for slip casting in plaster of paris molds using traditional slip casting techniques articles were formed by pouring the slip prepared as above in the plaster of paris molds and allowing suitable time for the molds to absorb the water the cast parts were then taken from the mold dried and fired the average strength of these articles with 72 void volume was approximately 4000 psi in general all the physical properties were the same as those described for the pressed material described in example 1 a highly porous zirconia toughened mullite article was prepared by mixing 38 4 weight percent zircon zirconium silicate 44 6 weight percent alumina with 15 weight percent water 1 weight percent nitric acid 1 weight percent pliable hollow polymer spheres and 0 01 weight percent of a defoaming agent the amount of water added was sufficient to produce a slurry suitable for slip casting in plaster of paris molds using traditional slip casting techniques articles were formed by pouring the slip prepared as above in the plaster of paris molds and allowing suitable time for the molds to absorb the water the cast parts were then taken from the mold dried and fired the articles formed were composed of approximately 70 void volume space in another instance solid substantially non.