The utilization of wollastonite as a flux in bone china was investigated in this work. Ceramic industry is always looking to optimize the use of fluxes as a way of reducing energy costs associated with firing. Wollastonite is added in ceramic formulations as an auxiliary flux and allows fast firing production. However, its use in bone china has not been tested before. In the present work, two formulations were prepared, comparing the traditional formulation to another in which the conventional flux (potash feldspar) was partially replaced by wollastonite. The firing behavior, technical properties, microstructure, and phase development were analyzed. A slip with wollastonite was also developed to analyze its rheology. Wollastonite body achieved a reduction of firing temperature, a large plateau for firing, and optimal slip stabilization for casting pieces.
Wollastonite is a calcium silicate. Both natural and synthetic minerals are used in ceramic formulations. Commercial wollastonite starts to melt at about 1450°C and cannot be considered a “flux” as alkali feldspar. For this function, it depends on the reaction with other raw materials. Its use is desired for rapid heating and cooling without cracking or warping. Other sources of CaO, such as limestone, enhance loss on ignition [
Bone china is known for its high technical and aesthetical quality. Bone china properties are superior to others porcelains in terms of whiteness, brightness, translucency, and high mechanical strength. As it is produced with 50% of a recycled material such as calcined cattle bones, its use must increase in near future, due to the recent awareness of environmental preservation.
Glassy phase determines the temperature and the firing range of ceramics formulated with silicate raw materials. Glassy phase control is a key factor to reduce firing temperature, which means a direct reduction in cost associated with the fuel. Realizing this aspect, whenever possible, industry chooses to use strong fluxes. However, in the case of bone china, strong fluxes are difficult to use because of the short firing range, limited by low thermal stability of parts [
The overall composition of bone china lies in the eutectic region of 11% tricalcium phosphate, 38% silica, and 51% anorthite, with a melting temperature of
In fast firing, the composition of the glass phase changes from point to point in the region under consideration, according to the works of Iqbal et al. [
The fact of not achieving an equilibrium composition of the glassy phase during firing makes more complex to predict the optimal experimental conditions of firing cycle. Hence it is very difficult to obtain a wide range for firing, and at the same time, to reduce the initial temperature of vitrification. In fact, a bad choice of firing cycle and body composition can easily lead to distortion, warping, or blistering of fired parts.
In the present study, wollastonite was chosen to partial replacement of feldspar, due to its successful use for fast firing of ceramic products. A higher level of CaO and SiO2 in bone china body may be favorable for reducing firing temperature, according to CaO-SiO2-Al2O3 ternary diagram, if it is properly designed. It was expected that CaO reacting with other oxides and silicates would form a liquid phase of higher viscosity, increasing the resistance to deformation on firing (pyroplastic deformation of parts).
Two bodies were formulated according to the following raw materials proportions (in weight percentage): traditional bone china: 50% calcined bone, 25% kaolin, and 25% feldspar; new body: 50% calcined bone, 25% kaolin, 15% feldspar, and 10% wollastonite.
Wollastonite (CaSiO3) was supplied by the NYCO Minerals. Table
Chemical composition and phase analysis of raw materials.
wt % | Kaolin | Calcined bone ash | Potassic feldspar | Wollastonite |
---|---|---|---|---|
SiO2 | 46.96 | 0.65 | 66.2 | 47.61 |
Al2O3 | 38.05 | 0.8 | 16.54 | 0.11 |
Fe2O3 | 0.46 | 0.12 | 0.15 | 0.14 |
MgO | — | 1.1 | — | |
CaO | 0.02 | 52.2 | 0.36 | 48.66 |
Na2O | 0.03 | 2.1 | 0.89 | |
K2O | 1.14 | 0.07 | 14.66 | 0.03 |
TiO2 | 0.03 | — | 0.03 | — |
P2O5 | 0.11 | 40.5 | 0.15 | — |
Others | 0.03 | 0.2 | 0.64 | 0.35 |
LOI | 13.2 | 2.3 | 0.38 | 3.1 |
Kaolinite | Quartz | |||
Major phases | Quartz | Hydroxyapatite | Microcline | Wollastonite |
Muscovite | Albite |
Raw material particle size distribution. After wet milling.
Raw material particle size distribution ( | |||
---|---|---|---|
Sample | 10%< |
90%< |
Mean particle size |
Calcined bone | 0.52 | 11.21 | 4.16 |
Kaolin | 2.35 | 14.26 | 7.89 |
Feldspar | 2.03 | 41.49 | 22.5 |
Wollastonite | 1.13 | 29.55 | 12.78 |
The bone powder used here is from a company that produces handmade items and is a waste from cutting and polishing of bovine bones, previously cleaned with caustic soda (NaOH). In laboratory, the bone powder was calcined at 1000°C and milled until 90% of the particles were smaller than 14
The body formulations of the raw materials were mixed into wet mill, dispersed with sodium silicate (solution in water), and passed through sieve 325 mesh. Next, the body was dried (110°C) in an oven, moistened with water (8 wt%), and granulated by sieving mesh 20 (0.84 mm). As a result, the ceramic bodies were formed in hydraulic press, comminuted by a mortar, dried, mixed by hand and again moist (8 wt%), granulated (sieve 20 mesh), and pressed (at ~30 MPa) to
The bodies were characterized for water absorption, linear shrinkage (per caliper), geometrical density (in an analytical balance and caliper), and four-point flexural strength tests [
A slip casting was formulated with wollastonite, and then the density was adjusted to the proper value with addition of water, to make it comparable to the traditional formulation. The slip was prepared with 70% of solid and 30% of deionized water with addition of 0.15% deflocculant agent (ammonium polyacrylate). 1% of ball clay was added to the batch.
The characterization of the slip viscosity was made in a digital viscometer (model LVDV-II with small volume device, spindle SC4-18, Brookfield, Stoughton, MA). The deflocculant agent was added until the minimum viscosity was reached.
Firstly, it the firing curve, deformation on firing, mechanical strength, and microstructure were analyzed. Then, the rheology study was shown. The tests were performed for both bodies described previously.
Table
Technical characterization of fired bodies.
Body/temperature |
Water absorption |
Shrinkage |
Apparent density |
|
---|---|---|---|---|
Wollastonite | ||||
| ||||
1180 | 2.33 | 11.62 | 2.29 | 53.3 ± 9.3 |
1200 | 0.01 | 13.67 | 2.43 | 62.4 ± 8.1 |
1220 | 0.04 | 13.23 | 2.46 | 73.9 ± 4.1 |
1240 | 0.02 | 13.10 | 2.34 | 58.8 ± 8.2 |
1260 | 0.03 | 12.66 | 2.28 | 53.0 ± 9.1 |
1280 | 0.00 | 12.47 | 2.25 | 52.6 ± 4.3 |
| ||||
Feldspar | ||||
| ||||
1240 | 1.22 | 8.80 | 2.17 | 41.1 ± 4.3 |
1260 | 0.06 | 9.65 | 2.22 | 54.0 ± 4.1 |
1280 | 0.05 | 9.59 | 2.16 | 48.1 ± 4.7 |
Linear shrinkage and water absorption (W.A.) as a function of firing temperature. Feldspar and wollastonite bodies.
Ceramic pieces must have water absorption lower than 0.5% to be classified as porcelains. Thus the firing curve shows the firing ranges for each body, which are formed by values of water absorption below 0.5%. This curve also correlates water absorption to shrinkage, which may be around 12%.
Feldspar body reached the desired vitrification only at 1260°C (Figure
Figure
Usually, commercial wollastonite melts at ~1450°C and cannot be regarded as a flux in pure form. Figure
Fusion cone test for compositions of 100% flux. Temperature ramp of 150°C/h; fired at 1240°C for 30 minutes.
Wollastonite body was designed according to CaO-Al2O3-SiO2 ternary diagram [
Feldspar (F) and wollastonite (W) bodies in CaO-Al2O3-SiO2 diagram.
The vitrification of wollastonite body could be explained by the presence of flux oxides even in low quantities (such as alkalis and FexOy). However, the amounts of these flux oxides are approximately the same as these of feldspar body.
According to literature, bone china firing behavior can also be explained by the presence of phosphorus oxide which forms metastable phases. In that case, the greater amount of calcium oxide and its presence in a more reactive form such as calcium silicate would contribute to reduce the firing temperature of wollastonite body. In this alternative way, CaSiO3 promotes the formation of bone china final phases, leaving a higher concentration of phosphorus oxide free. This oxide can lead to vitrification at lower temperatures, by means of phosphorus compounds with lower melting temperature, which do not appear in the final product, as proposed in Cooper [
The deformation on firing test (sag test) showed that no deformation occurred in the parts for the temperature range studied. Therefore, the surface tension of glass phase was high enough to avoid distortion of the pieces, for both bodies tested.
To explain the lower firing temperatures and, at the same time, the wide range of firing obtained, it is proposed that wollastonite, providing higher amounts of SiO2 and CaO in glass phase, facilitates sintering by viscous flow, but without significantly changing the viscosity of the glass phase. In fact, a higher amount of calcium in glass phase increases the surface tension of glass phase [
Figure
SEM micrographs. No chemical etching. Feldspar and wollastonite bodies fired at 1240°C. Wollastonite shows small and closed pores.
Table
It can be emphasized that the most important factor is the size of pore/crack in the determination of so-called critical defect, according to the theory of linear elasticity fracture mechanics. Besides, the larger the amount of porosity, the greater the probability to have pores of larger sizes [
In Figures
SEM micrographs. Chemical etching (HF 20%, 20 s). Feldspar and wollastonite bodies fired at 1240°C.
SEM micrographs. Chemical etching (HF 20%, 20 s). Wollastonite body fired at 1240°C. Details of the fractured microstructure.
Rheological curves of the both formulations are showed in Figure
Shear stress in function of shear rate.
Viscosity in function of deflocculant amount. Wollastonite and feldspar slip bodies at
Table
Slip properties.
Bodies | Feldspar | Wollastonite |
---|---|---|
Slip density (g/cm3) | 1.796 | 1.817 |
Thickness wall-forming in 1 |
3.58 | 3.58 |
Viscosity after deflocculation at 0.2 |
736.0 | 784.0 |
The use of wollastonite (partially replacing potash feldspar) in this work reached a lower firing temperature and an increased firing range, without occurring pyroplastic deformation of parts. Generally, bone china has short firing range. This adversity for bone china production was overcome with the utilization of wollastonite.
The reduction of the firing temperature obtained for wollastonite bodies is explained by a higher amount of glass phase according to CaO-Al2O3-SiO2 diagram, compared to feldspar body. The wollastonite flux improved the viscosity of the glass phase by increasing the amount of calcium oxide in it, which allowed a larger firing range. This is a consequence of a higher level of CaO (and a higher distribution of CaO in the microstructure) which increases the surface tension of glass phase. A higher homogeneity of CaO level in microstructure is a result of two sources of CaO in raw material batch: bone ash and wollastonite.
Feldspar and wollastonite slip casting bodies showed similar rheological properties, with stabilized suspensions, and typical Bingham curves, for the range of shear stress, shear rate, and viscosity analyzed.
The authors acknowledge CNPq and CAPES for financial support.