Breakage

[ronfire]

I added a wall mount controller to my 1027-3 kiln and now have more items crack, mostly large items. Some crack on the heat up and some on the cool down. I fire on medium slow program and using a slow cool down program on my  bisque as well as the glaze. The firing time to  cone 6 is close to the same as I did with the sitter, you would think that the heating would be more even with all the elements working together instead of the  on the sitter system. ( on medium 1/2 the elements run on high).

Here is the cool down I am using.

 

[Bill Kielb]

I am always curious when folks say it’s the kilns fault because normal clays fire just fine for bisque in the 200 degree per hour range and 400-500 degree per hour range for glaze. On cool down so many pieces In so many studios  are just fine in a normal kiln just cooling at the rate of the kiln losses.
I believe your stuff is cracking but would look elsewhere as well as just the schedule. If firing was this difficult for my stuff, I would just stop throwing actually. Fortunately custom schedules for me are for specific results, the premade cone fire schedules are pretty bulletproof for me.

Maybe there is another reason, I would look at all the possibilities.

[neilestrick]

I don't think it's necessary to be firing down at those temps. If things are cracking at those temps then I'm with Bill, look at other possibilities. I don't know anyone who controls the cooling rate below 1400F. Tell us more about the work- what clay body, what shapes, how big, etc. Photos would help.

[Min]

Those are my cooling rates when doing large pieces or re-fires of things like platters etc. Slows down the cooling through both inversion zones with some wiggle room. 480 - 380F @ 50F / hr isn't necessary unless it's a thin walled fast cooling kiln. Ron's sink cracking discussed in this thread.

[ronfire]

I am using M340 Plainsman cone 6 clay and bisque to  cone 04. The forms are sinks usually 16" across and maybe 3/8 to 1/2 " thick clear glaze both sides. The last set of cracks are these and appear to have a sharp edge and are not noticeable at first unless the light catches it.

 

 

[Mark C.]

Could be the horse kicking the bowl? Just a thought

[neilestrick]

Have you tried firing on waster slabs? Have you cracked one open to check the thickness and evenness?

[ronfire]

The ones that had the hairline cracks where fairly even with the clay . Have not tried with a waster slab but I do put them on 1/4 rollers to help with movement and isolate from the kiln shelf.

 

[Bill Kielb]

2 hours ago, ronfire said:

do put them on 1/4 rollers to help with movement and isolate from the kiln shelf.

 

Rollers are sharp catching point loads in the direction they don’t roll in (unless they are shaped like a round ball bearing). Might just try alumina on a smooth shelf. Expands and contract easily in all directions. The shelf helps heat evenly by conduction why create hot spots of contact?

[ronfire]

Would dry kiln wash work? Or would the Kaolin interfere, I don't have any alumina.

 

[Mark C.]

Could be the horse kicking the bowl? Just a thought

ok kidding aside the waster slab is the best

dry alumina  second best

all other coatings are lesser in my mind 

that said better than dry shelve

epk will not hurt anything

Really the waster slab you can make very thin and you have clay

[ronfire]

I will make a couple tonight on the wheel or is a rolled slab ok.. Would it be a problem to make the waster slab and use it in the glaze fire without bisque firing first?.

 

[Mark C.]

I always roll the slabs but I have an electric slab roller . Just make them thick enough to support the piece and stay flat. Put some flat stock on it so it drys flat drying

[Bill Kielb]

2 hours ago, ronfire said:

Would dry kiln wash work? Or would the Kaolin interfere, I don't have any alumina.

 

Alumina is time tested, grog is popular too. There are too many kiln wash concoctions to have an opinion. Something is definitely better than nothing though.

[ronfire]

When you refer to Alumina is that Alumina Hydrate? Looking to purchase some and try it.

I also have heard of using sand but found that is fused onto my shelf and left a mess.

 

[Bill Kielb]

37 minutes ago, ronfire said:

When you refer to Alumina is that Alumina Hydrate? Looking to purchase some and try it.

I also have heard of using sand but found that is fused onto my shelf and left a mess.

Hydrate is probably most useful as it can stay suspended in glaze mixes. About 35% of it is water so it’s roughly a 65% source of al2O3. Alumina oxide is more pure but just won’t mix in a glaze. Use of alumina in a glaze is a bit sketchy as distribution can be difficult but of the two the hydrate form is likely more useful to potters. Neither will melt in your firing.

Folks often mix up with wax and then carefully paint galleries to prevent the top from sticking to the pot during firing so useful in a number of ways to have some in hand.

and yes, sand is not grog. Grog is generally ground fired ceramics.

[ronfire]

Thanks Bill, think I will order some tomorrow and maybe Kaolin ep and Kaolin calcined to make my own kiln wash as well.

 

[Mark C.]

For wash use Alumina Hydrate

I have used the below for many decades -it costly to make but works better than lesser washes .

the best kiln wash mix I and. few other potters  feel is

50%  Alumina Hydrate

25% epk

25% calcined epk

apply the thickness of heavy creal with a paint rollor (smoother)or brush-let sun dry add another coat.

This can be scraped off or touched up

I never use alumina in a glaze-it can be used under pots in a fire to help slide . Firing it does not affect it at all still a dry powder after a cone 11 fire 2400 degrees 

I do use it with a wax sponge to wax my seats on my porcelain lidded forms-its also the best in this use as well. Careful when using it as where its ends up on a glaze it will be rough so be carefull to not let it get on glaze to be fired.

[ronfire]

To apply Alumina to shelves or under pots what is the best way.

Can it be mixed with water to coat the shelves or use a salt shaker.

My concern is haw to use it without getting it on the pot below and affect the clear glaze.

[Bill Kielb]

15 minutes ago, ronfire said:

To apply Alumina to shelves or under pots what is the best way.

Most folks I know carefully and gently sprinkle on shelf with their hand then smooth it out to the footprint of the pot making sure all is very lightly covered. Then they wash their hands. They try not to create any dust whatsoever (mask is good here) and try not to get any airborne so it doesn’t stick to other pots. Remember, it won’t melt. All done very carefully of course. Salt shaker might be good but can create a dust plume so I would rather place a small amount then gently spread to a very thin even coat. Some folks do this in  advance to a location of a shelf and then when time carefully set the shelf in the kiln and the ware in the spot. Still others simply paint the foot ring with medium  alumina / wax mixture.

[Mark C.]

alumina will ruin a glaze if it gets in it (flaws on it a stays) You want to be very carefull with keeping it only under the pots. I like the waster slab better although alumina under that slab will help it slide.You can put it on the shelve then load that shelve. or very carefully apply.Just do not let it get on the shelves /pots below.

I fired all my cone 10 sinks on a thrown  thick ring that took the abuse.

[oldlady]

well, this is perfect for me to say thank you to mark for the kiln wash recipe.

  THANK YOU, MARK FOR THE GREAT RECIPE FOR KILN WASH!

i made it years ago when you posted it here and have never had to scrape it or do anything but smile as i load each shelf. 

 for the 30 or so years before, i had flaking junk all the time.

if you have not tried it, do exactly what mark says and you can smile, too.

thank you,  mark.

[ronfire]

Is the kiln wash formula is by weight?

Think I will make some tomorrow and redo the shelves.

 

 

[Mark C.]

2 hours ago, ronfire said:

Is the kiln wash formula is by weight?

Think I will make some tomorrow and redo the shelves.

 

 

Yes

 

[ronfire]

I made up some kiln wash today on fresh cleaned bare shelves. I am getting lumps from or thick type runs when I apply with the roller. Seams the shelves suddenly grab the water out of the mix and harden the wash.Think I will now have to lightly sand the lumps off. Might go back to soft brush.