Start glaze chemistry by learning when you don't need it! A high feldspar glaze is a good example

Wednesday 16th October 2013

This reduction celadon is insight-live.com/trouble/2">crazing badly. Why? Its chemistry is dominated by feldspar, which supplies lots of K2O and Na2O fluxes. Scores of recipes being traded online contain 50% or more feldspar; they benefit from these powerful fluxes but inherit their high thermal expansion (COE). Many work only because the body happens to have sufficiently high expansion - often aided by significant residual quartz.

Crazing is more than a cosmetic problem in functional pieces. It reduces ware strength and durability, can compromise food safety and turn into a customer complaint. Let's fix it, starting without chemistry:

• Use recipe logic: Transplant the color mechanism into a transparent base glaze that already fits the body. In this case, the celadon color is largely produced by an iron addition under reduction, so there is no need to preserve the high-feldspar base itself.


• Use material sense: A 2:1 silica:kaolin mix supplies an Si:Al ratio similar to typical glossy glaze chemistries. A 10% addition thus shouldn't change the degree of gloss but will likely lower the COE, reduce melt fluidity and improve durability.


• Use glaze chemistry. Substitute some of the high-expansion KNaO with much lower-expansion MgO. Both are fluxes, and even a 0.1-mole shift can make a significant difference. Couple that with an increase in SiO2 where the melt can tolerate it, and glaze fit can improve dramatically.

Pages that reference this post in the Digitalfire Reference Library:

Feldspar, Na2O, No glaze chemistry needed At least not right away, Craze city: Pure Feldspar and Nepheline Syenite, High feldspar glazes craze on functional: Don't put up with it. Fix them., A high feldspar glaze is settling, running and crazing. What to do?, Cone 10 mug is crazing after a year. That's OK because it's high-fire, right?, ChatGPT was completely wrong about the cause of glaze crazing in 2023!, Glazy, Limit Recipe, Calculated Thermal Expansion, Celadon Glaze, Glaze Crazing

This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.