Jump to content
Sign in to follow this  
Ginny C

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

Recommended Posts

Hi Diane:

Have several questions, but will comment partly from your pics and partly from making certain assumptions. I will "assume" these plates were bisque fired before they were glazed fired. From that assumption: the cracks you see on the finished product occurred during the bisque firing. You did not see them after bisque firing because they were fissures in the body. I will venture a guess and say the edges of the crack are sharp.  When the kiln dropped below 1300F and down to 400F: the tension created by the clay cooling and the glaze cooling pulled the fissure apart. This also makes me believe that the COE of your clay and your glaze is not aligned properly.

Let's say I am wrong and you once fired these pieces. The above still applies, however the edges are smooth and not sharp. You blew through the quartz inversion temp (1063F) and blew the pieces apart and the glaze sealed the crack. Two problems causes that type of severe cracking: blowing through the inversion temp during bisque, or during once firing. Secondly, a COE differential between the clay and the glaze.

FIXES: program a slow ramp of 100F an hour between 1025F up to 1125F to stop fissures (bisque) or severe cracking during once firing. Secondly; double check your glaze calculations to bring the COE of the clay and glaze together. You have a unique issue because you are glazing both sides, so putting dry silica or wadding under the piece is not a viable option. If you are bisque firing, placing silica under the piece to allow it to move ( reduce contact tension with the shelf) will certainly help.

Simple Test: if you are bisque firing these larger/flat pieces on a regular basis then you need to start doing the "ping" test. Buy a cheap metal glass cutter with a small metal ball at one end. ($3 USA). Hold the bisque piece by the edge and gently tap it with the metal ball (GENTLY). If it "pongs", then the piece is solid and no fissures or cracks are in the body. If it "pings" (hallow sound) then fissures are present rather you can see them or not. If it does ping, you are wasting time and money trying to glaze fire it. As far as the glaze sticking to the shelf: that one is tricky.

Nerd

 

Warping concerns me as well: 1. not properly supported 2. over fired.

Share this post


Link to post
Share on other sites

John:

Ron and I had a lengthy discussion about free silica and cristobalite as well.  Cone 6 and under, neither of these are much of a problem: hard cone 6 and up  they do become a problem. The discussion was: above cone 6, should alternate fluxes be added instead of the standard potassium or sodium? Sodium and potassium are in a gaseous state at these temps, and calcium and lithium are liquid. How much of the potassium or sodium gasses off,  and is the remainder adequate to resolve the problem? By adding fluxes that are in a liquid state bind the free silica and avert cristobalite formation? Care to weigh in?

Nerd

Share this post


Link to post
Share on other sites

Thanks Nerd, that is helpful. I had bisqued prior to firing, next time i will downfire, after checking the fit between glaze and clay, but i thought that was OK- i am making my own glazes on some and not others. its a steep learning curve, so thanks for the advice

diane

 

Share this post


Link to post
Share on other sites

DSW -- when are you adding your art work? At leatherhard, after bisque? And, how are you supporting the plates while doing your art work? Is is possible that you could be stressing a thin plate/edge while adding your drawings and brushwork?

Share this post


Link to post
Share on other sites

Slow down the firing, and put the piece on silica sand. Your kiln paper won't do the job. Don't use silica that's 200 mesh or more- that's silica powder, which is used in clay and glazes. You need sand, like 70 mesh or larger. Just a dusting will do the trick. It will act like little ball bearings and allow the piece to move. Always a good idea for slab pieces, even in the glaze firing. Just make sure it doesn't touch the glaze. I prefer sand to grog because grog is sharp pieces, sand is round pieces. Round rolls better. But definitely slow down the firing.

Share this post


Link to post
Share on other sites

Flat firing fixes:

 

Roll any cone 10 clay you have to 1/2" and cut to desired length. Fire them to maturity before using. Dust with silica or alumina, and set your piece on it. The only thing that matters is that the top is smooth- do not have to get fancy.

 

FF02

 

For even larger pieces: if you have a broken shelf, or roll and cut 3/4" slabs for this purpose. I order kiln shelves and cut them down to fit my kiln;using the leftovers for slats. The larger and heavier the piece: the closer together the slat should be. By using thicker pieces, more space is given for heat to circulate. Dust the tops with silica or alumina.

Either method allows you to reuse the slats repeatedly: easier to  deal with than wadding (EPK & Alumina). Do not have to clean up after the firing.

 

Nerd

Share this post


Link to post
Share on other sites

 

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).

 

I make coils because balls roll and can get stuck on a glazed piece. I didn't see and explanation of your footing of the platter. If it had a foot I would have laid out coils in a radiating pattern, This allows even heat all around, top and bottom.

Meanwhile, since the platter has shrunk from the bisque. You might try gluing it back together with spooze/ paper clay mix. Dampen both the edges well apply the mix and compress together. The bisque will suck the moisture out of the mix quickly so get the edges good and were. rebisque it on coils , sand , balls, your preference.. sand it smooth. glaze with support coils.

Share this post


Link to post
Share on other sites

ok, have just read all of this.  right now is a slowish period for me and i planned to take all my shelves outside and grind the ancient kiln wash off.  that is underneath the silica sand that i put on the shelves several years ago.  all my shelves have silica sand on them because almost everything i make is flat on the bottom, no foot rings.  i got some new silica sand this winter and plan to toss the old stuff that has been there so long.  and all my shelves are full circles, rarely do i use the one half shelf.

 

my plan is to mix up some new kiln wash with 25% calcined epk, 25% epk and 50% alumina if i am remembering the recipe that is out in the studio.  what i wonder is this, should i try to sprinkle the sand on the wet kiln wash to stick it down in one place or continue to put it on top of dry kiln wash?  every firing i renew the grooves that happen when i use a toothed spreader to smooth out the sand.  i keep the sand in the center of the shelves, no closer to the edges than 3 inches so i do not find grains of sand in my glaze work underneath.  i am careful to place the shelves into the kiln without tipping sand all over the place.

 

but, sometimes i find that a kiln post has stuck to the bottom of a shelf.  i usually grind the tiny bits of kiln wash that sometimes stick to the posts so they are clean when i stack the kiln but occasionally one will get stuck underneath.  since the shelf is covered with silica sand, i do not turn the shelf upside down but just gently knock the stuck post off the shelf.  this cleanup will include grinding the bottoms of the shelves so any leftover tiny bits do not stay on the shelf to drop into something later.

 

question is, am i thinking this through correctly or have i missed something?  i have very little breakage and make things relatively thin so they do not have so much stress from supporting heavy weights of clay.  my work is about the same thickness as commercial china plates.  oh, yes, i single fire nearly everything.  hate putting glaze on bisque!

 

my usual firing speed it the L&L slow glaze to cone 6.  with a preheat and ten minute hold on the preheat.  firing usually 14 hours.

Share this post


Link to post
Share on other sites
On 4/29/2016 at 6:18 PM, Ginny C said:

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?? 

I had something similar happen with two 19b platters, and it made me want to cry....mine did not go clean through though.  I did some reading in old books and found some sections about different kinds of cracks.  Radial cracks, like this, that go from rim to center without following the S-curve pattern point to drying problem.

In my case, the platters took a ridiculous amount of time tobget to the point of being able to flip them.  Weeks in the damp Cali winter.  So the platter rim would harden first  even though the rest was still very wet.  In my case, the rim itself took a long time.  

It was at the studio, not at home, or I would have been able to leave it open to dry.  

Going forward, i plan to get some Hydrobats  ti ease the drying thing.  

In my mind, the cracks are from pressures as the clay shrinks and the clay being able to drag across the drying substrate.  I have a clear intuitive picture of how this would work in my mind and can't  think of any common thing to use as an analogy.  

I went ahead and glazed them on the off chance that the crack would not  open more.   I was not lucky.  I am sitting on it to decide if i will do something with the cracks.  They are neither functional, so I have opportunity 

 

HTH

Share this post


Link to post
Share on other sites

I had a colleague a few semesters back that had a bottle/vase form come out of the glaze firing with significantly sized cracks.  The cracks made an interesting pattern, so she filled the cracks with epoxy and coated the exposed area with a gold colored material - think Kintsugi.  

The cracks and her "salvage" technique strengthened the overall aesthetic of the piece.  Side effect: for a few weeks afterwards, other students were breaking stuff just to try to imitate her success. 

lt

Share this post


Link to post
Share on other sites

Try a little Kintsugi AKA Japanese pottery repair method? Glaze it and join it together with the glaze, and support it underneath with a waster so the broken edges touch during firing so they fuse. Afterwards, drill holes and  use 3 large copper staples to further join it, -or- do the glaze thing and use a metal leaf (gold or copper) over the repaired crack. 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×

Important Information

By using this site, you agree to our Terms of Use.