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Matt Katz

The truth about crazing

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Hi All, 

I've just posted my newest video about what really causes crazing, what are the issues with crazing and some solutions for avoiding it in the long term. 

 

I hope you like it! 

 

[Video removed by moderators, due to its advertising content. You can visit Matt's website to view the video.]

 

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Great video! Good, simple yet scientific explanation.

In regards to the image that shows the mold gunk growing in the crazed glaze, I have to wonder if the clay body is fully vitrified, of if it is still porous and that is what is actually holding the moisture, not simply the cracks in the glaze.

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29 minutes ago, neilestrick said:

Great video! Good, simple yet scientific explanation.

In regards to the image that shows the mold gunk growing in the crazed glaze, I have to wonder if the clay body is fully vitrified, of if it is still porous and that is what is actually holding the moisture, not simply the cracks in the glaze.

Thanks Neil. 

It is a possibility that the body has some porosity, Although the maker is extremely experienced, and I trust them when say say the body is well fired (although I haven't run absorption numbers on it).  As the issue is really water/residue remaining behind if it is in the body or the crazes, does not seem like it makes a difference, to me. As trapped water is trapped water. 

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4 minutes ago, Matt Katz said:

Thanks Neil. 

It is a possibility that the body has some porosity, Although the maker is extremely experienced, and I trust them when say say the body is well fired (although I haven't run absorption numbers on it).  As the issue is really water/residue remaining behind if it is in the body or the crazes, does not seem like it makes a difference, to me. As trapped water is trapped water. 

I guess I was thinking strictly in terms of the moisture from the environment being the only source.  Maybe if the piece was washed and wasn't totally dried I can see it remaining in the crazes. But if it was totally dried (like in a heated cycle in a dishwasher), then I don't think it's as likely that the moisture from a humid environment would be taken into the crazing, whereas I could definitely see a porous clay body absorbing moisture from the air.

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You seem to be implying that adding lithium will reduce the durability of a glaze, is that always the case? I would think adding an extra metal oxide in the right quantity would reduce crazing and increase durability. I found good success using Spodumene in small quantities, not that I ever qualified any durability.  

It's a really good crazing video, the only thing I thought was missing is a little part about oxides and their expansion not just glaze as a whole but I am probably over complicating things. 

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My grandmother used to have an old pitcher that she never would allow to be used with milk, only water. When I asked why she said that the fresh cows milk would always sour very quickly in it, but she did not know why. Years later, I realized that there was a spider web of crazing all over the interior even though there seemed to be little crazing on the outside. I surmise that is why milk soured so quick, especially fresh dairy milk on a farm.

 

 

best,

Pres

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Good video Matt, all the chemistry needed for the daily potter.

The additions of silica and kaolin is the better resolution as you suggested. Silica (35.09) and EPK (50.06) lowers COE quickly as you noted, without the side effects of color shift or fluidity that comes from spodumene (lithium carb). Remembering that silica adds glass only while lowering COE, but kaolin has 37% alumina content: that  increases the alumina content quickly. I prefer NZ kaolin as a glaze fix over a EPK because of it's purity, and a EPK next because of it's finer grain (28.56 SAS) for a better melt. Larger grain particle kaolin are not well suited for glaze because they effect the melt.

The other issue is modern clay formulation that is based more in price point rather than function. Typically stoneware averages 5.50 Coe and porcelain around 5.75. It is simple enough to raise both by 0.50 to a full 1.00 in Coe without adding much costs. It is easier to fit glazes to higher COE clay bodies than it is to lower a glaze to fit a lower clay COE body. The final issue is assigning broad firing range to a clay body such as: "for cone 04 to cone 10". Clay is still expanding up to 2050F, and begins to close after that. The Coe of that clay body would be higher at cone 04 than it would be at cone 10. If the flux molarity of that body was formulated for cone 10 vitrification, at cone 04 at best it would only be "fused." 

Tom

edit note: you will find that the clarity of the glaze is much higher with NZ over a EPK.

Edited by glazenerd
Info added

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Thanks for posting the info re how much crazed glaze weakens a claybody. It corroborates other data I have read regarding this. I'm guessing the finer the craze pattern the weaker the claybody? Or would the thickness of the crazed glaze be more of a factor? 

Agree that adding silica and kaolin will work in cases where the base glaze can take the amount needed to fix the crazing and yet still melt properly. When that fix alone doesn’t do it I find swapping out some of the higher expansion fluxes, while keeping the silica:alumina ratio constant, for ones with a lower expansion will usually get you there. Yes, there can be colour shifts with alternate fluxes but just talking crazing here. The combination of fluxes used doesn’t always represent an accurate coe/cte figure as some fluxes have a symbiotic relationship with others in regards to lowering the crazing while the coe figure might still be higher than a target one size fits all coe for any given clay. (referring to fully melted gloss glazes here, not mattes)

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Matt:

i was reading some papers from Prof. Brownell (1960- Alfred U) on sulfur, sulfides, and sulfates in clay bodies. Is there a link to the archive at Alfred that I can access his works? Never mind, found the info.

TY.. Nerd

Edited by glazenerd
answered my own question..

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