Band-aid solution #4 for crazing: Add silica to the clay body

It is not usually practical to adjust the body because most people use prepared clays. But if so, it is true that adding coarser sizes of high-expansion silica have the end-effect of compressing the glaze (because quartz particles contract 1% when going down through quartz inversion around 570C). Supposedly the added silica is not dissolving into the aluminum-silicate crystal body structure that the feldspar is creating and can impose its higher expansion on the surrounding matrix. Vitrified ceramics are brittle, does it sound good if individual silica particles are playing their own game, not expanding/contracting in harmony with their surroundings? Adding silica must be done at the expense of other materials in the recipe: If the clay component is reduced, the body will be less plastic; if the feldspar is cut vitrification is lost. This not only sacrifices strength but breeds another thermal expansion misfit: Cristobalite, a crystal phase of silica. Cristobalite suddenly expands/contracts on heat/cooling through 225°C, it actually develops within the body during firing when feldspar is lacking. Your firing and clay body may not be so scientifically balanced as all this, but it may still take a lot of silica to fix the crazing and temperature gradients that exist in firing bodies create expansion gradients that move like waves through the ware during cooling, causing dunting.

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