No problems with 04 bisque firing except flat pieces?

[Locosan]

Hi - I have cracks usually thin and through but not all the way broken on flat bone dry clay that I am bisque firing at 04... mostly terracotta clay. I am using an old Duncan kiln (manual). I am setting the flat pieces directly on kiln shelf (want to make sure they stay flat). I am sure the crack is occurring during fire not in cooling. I am firing with bowls and sculpture and they do not crack. All built 1/2" thick. All fired with peephole open 30min bottom low, 30 min top low, 45 min bottom med, 45 min top med, 1 hr both med high and then leave at high (no collar).  Total time of firing for 04 is  approx 6 to 7 hrs.   24 hrs to cool.....any ideas why my flat pieces only are cracking?

Thanks - Locosan

 

[Bill Kielb]

Post a few pictures of those that crack. 1/2” is pretty thick, which in turn is pretty heavy. Do you put grog or silica or some alumina on your shelf under the flat piece to make sure it can slide freely and evenly while it shrinks during firing? Is the shelf very flat and smooth? Does the flat piece have a foot? Pictures will likely generate some responses.

Edited January 29, 2023 by Bill Kielb

[oldlady]

is there a reason you want to make everything so thick?   i cannot imagine anyone wanting to use something so heavy.   a lot of beginner potters learned that they HAD to make wheel pieces 1/4 inch thick.   i guess that was ok as a complete beginner but that is nonsense after learning the skill of handling the shape on the wheel.   look at fine china in a store or your china cabinet.  quarter of an inch?   i don't think so.  even terra cotta is not usually so thick.   

many people use sand or fine grog  under flat pieces to allow the clay to shrink as it fires as bill says above.   if your shelves are perfectly flat that is probably all you need with a thinner plate.   if your shelves are not flat, level them off with a metal yardstick and fine grog.   flat pots do not stay flat on a warped shelf.   be careful not to go too close to the edge of a shelf,  some might fall onto a shelf below.   

your firing schedule seems very short to me, especially for some very heavy work.   are you aiming for a short firing period?  can you say why?    what is a "collar" ?   that is a new term for me. 

[Babs]

Hi, all of the above, and do you have cones or a pyrometer you are checking?

That is a very fast schedule.

Where are your large flat pieces placed?

How tight is your kiln packed?

Some folk place large flat pieces vertically.

Photos would help a lot.

[Locosan]

Thanks for all the responses - how am I supposed to send pics?  I don't have any less that 4MB? - not that computer literate haha

as for 1/2 thick.... I like them and started with blank terracotta bowl planters that I found cheap until I couldn't get them anymore  - and they were 1/2" thick -so I started making my own - these are not functional in the sense to use for food - they are my "canvas" for designs/art most are bowls, vases, and sculpture. I have been doing this for about 1 1/2 yrs now.  This question was prompted because it's only the 2nd time that I have made platter with a large flat bottom - both had a crack after bisque firing.. None of the others have had problems with "fast" firing - I know it's fairly fast - I have checked with standing cones, the shut off is mechanical with a cone sitter.  I try to always fire with shelves up as high as I need to in the kiln  but it is not packed - usually do not have more than 3 or 4 pieces in at a time because of sizes (don't make many things smaller than approx  6-8" high x 12-16" wide which takes up an entire shelf (almost). So far I have made about 35 pieces.   the 1st flat one was on the bottom (on a shelf 1/2 off floor) and the 2nd on top shelf with with a shelf over it just below the kiln top. Shelves are standard hex and only have kiln wash on them. No Feet on the pieces.  This is an old Duncan kiln and it had an option to enlarge it with a collar that raised the top - that is what the setting is labelled.  Thanks again - jerry/Locosan

[Bill Kielb]

4 hours ago, Locosan said:

I don't have any less that 4MB? - not that computer literate haha

Among many other methods, often if you email them to yourself, you will be asked if you would like to reduce the size prior to sending. The emailed pictures would be the resized originals. Don’t forget to lightly grog your shelf for anything heavy and large so it slides easily. 

Edited January 30, 2023 by Bill Kielb

[Locosan]

Thanks That is good advice and something new that I learned today - many thanks...Loco

[Babs]

Large flat pieces fire more safely in middle of kiln. Some people make a series of rolled coils then place them like spokes of a bike wheel under the pot. Let's air circulate under  plate and so heats ad cools more evenly.

 

[Locosan]

)Hi Babs - I was afraid you and others might say middle is better...I just try and fit all in and keep 1/2" away from walls (though I am pushing the limit on size with that one... my reasoning for not putting anything under flat pieces is fear of warp or waviness.   But y'all (and cracks) have taught me a lesson...;-) 

as for pics - I sent myself an image as suggested and changed it to small I tried to upload it and I tried to copy it and paste right out of my email and it is still too large.... 3.4mb - even when I send it as small it stays 3.4mb.....I don't have any camera that takes pics of 1mb and I'm not sure the pic would be clear enough if made that small either........hmmm?

[Babs]

7 hours ago, Locosan said:

)Hi Babs - I was afraid you and others might say middle is better...I just try and fit all in and keep 1/2" away from walls (though I am pushing the limit on size with that one... my reasoning for not putting anything under flat pieces is fear of warp or waviness.   But y'all (and cracks) have taught me a lesson...;-) 

as for pics - I sent myself an image as suggested and changed it to small I tried to upload it and I tried to copy it and paste right out of my email and it is still too large.... 3.4mb - even when I send it as small it stays 3.4mb.....I don't have any camera that takes pics of 1mb and I'm not sure the pic would be clear enough if made that small either........hmmm?

Not sure what you have but when you go to resize your image anyway , prior to trying to attach or even opening email stuff (not going to embed in message, ) options on size should appear .

Choose the one which  is less than original, usually mentions suitable for emails. Save as email image or sthing.

And select this to attach to your post here.

At1/2"" thick you won't get warping  if use about 8 coils radiating out from middle.

[High Bridge Pottery]

9 hours ago, Locosan said:

I don't have any camera that takes pics of 1mb and I'm not sure the pic would be clear enough if made that small either........hmmm?

There's a few ways you can make file size smaller without actually changing the size of an image. Different file types like .png .jpg .gif .webp all work in different ways and would give you different file sizes for the same image.

You can also change how many pixels per inch are in the image, the camera is probably around 300ppi but somewhere around 72ppi is suitable for the internet and it means there's a lot less data in the file. 

When exporting images in photo editing software you can also specify how much compression you want further reducing the file size.

 

It all gets a bit complicated but if you go to https://cloudconvert.com 

click 'select file' and choose the image you want to resize

change the 'convert to' dropdown so that you have webp selected (it doesn't have to be webp you can also stick with jpeg but the compression is a lot easier to see)

click the spanner icon and find where it says 'Quality' and input 60 into the box then click ok

Now click convert and download your image, if it is still too big reduce the number you enter into the quality box. You can go as low as 1 without being able to see much difference but your file size will be way way smaller.

 

Hope this helps.

 

 

Edited January 30, 2023 by High Bridge Pottery

[karenkstudio]

I fire flat tile pieces in a variety of sizes. Before loading each shelf, I sprinkle dry kiln wash on each shelf which helps the flat pieces slide on the shelf as the clay shrinks during firing.  I seldom have cracks in my bisque pieces, and where I place it on the shelf makes no difference.

[Locosan]

Thanks to all of you! figured out the pics - we'll see how they look on your end.

[Bill Kielb]

I would definitely make sure there is grog or silica or alumina under this so it can slide easily and freely in all directions. @Pres used a salt shaker I believe which is a perfect representation of how thin and evenly it can be applied. Having said that, the scallops are a perfect place for a crack to originate during construction. So definitely extra care when carving these.

[Babs]

+1 for crack there before firing but exacerbated by the firing.

How are you drying your pieces?

At what stage of drying are you carving?.

Are you applying moisture to smooth after carving at a dry phase?

Definitrly apply sthing to shelves to let these pieces glide as shrink in firing and when drying

[oldlady]

locosan, very nice design with appropriate sizing of the scallops.   to prevent the cracks forming, do not cut the scallop low point with a knife alone.  when you get to that area, make a curve not a point and then use a dowel, pencil or something of the right diameter to press the lowest point straight down.   the rounded edge becomes thicker and stronger with that compression and is much less likely to break.

edit   the bottom sharp angle can be eliminated by cutting it into a round shape with a metal tube, something like a 3/8 inch copper water line for a refrigerator  ice maker or one of the round tube cutters with a handle sold for making holes in pots.  

Edited January 31, 2023 by oldlady
clarify

[Callie Beller Diesel]

On 1/29/2023 at 5:22 PM, Locosan said:

how am I supposed to send pics?

Another method is to write your post on your desktop and save. To add photos directly from your phone, log into the forum via your phone’s browser and edit your post to add the photo. Most newer phones will give you an option to resize when you select the photo. 

[Locosan]

I've been off for a while and love all the suggestions because I can use all of them.  Only "real" problem I have had since last post. Thank you all - I have about 40 pieces so far that I am showing for the 1st time in a "studio tour" later March. - So Far So good....Jerry/Locosan

[Kelly in AK]

Congratulations on the upcoming studio tour! That’s a beautiful looking tray. I also agree that your cracks are starting before the firing. @oldlady’s suggestions are excellent. Of course, the steps to  alleviate stress during firing are worthwhile. For good measure I’ll add to never use or add water on the edges of work (use slip if you have to, the thicker the better). Some clay is very sensitive to this and it really shows up on the edges of slabs under tension.