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Mark369

What Made Plates Break?

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Mark369    6

4 of 5 plates in 08 firing Broke ???  Why????  

 

Plates where purchased bisque, glazed with commercial 06 clear fired ok.

Color glaze and under glaze applied and re-fired to 08 and they broke!   Any clue???post-62706-0-52798000-1400876170_thumb.jpgpost-62706-0-07999800-1400876213_thumb.jpgpost-62706-0-55965100-1400876235_thumb.jpg

post-62706-0-52798000-1400876170_thumb.jpg

post-62706-0-07999800-1400876213_thumb.jpg

post-62706-0-55965100-1400876235_thumb.jpg

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Mark369    6

Here is the fourth plate. post-62706-0-90588300-1400876443_thumb.jpg

 

The one that did not brake was the top plate in the kiln. 6 hours to fire. Let it cool 48hr.

post-62706-0-90588300-1400876443_thumb.jpg

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Wyndham    98

Refiring too quickly will crack/break/ screwup  pieces.

You've got too much heat too quickly and the fired piece can't relieve the stress the heat puts on the piece. There is a temp range between 900-1200 deg f that the glaze.glass and the claybody go through quarts inversion. This is the critical temp but there are other temps lower to consider.

Wide plates & uneven temp will also bust'em up

Long and slow 200 deg/hr to 900 deg then 100 deg/hr till 1200 then back t0o 200deg/ hr to end for refires and still you may loose some.

You don't know how the bisk was made or fired so there might also be issues there as well.

Chalk it up to learning.

Wyndham

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Pres    896

The others have pretty much nailed it I believe. You need to slow your firing time down.

 

I did take a minute to edit your title so that the search engine would have an easier time with it in the future.

 

Best,

Preston

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Mark369    6

I will try again and slow the firing time way down. I will update next week when done.  Thanks  I figured it was a stress issue, but not sure what.  Makes sense now.  I will be making my own plates soon with a hump mold. 

  Tired of learning in this manner. But I guess it will stick with me more profoundly.

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Babs    386

 

  Tired of learning in this manner. But I guess it will stick with me more profoundly.

There has been a lot written on firing schedules and the reasons for them. If you read up on these it would be a mere start only.... The kiln is a hard teacher, stuff up a little, change something and it will get you! Guys here who have been in the field for decades would be first to say there are no absolutes in this field .

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Mark C.    1,807

(Tired of learning in this manner)

Well thats the way with ceramics-experience trumps all other forms

Making mistakes often is the only way other than taking some advanced courses and even then ceramics will bite you. Its a long curve no matter how you slice it. Its lasts your whole life-trust me on this one.

Mark

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Mark369    6

Slowed 08 firing down to 11 hours and put plugs in peep holes when shut off to slow cooling down.

3 plates broke and a bowel cracked one plate make it.

 In looking at them I noticed the bowel cracked in the base and not on edges.

I think it is Dunting during cooling (sharp edged crack) from not being glazed the same on both sides.

Will try again next week and see if that corrects it.   This is getting costly time wise.

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Min    784

I think you have a couple things going on here. So just to clarify, you use bought bisque, glaze fire to ^06, apply underglazes and (coloured) glazes and refire to ^08. You put the plugs in when the kiln shuts off.

 

From looking at the star pattern of cracks on 2 of the plates plus the chunks of rim cracking off my thought would be you have too thick a glaze layer plus the kiln is cooling to quickly resulting in dunts.

 

You didn't say how long your kiln takes to cool down to approx a couple hundred degrees? If you have a rapidly cooling kiln then a firing down program might be necessary through the inversion temp zones.

 

You can put the plugs in the kiln once it is around 1100 (orange glow). You don't need to wait until the firing is complete.

 

just my 2 cents worth

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oldlady    1,323

dumb question..............why are you first glazing to 06 with a clear glaze at all?  why are you not just using underglaze and covering everything with clear and firing it together?

 

(I haven't done low fire work in 30 years so I do not know if this is normal today)

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Babs    386

I'm with you on this one Old Lady, only ever fired the underglaze and glaze together unless underglaze applied to raw ware.

What do the edges of the broken ware look like? Sharp?

If they are coming thro fine with te commercial clear glaze, why not try the underglaze, glaze and then fire routine, still require a slow firing, at least till equivalent of approx  600 d.Centigrade

In a glaze firing I put bungs in after wax is burnt off...

Do you know the clay body they are made of?

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Mark369    6

The edges of cracks are sharp.  The Bisque ware is from China. Who knows how it is fired or what is in it. It is supposed to be a substitute until I can make some plates.

Putting the under glaze and design glaze  over clear is easier to correct mistakes in application than on raw bisque. Putting the clear glaze over red glaze was burning out the red on smaller items. Clear glaze is poured on. Kiln takes about 16 hr to cool down to touch ware. First time using under glazes. Have now read up on proper application.

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Babs    386

Sorry thought the red was an underglaze. Could apply underglaze, fire to  06 to set then glaze for the first time and fire.

May have to consider firing down.

ANy manufacturing instructions for firing this ware?

Any local press molded stuff you could run a comparison to?

Could mention cheap imports :) but I wont :ph34r:

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alabama    144

Hey,

     It looks (in my limited experience in this) that the clay body of the Chinese plates just failed.  We had students throw cone 10 Amaco clay from

Hobby Lobby fail somewhat in this same manner.

     I've had some success going to Thrift shops, buying 25 cent white china plates, decorating them with cobalt cone 06 underglazing and firing to cone 06.

When things go right the underglaze fires to the bright shiny glaze its suppose to. 

     Plates and platters seem suicidal and look for a reason to crack. 

Hope this helps,

Alabama

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