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Can A Glaze With A Higher Coe Have Less Crazing Than A Lower Coe?


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Working on a new glaze and I decided to look at their COE's as I had slight crazing. First I put in the one that had the least crazing.

 

Cornish Stone - 50

Whiting - 50

COE = 848

This recipe has the least crazing.

 

Cornish Stone - 45

Whiting - 45

China Clay - 10

COE - 769

This recipe had the middle amount of crazing, only small but still there.

 

Cornish Stone - 40

Whiting - 40

China Clay - 20

COE - 691

This has the most crazing.

 

So I thought that must be right, so to check I put in a glaze I have that I know doesn't craze. That came out to be 400ish. Can't remember the exact figure as I didn't right it down. Tested another glaze I know doesn't crazy with my clay. Same thing, around 400ish.

 

Why is the first glaze test with the highest COE got the least crazing  :unsure:

 

To my knowledge you want a glaze with a slightly higher COE than your clay to have slight compression in the glaze. Too high and it crazes, to low and it shivers.

Correct me if I have F'd up somewhere.

I don't know the COE of the clay.

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Couple things, do you have a gloss glaze that fits your body without crazing? If you do, what is it's COE (without colourants or opacifiers)? The silica to alumina ratio's suggest your tests are not gloss, COE figures don't work with matte glazes.

 

Did the tests come from the kiln crazed or did you put them through accelerated craze testing? Glaze thickness all the same?

 

Way too little silica in all those ones you tested to be durable plus the first one is really low in alumina also, the second quite low too. If these are the ingredients you want to play with maybe start with about 80 cornwall and 20 whiting but I think you will still get crazing.

 

To put the glaze under compression glaze should have an expansion that is slightly lower than the body but there is a risk of shivering when you do this too much.

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Yea I use John Britt's craze free clear cone 10 which gives me a COE around 400. I didn't realise COE only really worked for a gloss glaze, it is a very matt glaze.

 

They came out of the kiln crazed, it is only very slightly. When I start getting the right amounts of silica/alumina I start losing the qualities that I am after. You can't really see the crazing on the picture  :unsure: 1, 2 and 3 are the same order as the recipes above.

 

post-23281-0-79122700-1417031409_thumb.jpg

 

I thought glaze should be under compression not the clay?

post-23281-0-79122700-1417031409_thumb.jpg

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Have you read this article: http://ceramicartsdaily.org/ceramic-glaze-recipes/glaze-chemistry-ceramic-glaze-recipes-2/technofile-glaze-fit/

It says it more succinctly than I could. I like the tight pants versus baggy pants analogy.

 

For your flux, I would try one of the low expansion ones in place of the whiting. Magnesium from talc, dolomite, magnesium carb or lithium from spodumene, a lithium frit if you can get one in GB that's not to expensive, or even some lithium carb with some source of magnesium. Keep an eye on the alumina to silica ratio. I don't fire ^10 so hopefully someone else can chime in here and give you a hand with other suggestions. 

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Yea that explains it really well, thank you for the link. Now I have to remember all my pots wearing baggy pants and I won't forget :D I have some more tests firing tonight with a mixture of dolomite/clays/silica. Don't have any lithium around. I did have a few with zinc oxide and some others with Gerstley Borate substitute.

 
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Ok, I am still confused. Here are two glazes.

 

post-23281-0-50795400-1417180295_thumb.jpg

 

3

Cornish stone - 40

Whiting           - 40

China Clay      - 10

Gerstley Borate - 10

Si/Al - 6.06     COE - 736      SiO2 - 1.164      Al2O3 - 0.192

 

4

Cornish stone - 40

Whiting            - 40

Gerstley Borate - 20

Si/Al - 8.78       COE - 783     SiO2 - 0.967       Al2O3 - 0.110

 

In my head number 3 should be less crazed as it has a lower COE and more SiO2 and Al2O3 but that is not the case. Number 4 looks like a nicer glaze. Also with a Si/Al of 6.06 I would have thought it would run less but yet it runs more than number 4. Even though number 4 has more Gerstley Borate.

 

Too many glaze tests and not enough knowledge. Maybe B2O3 is confusing me as it is a sort of glass former or something. They have still strayed too far from the original glaze that was 50.50

post-23281-0-50795400-1417180295_thumb.jpg

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Guest JBaymore

Where are you getting the "numbers" for your glaze calc program?

 

The data you generate is only as good as the data you are using to work with.  (Remember the old computrer adage, "Garbage in, garbage out".)

 

If you are using "stock" data on the raw materials that came with the program... or something like the "teaching database .... which is often simplified so that students can learn how/why the calculation work........ then your accuracy of the calculation you are using is potentially off. 

 

Get the typical analysis sheets from your supplier for the SPECIFIC raw materials that you are using and load that material data into the materials data table in your program.

 

Also note that those pieces of printed information are just that .........."typical analysis" sheets.  The data that they supply is only as good as the complaince between them and what is actually IN the bag you got the material out of.

 

COE calcs assume a full melt and all of the oxides participating in the melt.  If some precipitate out.... they atre not part of the melt anymore.  So the COE part of the program cannot account for them anymore.  So glazes that have crystalline strucure as a part of their composition........ screw up the calcuations for not only COE... but also the general molecular formula.  Matt glazes are fully in this category.

 

No one has been able to model the performance of a conglomerate of crystalline and glassy phase material yet.  THAT is why you can't model the COE of a clay body.

 

best,

 

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

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You are right, I was using the stock data that came from http://glazecalculator.com/default.aspx. I thought a ballpark figure is better than no figure at all.

 

I really like matt/semi-matt glazes but they do seem to make things a bit more complicated. I feel like I know just enough to understand that I really have a lot more to learn. I need to find a good white liner glaze so I can be happy with its performance and experiment more with matts on the outside of pots.

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  • 2 weeks later...
 

You can make matte glazes that are well formulated, with good glass, that are food safe. Do some test with magnesium mattes, experimenting with cooling rates.

 

What is a good starting point for experimenting with cool downs? I have never tried or really read much about it. When I got the new kiln I did get a controller that can do a cool but I haven't had much time to tinker with it yet. My only semi-matte has a reasonable amount of dolomite in so maybe it could improve with a cool.

 

 

No one has been able to model the performance of a conglomerate of crystalline and glassy phase material yet.  THAT is why you can't model the COE of a clay body.

 

best,

 

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

 

Does this give any hope that one day it could be possible? To me it sounds like they could try but I am probably ignorant of many facts.

http://www.bbc.co.uk/news/science-environment-30397509

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