficient. The problem is compounded by the sheer weight of this piece: it is large.

Two remedies: 1. Program an additional ramp in your bisque cycle from 1025 to 1125F at 100F an hour. This covers the peak inversion temp when the piece is  moving at a molecular level. 2. Ditch the paper and dust the shelf with dry silica (preferred) or alumina: this will allow the piece to move unrestricted. Alumina is preferred but can become air borne as kiln gets hotter. Silica works well, and stays in place for the most part. Since incorporating these two items into my bisque firing, it is a rarity to loss a piece to cracking.

Nerd

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Glaze nerd ... Just a quick question re your reply.

I thought cracks on the way up were smooth and on the way down are sharp.

That crack looks sharp so I tend to think it happened on the way down ...

 

So does passing through quartz inversion time during cooling produce just as much stress as on the way up?

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Chris, Yes, it's sharp! Would take a lot of grinding to not look like just a broken piece! And, Babs...maybe a garden piece is a good idea. :D

Nerd & Chris, thank you for all that explanation!  I've never tried to use my kiln without just the built-in settings. It's an Excel with a Select Fire board.  I set it for cone 5 (but when I use a witness cone it shows cone 6 was reached) and I usually choose slow or medium speed, but this time I used Fast...maybe that was the problem!!?? 

 

It looks like about 6 inches of the platter rested on the shelf. (No foot ring on this piece.)

 

I will order silica for future attempts. Please tell me which kind to get, as I see 4 different kinds or particle sizes on Bailey's website: silica sand or silica flint (200, 325, or 400 mesh). Since I use pre-mixed glazes, I've never ordered any of the ingredients.

 

I'm about to load another bisque batch. Guess I'll set it for slow! No platters, but one large utensil jar, which I'll prop on stilts.

Thanks!

ginny

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I would consider firing the next piece on a tray of grog or sand. This would allow support, and movement. I think a second thing would be to make certain to fire down slowly to below 1100F. to pass quartz inversion, as Chris is alluding to. It looks like to me it is a mechanical break, possibly aggravated by not enough support for the size.  All my own opinion based on what I see, and know from firing kids work for 35 years.

 

best,

Pres

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When I am teaching my colored clay/neriage/nerikome classes, my number one piece of advice is ... SLOW DOWN.

 

Consider all the things you are asking your clay to do ...

you have colored it, shaped it into patterns or designs ... it has a hundred places it needs to connect ... you have rolled it, stretched and molded it into your idea of a good form.

That clay has had it!!!

Give it time to settle, treat it gently as you go forward.

You have already invested so much time and attention in the production that it is a shame to lose it all by hurrying the firing.

 

I don't fire slow, but I never fire colored clay on 'fast'. Medium is good.

I usually use the ramping programs to fire up and do controlled cooling down to 1100. Electric kilns cool much too quickly for many kinds of work.

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Guest JBaymore

Alpha/beta quartz inversion is at 1063 F.  It is sudden... at 1062 nothing...... at 1064 expansion.  Reversible on all crystalline silica (not glassy phase nor the cristobalite form).

 

Given the inaccuracy of most pyrometers folks are using... (not to mention thermal lag in the load) the safe bet is to go slowly up and down between about 1000 F and 1100 F.

 

best,

 

...............john

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Chris:

This was a bisque fire, not a glaze fire. Although the general rule of cracking up or cracking down still applies (mostly). In glaze, if it breaks on the way up, then the glaze will slump over the break line (smooth), and if breaks on the way down (below fluid temp), then the line will be sharp because the glaze is not able to slump over the line. In bisque, 99% of the time the break is on the way up and a sharp line will always be present. The break pattern will follow the foot ring: by which I mean wherever the pressure is most prevalent: the break will follow.

If you are bisque firing a large format (mostly flat) piece: the mechanical stress increases by the amount making contact with the shelf. The problem shows up differently with heavy thrown pieces on a small foot ring. In those cases, then inversion stress travels up the cylinder and forms fissures in the body. Sadly the problem does not show up until glaze firing. At this point, the piece now has the additional mechanical stresses of the glaze to deal with. It is more common to see sharp lines in this case: because the COE of the glaze at cooling creates the stress on the clay at another COE: opening the stress fissures created during bisque.

If you are firing large format pieces: slow down the entire cycle if you cannot control all segments. If you have a programmable controller: slowing down from 1025 to 1125 is the primary target.

Ginny: buy the cheapest silica money can buy for this purpose: 120mesh is perfect, or just use what you have.

Nerd

 

Forgot something: Chris, inversion is the biggest problem during bisque firing. As you know, the change is from alpha to beta quartz: forever changing clay into ceramic. That said, there is still movement during the cooling, and during glaze firing: but not enough to produce cracks. Think of inversion during bisque as a molecular earthquake, and subsequent firings/cooling as minor tremors.

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So if I understand you correctly, Quartz inversion is stressful every time you fire a piece. Or is it the COE in later firings?

Sometimes I fire high (Cone 8) then fire low several times after ... I get very casual with the later firings, but it seems like I shouldn't?

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Quartz inversion is very stressful during bisque firing. After that, COE becomes more of the problem; but unseen problems created during bisque firing will show up during glaze firing. The COE stresses during glaze firing will magnify the problems created during bisque: if any exist. Yes, there will always be movement during inversion temp: but after bisque they do not create problems unless the pieces are extremely large or large surface contact.

The other problem which applies to large format flat surfaces making contact with the shelf. In this scenario: the top of the piece cools at one rate, and the area making contact with the shelf cools at another due to thermal mass. In this case, a thermal COE issue is created. Around 400F: the air temp in the kiln is correct: but the shelf temp under the piece can be very much hotter. I am sure everyone has opened a kiln around 120F to fetch out pieces. While the piece is very warm: touch a shelf and it burns!! So when firing large flat pieces: it is still advisable to dust the shelf to deal with this issue as well.

Nerd

 

Yeah, I know.. information dump...sorry!

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Certainly not an information dump! Very informative and full of useful information that I for one will be using very shortly as I attempt some large (relatively) flat pieces. The chances of disaster are high, but this will help. Thanks Nerd - always interesting to hear your insights.

 

Girts

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Guest JBaymore

As you know, the change is from alpha to beta quartz: forever changing clay into ceramic.

 

Actually the change from "clay" to "ceramic" is due to the expulsion of the chemical H2O from the clay crystals (hydroxyl actually) as clay converts to the meta form. Happens starting at about the 990 F range and takes some time up to about 1600 F.  Also called the "water smoking period" by some folks.  Continuing up, at about 1700F ... the de-watered crystals change further and eject a molecular sized silica molecule which instantly is in the beta configuration.  If not caused to melt by fluxes present in the body... these molecules are one prime source for cristobalite development in bodies fired over 2200F for long periods.

 

best,

 

...............john

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Guest JBaymore

So if I understand you correctly, Quartz inversion is stressful every time you fire a piece.

 

Only crystalline silica goes thru the inversion.  Silica already involved in a melt (including in a clay body), being in the glassy phase, does not.

 

best,

 

.............john

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I set it for cone 5 (but when I use a witness cone it shows cone 6 was reached) and I usually choose slow or medium speed, but this time I used Fast...maybe that was the problem!!?? 

 

 

Are you saying you fast-fired to ^5 for a bisque firing?  Or do you mean to ^05?

 

If you fast-fired to ^5 from greenware I'm not surprised it cracked.

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John:

Ty you for the added clarity: was trying not to dump anymore than I did. That said, there is an excellent paper out from the Silica group that goes into inversion temps, and subsequent "movement" above and below the 1063F point. Actually the movement below that temp during cooling has long made me wonder if it is also involved in crazing issues. No  proof, but it does strike me as odd that the temp is close to the temp people report hearing "pings" coming from their kilns. If I only had the X-ray diffraction equipment to take a closer look. Think I will change my name from Glazenerd to Chronically Obsessed Experimenter.

http://www.quartzpage.de/gen_mod.html

Nerd

 

There are additional tricks to firing large flat, or large flat and heavy pieces that I can share if they continue to be a problem. Slowing down during inversion temps and floating the piece on silica should cure 90% of the problems.

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I set it for cone 5 (but when I use a witness cone it shows cone 6 was reached) and I usually choose slow or medium speed, but this time I used Fast...maybe that was the problem!!?? 

 

 

Are you saying you fast-fired to ^5 for a bisque firing?  Or do you mean to ^05?

 

If you fast-fired to ^5 from greenware I'm not surprised it cracked.

 

Chilly,  Oh good grief. Sorry !  I was just talking about my glaze firings there..and the fact that the kiln slightly overheats.  The bisque firing under discussion here was to cone 04!  Sorry for the confusion.

ginny

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Most of the  flat slab objects I have seen cracking in the last 10 years have been due to uneven cooling due to the temperature differences between the top and edges and the parts in contact with the kiln shelf. 

 

When I switched to placing the objects on balls of wadding (half and half by volume alumina hydrate and epk) such that "slab" is at least a fingers thickness distance from the kiln shelf, cracking failures dropped significantly.

The logic is that the edges and upper surfaces are the first to cool and the shelf and the bottom surface is the last to cool. Since the bottom surface of a flat object is in contact with the shelf over a large area the rate of cooling of the bottom is determined by the mass of the shelf, not the mass of the object. The wadding balls disconnects the ware from the kiln shelf and allows the ware to almost independently from the shelf. 

 

You could use the very short kiln posts instead of wadding if you have enough.  In the bisque firing you could also use bone dry clay balls. 

 

 

LT

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OH!  Just make little balls of clay and put them underneath?!  What fun!  How far apart...I'm guessing about 1 inch apart...so 8-10 for a platter with about 4-5 inches diameter  flat area on the bottom?

ginny

( I love the interest in this, and the advanced scientific discussion some of you have brought into it!)

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OH!  Just make little balls of clay and put them underneath?!  What fun!  How far apart...I'm guessing about 1 inch apart...so 8-10 for a platter with about 4-5 inches diameter  flat area on the bottom?

ginny

( I love the interest in this, and the advanced scientific discussion some of you have brought into it!)

 

Not little balls of "clay"; little balls of a mixture of 50% alumina hydrate and 50% EPK.  For a platter, five -- one in each corner and one in the middle.  Roll the wad and attach to the bottom of the pot with a bit of Elmer's glue.  When attaching them, avoid touching other areas of the pot with the hands/fingers used to make/attach the wad as any transfer of the alumina could result in a white spot where you touched.  I usually keep a damp hand-towel nearby and wipe my hands between placing the wads on the pot and then putting the pot on a kiln shelf.  Left over wadding can be kept in an airtight zip-lock bag; some keep theirs in the refrigerator/freezer if the wadding includes organic materials (in salt kilns, some add sawdust, flour, and other organics to burnout of the wadding and make it easier to release from the pot; I generally use the 50/50 mix and have not had problems with sticking).

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I had a similar problem - One  warped - post-68505-0-38132900-1462232717_thumb.jpgpost-68505-0-33134400-1462232742_thumb.jpg (the diprotodon) and  one cracked (kangaroo) - these are in a series of Australian kitchenware  Kangaroo plate nearly cracked in half, while others in the fire were ok, except for the one which warped- i had that one on stilts-- yet 6 survived ok. Cone 6 firing.

So if i want to do more- should i change my firing schedule (4 Ramps, but no downfire) to lessen the likelihood of happening again?

 

I had glazed both sides for kitchen use, which is why i used stilts- with alumina on the shelves, i assume the glaze will not stick to the shelf at cone 6?

diane

 

post-68505-0-38132900-1462232717_thumb.jpg

post-68505-0-33134400-1462232742_thumb.jpg

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