Encapsulated stains are firing with a mass of bubbles and pinholes
This is happening on five different stains at 8% concentrations. The body: A fritted porcelain. Temperature: Cone 03. The glaze: 85% frit. The solution? Documentation for inclusion frits notes that adding 2-3% zircon can brighten the color. Although this does not seem intuitive, we added 2% anyway and refired another sample. You can see the dramatic difference on that tile below. The color is brighter because the micro-bubble clouds that were diffusing it are gone! Of course, it is apparent that the percentage of stain also needs to be increased to get more intense color. What happened to the bubbles? It could be that the particles of zircon that float, unmelted in the glaze melt, act as seed-points for bubble agglomeration and the bigger bubbles then break the surface and it heals behind them. But where do the bubbles come from? I do not know.
Stain Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.
Encapsulated Stains This is a type of stain manufacture that enables the use of metal oxides (like cadmium) under temperature conditions in which they would normally fail.
Pinholing Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.