cracked bisque -- will it break with ^5 firing?

[Susan Herre]

Please see the crack near the bottom of three sides of this piece.  The crack occurred with bisque firing (I am in a group studio.)  I think perhaps the walls were too thin. . .??  Can anyone suggest other reasons for the crack? 

The clays are Laguna (WC871 Calico and WC401 BMix-5) Cone 5.  I would like to clear glaze the piece and fire it at Cone 5.  Do you think the crack will break open / break the piece completely? 

Thank you for your input. 

[liambesaw]

I think it will probably break if you fire it again that's quite the crackitude.  Doesn't hurt to try though!  

[glazenerd]

Hi Susan:

To answer your question: yes the crack will get worse in a glaze fire. The added contraction/compression exerted when the glaze cools will ruin these pieces. 

Quartz Inversion: do a forum search on this topic: it is the most likely cause of these cracks during the bisq fire. If you noticed, the crack extends from the corner and upward on your sample pics. When pieces have surface contact with a shelf: this creates drag, which in turn fractures the piece because it does not move. Placing sand, wadding, or suitable material under these pieces allow them to move during firing. Quartz inversion occurs at 563C, slowing the kiln down 50 degrees before, continuing 50 degrees after this temperature is also necessary.

T

[Susan Herre]

Thanks for your comment. 

I understand that our group studio does this: Bisque fire (06) overnight for 12 hours.  Because of the conversion chart attached, I assumed this is 1828 or 1855 F. 

Is the 563C that you mention Celsius?  = 1045.4 Fahrenheit.  Is there a range of temps considered "bisque firing?"  Thx

 

[Susan Herre]

I see... Quartz Inversion

After dehydration, the next change that happens during the firing process is Quartz Inversion, which happens at 573 degrees C (1064 degrees F). At this point, quartz crystals rearrange themselves into a slightly different order. A slight and temporary increase in volume occurs at this point. This is why you always need some space around pieces during firing, as they will expand somewhat. Firing should proceed slowly during this Quartz inversion. A large percentage of ware that is cracked during the clay firing process happens from fast firing through this stage. The factory set program on electronic kilns usually slow down the firing at this stage for you.

[Mark C.]

You could try amaco's bisque fix but that crack in my brain is going to open more not less due to the form. Whats to loose?? trying

[Babs]

Mark please don' use bisque fix for that crack in your brain!!!!

 

[Bill Kielb]

6 hours ago, Susan Herre said:

I see... Quartz Inversion

After dehydration, the next change that happens during the firing process is Quartz Inversion, which happens at 573 degrees C (1064 degrees F). At this point, quartz crystals rearrange themselves into a slightly different order. A slight and temporary increase in volume occurs at this point. This is why you always need some space around pieces during firing, as they will expand somewhat. Firing should proceed slowly during this Quartz inversion. A large percentage of ware that is cracked during the clay firing process happens from fast firing through this stage. The factory set program on electronic kilns usually slow down the firing at this stage for you.

While quartz inversion happens, it really always happens. Ideas about slow warm up to 250 degrees to absolutely dry something are  very valid. With respect to quartz inversion it happens at a  temperature but the pots in the kiln are certainly not exactly the same temperature from top to bottom so parts are inverting and other parts are not. Talk about stress! So lots and lots of wares go through quartz inversion every minute of the day from firing speeds of 200 degrees per hour to 570 degrees per hour.

use common sense sculpture work rules and construct everything well, including reinforced joints etc...  in the picture the corner joint appears to overhang a bit in one of the views and likely was a source of origination. 

 

[Bill Kielb]

3 hours ago, Susan Herre said:

Thanks for your comment. 

I understand that our group studio does this: Bisque fire (06) overnight for 12 hours.  Because of the conversion chart attached, I assumed this is 1828 or 1855 F. 

Is the 563C that you mention Celsius?  = 1045.4 Fahrenheit.  Is there a range of temps considered "bisque firing?"  Thx

 

To answer your question common bisque temperatures are cone 04 - 05, however folks can bisque several cones up or down for artistic reasons. A bisque firing needs to permanently dehydrate your work which means it cannot go back to being clay by adding water, the chemical H20 has been removed. Additionally the bisque firing needs to burn out any organics and impurities in your clay so bisque firings generally take longer than glaze firings to assure all organics have been effectively removed. Finally the firing should firm up your work while still allowing it to be porous enough to accept your glaze. 

Folks that sculpture sometimes glaze fire to lower temperatures like cone 2 or 3 to preserve the colors of their underglaze. They sacrifice some ultimate strength as the final product will not be fully vitrified in favor of preserving their colors. Many underglazes and stains change color the higher they are fired.

It is a great piece so two ideas strike me

• Little risk - Leave it as is, infill with epoxy artistic touchup or not and paint or seal with some clear product. (Urethane, acrylic …..) It will still not be as strong as fired to a high temperature
• More risk - Fire it to some lower cone to gain some strength which also allows you to try the bisque fix with a little less risk of the crack getting much larger and you could try glazing with a low fire glaze after artistically hiding the crack.  If it makes it through this but the crack is still visible then epoxy and paint are still a possibility on a stronger piece.

Best of luck!

[Susan Herre]

many thanks for your message!  

[glazenerd]

Just now, Mark C. said:

You could try amaco's bisque fix but that crack in my brain is going to open more not less due to the form. Whats to loose?? trying

In 1894, after Taxtile Doat opened his three story bottle kiln : he held a vase up to his aide and exclaimed: " crack pot." 

This history lesson bought to you by: NoFixin ..cement based porcelain clay. 

back to the serious side: yes bisq fix is possible. Still need to put sand under the pieces so they can move. Fire on slow.  The more you can control movement, and the slower expansion/ contraction occurs: the higher the chance of survival. Actually I would program a cool from 2000 down to 1000 @ 100F an hour. 

[Gabby]

It's a very cute piece, Susan.

I have a large, funky-looking bisqued piece in my basement, also hand-built and which I call "the tote bag."  Mine has a maybe half inch vertical crack at the top, on a seam. 

It is not intended as an eating vessel or vase or anything for which a crack would present a hazard.

So my plan is to glaze and fire it and see what happens.  I will measure before and after this experiment.

If you measure yours before and after we can compare notes.

My plan beyond experiment is to patch it up at the end if I can.

 

[Denice]

If I have a large crack in a piece like that I would make a new one using that one as a model,  it usually comes out much better the second time around.    I would also do some clay bracing inside of it especially on the lower half.   You are asking that clay to support a lot of weight from the top half.     Denice

[Magnolia Mud Research]

Incorporate the crack into the design of the sculpture and move on with the piece.  

 

when making your next piece,  maybe a reengineered version of the one that cracked, improve the construction to avoid the stress concentration in the area.   

LT

[Susan Herre]

thanks for all the comments.  I really appreciate this forum.