Once-Fire

The practice of once-firing ceramics (no bisque) to produce fully glazed products. Considerable technical expertise and good testing are required to make it work. This is not a shortcut to skipping bisque firing.

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Details

The practice of applying glazes to dried, rather than bisque-fired ware, and firing in one operation. Tradition is an important driver; some cultures have never used any other method. In industry (e.g. tile, sanitaryware, porcelain insulators), the advantage of the practice is saving energy and money. Understandably, many potters have or are considering single fire. But the difficulty of once firing is not to be underestimated, nor should it be considered a shortcut for skipping the bisque. Once-fire is a body-glaze-application-firing system.

Factors that give the greatest chance of success

-Use ware having good dry strength, uniform wall thickness and adequate permeability. Avoid thin spots, abrupt changes in section and sharp inside corners.
-For bone-dry glazing, use the lowest practical amount of water that will produce the needed viscosity and thixotropy.
-Establish a suitable specific gravity and rheology for each individual glaze.
-Use enough raw clay, bentonite or binder to produce a glaze layer that remains attached during drying, handling and firing. Too little can cause powdering or flaking; too much raw clay can produce excessive drying shrinkage, cracking and subsequent crawling.
-Where additional clay is needed to supply Al₂O₃, replace part of it with calcined clay if drying shrinkage needs to be minimized.
-Apply the minimum thickness needed.
-Spraying is especially effective because the coating can be built in thin increments with little penetration of water into the ware.
-Quick dipping or pouring can also work where the shape is strong enough and can be supported without concentrating pressure in one area.
-Experiment with the amount of pre-warming of bone-dry to shorten dry but avoid flash-drying that might cause uneven deposition or leave a poor bond.
-Minimize gums and other additives that unnecessarily slow water removal, but do not assume that all binders are undesirable. A small amount may be essential for adherence and handling strength.
-Glaze the inside and outside in quick succession where uneven wetting could place the wall under stress. Avoid repeatedly wetting one area or applying several water-rich layers by dipping.
-Dry glazed ware uniformly and completely before loading it into the kiln. A surface that appears dry can conceal moisture in thick walls, bases or attachments.
-Glazing at the leather-hard stage using glazes of high clay content enables slower drying (leather-hard clay does not slake, but it can still soften, swell or warp. An advantage is that the body and a clay-bearing glaze can continue drying and shrinking together. A disadvantage is the drying time needed to equalize moisture.
-Make sure re-drying is complete before 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 muddying of the body below, 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. If ware is glazed at leather hard stage, extra drying times require extra floor space or special driers.

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).

Firing

A single firing is not simply an ordinary glaze firing containing unfired pots. Once firing brings the body-decomposition (bisque) and glaze-melting processes together in the same firing. Water vapour and gases from organics, clay minerals, carbonates, sulfates and other body or glaze materials must escape before the glaze seals the surface. If gas evolution continues after the glaze has begun melting, bubbles can be trapped or can break through too late for the surface to heal, producing microbubbles, pinholes, pits or blisters. Both the timing of gas evolution and the ability of the glaze melt to heal need to be considered.

Testing

A practical testing program should examine the merits of glazing leather-hard or bone dry, determine limits of wall thickness and construction methods, discover a glaze specific gravity and flow behaviour that work best, a suitable application method, limits of glaze thickness and layering, best preglaze preheat and drying time, best firing schedule and kiln atmosphere and factors that produce fired pinholing, blistering, crawling, or ware warping.