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dom92

odd warping during glaze firing

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Hello -

I've been experiencing some odd warping during glaze firing and I wanted to ask the experts here for assistance.  I use a porcelain slip purchased from a well regarded local ceramic supplier (they make and sell their own line of slip).  It's labeled as Cone 6 slip and is supposed to be slightly translucent.  The label goes on to recommend bisque firing at 1940 F and glaze firing at 2200 F.  Everything is fine through bisque firing.  Once it cools, I apply a Cone 6 glaze and then fire again to 2200 F.  It's after the glaze firing I'm getting the warping:

https://drive.google.com/open?id=1SeMLeRahIeQyISG0r7ck3B0Q-3MCiYQX

Here's the weird thing.  I've made these cups many times before with the exact same slip and I've never experienced this before.  The only differences I can pinpoint are that this is a new batch of slip (the exact same type of slip though) I just purchased and I'm also using a new glaze I haven't used before (although I doubt glaze can have anything to do with it).  I have a fully manual kiln with no sitter so I basically just slowly ramp it up to temperature over about 8 hours and watch my pyrometer closely for temperature.  Is it possible this is just a difference between porcelain slip batches?  I suppose my pyrometer could be off as well and I'm simply firing too high and just don't know it?  Using cones isn't an option for me currently as my kiln has no viewing window or peep hole.  I'm open to suggestions as to what could go going on here. 

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Not my field of expertise, but I would guess that it is the different batch of slip. The warping seems to be a slump in the direction of what appears to be a large handle which is pulling the mug out of shape in that direction. I seriously doubt that it's the glaze, but more the temp that is causing the mug to soften and the weight of the handle is pulling it out of shape.

As for the lack of a peep hole, if you are handy enough with a drill and a hole saw, you should be able to make your own...

JohnnyK

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I'm far from being an expert, and have never worked with porcelain, but I think a new glaze could have something to do with it.  A few years ago I was trying to turn a locally dug clay into a ^6 body.  In the process, I would make several 1" wide x 4" long strips from each test batch, and form them into half-round arches.  Then, after a ^04 bisque, I would fire at ^6, one un-glazed, and each of the others with a different glaze I was hoping to use once I got a 'good' clay.   More than once, I had one piece out of the bunch that melted to the point that the arch became an 'M'.  All of the pieces were close together, on the same shelf - so the only difference was the glaze.   

Attached pic is one example from my tests.  An un-glazed piece, that was fired right next-to the glazed piece.  Notice the one that slumped has two different glazes, and most of the melting was on the half with the darker glaze.  Since everything else was equal, I'm pretty sure this indicates the darker glaze interacted with (fluxed) the clay a lot more than the lighter glaze.

One more thing:  Do you fire with witness cones on each shelf ?  If so, were they consistent with past firings ?  If not - I would strongly recommend doing so.  You may find that you have substantial differences from one shelf to the next.

Img_1476b.jpg

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Rockhopper - Interesting theory about the glaze, and your tests definitely show a difference.  My kiln is very small and has only one shelf.  I'm not able to use cones because I have no way to view them. 

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23 minutes ago, dom92 said:

I'm not able to use cones because I have no way to view them. 

You don't have to be able to view them while it's firing, for the cones to be helpful.  Even if you can't see them during firing,  to tell when to turn the kiln off, they'll still tell you what cone you actually reached.  Put one of the cone you're aiming for, plus one higher and one lower, on the shelf.  (If you're firing ^6 - use ^5, ^6, and ^7.)   When you open the kiln after it's done, you'll be able to tell whether you're over, under, or right on the desired cone.

Edited by Rockhopper

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Glazes could definitely be a part of the issue; certain oxides act as fluxes, in certain percentages, over certain temperatures, certain atmospheres, etc. Easy way to determine if its the glaze, or another variable; make a batch of cups, using the same slip, and try to eliminate any other variables that you can (cast for the same time, same wall thickness, empty your molds in the same direction......) and then glaze one, and dont glaze the other. Load them into your kiln on the same shelf, trying to keep them about an equal distance from the perimeter of your kiln, with the handles facing the same direction (so that the heat is even, and the portion of the pot which is facing the heat is the same), fire and see what the results are.

I believe its the weight of your handle pulling the mug out of round. It could be that the materials for the slip recipe have changed somewhat, which might add to the issue, along with maybe the glazes. If the slip isnt properly deflocculated and specific gravity is out of whack, you could be building uneven wall thicknesses, and or building walls that have different amounts of each specific material in the recipe. Remember, a slip is a suspension of clay (& other stuff) in water. If the suspension is off, the particles will settle differently and will impact your castings.

If the recipe's materials havent changed, and your process hasnt changed, and your glazes arent the issue, why you would be experiencing this issue all of a sudden is beyond me. I dont have enough experience with slip casting to really nail down all the variables of slip casting.

I also agree with other above posters, that in the effort of eliminating variables, you should be placing witness cones in your firings to make sure you are getting the same temperatures. Your kiln's elements could be failing and changing the amount of heat you are actually getting to each zone of your kiln. The pyrometer only measures temp in one spot; my gas kiln has some zones which are a couple of cones out of sync with the rest of it and no alterations to firing will even them out. For me, its just something I live with, and adjust accordingly, but if there are some simple fixes you can do to your kiln it might help. For example, almost all kilns are going to be hotter at the top of your kiln (heat rises), and if your pyrometer is measuring temp at the middle or towards top only, you could have elements which are weaker at the top, which isnt getting as hot as quickly, and your bottom elements could be working stronger, and providing more heat. The longer your clay body is in a state of melt, the more its going to move around, so by the time that your pyrometer shows 2200 at the top, your bottom may have been at that temp or higher for longer.

Translucent porcelains are going to get glassy and melt; that is what they are designed to do. When somewhere around 30-50% of your clay body is flux/feldspar and glass maker, its going to move around. If translucency isnt a desired outcome (especially if you're covering it with a non translucent glaze), then maybe switch slip recipes to something that is either a white stoneware, or less translucent porcelain. If your seller doesnt carry a lot of options maybe look into making your own slip (LOTS cheaper), or having them mix a recipe of your liking (once you find one you like) and you wont have the mess in your studio (if dust is a concern/issue).

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Dom:

have any pics of unglazed handles @ c6?  Expanded your pics trying to see any area of unglazed to check color. Translucent porcelain slip @ c6 would have 28-32% feldspar in a grolleg based recipe. I am seeing a white stoneware- off white ( where visible.) it would be tough to over fire a c@6 body to produce that type of slumping. I will go with..oops, too much feldspar when blending. Kaolin has a cone 32 rating without spars: so it can handle heat. Really need to see one unglazed at 2200F. If it shows signs of self glazing; that will speak the loudest.

Tom

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Unfortunately I don't have any photos of unglazed handles.  I'll try to do that the next one I make.  I appreciate all the tips everyone.  Definitely a few things to consider next time around. 

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