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kraythe

How to fit a glaze to my Cone 5 porcelain thermal expansion?

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I am using a cone 5 porcelain from my local supplier and there are lots of cone 6 glazes out there and I should be able to fire the clay to cone 6. The question I have is about fit. My supplier says the Coefficient of thermal expansion is 6.0 which is VERY low. But I want to use a glaze called floating blue with this recipe. That glaze has a CoTE of 7.9 according to digital fire so I am thinking to change its CoTE to the 6.0 neighborhood I can add silica or perhaps take something away. But I need to add a LOT of silica to get it to 6.0 so I am wondering if what I am doing is even rational. 

1) Am I going about this the right way? 

2) Is there something else I can do to the glaze to not mess up its look but lower the CoTE ? 

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Floating Blue is finicky on a good day, so anything you do to it will affect the color. The good thing about floating blue is that the surface it mottled enough that you probably won't  notice any crazing, and on a vitrified porcelain you won't have to worry about weeping. Someone more familiar with the glaze will be able to tell you if it even works on porcelain. If I remember correctly it's happiest on brown stoneware bodies.

Some glazes rely heavily up their chemistry to create their look, and you can't do much to change them without ruining the thing that makes them great. Others you can change a lot, with little effect on the final result.

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I have tested a couple at 7.9 and they crazed. I suppose I can keep going down. Is there a way I can measure CoTE myself without help from the supplier? Its not like I can pop open the kiln at 2k temp and measure it with a ruler. 

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3 hours ago, kraythe said:

I have tested a couple at 7.9 and they crazed. I suppose I can keep going down. Is there a way I can measure CoTE myself without help from the supplier? Its not like I can pop open the kiln at 2k temp and measure it with a ruler. 

Unfortunately no, you can't do this but you can figure it out. 

Make up a series of clear glazes with COE/CTE figures ranging from very low to high. Don't add any opacifiers or colourants to them. Fire them same way you fire your pots then do stress testing to test for crazing and  shivering/dunting for the low coe ones. You can see what the range of COE's will be that will be craze free and yet not so low that they shiver or dunt. What does complicate things is that the combination of fluxes used will have an effect on crazing or not but the clear glazes test will get you most of the way to figuring it out. Example of this would be zinc oxide, it can actually decrease crazing in some recipes and yet with glaze calc it looks to raise the COE. (pm me if you want my list of clears with increasing COE figures)

When you look at COE figures for any given glaze don't include opacifiers or colourants. Many can look like they are increasing the COE figures but in fact the crazing will actually be reduced. Zircopax would be an example of the former and titania from rutile or titanium dioxide examples of the latter.

My porcelain plus a red clay I use have COE's below 5, it's doable to make the low COE glazes craze free but it does take a lot of work and like Neil said not all can be lowered without drastically changing the qualities of the glaze.

 

 

 

Edited by Min

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Min: I think there is more than a glaze COE issue here: strongly suspect the clay is unstable for numerous reasons. Bowing out of the glaze discussion: you are better at explaining it than I am.

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I'm going to recommend steering clear of Armadillo clays--the supplier I'm guessing you're using.  Their COE's are all bizarrely low on the mid-fire stuff they sell(sub 5's in some cases).  It's probably easier to find some more usual mid fire porcelain than to reformulate glazes--some of which are very chem dependent.  Track down some Laguna or something nationally available and enjoy. :)  Frost cone 5-6 just happens to have a much more manageable coe of 6.99ish.

I should add, though, that COE is woefully inaccurate.  B-Mix cone 6 is posted at anywhere from 5.25-7.25 x 10-6. 

Edited by Tyler Miller

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5 minutes ago, kraythe said:

I guess the question becomes when do I ditch the supplier and formulate my own clay body that I can be more sure of the COE.

It would be easier and faster to test some clay bodies from other suppliers. Where are you located? We all might be able to make some suggestions for nearby options.

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4 hours ago, neilestrick said:

It would be easier and faster to test some clay bodies from other suppliers. Where are you located? We all might be able to make some suggestions for nearby options.

I am in Austin Texas and yes, I would love to know a good cone 6 porcelain with say a manageable 7.2 COE but everyone keeps their recipes close to the proverbial vest and there isnt much I can do about it. And no, I dont like stoneware, tried it a couple times but never got into it. I have gotten good with porcelain for a beginner. Below is a pic of my last Bisque result with not one failure. (I use cut in half cylinders as test tiles so they are that way on purpose.  I have been looking for such a recipe but also I would love to actually have some glazes that work on my current items. Just found this glaze below but annoyingly my supplier doesn't carry frit 3249 and I don't know how to substitute it. Every glaze I go to I run into roadblocks.

Last bisque run ... 100% success NOTHING cracked in the slightest

 

59ebcda7348c0_FileOct21173650.jpeg.60a364b9f12da28f3796b53a70040c8c.jpeg

 

 

Edited by kraythe

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