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Journal of The Australian Ceramic Society Volume 51[2], 2015, 75 – 83 75
© Copyright 2015, The Australian Ceramic Society ISSN 0004-881X
Influence of Bone Ash on Bone China Characteristics at
Constant Firing Temperature
Ali Arasteh Nodeh1*, Talat Zakeri2, Zahra Hejazi3, Solmaz Boroomandi Bararti4
1) Chemical Engineering Department, Quchan Branch, Islamic Azad University, Quchan, Iran
2) Advanced Material & Ceramic Research Center, Council of Scientific & Industrial Research (Maghsoud Kavan
Shargh), Mashhad, Iran
3) Material Engineering Department, Shahrood Branch, Islamic Azad University, Shahrood, Iran
4) Chemical Engineering Department, Ferdowsi University of Mashhad, Mashhad, Iran
Email: Ali.arastehnodeh@gmail.com
Available Online at: www.austceram.com/ACS-Journal
Abstract
In this paper, the effect of various amounts of bone ash on bone china mechanical and physical properties under
constant firing temperature has been investigated. The features that were studied are shrinkage, modulus of rupture,
bulk density and thermal expansion coefficient. The samples were characterized by XRD, XRF and SEM images. It
was observed that increasing the bone ash content leads to more intensity of β-tricalcium phosphate (β-TCP) as well
as anorthite peaks but less formation of intense SiO2 peaks. Moreover, by increasing the bone ash content from 25%
to 45% the total shrinkage increases from 9.5% to approximately 12%, modulus of rupture from 400 to 700 Kg/cm2,
and bulk density from 2.34 to more than 2.55 gram/cm3 while the water adsorption remains constant at zero. In
addition, in almost all temperatures by increasing the bone ash content the thermal expansion coefficient increases
by the slope of near 1.3%. The brightness also has a direct relationship with the amount of bone ash. In other words,
by increasing bone ash percentage from 25% to 45%, the brightness index goes up from its initial value at 89 to
more than 96 and the green color diminishes. Concluding that increasing the bone ash content in the bone china
bodies resulted in a brighter and white body with less colorful tint on the appearance of samples.
Keywords: Bone ash, Bone china, Tricalcium phosphate, Anorthite
1. Introduction
The addition of bone ash to china body was first
developed and commercially used around 1800 A.D.
by J. Spode. This change made a distinct difference
in china bodies due to containing calcium phosphate
as its main component. The particular characteristics
of bone china that appeal are its whiteness,
translucency, high glaze and decoration quality and
high strength [1].
High strength of this product is related to small size
of crystals and good matching of thermal expansion
of crystals and glass [2]. According to the definition
of American Society for Testing and Material
(ASTM), bone china is soft porcelain with high
translucency, which contains 25% bone ash in body,
and its water adsorption is less than 2%. Bone ash is
used as a constituent in the body of bone china and
this addition makes this product have unique features.
Traditional composition of bone china body contains
50% bone ash, 25% plastic clay (as plasticizer) and
25% Cornish stone (as flux), however this
composition may vary from one company to another.
The microstructures of bone china are known to be β-
tricalcium phosphate (β-Ca3(PO4)2)(β-TCP), anorthite
(CaAl2SiO8), α-quartz (SiO2) and calcium
aluminosilicate glass.
Production of bone china has always been a very
difficult process and needs severe control on a
number of processing parameters such as particle size
distribution, the temperature of dryer, density of
slurry and biscuit firing temperature. One of the most
important problems of bone china production is its
narrow firing range, because bone ash contains
Nodeh et al. 76
calcium carbonate as a powerful flux, which makes
an extremely fusible body. Depending on particle
size distribution and final composition of prepared
bodies, biscuit firing temperature varies between
1220-1250oC. The ware shrinks significantly during
firing and is very sensitive to over-firing. If the
temperature rises slightly above the maturing point, it
causes the ware to form bubbles and cavities which
leads to form a spongy body. Another matter, which
causes problems to the manufacturer of this product,
is its tendency to go off-color [3]. Using DTA and
XRD results, Pierre [4] concluded that final phases of
bone china after firing process are tri-calcium
phosphate, anorthite and quartz.
As bone ash is expensive, many producers are eager
to know the results of reducing bone ash in their
products [5]. Gratton attempted to reduce the bone
ash content in bone china body and concluded that it
has a defective effect on color of the product [6].
Jackson replaced bone ash in a body with equal parts
of Cornish stone and china clay and reached this fact
that a mixture of 15% bone ash and 42.5% of china
clay and Cornish stone has the lowest fusion range
[7]. The goal of this paper is to know the effect of
changing the amount of bone ash in bone china wares
while the firing temperature is kept constant.
2. Experimental
2.1. Material and methods
Bone china is produced traditionally from the raw
materials including bone ash, china clay and Cornish
stone. In this research, a typical kind of bone china is
being produced by addition of about 46% bone ash,
29% clay, 15% potassium feldspar and 10% silica.
Table 1 shows the chemical composition of raw
materials used in this study and Table 2 and 3 shows
the components of mixed clay (a mixture of ballclay
and three kinds of kaolin) and milled mix (a mixture
of feldspar and silica) respectively.
Mixed clay, milled mix, and bone ash were mixed as
is shown in table 4. In this study, for the preparation
of bone china body mixed clay was kept constant and
milled mix was replaced by bone ash.
In each test, 2500 gram of the raw material were
loaded with 1250 cc water in a 5 litter jar and milled
for 390 min to attain a particle size of 58% less than
15 micron. The slurry was transferred to a storage
tank, passing over a 140 mesh sieve and magnet.
Then, viscosity and density were adjusted to
180O (measured by Brook field viscosity meter) and
1.8 gram/cm3 respectively.
The slurry was dewatered up to 20% moisture on
plaster plates, extruded to 2*1*10 cm sized bars by
experimental extruder (Netzsch, D-95100) and dried
in a dryer at 100±5 o
C. Some of the dried bars were
used for measuring modulus of rupture (using
Netzsch, Bending strength tester 401), some for
measuring fired shrinkage, and other milled to make
some tablets (D (diameter):5cm, z (height):5mm)
using hydraulic press of make Carver, 8664CE. All
the samples were fired at a temperature of 1250oCin
an industrial gas kiln with 2m*2m*2m dimensions
and with a defined temperature curve as shown in
Fig. 1. As the firing procedure of bone china bodies
depends very much on the location of wares in the
kiln and also on the type of kiln [4], in this work, to
eliminate the effect of these parameters [8] all the
samples were fired in the middle deck of one single
biscuit kiln.
Table1. XRF analysis of raw materials
component Bone ash
(Glubal) Feldspar
(Gp200) Silica
(Malayer) Bal clay
(puraflo) Kaoline
(SP) Kaolin
(OKA) Kaolin
(Zswnk1)
SiO2 1.18 65.2 99.5 47.3 46.37 52 62
Al2O3 0.04 18.7 0.2 31.9 37 34 25.7
P2O5 41.07 - - - - - --
CaO 52.95 0.27 - 0.2 0.29 0.18 1.02
K2O 0.05 12.2 - 2.0 2.22 0.24 0.68
Na2O 0.73 2.68 - 0.2 0.01 0.01 0.2
MgO 1.18 0 - 0.3 0.38 0.23 0.3
Fe2O3 0.04 0.12 0.2 1.2 0.76 0.18 0.5
TiO2 0.01 0 - 0.9 - - -
Loss@1100C 1.22 0.31 0.1 15.9 12.4 13.16 2.9
Journal of The Australian Ceramic Society Volume 51[2], 2015, 75 – 83 77
Table 2. The ratio of mixed clay used in bodies
Clay %
Zswnk1 44.83
Sp 24.14
OKA 24.14
Ball Clay (Puraflo) 6.9
Table 3. The ratio of milled mix used in bodies
Materials %
F-Gp200 71
Silica 29
Fig. 1. The variation of temperature versus time for
firing bodies
Fired bars were used to measure the modulus of
rupture (M.O.R), fired shrinkage, thermal expansion
coefficient and molten viscosity. Molten viscosity of
different bars can be compared by measuring
deformation of bars, which were fixed by one of their
end to the wall as shown in Fig. 2. On the other hand,
the fired tablets were used to investigate the water
adsorption, bulk density, whiteness, XRD patterns
and SEM images etc.
Fig. 2. Molten phase Viscosity Measurement
2.2. Experimental techniques
2.2.1. X-ray diffraction (XRD)
X-ray diffraction (XRD) was used for determination
of the crystalline phases present in the fired bone
china bodies with different percent of bone ash
addition. X-ray diffraction patterns were recorded on
a Unisantis (XMD-400) diffractometer with a power
of 30 kV×20 mA. Monochromatic
CuKα radiation, λ=0.154060 nm was employed. The
X-ray scan was made over a range of 2θ values of 10-
90◦.
2.2.2. Scanning electron microscopy (SEM)
The SEM used was a Cambridge S360 analytical
scanning electron microscope run at 19.9 kV. This
technique was used for the biscuit fired bone china
bodies with the maximum and minimum bone ash
content. The goal of this test was to study the
microstructure of these two samples. Gold coating of
the samples was carried out using a Sputter coating
instrument.
3. Result and discussion
3.1. Chemical analysis (XRF)
Fig.3 illustrates the variation of chemical
composition of bodies following the replacement of
milled mix by bone ash.
It is obvious that by increasing the bone ash and
subsequently decreasing the milled mix content
(milled mix has been replaced by bone ash), CaO,
P2O5 increase and Silica (SiO2), Potassium oxide
(K2O) and Alumina (Al2O3) decrease. Phosphorus
penta oxide (P2O5) like silicon oxide (SiO2) is the
main glass former in ceramics and represents the
essential component present in all vitreous china. The
increase of these oxides in body gives a higher
melting temperature, a lower thermal expansion
coefficient and lower molten glass fluidity. Potassium
oxide (K2O) is a glass modifier; it lowers the melting
temperature, thermal expansion coefficient (TEC)
and molten glass viscosity. Then, decreasing K2O by
increasing the bone ash content (and subsequently
decreasing the milled mix content) is expected to
raise the melting temperature, TEC and molten
viscosity. As mentioned in section 1 Calcium oxide
(CaO) reacts easily with other components and is a
good flux. Its action provokes rapid melting and
subsequently, rapid lowering of the molten glass
viscosity. An increase in calcium oxide reduces the
thermal expansion coefficient and provokes
devitrification. Then, by reducing K2O, milled mix
amount and consequently increasing the bone ash and
calcium oxide cannot expect a predictable behavior
of viscosity, expansion coefficient and changes in
Nodeh et al. 78
melting point. Alumina (Al2O3) is another
fundamental component of ceramic bodies, it reduces
the possibility of devitrification, raises the melting
temperature and viscosity of molten body [9].
3.2 X-ray diffraction (XRD)
A representative XRD spectrum of the biscuits of
fired bodies is illustrated in Fig. 4. It can be seen that
the only crystalline phases present in the fired bodies
are TCP and anorthite, with a small amount of quartz.
Due to the similarity in crystal lattices spacing of
TCP and anorthite the peaks of their phases
frequently overlap in spectrum.
During firing process, reaction between bone ash and
clay leads to produce tri-calcium
phosphate (Ca3(PO4)2) around 1000oC and then
anorthite (CaO.Al2O3.2SiO2) is formed from excess
calcium oxide (Eq. 1) [4].
[Ca(OH)2.3Ca3(PO4)2]+[Al2O3.2SiO2] 3Ca3(PO4)2+
(CaO.Al2O3.2SiO2)+ H2O (1)
Anorthite (A) diminishes as temperature increases,
but tri-calcium phosphate (TCP) remains up to
1370oC [4]. Comparing the XRD patterns illustrates
that by decreasing the bone ash content in 5 different
bodies, the intensity of peaks related to Ca3(PO4)2 and
CaO.Al2O3.2SiO2 decrease while the peaks related to
SiO2 increases. As Al3+, Si4+ and Ca2+ fall into the
same lattice, the formation of Ca(Al2Si2O8) under the
calcination condition is a possibility. This was
observed with the corresponding peaks at 2θ of
11°,14°,17°, 22°, 28°, 32°, 36° and 40°.
Fig. 3. Variation of chemical analysis of body
As mentioned above, by reducing the bone ash in
china bodies, the intensity of peaks, which are related
to Ca3PO4, decreases by decreasing in bone ash
content. As bone ash contains Ca3PO4, by reducing
the percentage of it in bone china body, it is
reasonable that bodies with higher amount of bone
ash have more intense Ca3PO4 peaks. A measurement
of the particle size was carried out using the Scherrer
equation (Eq. 2):
D = (R× λ)/(β×Cos θ) (2)
Where D is the active particle size, R is the Scherrer
constant, λ is the wavelength of X-ray (1.5408◦A)
and θ is the half peak width (radian). However, it is
interesting to note that the crystalline size increases
with increasing bone ash content. However, this
change is not significant as it is 13nm for body with
46% bone ash where it contains higher bone ash and
is 11nm for body 26% bone ash, which has the least
bone ash content. The other point that can be noted is
that, increasing the bone ash results in increasing the
crystal content of bone china. The crystals are mainly
tricalcium phosphate and anorthite and it is obvious
in XRD patterns that peaks related to these two
components diminish by decreasing the bone ash
content. The high crystal content of bone ash
accounts for its good mechanical strength [10]. By
decreasing the bone ash, the SiO2 peak at 2θ=68° is
intensified. On the other hand for body 46% bone
ash, which has the most bone ash content, no
SiO2 peak was observed at 2θ=87° .While by
decreasing the bone ash in other bodies, the
SiO2 peak appears at 2θ=87°. These evidences show
that by replacing bone ash with milled mix, a part of
SiO2 cannot be melted and remains as SiO2 crystals
in the structure of china body. This leads to lower
vitrification and consequently less translucency.
On the other hand, the phase transition of quartz to β-
quartz at 573ºC accompany with a drastic change in
the volume. Depending on particle size this volume
change will lead to some tiny cracks around the silica
crystals and reduce mechanical strength. This result
was also confirmed by SEM images (Fig. 5).
Journal of The Australian Ceramic Society Volume 51[2], 2015, 75 – 83 79
Fig. 4. XRD spectrum of the biscuits fired body (a) no. 1 (b) no. 2 (c) no. 3 (d) no. 4 (e) no.5 (C:Cristobalite ,
A:Anorthite , W:Whitlockite)
3.3. Scanning ElectronMicroscopy (SEM)
Fig. 5 shows the SEM images of microstructure of
biscuit fired bone china bodies with different amount
of bone ash. Comparing Fig. 5a to e shows that
increasing the amount of bone ash from 26% to 46%
leads to decrease the amorphous phase and increase
the crystalline phase. This may be due to increasing
the bone ash content leads to decrease silica and
feldspar content while aluminum oxide of kaolin is
remained constant. As a result, there is enough
aluminum oxide for converting SiO2 to anorthite. But
by decreasing bone ash, the amount of Silica and
feldspar increase and the amount of silica that has not
been converted to anorthite cannot be melted
completely and consequently some crystals will
remain in the glassy phase.
The EDX spectrum confirmed that the white and gray
areas represent the quartz and tri-calcium phosphate
respectively and the discrete bright areas contain
anorthite crystals. It can be inferred that by increasing
bone ash, the amount of glassy phase, quartz crystals,
anorthite and tri-calcium phosphate increase. The
presence of these crystals results in higher strength in
bone china body.
Nodeh et al. 80
Fig. 5. SEM images of the microstructure of biscuit fired bone china bodies (a) with 26% bone ash (b) 31% (c) 36%
(d) 41% (e) 46%
Journal of The Australian Ceramic Society Volume 51[2], 2015, 75 – 83 81
3.4 Physical Properties
Fig. 6 shows the variation of fired shrinkage versus
bone ash content of bone china bodies. It can be
observed that as bone ash increases from 26 to 46
percent, the shrinkage changes from 8.9% to 11.7%,
however, the largest change occurs at 26%.
Fig. 6. The variation of fired shrinkage
Fig. 7 illustrates the effect of bone ash content on
bulk density (using Sartorius LA230S). It can be
observed that bulk density increases with the increase
in bone ash content and consequently shrinkage
increases. The value of bulk density depends on
many factors like carbon content of bone ash and
temperatures. As it was mentioned before, when
temperature rises above firing point, bubbles form
and bulk density decreases. The best firing
temperature is the temperature that attains maximum
bulk density, in which the water adsorption of body is
around zero and sintering process completed.
Fig. 7. The variation of bulk density
The water adsorption of bodies with different bone
ash content is zero, while according to fig. 7 the bulk
density varies from one sample to another. These
results convey this fact that bodies melted at 1250oC
and went to vitreous phase where all the pores are
closed. Closed pores can change the bulk density but
are not able to adsorb water, and this is the reason of
different bulk densities (Fig. 7) in spite of the same
water adsorption. Fig. 8 shows fired modulus of
rupture versus bone ash contents. It can be easily
seen that body with 46% bone ash showed increase in
strength up to 66% as compared to body with 26%
bone ash. Similar observation has been reported by
Spode as well, as he also concluded that by
increasing the bone ash content in bone china bodies,
the strength of fired biscuits increase [11].
Fig. 8. Variation of fired MOR versus bone ash
content
In Fig. 9 the deformation of each sample in 1250oC is
shown. As it is expected, for samples with lower
amount of bone ash and higher amount of fluxy
materials (according to XRF results) the bars bend
more and the viscosity of molten phase is lower.
More technically, there is also another reason for
more deformation of bodies containing less than 25%
bone ash. The key point is that tricalcium phosphate
acts as flux up to 25%, but in more percentages, it
acts like a refractory component [10] and increases
the firing temperature and the viscosity of molten
phase as well, and consequently reduces the
deformation of bar-like samples.
Fig. 9. Deformation of bodies with different bone
ash content (left hand is body with 46% of bone ash)
Nodeh et al. 82
Therefore, we can conclude that in bodies, which
contain high amounts of bone ash, tricalcium
phosphate has exceeded the 25% and has acted like a
refractory component, which has leaded to less
deformation
3.5 Color test
In the present study, the color was measured by using
a spectrometer (COLOREYE XTH, Gretagmacbeth
Co.) which was used in the reflection mode. The
color equation chosen for the instrument is CLAB
color equation in which the color descriptors are L*,
a* and b*. The L*, a* and b* system is one of the
colorimetric systems which is determined by the CIE
standard. L* is an index of illumination, +a* is an
index of redness and –a* is an index of greenness,
+b* is an index of yellowness and –b* is an index of
blueness [12].
Fig. 10 shows the illumination of bodies in the term
of L*. It can be observed that brightness decreases by
reducing the bone ash content. The curve has a slight
slope for bodies 1, 2 and 3 but an intensive slope can
be observed in bodies 4 and 5, which contain 31 and
26% bone ash. It can be concluded that decreasing
the bone ash content up to 30% does not affect the
brightness of china bodies but by decreasing it to
values less than 30% a drastic negative change can be
observed in the brightness of bodies.
Fig. 10. Illumination of bodies
It can be observed in Fig. 11 that bodies with more
phosphate are whiter and increasing bone ash causes
the bluish tint to disappear so the color changes to
creamish white. In other words, as Fe2O3 is the only
chromophore in the structure of bodies, it plays an
important role about the color of the samples. It
means that bodies with less amount of bone ash and
high amount of feldspar, as feldspar contains more
Fe2O3 in comparison with bone ash, tent to have more
colorful appearance rather than a creamish white
appearance.
Fig. 11. The color of bodies
3.6 Thermal expansion coefficient (TEC)
Fig. 12 shows thermal expansion coefficient of
bodies in various temperatures. This parameter was
measured by using Netzsch Dil 402pc instrument. It
can be seen that by lowering the amount of bone ash
and increasing K2O according to XRF results,
thermal expansion coefficient decreases. It has been
reported previously that TEC of anorthite from 20 to
500°C is 4.3×10-6 °C-1 [13], while TEC for glasses of
the compositions detected in bone china from 20 to
350°C were calculated to be 3-4.5×10-6 °C-1and the
approximate TEC of β-TCP from 50 to 400°C is
12×10-6 °C-1[14]. Decreasing bone ash in bone china
bodies leads to decreasing the amount of tri-calcium
phosphate which has a high thermal coefficient.
Then, reducing bone ash results in increasing K2O
and CaO and thus a body with lower thermal
coefficient. This can show a more stable thermal
behavior. Although, decreasing the bone ash seems to
have a positive effect on TEC but is contrary to some
other features such as color and strength (Fig. 8, Fig.
10 and Fig. 11).
Fig. 12. Thermal expansion coefficient of bodies in
various temperatures
Journal of The Australian Ceramic Society Volume 51[2], 2015, 75 – 83 83
4. Conclusion
Five bone china bodies were prepared with different
percents of bone ash with milled mix. XRD patterns
showed that by replacing bone ash by milled mix
resulted in less intense β-tricalcium phosphate (β-
TCP) and anorthite peaks. In samples with lower
amount of bone ash, SiO2 peaks at 2θ=68° were
intensified. For body with most amount of bone ash
(body number 1) no SiO2 peak was observed at
2θ=87° while by decreasing the bone ash in other
bodies, the SiO2 peaks appeared at 2θ=87°. Increasing
the milled mix results in increase in SiO2 content
caused a body with high amount of quartz crystals,
which had not been melted in the structure of the
body. The bodies with lower amount of bone ash,
showed less translucency, which was due to
formation of less glassy structure. No variation in
Water adsorption has been observed with the change
in percent of bone ash, this may be attributed to the
formation of closed pores. Increase in bone ash
content increases the formation of β-tricalcium
phosphate and thus results in higher thermal
expansion coefficient as it has higher value of
thermal expansion as compared to other bone china
components.
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