- THE MAGAZINE
High-quality household fine porcelain, including bone and talc china and quartz-enriched porcelain, is very popular because of its attractive appearance and sound performance properties. In the past, glazes for these products included lead-containing frits, but increasing health and environmental concerns have led many countries to develop more stringent standards to control the use of heavy metals, including lead and cadmium.
As an alternative to lead-containing glazes, leadless fritted glazes do not contain any lead compounds. These types of glazes can not only protect public health but also improve the working environment by eliminating lead damage from the source.
Lead SubstitutesLead is an important network modifier in the glass network structure of lead-containing glazes. Its strong solvent effect and high-density characteristics provide lead-containing glazes with low surface tension, relatively low high-temperature viscosity, a wide burnt range, a high refractive index and good fragmentation prevention. It is vital that any substitute or combination of substitutes provides similar characteristics.
Leadless frits such as Li2O, SrO, ZnO and BaO are often used to replace PbO; they are typically introduced as Li2CO3, SrCO3, ZnO and BaCO3. The meltability of these components (in descending order) is: Li2O > Na2O > K2O > BaO > CaO > SrO > MgO > ZnO.
Lithium is widely used in glaze production as a strong solvent. Aluminum oxide and silicon oxide can be used when lead is replaced with lithium to obtain more stable high-intensity glazes and also to increase surface hardness. Adding a small amount of a lithium compound during frit processing rapidly decreases high-temperature viscosity and substantially reduces the glazes’ expansion factor.
Mostly used for porcelain production, lithium carbonate (Li2CO3) is slightly soluble in water and does not absorb moisture or agglomerate. It is usually added to frits to decrease high-temperature viscosity, is easily stored under normal conditions, and can be directly used for the production of raw glazes. A large addition of lithium minerals can lead to crystallization, and lithium products are also relatively expensive, so the amount added is usually less than 3%.
Non-toxic and harmless to humans, strontium is the best-recognized lead substitute. Strontium oxide (SrO) accelerates the formation of low-temperature glazes to increase surface glossiness, and it aids in intermediate-layer body-glaze generation to reduce surface alligator cracks. Strontium glazes provide better smoothness and higher surface hardness than lead glazes.
Strontium carbonate (SrCO3) is non-toxic and slightly soluble in water, and it can be directly added to the raw glaze (usually not more than 10%). The addition of raw strontium material to frits reduces high-temperature viscosity. To date, SrCO3 has not been widely applied in the porcelain industry due to its limited development history and relatively high price.
Zinc oxide (ZnO) is mostly used in high-temperature glazes. A small amount of ZnO can be an important assistant fluxing agent during the high-temperature stage, since it can expand the glaze’s fusion range, increase its elastic ratio, decrease its expansion factor, and reduce alligator cracks. Zinc oxide is a good raw material for crystalline glazes, but its addition should be limited to no more than 10% to avoid over-crystallization. In most cases, ZnO is melted into frits during production; calcination is needed when ZnO is used directly to reduce surface flaws.
With characteristics similar to lead, barium glazes have a relatively high refractive rate that can increase surface glossiness and help improve elasticity and mechanical robustness while reducing atmosphere-related effects on the glaze’s surface. However, barium compounds are toxic, so their application should be minimized. Barium carbonate (BaCO3) is generally added directly to frits during production, typically in amounts of no more than 6%.
Table 1 illustrates the chemical compositions of representative leadless fritted glazes. No good empirical equation currently exists that can be used as a reference for the design of leadless fritted glazes, so improvements on existing formulation are usually made. The frit’s water solubility and melting temperature should be fully considered, as well as surface glossiness, hardness, expansion factor and elasticity.
A good frit formulation must be fusible under high temperature and insoluble in water. An excessively high melting temperature will lead to the volatile loss of effective components such as boron and strontium, while high solubility in water will lead not only to poor liquidity of the glaze slip and difficulties with storage, but also marginal bubble flaws on the finished product.
Processing ConsiderationsQuality raw materials are a prerequisite for the production of quality frits (see Table 2). Smaller raw material grain sizes are generally better for frit preparation since smaller grains and larger specific surface areas represent lower melting temperatures and higher melting speeds. A 60-mesh screener (at least) should be used for refractory substances such as quartz and feldspar; quartz should not be finer than 200 mesh to avoid glomeration or lack of fusion.
Dry mixing methods, such as multiple manual screenings or the use of mixing equipment, are often used for raw material mixing. First, a small amount of water is added to a wet refractory raw material, such as quartz. A fluxing raw material (such as borax or boric acid) is then added, mixed and screened. Finally, the remaining raw materials are fed with intensive mixing to make sure that the surfaces of the refractory components are fully covered by the fluxing agent, which helps accelerate the formation of eutectic substances under low temperature while achieving rapid fusion. Another approach involves the preliminary mixing of large amounts of easily dispersive raw materials, followed by full mixing with the remaining raw materials to maximize uniformity.
Leadless frits have no fixed melting point during heating, but utilizing a reasonable melting temperature, timeframe and atmosphere is critical to frit quality. If a too-low melting temperature is used, insufficient frit melting can lead to flaws such as surface pinholes and glaze bubbles. An excessively high melting temperature and/or long melting period can lead to the volatilization of fusible components, such as borax, boric acid and calcium bicarbonate, which not only will result in increased energy consumption and lower production but also will change the frit’s composition and affect quality. Generally speaking, melting temperatures for leadless glazes vary from 1350 to 1450°C.
A phased grinding process is most commonly adopted when preparing fritted glazes. The frit is ground first, and then clay or other additives are incorporated. A general leadless glaze formulation would include (by weight) frit, 90-95%; kaolin, 5-10%; and other additives, 0-0.5%. The ratio of raw materials, ball clay and water is generally in the 1:1.5-2.0:0.4-0.6 range. The fineness of the glaze slip should be around 250 mesh, with 0.01-0.02% of screen residue or with particles smaller than 10 µm accounting for 75-80% and the largest particle not larger than 30 µm.
Suspending agents can also be further categorized as belonging to clay-mineral-suspending or organic-substance-suspending methods. In the clay mineral method, a raw clay material such as kaolin (5-10%) is used as the suspending agent, while 0.1-0.2% gum arabic or carboxyl methyl cellulose are added to increase the bond strength of the glaze layer to reduce spalling and sticking. Trimeric sodium phosphate is also added to adjust the liquidity characteristics to achieve a uniform and smooth glaze slip layer. In the organic-substance-suspending method, biologic polyoses and xanthan gum are generally applied.
Application and FiringThe leadless fritted glaze application process is generally similar to that of a lead-containing fritted glaze, in which spraying and soaking methods are both used. However, since leadless frit has a lower density than lead frit, the density of the leadless glaze should be slightly lower to obtain the same glaze layer thickness under the same conditions. The density of the glaze slip should be 1.70-1.75 g/cm3 for spraying and 1.52-1.56 g/cm3 for soaking, and the glaze thickness should be controlled to within a range of 0.2-0.3 mm.
Leadless fritted glazes have a slightly worse moistening performance than leaded fritted glazes, so strict quality control in the porous body is necessary to obtain good surface quality. Bone china and talcum ceramic both have a narrow burnt range, and most manufacturers therefore select the minimum temperature required for ceramic formation to help reduce over-firing. In addition, porous bone china and talcum ceramic bodies feature good luminosity, even with 2% of water-absorbing capacity, which would be subject to undetected poor-fired porous bodies. Leadless fritted glaze application on bodies with relatively high water-absorbing capacities might lead to a thin glaze layer and bubbles, so adequate ceramic formation is an important condition for ensuring the quality of the glaze surface.
The glaze firing temperature for fritted glazes is usually lower than the biscuit firing temperature by 100-150°F. Kaolin and small amounts of organic substances are added to fritted glazes, and gases generated by these substances during the glaze firing can lead to flaws such as glaze surface bubbles. Therefore, these gases (as well as those from other sources) should be eliminated as much as possible prior to the fusion of the glaze layer. The firing period should be appropriately extended within this temperature range when developing glaze firing procedures.
Sustainable GlazesThe use of leadless fritted glazes can become an effective element in a sustainable manufacturing process. This approach to the development of high-grade double-fire ceramic products harmonizes industry and the environment, and can help manufacturers improve their traditional processes by applying new technologies. Ongoing studies exploring ways to widen the burnt range, increase surface hardness and improve surface characteristics will also provide additional benefits.
For additional information regarding leadless fritted glazes, contact the Shandong Institute of Advanced Ceramics Co. Ltd., (86) 533-3582117; fax (86) 533-3582244; or visit www.sicer.com.