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What A Crack! ( In Pottery )


ronfire

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I have an order for a crock pot lid about 13". Well my first attempt came out well but did not shrink as much as planed and was a ¼" to big when finished. Back to wheel again with the new calculations and after the bisque fire was ¼ to big so I was happy that it should fit when done. 

The good news is that after the glaze firing it would have fit.

The bad news is that it cracked. Not just cracked but found that the lid cracked strait across side to side, no spiral crack. The 2 pieces where spaced 1 ½" apart on the 1 piece shelf at the bottom of the kiln. The break must have happened on the heat cycle as the glaze did not have a sharp edge but how and why did it separate so much. Wish I took a photo of it in the kiln

My thinking is that the weight of the lid did not allow it to slide on the shelf during the glazing but the gap is much larger than the shrinkage was. There was no glaze on the bottom of the lid and was about ⅜ thick with a ½" rim on the inside of the lid.

 

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In future either say 'No" or when making crock pots make a few extra lids at the time of making a batch.

Think ther ewas a poter who even sold spare lids for vessels at he time of initial sale.

if Mark ever gets down out of his tree, he will have a solution for sure!!

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I would never make a lid for an existing pot, because as you're finding out, it's not worth the hassle. You've already had 2 failures, which means at best your profit will be 1/3 of what you quoted the customer. In the end you could end up basically doing it for free. I would also never make a lid for a pot that I didn't make in the first place. You have to decide whether you want your customers to by YOUR pots, or if you're just there to make what they demand. Would you ask Warhol to replace your damaged Picasso?

 

As for why the lid cracked, big wide lids like that love to crack. You're probably right that it wasn't able to shrink the way it wanted to, or it heated unevenly. 3/8" is a little thick IMO, so that may have contributed as well. Try putting some silica sand on the shelf next time. It acts like little ball bearings and allows the piece to move.

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That is the problem with having friend, hard to say no to. Think I will try one more time, hate to fail. I will have to build a pot for the first lid now, I have the original size so the new pot should fit the lid ( I hope ). Might start a vicious circle   <_<

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That is the problem with having friend, hard to say no to. Think I will try one more time, hate to fail. I will have to build a pot for the first lid now, I have the original size so the new pot should fit the lid ( I hope ). Might start a vicious circle   <_<

Might?? That horse has already left the barn!

(Could not resist a horsey quote!)

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Mark, when he gets out of his tree will tell you to never take custom orders. Something always goes wrong. You have to make three of everything and MAYBE you'll get one that fits. Go stand in front of a mirror and practise saying "no". Feels great, doesn't it?

TJR.

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I have a old crock pot lid with cobalt trim on my fence-its a custom order from 1974 that went sideways--just like yours-1st one did not fit

The 3rd one did after the second one cracked-I made minus 25$ at that time in 1974.

No is alot easier to say -and you will be so much ahead at that point. You can learn this the easy way or the school of hard nocks way.

Me I learned the hard nocks way-right now you have a choice-hey you want my lid off fence?I keep it there as a reminder of how to say no.

I said no this week to an 2 gallon special crock order-saved about $200 right then.

Mark

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If you are really insisting on doing this. . . 

 

A 13" lid is quite a problem to fire flat. Seems like the shrinkage stresses and movement in the kiln are going to be quite a bit, so first I would do as Neil says. Another solution that may seem like extra work is to throw a collar/rim for the pot to be bisque and glaze fired on. This would give even support on the places where the lid should really be supported-outer edges with no support on gallery base. Hope you are also allowing a little extra thickness at the outer areas of the lid.

 

best,

Pres

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There's also a possibility that there was a hairline crack in the lid before it went into the kiln, that widened.

I'd be inclined to think

A) knocked in the green or bisque state

B) dried unevenly, specifically too fast around the edges like in a DFAC stress test http://digitalfire.com/4sight/education/the_black_art_of_drying_ceramics_without_cracks_196.html

C) heated unevenly

D) any combination of the above.

 

 

And that should be a "B", not a sunglasses dude.

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I have a old crock pot lid with cobalt trim on my fence-its a custom order from 1974 that went sideways--just like yours-1st one did not fit

The 3rd one did after the second one cracked-I made minus 25$ at that time in 1974.

No is alot easier to say -and you will be so much ahead at that point. You can learn this the easy way or the school of hard nocks way.

Me I learned the hard nocks way-right now you have a choice-hey you want my lid off fence?I keep it there as a reminder of how to say no.

I said no this week to an 2 gallon special crock order-saved about $200 right then.

Mark

Mark;

I am with you. Sadly, I had to learn it the hard knocks way. Three casseroles with lids. Third one finally fit. I don't even know why I attempted it. I don't make casseroles.

TJR.

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