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Spiral Crazing On Base


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Clear glaze on base of earthenware pots, two pots are absolutely fine and (so far) uncrazed, but this one (pic attached) has crazed in a spiral pattern on base and a little up the sides too. Is this down to poor application, throwing or just bad luck?

 

I'm not sure if it even matters as I see more and more potters' work than is crazed to some degree?

post-61816-0-82002200-1493210971_thumb.jpg

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I always thought spiral cracks were caused by not enough compression or not keeping excess water out of the bottom of the pot.  It looks like a nice piece of work to bad you have to remake it.   Denice

Just to be clear it's the glaze on the base that has crazed in a spiral, the clay body itself seems to be okay ; )

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Crazing might not be showing up yet but I think the other pots will show it in time. Glaze is either thicker on the ones showing crazing now or the other non crazed ones might have been in part of the kiln that fired a bit hotter. The craze lines showing now will likely fill in over time with craze lines roughly perpendicular to those lines. It’s one thing for crazing on a vitrified clay pot and another on earthenware which is typically quite porous. Moisture will soak into the clay and exacerbate the crazing. Craze lines can hold bacteria, the pots will leak and the strength of the pot is decreased. It’s a lot of trial and error work to come up with a glaze that fits without crazing but easier to toss test tiles than pots. To test for crazing apply glaze to test tiles made to about the same thickness as you make pots, heat them up in the oven to 150C (leave them in the oven for around 20 minutes to get heated all the way through) then plunge them into ice water. Repeat this 3 times and fingers crossed no crazing. If you are interested here is a short article on crazing and simple first steps to eliminate it. 

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I've made many an earthenware pot with crazed glazing in my time, but never one with that pattern! Very well done!

I can only assume that the crazing has followed some stress lines in the body, but that's just a guess.

If the glaze is prone to crazing, you'll probably find that the other pots follow suit shortly. Is it a new glaze, or perhaps a new body? Have you tried stressing the glaze with boiling water?

I think every single (glazed) earthenware pot in my possession is crazed - new, old, it doesn't matter; some of those are made by very well known potters, as well as by me.

Do I care? No. Is my opinion universal? No.

 

 

 

Crazing might not be showing up yet but I think the other pots will show it in time. Glaze is either thicker on the ones showing crazing now or the other non crazed ones might have been in part of the kiln that fired a bit hotter. The craze lines showing now will likely fill in over time with craze lines roughly perpendicular to those lines. It’s one thing for crazing on a vitrified clay pot and another on earthenware which is typically quite porous. Moisture will soak into the clay and exacerbate the crazing. Craze lines can hold bacteria, the pots will leak and the strength of the pot is decreased. It’s a lot of trial and error work to come up with a glaze that fits without crazing but easier to toss test tiles than pots. To test for crazing apply glaze to test tiles made to about the same thickness as you make pots, heat them up in the oven to 150C (leave them in the oven for around 20 minutes to get heated all the way through) then plunge them into ice water. Repeat this 3 times and fingers crossed no crazing. If you are interested here is a short article on crazing and simple first steps to eliminate it. 

 

 

Thanks folks. So far the other two pots seem completely craze-free, so I'll keep fingers crossed. The glaze is a pre-mixed powder from reputable supplier, the clay a top-quality 'professional' earthenware, so I've been trying my best to eliminate problems (I think). I'm prob going to bisque to higher temp next time and see how that works out. 

 

Just looking at my collection of others' earthenware pots and yes, most are crazed to some degree.

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Guest JBaymore

A glaze either fits the body or it doesn't.  To "fit" the COEs (or COTE .... or CTE) of the clay body and the glaze need to be close enough so that the tensile strength of the glass produced by the glaze formula is not exceeded.  COE is abbreviation for "Coefficient of Thermal Expansion".  Note that this "thermal expansion" in this case is NOT directly related to the large amount of permanent firing shrinkage that a clay object goes thru when fired. 

 

I also like to add the word "reversible" in that title for clarity of concept: "Coefficient of Reversible Thermal Expansion".

 

All materials expand a bit when they are heated and contract when cooled.  In the kiln, once a glaze has "frozen" after the melting process is completed.......... it can be considered a "solid" (even though is really is not exactly that).  The clay body is a solid (even though it has some glassy phase within it).  Each of those things will shrink a certain percentage of their linear size as the kiln cools. 

 

If the clay shrinks LESS than the glaze, and hence you could also say the glaze shrinks MORE than the clay, as the object heads toward "room temperature" it puts the glaze in tension.  It "pulls" on the glaze layer.  If this pulling exceeds the ability of the glaze to resist... it cracks to relieve the tension.  This is crazing.

 

Crazing is when the glaze can't "resist" the tensile pull.  Where the glaze is thinner in application... the is not as much glaze "pulling" on the rest of the glaze layer.  But where thicker....... it can reach that, "I canna' hold er together Cap'n.  She's breakin' up" point.  And the thick areas show crazing.  This tells you that the glaze does NOT fit the clay body... and delayed crazing will happen on this object.

 

If the fit between the glaze and the clay is CLOSE... but not close enough... you get what is called "delayed crazing".  This is displayed when the pieces come out of the kiln looking just fine without any crazing, but in use, the crazing develops.  This is because the glaze is sitting there in tension close to its breaking point.  Heating and cooling in daily use or just from atmospheric temperature changes finally causes the glaze to say "enough!", and let go.  Cooking and food use is a great way to stress the pieces to have them let go.

 

I've seen this "spiral" business on the feet of earthenware before.  And yes, it is showing the radial stresses in the forming of the wet clay piece... as the fired clay "remembers" some of the alignment of the clay particles that happened during forming.

 

best,

 

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

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A glaze either fits the body or it doesn't.  To "fit" the COEs (or COTE .... or CTE) of the clay body and the glaze need to be close enough so that the tensile strength of the glass produced by the glaze formula is not exceeded.  COE is abbreviation for "Coefficient of Thermal Expansion".  Note that this "thermal expansion" in this case is NOT directly related to the large amount of permanent firing shrinkage that a clay object goes thru when fired. 

 

I also like to add the word "reversible" in that title for clarity of concept: "Coefficient of Reversible Thermal Expansion".

 

All materials expand a bit when they are heated and contract when cooled.  In the kiln, once a glaze has "frozen" after the melting process is completed.......... it can be considered a "solid" (even though is really is not exactly that).  The clay body is a solid (even though it has some glassy phase within it).  Each of those things will shrink a certain percentage of their linear size as the kiln cools. 

 

If the clay shrinks LESS than the glaze, and hence you could also say the glaze shrinks MORE than the clay, as the object heads toward "room temperature" it puts the glaze in tension.  It "pulls" on the glaze layer.  If this pulling exceeds the ability of the glaze to resist... it cracks to relieve the tension.  This is crazing.

 

Crazing is when the glaze can't "resist" the tensile pull.  Where the glaze is thinner in application... the is not as much glaze "pulling" on the rest of the glaze layer.  But where thicker....... it can reach that, "I canna' hold er together Cap'n.  She's breakin' up" point.  And the thick areas show crazing.  This tells you that the glaze does NOT fit the clay body... and delayed crazing will happen on this object.

 

If the fit between the glaze and the clay is CLOSE... but not close enough... you get what is called "delayed crazing".  This is displayed when the pieces come out of the kiln looking just fine without any crazing, but in use, the crazing develops.  This is because the glaze is sitting there in tension close to its breaking point.  Heating and cooling in daily use or just from atmospheric temperature changes finally causes the glaze to say "enough!", and let go.  Cooking and food use is a great way to stress the pieces to have them let go.

 

I've seen this "spiral" business on the feet of earthenware before.  And yes, it is showing the radial stresses in the forming of the wet clay piece... as the fired clay "remembers" some of the alignment of the clay particles that happened during forming.

 

best,

 

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

 

Thanks John. So far although three pots were same clay and glaze and on same kiln shelf, just checked again and only this one shows any sign of crazing!

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