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Once fire glazing

The practice of applying glazes to dried ware and firing in one operation. Obviously this is going to save money on energy. But it introduces extra problems also. In general, the thicker and heavier the ware and the greater its dry strength the greater the chance that it can be glazed easily in the dry state. Unfortunately, the single-fire process can expose a melting glaze to a hoard of bubbles escaping from decomposition and dehydration happening within the body (these are all expelled in a bisque firing).

A major challenge is to maintain the glaze slurry properties within an often narrow range of parameters (within which shrinkage can be tolerated, adherence is good, drying is efficient and bonding is good). Each glaze must be tested thoroughly to discover this range and the methods needed to successfully apply and fire it. Often glaze recipes need to be adjusted (often involving finding ways to increase the clay percentage while maintaining the chemistry). Common problems with the glaze itself include cracking and flaking during drying, crawling and blistering during firing, inadequate thickness and surface defects. Handling of the more fragile unfired ware (for application, cleaning, drying and setting) presents challenges. Drying time can be increased considerably and this requires extra floor space. Since single-fire glazing is often done by spraying, we have included a video showing a technique (see link below).

Once-fire is obviously popular in industry, especially for tile, sanitary ware and porcelain insulators. They have also developed ways to single-fire even table ware.

A once-fire mug vs. a bisque-fired mug

A once-fire mug vs. a bisque-fired mug

The mug on the right was bisque fired and then glazed, the one on the left was glazed in the green (dry) state using our standard meet-two-colors-at-the-rim glazing method. This method lends itself well to single fire glazing. Notice the glaze did not go on as thick on the once-fired piece (extra attention is needed to make sure it gets on thick enough without cracking the piece). In addition, there are a few pinholes whereas the bisqued piece is flawless. Single firing ware requires extra attention to firing, climbing to a point just before the glaze begins to melt and soaking there to enable hydrates and carbon to escape.

Bisque-glazing vs. green-glazing in medium temperature porcelain

Bisque-glazing vs. green-glazing in medium temperature porcelain

The mug on the left was bisque fired and then glazed, the one on the right was glazed in the green (dry) state. The glazes are the same inside and out but the porcelain one the right is based on New Zealand kaolin (vs. American kaolin on the left). Three secrets for success for the one on the right were: It was glazed inside and out in two operations with a drying phase between, it was heated to about 150F before each application and it was fired with a soaking period (at about 1900F) on the way up to top temperature (cone 6).

18 hours from thrown on the wheel to glazed and out of the kiln!

18 hours from thrown on the wheel to glazed and out of the kiln!

It took 2-3 minutes to get this mug to soft leather hard for trimming using a heat-gun (not a blow drier). It took seconds to stiffen the handle for attachment after. I am now taking it to stiff leather hard to prepare for glazing (left). I dry it evenly by judicious technique. Then I pour-glaze the inside and immediately push it lip-down down into the glaze to do the outside. I re-gun it a couple of minutes and then re-dip the outside bottom (up to the previous glaze boundary). Last I gun it another 3 minutes an put it in the kiln. The lesson: The key is not drying speed. It is how even the drying is (I watch the color change and focus on the wettest parts). Finally I fire 400F/hr to cone 6 (with an hour soak at 250F for final water smoking). The clay: Plainsman M370.

Out Bound Links

In Bound Links

  • (Glossary) Spray Glazing

    In the production of smaller bisque fired ceramics...

By Tony Hansen

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