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Chris Campbell

Help with high fired bowl bottom problem

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Guest JBaymore

So, question two ... why are the castable molds problem free? They are not moving or shrinking at all ... maybe because they are thicker and spreading the heat better??

 

 

Maybe because those castable ones are keeping the porcelain in those molds a tinny teeny tad lower fired. So it is not fluxing quite as much??

 

Maybe take a quick "waster piece" -straight porcelain no coloerd clay -, place it in a castable mold and put an end point witness cone inside it in the bottom. Do the same with a clay mold one. See if there is a difference in the bend.

 

For the simplicity of the idea, the approach of of "the piece shrinking into the mold" is why I was asking in my other postings if the pieces actually touched the bottom when they were loaded. I assumed that you had taken that into account.... but wanted to make sure about it. I was (and still am) thinking that the best solution would be that the firing mold and the making mold be slightly different sizes. So that the porcelain form actually IS being supported by the firing mold during the firing.

 

I was assuming here that the bowl is fully supported in the refractory mold in the kiln from Chris' description of how she fires her work in other postings/discussions. The bowl will shrink as the firing progresses and the clay will hopefully "slip" freely down the walls of the slump mold/saggar settling slightly more toward the bottom.

 

 

And back to that question of ......... does the floor of the bowls touch down on the "floor" of the miolds when they are being stacked into the finish fire?

 

 

You are getting into some sophisticated mold work there to do this ........ but it might be worth it in the end. Not having to make one use "firing molds". The time invested will eventually pay back.

 

 

And no, the dense castable will be just about as much of a pain in the butt as the insualting castable is. Sorry. ;)

 

best,

 

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

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I hear you ... plus, that explains why porcelain people do not re-use their clay molds. Some of these were on multiple use.

 

So, question two ... why are the castable molds problem free? They are not moving or shrinking at all ... maybe because they are thicker and spreading the heat better??

 

 

I was thinking it was combination of the shape of that particular piece and the shape of the mold.

 

I think, now my brain hurts. :P

 

 

 

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I was assuming here that the bowl is fully supported in the refractory mold in the kiln from Chris' description of how she fires her work in other postings/discussions. The bowl will shrink as the firing progresses and the clay will hopefully "slip" freely down the walls of the slump mold/saggar settling slightly more toward the bottom.

 

 

And back to that question of ......... does the floor of the bowls touch down on the "floor" of the miolds when they are being stacked into the finish fire?

 

I see you've been mentioning this all along. So, now it's time to test the hypotheses right. ;)

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Yes, testing time again and I gotta tell you that I totally dread making the castable molds again ... unlike clay, they can be re-used so now I see why people take the time to make them.

Problem is you have to buy the stuff in a 55 lb bag and have to mix the whole bag and use it fairly quickly. Solo job. Mask, gloves, plastic, wheel barrow, icky mess, garden hose clean up ..... after a week of making enough mold forms to use up a 55 lb. bag of cement like material. I will post images and whine a lot when I do it!!<_<

 

Just to be sure ... the castable I should be using is the dense, not the insulating even though the insulating has not been a problem ... yet???

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Guest JBaymore

Just to be sure ... the castable I should be using is the dense, not the insulating even though the insulating has not been a problem ... yet???

 

Chris,

 

We don't absolutely KNOW that yet. In fact ... the insulating ones used on the NON BUBBLING UP pieces may have been preventing it due to the slight temp lag. IF that theoretical lag is actually happening. It might be all in my head. Do a test firing.

 

I think you've got some testing to prove out the theory(ies) to do here before you go and make a lot of new refractory molds. Becasue it will be a lot of work...... you want to be pretty sure that you've found the issue(s). "Measure twice, cut once".

 

I have had success in dividing bags of castable even though you technically are NOT supposed to do that. It first requirees a BUNCH of time in doing a thorough dry mix as best you can. Dusty work. It won't be a perfect even distribution of the components... but in THIS application... you do not NEED all the best properties of the material. You usage is pretty well below its normal top end capabilities.

 

And yes.... dense castable will give you better heat transmission THROUGH the wall cross section of the mold itself.... thereby firing the piece "better". I've never heard of anyone using INSULATING castable or INSULATING refractories as kiln furniture. The goal is to get heat energy to move through it. If you could fabricate the proprietary Advancer SiC mixture.... you'd be using that ;) .

 

As a test, I'd make a mold from a FIRED porcelain piece of the exact shape and size you want, and use that master mold to form the working setter molds for the actual pieces to be fired. Once you have the shape size down.... production using an intermediate mold form should speed up the castable process.

 

best,

 

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

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Guest JBaymore
I see you've been mentioning this all along. So, know its time too test the hypotheses right. ;)

 

For sure, Matt.

 

best,

 

.....john

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I see you've been mentioning this all along. So, know its time too test the hypotheses right. ;)

 

For sure, Matt.

 

best,

 

.....john

 

 

Holy spelling errors on my part: too to, know now

where is my editor

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SO .... here is the plan after discussing with the experienced person who used the insulating castable first ... we are going to try making our new molds from the dense castable and report back ... I suspect we are both hoping the other will find the time to do this first!:D

 

This discussion has been so productive ... THANK YOU ALL ... I now know why I should not re-use the clay molds and why the castables are truly worth the (UGH) time and effort.

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Guest JBaymore

No probklem, Chris. HOwever..... don;t say "thanks" until we are sure that something worked to solve it. ;)

 

best,

 

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

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The way i work, the challenges never end ... all solutions inevitably lead to new challenges ... So off I head off into whatever awaits. :D

 

 

Chris,

I've been following your discussion with interest because my porcelain

plates also bulge in the middle. sometimes. so it's helpful to see a group of

experienced professionals work out a likely solution. Matt's idea of the

peripheral pressure levering the center up seems to fit my experience,

as my pieces were 'fixed' by making the foot ring slightly wider. (I know

that you don't want to change your shape, so I'm not suggesting that,

only commenting that in this case, his reasoning seems to fit)

 

 

So, I know that you are not supposed to break up a bag of castable

refractory and use only a portion because of possible inhomogeneity. BUT!

The nice people at Minnesota Clay do sell 5 lb paper sacks of the of the 2600

regular castable refractory (KS4) (as well as the Kastable Lite). This might

more pleasant than opening a 55 lb bag yourself be reasonably close

enough in formula for you to test with. And once you've tested the concept,

you could pour a whole bag of good quality pieces :-).

When I ordered from them, they were very prompt and reasonably

priced for the KALite.

 

https://webstore.mnclay.com/

 

warmly,

Lily

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