are those of samples taken as indicated above the skilled person will be aware that the proportion of zirconia tends to be greater at the bottom of the block because of segregation in the block the characteristics of the microstructure were evaluated over an area of 64 mm 2 to the rear of the saber cut area level of the free surface of the glass all zirconia crystals whose surface area is greater than 640 u03bcm 2 were regarded as free zirconia crystals zirconia crystals with a surface area less than 640 u03bcm 2 were encountered only at the edges of the eutectic areas they were very few in number and had no major influence the characteristics of the microstructures given in table 1 relate only to free zirconia crystals t is the total number of free zirconia crystals counted over the area studied d is the total number of dendritic free zirconia crystals for which l i is therefore greater than 5 d300 is the number of dendritic free zirconia crystals longer than 300 u03bcm d500 is the number of dendritic free zirconia crystals longer than 500 u03bcm ic is the corrosion index the index 100 is that of the reference product which was our er 1681 product eutectic crystals of the materials according to the invention had different morphological characteristics to the reference products the fig5 microphotographs show that the appearance of eutectic crystals is close to that of free zirconia crystals one dimension of these crystals is generally very much greater than the other which gives an elongate appearance the values of the corrosion resistance indices and the fig2 graph show clearly the general tendency of the materials.