|Monthly Tech-Tip |
Two transparent glazes applied thickly and fired to cone 03 on a terra cotta body. Right: A commercial bottled clear, I had to paint it on in layers, I ended up getting it on pretty thick. Left: G1916S, a mix of Ferro frits, nepheline syenite and kaolin - one dip for 2 seconds and it was glazed. And it went on more evenly. Bubbles are of course generated by the body during firing. But also in the glaze. Raw kaolin loses 12% of its weight on firing, that produces gas. Low temperature glazes melt early, while gassing may still be happening. Keeping raw clay content in a glaze as low as possible is good, but at least 15% is normally needed for working properties. Improvements? Both of these could have been applied thinner. And I could have fired them using a drop-and-hold and a slow-cool schedule.
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
Many ceramic glaze benefits and issues are closely related to the thickness with which the glaze is applied. Many glazes are very sensitive to thickness, so control is needed.
The purest of all clays in nature. Kaolins are used in porcelains and stonewares to impart whiteness, in glazes to supply Al2O3 and to suspend slurries.