Many years ago, sanitaryware and plumbing products were produced in a traditional two-firing process-the first fire to harden the clay body and the second to vitrify the glaze. However, re-firing large, complex shapes was expensive, labor-intensive and energy-inefficient, and it yielded a low production rate.
The double-fire process eventually gave way to single-fire. Single firing produces a better article at a lower cost but is a much more difficult process. The glaze is applied to the dry but raw clay body. Both have to mature together, a process that can take between 12 and 20 hours at temperatures rising to 1250°C (2280°F). The glaze must both develop a glossy surface and remain "open" (gas permeable) long enough to allow the body to mature and de-gas without causing bubble-related defects. If such defects occur, they announce themselves only when the piece appears from the kiln finished and glazed, but unready for the market-not necessarily scrap, but in need of treatment.
A manufacturer can congratulate itself if its reject rate is only 25%. Sometimes the reject rate can be as high as 60% if best-quality pieces are the goal. Glazes often have iron speckling from conveyor or kiln contamination, "crawling" where there is incomplete coverage, or body pinholes visible as holes in the glaze. The article must then be sent back to the kiln for defect treatment, sometimes more than once. Some top-quality manufacturers re-fire up to three times to achieve the desired result, and sometimes the piece has to be completely re-glazed.
The problem with this process is not just the extra time, money and energy used. Since re-firing is usually at a slightly lower temperature (by 50-100°C or 122-212°F), devitrification of the glaze often occurs. This means that crystals form and dull the surface, lowering the piece's quality. More re-firing equates to less gloss, a rougher surface, lower quality, and worse, a less hygienic product.
Boric oxide is well known to glaze technologists as being a multifunctional batch ingredient with many useful properties. However, in sanitaryware, in which a raw glaze is used, the solubility of many common borates has meant that they have been discounted in spite of their potential benefits. Equally, they have sometimes been suspected of promoting glaze bubbling at the temperatures used.
Insoluble borates (borates that have minimal solubility in water), on the other hand, don't contribute any negative effects. Such borates are available as frits, calcium borates and zinc borates, and all are custom-made to avoid devitrification problems.
Using advanced computer modeling techniques and laboratory tests substantiated by a leading U.K. industry association, researchers have confirmed that including borates in the glaze in exactly the right amount deals with the devitrification problem, and does not cause glaze bubbling. (See sidebar: Experimental Glaze Development Using Borates.)
The glaze glossiness, measured in the laboratory after the first firing, was slightly less than the standard glaze, but the difference was imperceptible to the eye. In subsequent re-firings, virtually no reduction of gloss occurs. In contrast, the gloss of a non-borate glaze will have fallen by almost 40% after three re-fires. Additionally, only a very small percentage (2% by weight) of B2O3 was needed in the batch to produce these benefits.
Use of borates also provides good high-temperature fluxing, replacing the need for barium and zinc oxides. Borate glazes also result in far better chemical resistance in this notoriously harsh environment. And perhaps most importantly for most manufacturers, recent work at Rutger's University in New Brunswick, N.J., has demonstrated that highly significant cost savings can be won by the use of borates. By being able to avoid the need for other batch ingredients, the cost of the glaze can fall by up to 25%.
Glaze Preparation and Evaluation. Glazes were ball-milled (62.5 wt.% solids, 1% CMC binder) to a particle size of about 70% < 10 micrometers and sprayed onto bisque tiles and sanitaryware to a (dry) weight of 1.25 kg/m2, which corresponds to a fired thickness of 0.5 mm. First firings used a heating rate of 100°C per hour and a peak temperature of 1200°C per minute followed by natural cooling. Re-firings used the same cycle with a peak of 1100°C.
The glaze properties of interest were gloss (first fire and re-fire), sealing temperature and liquidus temperature (the temperature below which devitrification occurs). Gloss was measured with a 60° gloss meter, and sealing temperature was measured by firing a glazed bisque tile in a gradient furnace and then impregnating the piece with a blue dye. Liquidus temperature was measured by placing strips of glazed and fired bisque in a gradient furnace for two hours. The sample was then examined visually and with a reflected light optical microscope to detect any devitrification. These tests were repeated on sanitaryware body slabs using the best glazes from the tile experiments.
Increasing the boron content caused the first fire gloss to decrease, but the change was imperceptible, and all first fire glazes were similar in appearance. (A change in gloss of greater than three units is visible to the naked eye.) On re-firing, however, the boron-free glaze devitrified, particularly on the second and third re-fire. The gloss of the 4% B2O3 glaze changed on re-firing-in some samples, the surface became disrupted by pinholes. The 2% glaze did not change its appearance, and after two re-fires was identical to the first firing. (Boron-containing glazes were not re-fired a third time.) (See Figure 1.)
Sealing and Liquidus Temperature
Adding 2% boric oxide reduced the sealing temperature by 40 to 55µC, while 4% boric oxide reduced the sealing temperature an additional 35 to 65°C. Adding B2O3 also reduced the liquidus temperature, which reduces the chance of devitrification on re-firing. Glazes with no B2O3 showed severe devitrification, with many large crystals visible to the naked eye. Adding 2% B2O3 reduced the devitrification so that only a slight amount could be seen, and only under a microscope. No crystallisation occurred with 4% B2O3.
The standard glaze and B2O3-containing glazes were evaluated for resistance to chemical attack (50% HCl, 10% acetic acid, Teepol, 5% NaOh, 3% H2SO4, 10% citric acid, 0.15% Na-stearate) and staining and burning, both before and after re-firing. The first fire glazes were also subjected to craze resistance testing. All samples passed the tests. An additional chemical durability test was performed by immersion in an alkaline detergent (0.4% IEC 436 solution at 77°C for 32 hours). This showed differences between the glazes-the standard was badly attacked, the calcium borate-containing glaze was slightly attacked, and the glaze made with the high boron content frit was unaffected. The presence of boric oxide in the glaze therefore significantly improves the resistance to chemical attack by alkaline detergents.