Underglazes require a fluid melt transparent But extra melt fluidity comes at a cost

Wednesday 19th October 2016

These porcelain mugs were decorated with the same insight-live.com/url/3409">Amaco Velvet underglazes (applied at leather hard), then bisque fired, dipped in clear glaze and fired to cone 6 (using a drop-and-hold schedule). While the G2926B clear glaze (left, A) is a good glossy transparent for general use, its melt fluidity is not enough to clear the LOI micro-bubble clouds that dull the colors below (strangely, underglaze colors can generate them too). However, the G3806C recipe (right, B) has a more fluid melt, which is one factor that better enables bubble escape. Its melt also has a lower surface tension. An additional factor is thickness. A thinner layer of B would be even better (brushing glazes enable thin layering).

But B has downsides. Running is one, but not a factor here because we want it thin. Its high flux content also means it fires less durable. And, its high KNaO content raises the thermal expansion (COE) and thus the danger of crazing. Although it fits this porcelain, it crazes on others.

Pages that reference this post in the Digitalfire Reference Library:

G3806C - Cone 6 Clear Fluid-Melt transparent glaze, Transparent over-glaze for cone 6 stoneware: Is a good one even possible on brush-decorated ware?, 2% Copper in two cone 6 transparents: One does not bubble and orange-peel. Why?, Underglaze, Transparent Glazes, Glaze Bubbles

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.