Bisque Firing

Why we Bisque:   Bisque firing is done to change the “bone dry” greenware into a different, better type of ware suitable for glazing.  The greenware is heated to 1728 F, during which we:

  1. Burn away organic material, vaporize trapped water, and drive off chemically-bonded water, all of which can disrupt the glaze surface by pushing out gas/ vapor during the glaze firing.
  2. Lighten and sinter the piece to improve handling for glazing.
  3. Increase porosity to allow glazes to absorb into the piece.
  4. Normalize the ware to allow more consistency in the firing results.

Pink Color: Part of the reason clay has a brown color is due to organic materials in the body. When these burn away during bisque firing, only the base mineral and oxide colors are left.  Aluminosilicates are white and iron oxide is red; these mix to create the light pink color of bisqueware.

Venting : As organics burn away during the firing, toxic fumes are generated: particularly sulfur dioxide and carbon monoxide. That’s why bisque firing is done in a separate room with the door closed, and with our exhaust fan running.

Trapped Water :     Even in a bone dry piece, some water remains trapped between the clay platelets.  Trapped water vaporizes at 212 F, and escapes from the ware as steam. The thicker the ware, the more the chance that moisture can be trapped deep inside. If the deep steam can’t escape without building up pressure, the piece can violently burst (Bisque explosion).  Be sure your ware is Bone Dry before placing it onto the Bisque shelf.

Oxidation Firing: Bisque is done in an “Oxidizing” atmosphere (that is: heated with plenty of ordinary air present) to burn away organic materials:

Generally, the Skutt kilns are used because electric heat creates a purely oxidation atmosphere, the kilns are programmed for automatic firing, they are small enough to allow for frequent firing, and can be quickly loaded and unloaded.

Stages of a Bisque Firing:                                                                                        Temperature ______________Events Occurring__________________

75 to 300 F ………Water Smoking: Pore water is converted to steam and driven off.
200 to 400 F        Burning of organic matter.
850 to 1100 F       Water is decomposed from the clay molecule.   (see below)
~ 1060 F               Quartz Inversion:  2% size change due to structure of silica molecule.
400 to 1650 F       Burning of all impurities (carbonates, sulfurs) driven off as gas/vapor.


Bisque permanently alters Chemical Structure of Clay: In the tempeature range of  840 – 1110 F, clay breaks down into a dehydrated aluminosilicate known as Metakaolin, and free water:

 

Metakaolin’s characteristics compared to clay:
– Overall weight loss of 13.95 %
– Hexagonal platelet shape retained.
– Reduction in platelet size.
– Loss of plastic properties.
– Color change to light pink.
– Increase in ware strength.
– Increase in porosity.
.
 

Bisque Strength: Bisque is stronger than greenware because the tips of the clay platelets melt and weld (sinter) to one another as illustrated below:

 

 

 


 


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