|Monthly Tech-Tip |
Refers to the practice of firing ceramics in one firing (rather than two) to produce a fully glazed product. This practice requires more technical expertise.
Key phrases linking here: once fire glazing, once-fire - Learn more
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, the greater its dry strength, the higher the specific gravity of the glaze and the quicker it dries the greater the chance that it can be glazed effectively 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 mostly 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 (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 or special driers. 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.
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.
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).
The cone 6 porcelain mug on the left was successfully green glazed by first pouring the inside (using a dipping glaze) and allowing that to dry thoroughly. The lip was then waxed and the excess glaze that went over it was trimmed off. Finally, the outside was glazed by dipping the rim and then turning it over and dipping down to the rim. The mug on the right was glazed on the inside and outside at the same time (by pouring, then dipping). As a result, it was over-whetted and the body laminated within and bubbled during drying. An alternative method would be to use a brushing glaze, applying it one coat at a time and drying that thoroughly before the next.
This is G2926B clear cone 6 glaze deflocculated with Darvan. Because the Darvan is thinning it, 2.5kg of glaze powder is suspended in only 1100g (1100ml) of water (half the normal amount). While the slurry in the bucket flows well and appears like it should work, a one-second dip produces twice the desired thickness. It dries slowly and it is very difficult to prevent runs. The lesson: Make sure the specific gravity (SG) of your glazes is right. What should the SG be? Measure it when your glaze is working well. Or take note of it in instructions that come with the recipes you use. For bisque ware: 1.43-1.45 with a flocculant (like Vinegar or powdered Epsom Salts) added to gel the slurry slightly.
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.
Creating a Non-Glaze Ceramic Slip or Engobe
It can be difficult to find an engobe that is drying and firing compatible with your body. It is better to understand, formulate and tune your own slip to your own body, glaze and process.
In ceramics, this term refers to the flow and gel properties of a glaze or body suspension (made from water and mineral powders, with possible additives, deflocculants, modifiers).
In ceramic industry glazes are often sprayed, especially in sanitary ware. The technique is important.
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
|By Tony Hansen|
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