Low Expansion Base Glaze Development

This is the process I followed to make my current clear / base glaze.

This is why I got into glaze mixing and glaze chemistry in the first place. The studio clear glaze and some random recipes I found online would craze and blister on my porcelain bodies that was otherwise quite enjoyable to use.

The goal is to make a well melting low expansion clear glaze. The clear glaze can also be used as a base glaze and should accept colorants well. This means that certain fluxes are off the table as they negatively affect the colors of some stains and oxides.

MC6 Glossy Liner

This glaze comes from Mastering Cone 6 Glazes by Hesselberth and Roy.

• The now discontinued G200 Feldspar is replaced with Custer Feldspar
• The wollastonite brand is Vansil
• The talc brand is AmTalc

• Silica-Alumina Ratio says this glaze should be glossy, but it isn't
  • Surface is matte and bumpy
  • Glaze is not clear
  • The culprit is likely talc
    • Too much magnesium can matte glazes

Talc does not melt easily and its magnesium can decrease gloss. The specifics may depend on your brand of talc (particle properties, wollastonite contamination, etc). The overall magnesium content (0.15 MgO) is also somewhat high - I read somewhere (note to self: find out where) that a MgO level above 0.1 begins to noticeably matte glazes.

The good news is that this glaze does not craze on cone 6 porcelains.

Porcelain Base Glaze

This version was chemically matched to the previous one with a glaze calculator. I needed to make substitutions due to an EP Kaolin shortage in 2023.

Version 2

There are some minor issues with this clear glaze:

• The above recipes are slightly matte due to the high silica content
• The glaze is stable for underglaze, but has slight issue passing bubbles

To solve the two issues, I'll

• Decrease some silica to increase gloss
• Decrease boron (frit) content to delay sealing of the surface

Created 2023-05-10.

Home > Q Science > QD Chemistry > Low Expansion Base Glaze Development