I threw a chip and dip set.  I used a buff clay from Highwater.  I fired to cone 04.  The two pieces went through the bisque firing fine.  I used a combination of Amaco Potters Choice glazes which are very stable glazes.  I fired to cone 5.  When I unloaded the kiln, the larger piece had a large crack down the middle.  What would cause that?  Any help would be much appreciated.  Thanks so much!!!!

Views: 105

Reply to This

Replies to This Discussion

Bisqued ware fired to ^6 which cracks, with or without a glaze, is usually relieving stresses, from throwing or hand forming, as the clay becomes more plastic at temperature. Photos of the crack and where it is on the bowl would be helpful for others to suggest a cure.

Highland Clay does not post COE numbers for their clay, so it's just a guess to think there's a COE mismatch between glaze and clay.  But I think this is unlikely to be the problem just based on the way your describe the damage to the body rather than the glaze.

Norm,

Thanks for responding.  I showed the piece to someone locally and they said it was because the clay was marbled.  It cracked where the two types of clay were joined.  I recycle clay all the time and that was the first time that happened.  Thanks again!

Mixing clays with two different COE or shrink ratios can certainly build up enough stress in the kiln to develop cracks.  Sometimes even before firing this stress can develop cracks.

I think you found your problem.

Reply to Discussion

RSS

Videos

  • Add Videos
  • View All

Use These Links to Support Us

Low cost flat lapping disc can be used on you potters wheel if you, drill bat pin holes in it, and provide a trickle of water to cool it. At amazon.com, 120 grit for aggressive material removal. Click the image to purchase 

Members have had great things to say about John Britt's new book, Mid-Range Glazes. Click the image to buy from Amazon.com

Purchase Glazes Cone 6 by Michael Bailey, The Potters Book of Glaze Recipes by Emmanuel Cooper, or Making Marks by Robin Hopper, all available at amazon.comMastering Cone 6 Glazes by John Hesselberth & Ron Roy is now out of print.

Harbor Freight is a great place to find unbeatable prices for better HVLP spray guns with stainless steel parts and serviceable economy models, as well as detail guns, all tested by our members for spraying glazes, as well as compressors to power the guns. As yet no one has tested and commented on the remarkably inexpensive air brushes at harbor freight.

The critter siphon gun is a spray alternative that is well liked by some of our members, and is available at amazon.

Amazon is also a competitive source for photo light tents for shooting professional quality pictures of your work. They also have the EZ Cube brand favored by several of our members. You might also want to purchase the book Photographing Arts, Crafts and Collectibles . . .

If you are up to creating videos of your work or techniques you might want to invest in a flip video camera

Following are a few scales useful for potters. Your final price could be less or more - things change.

American Weigh Black Blade Digital Scale, 1000g X 0.1g $11.08 

For the non-digitally inclined the old standard Ohaus Triple Pro Mechanical Triple Beam Balance, 2610g x 0.1g, with Tare $169.00

And finally a low cost clone of the OHaus. The Adam Equipment TBB2610T Triple Beam Mechanical Balance With Tare Beam $99.62

ebay is a great alternative for many tools and the equipment used in the ceramics studio - kilns, wheels, extruders, slab rollers are often listed there both new and used.

Tips for Members

If you just want to spout off, it is best accomplished as a blog posting. If you want to get more guidance and ideas from other members, ask a question as a new discussion topic. In the upper right corner of the lists for both types of posting, you will find an "+Add " button. Clicking it will open an editor where you create your posting. 4/16/2014

© 2021   Created by George Lewter.   Powered by

Badges  |  Report an Issue  |  Terms of Service