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Refiring Glaze - Food Safe?


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I have a nice size bowl ( 12" wide, 5 inches in height).  Bisque fired to cone 04, glazed fired to cone 5, commercial glazes ( amaco potter's choice)  I liked the shape, the glaze turned out great.... but after the glaze firing there was a noticeable crack on the bottom, inside the foot.  The crack did not go through to the glaze on the inside of the bowl.  I have repaired bisque ware before with paper clay....  so I thought I would try it.  I refired at cone 04. My thinking was i just wanted to get the paper clay to the bisque stage but not risk defects on the surface of the glaze.  The paper clay worked great.  The glaze color was much subdued but still beautiful. A little bit of a gold sheen in the glaze. The glaze surface was fine.  The commercial glaze is food safe when fired to cone 5. Should I worry about a difference in the food safe quality of the glaze because of the retiring to a lower temp.?  The bowl is certainly large enough that no one is going to eat cereal out of it... but I am hesitant to sell it with my other functional work?  what do you think?

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As it was fired to the recommended temperature, the first time, the second, lower firing, doesn't undo that.    Remember, if a commercial glaze, lists itself as "food safe", it generally means that it won't leach materials into food/ drink.

 

However, if the glaze crazes a bit, liquids will seep through.  If the clay body is vitrified, this isn't an issue.  but the portion you patched with the paper clay, is not vitrified.  So liquids would seep into those portions, creating an area that mold/ bacteria can grow.

 

Also, even though the crack is patched, it's still there structurally.  If that bowl is heated an cooled, in a microwave or dishwasher, it could get worse.

 

So, basically I'm saying, that I'd just sell it as decorative.  Tell customers they can put fruit in it, or candy, but advise against anything much more than that.

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Guest JBaymore

My take on this is ...... if you have to ask.... don't sell it.

 

"Right on" about the concerns about the "it is patched" aspect.

 

The problem with selling pieces that you indicate are "decorative" or "not for food use" (but look like they COULD be) is that the instructions at BEST only go with the first purchaser.  Hang tags get lost, pieces get given as gifts.  Memories fade.  Stuff gets inherited.  Pieces go to thrift shops and flea markets.  The INFO does not go with it.

 

There is a reason that the FDA requires FIRED ON documentation for lead glazed pieces that are "decorative only".

 

Make another one...... it'll be better and you'll sleep easier.  And it won't come back to bite you later.  Or "bite" anyone else.

 

best,

 

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

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I agree with John, but if you like the piece, fix it, and use it at home.  I have some bowls that we use at the house from earlier years, no cracks in bowl, but cracked underneath. If I tried to remove them now, it would be grounds for divorce as my wife loves them, and uses them to mix up all sorts of cookie dough at Christmas time and uses them for marinating at other times. They really won't be a problem for your home use if not cracked through.

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