November 1983
Effect of Reducing Atmosphere on Minerals and Iron Oxides Developed in Fired Clays
With a static load time of 4 h, the fatigue limit was determined to
be 1.25 MPa.ml/z at 1400” and 1.75 at 1200°C. Strengths were
observed to increase with initial applied stress intensity at 12OO0C,
indicating a stress-dependent flaw-blunting mechanism is operativc bclow the fatigue limit.
For the as-machined sintered a-Sic, the fatigue limit was determined to be 1.75 MPaml/z at 1400” and 2.25 at 1200°C in a
nonoxidizing atmosphere. The blunting behavior was found to be
more pronounced at 1400” than at 1200°C for the as-machined
sintered material.
Crack-growth bchavior in the oxidized sintered material had
prcviously becn explained by the viscous grain-boundary separation model of Lange,19 and it was found that the predictions of
Lange’s model for the value of the fatigue limit agreed well with
that determined. This crack-growth behavior, coupled with the
blunting observed in the modified static loading tests, suggests that
flaw blunting is the result of viscous relaxation near the crack tip.
The process of crack growth and the prediction of a static fatigue
limit for the as-machined sintered material is best fitted to a grainboundary diffusion model. The blunting mechanism may be the
rcsult of diffusive creep deformation with grain-boundary diffusion
along local stress gradients reducing the stress concentrations in
the crack-tip region.
References
‘ A . G . Evans and F . F . Lan e, “Crack Propagation and Fracture in Silicon
Carbide,” J . Mater. Sci., 10 [lo! 1659;64 (1975).
’K. W . McHenry and R. E. Tressler, Fracture Toughness and High-Temperature
Slow Crack Growth in Sic,” J . Am. Cerum. Soc., 63 [3-41 152-56 (1980).
3 K . D. McHenry and R. E. Tressler, “High Temperature Dynamic Fatigue of HotPressed SIC and Sintered 0-Sic,” Am. Ceram. Sac. Bull., 59 [4] 459-61 (1980).
4 M . A. Walton and R. C. Bradt, “Dynamic Fatigue of Oxidized Silicon Carbide,”
Proc. Br. Ceram. Soc., 32, 249-60 (1982).
773
5E. Minford, J. A. Costello, I. S . T. Tsong, and R . E. Tressler, “Oxidation Effects
on Crack Growth and Blunting in SIC Ceramics”; pp. 51 1-22 in Fracture Mechanics
of Ceramics, Vol. 6. Plenum, New York, 1982.
6B. J. S. Wilkins and R. Dutton, “Static Fatigue Limit with Particular Reference to
Glass,” J . Am. Cerum. Soc., 59 [3-41 108-12 (1976).
7T. E. Easler, R. C. Bradt, and R. E. Tressler, “Strength Distribution of S i c Ceramics After Oxidation and Oxidation Under Load,” J . Am. Cerum. Soc., 64 [I21
731-34 (1981).
*S. C. Singhal, “Thermodynamic Analysis of the High Temperature Stability of
Si N, and Sic,” Ceramurgiu Int., 2 [3] 123-30 (1976).
$D.M. Kupp, “Behavior of Microflaws in Oxidized Sintered Silicon Carbide Under
Static Load at 1200°C”; B. S. Thesis, the Pennsylvania State University, August
1982.
‘OFF.F. Lange, “Healing of Surface Cracks in S i c by Oxidation,” J . Am. Cerum.
Sue., 53 151 290 (1970).
” A . G. Evans, “High Temperature Failure in Ceramics”; pp. 55-133 in Recent
Advances in Creep and Fracture of Engineering Materials and Structures. Pineridge
Press, Swansea, U. K., 1982.
12E.J. Minford and R. E. Tressler, “Determination of the Threshold Stress Intensity
for Crack Growth at High Temperature in Silicon Carhide Ceramics,” J . Am. Cerum.
Soc., 66 [5] 338-40 (1983).
])A. A. Griffith, “The Phenomena of Rupture and Flow in Solids,”PhiL. Trans. R .
Soc. London, 7221, 163-98 (1920);‘
I4W. B. Hillig and R. J. Charles, Surfaces, Stress-Dependent Surface Reactions,
and Strength’; pp. 682-705 in High Strength Materials. Wiley & Sons, New York,
1965.
I5C. E;, Inglis, ‘‘Stresses in a Plate Due to the Presence of Cracks and Sharp
Corners, Trans. Inst. Naval Archit., 55, 219 (1913).
I“B. R. Lawn and T. R. Wilshaw, Fracture of Brittle Solids, Ch. 7. Cambridge
University Press, Cambridge, England, 1975.
”8. R. Lawn, “An Atomic Model of Kinetic Crack Growth in Brittle Solids,”
J . Muter. Sci., 10, 469-80 (1975).
“R. Dutton, “The Propagation of Cracks by Diffusion”; pp. 647-57 in Fracture
Mechanics of Ceramics, Vol. 2. Plenum, New York, 1974.
I9F. F. Lange, “Non-Elastic Deformation of Polycrystals with a Liquid Boundary
Phase”; pp. 361-81 in Deformation of Ceramic Materials. Plenum, New York, 1975.
2uB.C. Allen and W. D. Kingery, ‘‘Surface Tension and Contact Angles in Some
Liquid Metal-Solid Ceramic Systems at Elevated Temperatures,” Trans. AIME, 215
[2\30-37 (1959).
‘K. D. McHenry, “Elevated Temperature Slow Crack Growth in Hot-Pressed
and Sintered Silicon Carbide Ceramics”; Ph.D. Thesis, The Pennsylvania State
University, November 1978.
22J.D. Hong, M. H. Hon, and R. F. Davis, “Self-Diffusion in Alpha and Beta
Silicon Carbide,” Cerumurgiu In?., 5 [4] 155-60 (1979).
Effect of Reducing Atmosphere on Minerals and Iron Oxides
Developed in Fired Clays: The Role of Ca
Y. MANIATIS, A. SIMOPOULOS, and A. KOSTIKAS
Nuclear Research Center Demokritos, Aghia Paraskevi, Attiki, Greece
V. PERDIKATSIS
Institute of Geology and Mineral Exploration, Messogion 70, Athens, Greece
The transformations induced in two clays differing in Ca
content, by firing under reduced conditions up to 1080°C,
were studied by X-ray diffraction, scanning electron microscopy, Mossbauer, and magnetization measurements. In the
calcareous clay, gehlenite forms at lower temperatures
(900°C) and, in addition, wollastonite forms at higher temperatures (1080°C). Ferric iron persists even under strongly
reducing conditions and its presence is attributed to trapping
in gehlenite. Extensive vitrification is observed in the noncalcareous clays. Ferrous iron, produced by dissociation of
iron oxides, is partly dissolved into the vitreous matrix and
partly incorporated into the spinel mineral hercynite. The key
role of Ca in controlling the above transformationswas verified
by studying the clays after removal or addition of calcite. The
interaction of Ca with the clay constituents and its progressive
Received March 8, 1983; revised copy received June 23, 1983; approved June 29,
1983.
attack on the quartz grains forming wollastonite zones was
observed with the electron microscope. The bulk magnetic
properties of the samples depended principally on the amount
of iron oxides present, which in turn were strongly affected by
firing temperature and type of clay. Metallic iron was detected
in strongly reducing atmospheres.
I. Introduction
A
N UNDERSTANDING of
the refractory properties of natural clays
fired under reducing conditions is of great interest to brick and
pottery manufacturers. Furthermore, understanding the transformations induced in the state of iron and the iron oxides under
these conditions is of major importance with regard to the coloration of the finished product.’ Apart from the importance for
modem industry, there is growing interest in the understanding of
the ancient ceramic te~hnology,~
which involved primarily the use
of natural clays fired under oxidizing, but also, for several characteristic types of pottery, under reducing conditions. Finally, deter-
114
Vol. 66, No. 11
Journal of the American Ceramic Society-Maniatis et al.
Table I.
Illitc
Chloritc
Kaolinite
Feldspars
Quartz
Calcite
r--
Chemical and Mineralogical Analyses*
of Raw Clays
Karfi clay
Connth clay
0.38%
2.40
20.80
54.20
2.45
8.70
0.98
9.90
99.81
0.60%
2.50
9.70
36.10
1.70
4.80
22.20
22.10
99.70
++
++
+
+
+++++
-
+
+
+
++++
I
I
I
-
38%
* B y atomic absorption and X-ray diffraction, respectively.
mination of the amount and kind of iron phases developed in fired
clays in association with the bulk magnetic properties is of great
interest to archaeomagnetism, which involves the study of the
recorded history of the magnetic field of the earth in baked clay.5
Up to now the bulk of the investigations on the changes that
occur in natural clays during firing has dealt with firing under
oxidizing conditions. For example, Grim,6 Segnit and A n d e r ~ o n , ~
and Peters and Jenni8 studied the development or disintegration of
the mineral phases for a range of different starting materials.
The important role of Ca was investigated by Freeman' and West'"
and its influence on the microstructure of ceramics by Tite and
Maniatis." A detailed study of the iron phases which develop in
natural clays during firing in oxidizing conditions was reported
earlier (Maniatis et ul.").
Firing in reducing conditions has also been studied, to a limited
degree. Harrell and Russell13suggested that the mechanism which
operates on the iron oxides in reducing atmospheres:
(1)
Fe2O3-tFe3O4+FeO+Fe
produces, above 700"C, mainly FeO which is a very reactive flux
assisting the solid state reactions. W e d 4 suggested that the reaction of FeO+CaO+A1,03+ SiOz produces black glass in calcareous clays under strongly reducing conditions, whereas calcium
fcrrites are produced in slightly reducing conditions. Maniatis and
Tite" showed that a low-viscosity glass with a lot of bloating pores
develops in the microstructure of noncalcareous clays, whereas the
effect is much less dramatic in calcareous clays. Heimann et uZ.l6
gave a range of iron oxides and iron-bearing minerals which are
formed in a calcareous clay under various reducing conditions.
Finally, Chevalier et ul .l7 studied the iron phases in a calcareous
Fig. 1. Typical magnetization and hysteresis curves of (a)Corinth
clay fired at 900°C and ( b ) Karfi clay fired at 900°C. Insert is
magnification of region around zero field.
clay under reducing conditions using Mossbauer spectroscopy and
showed the progressive increase of Fez+ diluted in silicates at the
expense of F$+ and of the poorly crystallized iron oxides.
The available evidence, cited above, does indicate that Ca plays
a dominant role in determining the ceramic properties end mineral
phases in clays fired under reducing conditions, as it does in
oxidizing atmospheres. However, the mechanisms operating on a
microscopic level, from a chemical and mineralogical point of
view, have not been investigated systematically. Furthermore,
very little information is available on the transformations in the
iron-bearing minerals in clays fired under reducing conditions and,
in particular, on the iron oxides. There is no information on the
possible interaction between Ca and Fe. Finally, there is no available information on the bulk magnetic properties of clays fired
under reducing conditions at high temperatures and their cor-
Table 11. Distribution of Iron Phases Derived from Mossbauer Spectra
Karfi Cldy
Corinth clay
Fe'+
Oxides
Fez+
Fe3+
Oxides
7
4
3
67
28
22
26
68
75
23
18
15
69
39
38
8
43
47
RT
LN
LHe
46
43
30
21
23
10
33
34
60
41
39
37
33
27
12
26
34
51
900
900
900
RT
LN
LHe
100
100
0
0
0
0
16
26
42
31
42
43
1080
1080
1080
RT
LN
LHe
100
100
0
0
0
0
42
46
36
53
44
26
5
10
38
Firing temp ("C)
Temp of measurement*
Raw
Raw
Raw
RT
LN
LHe
700
700
700
Fez'
*RT=room temperature (293 K), LN=liquid nitrogen temperature (80 K), and LHe=liquid helium temperature (4 2 K)
Effect of Reducing Atmosphere on Minerals and Iron Oxides Developed in Fired Clays
November 1983
relation with the presence of iron oxides. Therefore we have
undertaken a detailed investigation, by a variety of experimental
techniques, of two clays, one calcareous and one noncalcareous,
fired under reducing atmospheres up to 1080°C. This study
parallels an earlier investigation of these two clays fired under
oxidizing atmosphere.
775
Table 111. Magnetization Results for Fired Clays
Value
Magnetic parameter
Raw
700°C
900°C
900'C*
1080°C
Karfi clay
M , (Am2/kg Fe) 1.22
18.71
M , (Am'/kg Fe) 0.28
5.75
Bc (TI
0.0097 0.0119
0.31
M,IM,
0.22
11. Materials and Methods
1.0
0.33
0.0206
0.33
7.88
1.32
0.0400
0.17
0.24
0.07
0.0091
0.29
9.52
46.73
22.32
2.68
7.76
14.28
0.0130 0.0163 0.0371
0.28
0.30
0.33
4.14
1.19
0.0231
0.29
Corinth clay
Two naturally occurring clays in Greece were selected for the
present study: a calcareous clay from a bed near the archaeological
site of Corinth and one noncalcareous from a bed near the archaeological site of Karfi in Crete. The results of chemical and
mineralogical analyses of the raw materials are listed in Table I.
In a previous study, these clays were found to be representative of
two groups of calcareous and noncalcareous clays, respectively.
The firing was done in an electric furnace with a heating rate of
200"C/h and a I-h soaking time at the top temperature. The cooling rate was also 200"C/h. A continuous flow of nitrogen provided
a neutral atmosphere which, with the help of the organic matter in
the clay, created reducing conditions in the clays. When stronger
reducing conditions were investigated, the samples were placed in
a boat filled with sawdust. The firing temperatures were 700°,
900", and 1080°C. These values were selected as characteristic of
ranges where mineralogical changes occur.
In addition, samples of the Corinth clay were treated for 30 min
with 10% acetic acid (pH= 1.5 to 2 ) at room temperature in order
to dissolve the calcium carbonate content. The samples were
washed after treatment. Mossbauer and XRD spectra of the treated
samples taken prior to firing showed that all iron and mineral
phases were intact. Further chemical analysis of the treated samples showed a small decrease in Mg by approximately 6% and in
Si by 3%. The decrease in Si could be due to loss of some of the
larger quartz grains due to sedimentation during washing. However, there was no effect on Fe or Al, whereas CaO was down
to 0.91 %, indicating complete removal of calcite.
Fine-grained, chemically pure CaCO, was added to the Karfi
clay in two ratios: 13 and 33%.
Chemical analyses were done by atomic absorption and the
water and organic content were determined using a thermoanalyzer. * X-ray diffraction analysis was done using a diffractometer' with a vertical goniometer. A commercial analyzer*
was used to study the microstructure and for the microanalysis.
M,v (Am'/kg Fe) 0.18
M , (Am'/kg
. - Fe) 0.05
B, (TI
0.0099
M,IM,
0.25
I
*Clays fired with sawdust
The beam diameter, according to manufacturer specifications
and our measurements, was 1 pm. Mossbauer spectroscopy was
applied to identify the iron-bearing minerals and determine the
iron distribution in the various phases. Spectra of 150-mg samples
were obtained at room, liquid nitrogen, 50 K, and liquid helium
temperatures with a conventional constant acceleration spectrometer. The source was 100 mCi 57Co(Rh)at room temperature.
Finally, magnetization measurements were performed on a vibrating sample magnetometer.'
111. Results and Discussion
(1) Karfi Clay
This noncalcareous clay contains in its raw form basically the
clay minerals chlorite and illite (Table I). The iron-bearing phases
are determined best by liquid helium Mossbauer spectra where
superparamagnetic phenomena have relaxed. These spectra show
that 75% of the total iron is in the form of cr-Fe203iron oxides and
oxyhydroxides, whereas the remaining 25% is in paramagnetic
form as ferric (Fe3+)and ferrous (Fez') ions in the clay minerals"
at a ratio of 7:1 (Table 11). The spontaneous magnetization ( M s ) ,
the remanence magnetization ( M r ) , and the coercive force are
determined by room temperature (isothermal) magnetic measurements (Fig. 1). The values of the above three parameters (Table 111)
for the raw Karfi clay agree well with hematite particles (a-FeZO3)
which are magnetically unsaturated even at applied fields of
1.8T.l 9 On firing at 700"C, all the XRD lines of the clay minerals
disappear (Table IV), indicating that dehydroxylation is completed
___
'Mettler Inatrument COT., Hightstown, NJ.
+Philips Gloeilarnpenfabrieken N. V., Eindhoven, The Netherlands.
'JEOL Super-probe 733, Japan Electron Optics Co. Ltd., Tokyo, Japan
%Princeton Applied Research Corp., Princeton, NJ
Table IV. X-Ray Diffraction Data
Clay
Karfi
Corinth
Karfi
Corinth
Karfi
Corinth
Karfi (sawdust)
Corinth (sawdust)
Karfi
Corinth
Corinth
(no CaC03)
Karfi
(+ 13% CaCO1)
Karfi
(133% CaCO?)
Illite
++
+
Chlorite
++
+
Kaolinite
+-
Quartz
+++++
++++
+++++
++++
+++++
++++
++++
++++
+++++
+++
+++t
Feldspars
Raw clay
+
+
at
+
+
Fired
Fired
at
-
Calcite
Hercynite
Gehlenite
Ca(OH),
Anorthite
-
38%
700°C
-
+
900°C
-
-
++
++
-
+
-
-
++
++
-
++
-
+++t
+++t
Wollastonite
t
++
+
116
Journal of the American Ceramic Society-Maniatis et al.
Vol. 66, No. 11
Fig. 2. Backscattered electron micrographs of Karfi clay (A) fired at 700°C (elongated particle is mica flake and large particle at bottom is quartz
grain), ( B ) fired at 900°C (large grain at top left is feldspar particle), and ( C ) fired at 1080°C (bar=10 pm).
Fig. 3. Mossbauer spectra of K a f i clay (a) fired at 700"C,
mcasured at room temperature (stick diagram indicates
Fe'' component (I), Fe2+component (11), and iron oxides component (111)); (6) fired at 1080"C, measured at liquid helium
temgerature (stick diagram indicates hercynite lines); (c) fired at
900 C under strongly reducing conditions (buried in sawdust),
measured at room temperature (stick diagram indicates metallic
iron lines).
ncar this temperature.' A peak persists however at 0.449 nm,
which may be attributed to the (110)reflection of illite arising
probably from the dehydrated illite structure, an "anhydrous
modification" which is only slightly different from the hydrated
onc. Examination of polished surfaces by SEM and microprobe
reveals that the microstructure is compact but not vitrified
(Fig. 2(A)). It can be seen that there is a range of iron oxide
particles (bright particles) of irregular shapes, varying in size from
5 p m down to much below 0.5 pm. Mossbauer results of this fired
clay indicate that there is a general decrease in the amount of
original iron oxides from 75% in the raw to 60% in the fired form
and a corresponding increase in the paramagnetic iron forms from
25% to 40%. The Fez' component dominates, now giving an
Fe3+/Fe2" ratio of 1:3. The room-temperature spectra (Fig. 3(a))
display broad magnetic lines, indicating a range of magnetic hyperfine fields and therefore oxide particles with varying sizes. An
analysis of those spectra with two magnetic components gave
isomer shifts and quadrupole interactions (Table V) which are
similar to magnetite but with lower values for the hyperfine fields,
indicating poor crystallization. The presence of magnetite in this
clay is further supported by its magnetization data. The magnetization of the sample increases drastically from the low M, value of
1.22 Am2/kg Fe in the raw clay to 18.7Am2/kg Fe after firing
at 700°C (Table 111). In addition the ratio M,/M,=0.3,which is
near the value for pure magnetite." The M, value of this sample
on the other hand is much lower than that of the pure magnetite,
presumably due to poor crystallization. Poor crystallization and
small particle sizes can also explain the absence of magnetite lines
in the XRD data. The color of the sample is gray.
Firing at 900°C produces much more pronounced changes in the
clay body. The SEM picture (Fig. 2 ( B ) ) shows that the sintering is
quite advanced and much vitrification has been produced. Within
this glass matrix a network of fine bloating pores has developed,
ranging in size from 0.1 to 5 pm. The comparison of the 900°C
picture (Fig. 2(B)) with that at 700°C (Fig. 2(A)) shows a somewhat brighter matrix at 900°C.This indicates disintegration of iron
oxides and diffusion of iron into the glass matrix, a conclusion that
is clearly substantiated by the Mossbauer spectra (vide infra). The
resulting increase of iron content in the glass could, in principle,
be detected by microprobe analysis. In this particular case, however, the small size of a large fraction of iron oxide particles at 700°C
70.03 pm), which are dispersed in the matrix, prevents a differentiation from iron diffusion in the glass at 900°C in view of the
cross section of the electroprobe beam (1 X 1 pm2).
The distribution of iron and the changes in the mineralogy of the
clay at 900°C are understood better with the Mossbauer measurements. They show that all the iron oxides have disintegrated
completely and all the iron has been reduced into ferrous form. The
spectra at temperatures down to 50 K show only a doublet with
large linewidth. The best fit was obtained with three superimposed
ferrous doublets (Table V), although it is possible that a distribution
of more than three sites exists. When the temperature is decreased
to that of liquid helium, a spectrum appears with magnetic hyperfine interaction typical of Fe2+ complexes and a ferrous doublet with quadrupole splitting of 2.2 mm/s and isomer shift of
1.1 mm/s. The fractional area of the two components is estimated roughly at 60 and 40%, respectively. The absorption peaks
of the magnetic components correspond to those of the mineral
hercynite (FeA1204),20which undergoes a magnetic transition at
8 K." This phase develops further by firing at 1080°C (Fig. 3(b)).
The presence of hercynite in this sample is also verified by the
XRD measurements, which show diffraction lines at 0.143 and
0.245 nm (Table IV). They show also a lot of quartz (which
masks the rest of the hercynite lines) but no other minerals. The
November 1983
Effect of Reducing Atmosphere on Minerals and Iron Oxides Developed in Fired Clays
Table V.
Component*
Hyperfine Parameters and Relative Absorption of Fe Phases Derived from Room-Temperature
Mossbauer Spectra of Karfi Clay
Firing temp. (“C)
SI
Raw
700
900
1080
S?
Raw
700
900
1080
DI
(Fez+)
D3
(Fez+)
777
H (T)
eZqQ /4 (mm/s)
6’ (mm/s)
r/2 (mmls)
Fraction (%)
49.8
47.3
-0.12
-0.02
0.3 1
0.37
0.46
0.43
26
22
40.9
-0.06
0.60
0.42
11
Raw
700
900
1080
1.38
1.04
1.19
0.84
1.04
1.09
1.23
0.95
0.24
0.37
0.25
0.23
7
46
23
22
Raw
700
900
1080
0.31
0.48
1.10
0.52
0.30
0.36
0.94
0.95
0.27
0.33
0.24
0.27
67
21
13
49
Raw
700
900
1080
0.64
1.10
0.95
0.98
0.42
0.21
64
29
* S , and S, represent Fe oxides and D , , D 2 . and D, paramagnetic iron ions. ‘With respect to Fe at room temperature.
lack of any other mineral phases suggests that the paramagnetic
component observed in the liquid helium Mossbauer spectra consists of Fez’ ions diffused or diluted in the glass matrix. This component is probably responsible for the dark gray color that the clay
achieves after firing at 900°C.
It is worth noting that we have detected hercynite in the Mossbauer spectra of a number of ancient ceramics of a special group
of gray pottery called “Grey Minyan” manufactured in Lerna of
Argolis (Greece) between 2000 and 1600 BC. Also this mineral
was found to be present in black decorations of Campanian pottery
of the 4th to 1st centuries BC.”
Further experiments under stronger reducing conditions, i.e.
samples sitting on a layer of sawdust or buried in it and fired at
900°C, revealed again the formation of hercynite together with
metallic iron (Fig. 3(c)). From the room temperature spectra it was
observed that the amount of metallic iron formed was greater in the
samples buried than in those sitting on sawdust. The presence of
metallic iron indicates extremely strong reducing conditions inside
the clay body of the order of =lo-’ Pa of oxygen.23
The magnetization data for the Karfi sample fired at 900°C show
a sharp decrease in the values of M , and M , relative to the sample
fired at 700°C (Table III), implying almost complete paramagnetic
behavior. This result is consistent with the disintegration of iron
oxides and the formation of paramagnetic ferrous iron and hercynite, which is paramagnetic at room temperature. The higher
value of M , for the sample fired in a strongly reducing atmosphere
can be attributed to the presence of metallic iron. The values of
0.30 for the ratio M,/M, for both firing temperatures indicates the
presence of pseudosingle magnetic domains or a mixture of single
and multidomain
However, the value of 0.17 obtained
for the sample at 900°C with sawdust approaches the clear multidomain value, which may indicate that metallic iron, the only
magnetic phase here, has been crystallized mainly in a multidomain pattern.
Firing at 1080°C results generally in an enhancement of the
features observed in the sample fired at 900°C. The SEM picture
(Fig. 2(C)) shows a pronounced increase in the amount of glass
and in the size of the bloating pores which are now in the range
of 5 to 100 pm. Higher brightness in the backscattered electron
picture relative to 900°C (Fig. 2(B)) indicates further dilution of
iron into the glass. The quartz grains are surrounded by glass and
the small ones appear rounded, indicating their gradual fusion into
the glass. No iron oxides are visible. The XRD results indicate a
much better crystallization of hercynite, with most of the stronger
diffraction lines present. Apart from quartz, this is the only crystalline structure detected (Table IV). The Mossbauer results show
an increase in the amount of hercynite and a corresponding decrease of the paramagnetic Fez+ component.
The bulk magnetic properties of the sample fired at 1080°C
display clearly paramagnetic behavior at room temperature, the
magnetization being almost linearly proportional to the applied
field. This agrees with the presence of iron, mostly in hercynite,
which is paramagnetic at room temperature, and as ferrous iron
diluted in the glass matrix. Macroscopically the sample appears to
be dark gray and swollen.
It is worth noting that phases like fayalite (Fe2Si04) or iron
cordierite Al3FezSi5AlOl8,stable at these condition^,^^ have not
been formed in the Karfi clay, in contrast to the observations of
Heimann et al., l 6 who reported the formation of these two minerals
above 1000°C under strongly reducing conditions (fez< 10 Pa).
The failure of this formation may be attributed to the high amount
of A1203present in the Karfi clay, which does not favor the formation of f a ~ a l i t e . *On
~ the other hand, the very narrow space of
the phase diagram occupied by iron cordierite makes the formation
of this mineral rather difficult, especially under partial equilibrium
conditions as they are in the present work.
(2) Corinth Clay
This is a calcareous clay containing initially 30% calcite. The
basic clay minerals are again illite and chlorite (Table I). The iron
is distributed almost equally between the clay minerals and the
oxides. In fact it is found from room temperature and liquid helium
Mossbauer measurements (Table 11), that 47% of the iron is in the
form of oxides, of which 39% are smaller than 0.03 pm. The
remaining 53% of the iron is ferric and ferrous paramagnetic ions
incorporated in the minerals chlorite and illite, the ratio ferric to
ferrous being 2.5:l. Comparing with Karfi clay, we observe that:
( a ) There is a smaller total amount of iron (Table I) and ( b ) there
is a smaller percentage of iron in the form of oxides which now are
predominantly of small particle sizes.
The magnetic measurements (Table 111) of the raw Corinth clay
show paramagnetic behavior which is justified by the superparamagnetic state of the iron oxides manifested by the Mossbauer
Firing at 700°C results in decomposition of the clay minerals
illite and chlorite. All lines disappear from the XRD spectrum
(Table IV), with the exception of the second-order weak line of the
“anhydrous modification” of illite, as in the case of the Karfi clay.
Vol. 66. No. I1
Journal of the American Ceramic Society-Maniafis et al.
I18
Fig. 4. Backscattered electron micrographs of Corinth clay (A) fired at 700°C (lighter particles are calcite and darker ones quartz), ( B ) fired at
900"C, and (C) fired at 1080°C (bar= 10 wm).
Table V1.
Hyperfine Parameters and Relative Absorptions of Fe Phases Derived from Room-Temperature
Mossbauer Spectra of Corinth Clay
Firing temp. ("C)
ff (TI
eZqQ14 (mm/s)
fit (mm/s)
r/2 (mm/s)
Fraction (%)
SI
Raw
700
900
1080
51.4
47.0
49.5
-0.06
-0.11
-0.06
0.25
0.30
0.32
0.29
0.79
0.37
8
26
20
S2
Raw
700
900
1080
0.05
0.33
0.86
22
Component*
45.0
D,
(Fe")
Raw
700
900
1080
0.31
0.52
0.57
0.47
0.21
0.37
0.30
0.43
0.27
0.45
0.40
0.41
69
33
42
56
.&
Raw
700
900
1080
1.30
1.03
1.16
1.24
0.96
1.05
1.17
0.94
0.24
0.36
0.43
0.28
23
41
16
44
(Fez'-)
"S,and Sz represent Fc oxides and D , and D , paramagnetic iron ions. +With respect to Fe at room temperature.
Calcite is still present since it decomposes at higher temperatures.
Thc micrographs (Fig. 4(A)) show clearly that the calcite particles
are intact and they range in size from 1 to 20 pm. Iron oxides are
rare and small, the visible ones being between 0.5 and 1 p m (sce
small bright particle at bottom left side of Fig. 4(A)). It is evident
that no solid statc reactions have taken place as yet. Comparing
the microstructure with Karfi at 700°C shows that Corinth is generally a coarser clay. The iron oxide particles however, are generally finer.
The Mossbauer results at room temperature (Table 11) indicate
that the small amount (8%) of the larger iron oxides existing in the
raw clay has increased to 26% at 700"C, but the effective field
(Tablc V1) has decreased from 51.4 to 47.0 (T), indicating a change
in the nature of the oxides. The total amount of the oxides has not
changed significantly (liquid helium data in Table 11). Therefore,
thc small change in the total amount of the iron oxides present
implies that there is not a significant diffusion of iron from the
oxides to the clay minerals, as is the case with the Karfi clay at
700°C. The amount of iron in the dehydroxylated clay minerals
has rcrnained the same, but more than half of the ferric iron has
been converted to ferrous (Table 11). As with the Karfi clay, it is
difficult to identify the kind of iron oxides due to the broad features
of the spectra. However, the decrease in the average hyperfine
ficld, thc increase in the spontaneous magnetization of the sample
(Tablc IIl), and its tendency to saturate at increased magnetic fields
indicate possible production of magnetite. The sample is gray,
similar to Karfi at the same temperature.
Firing at 900°C produces dramatic changes in the Corinth clay.
The SEM micrographs (Fig. 4(B)) reveal that solid state reactions
are well under way. The fiberlike clay particles present at 700°C
have now disappeared and much sintering has taken place. The
calcite particles have practically all decomposed and Ca has diffused into the matrix, which appears brighter in the backscattered
pictures compared to 700°C. Microprobe analysis indicates an
average increase of Ca in the matrix compared to 700°C, although
the large variation from site to site makes the analysis somewhat
inconclusive. Some small ( ~ 0 . 5
pm) spherical bubbles have been
formed, indicating the presence of low-viscosity glass, although
they are not as extensive as in the case of Karfi clay fired at
900°C. The edges of quartz particles (Fig. 4(B)) appear to be
brighter and the microprobe analysis shows a fine layer rich in
Si and Ca. In fact qualitative energy dispersion spectra suggest
the formation of wollastonite (CaSi03).
The XRD results (Table IV) verify the complete dissociation of
calcite and show two very weak lines of Ca(OH)2, which apparently is a postdissociation product arising from reaction of the
remaining CaO with water vapor. The XRD spectrum also indicates the formation of a substantial amount of gehlenite and the
beginning of formation of wollastonite, as indicated by the presence of its strongest line. Gehlenite must have been formed inside
the sintered material after the reaction of CaO with the A1,03 and
Si02 of the clay minerals. The crystals of gehlenite and wollastonite must be very small since they are not discernible by SEM
even at higher magnifications. No iron-bearing minerals were
observed with XRD. This may be due to the low iron content of
this clay.
The presence of components with intermediate spin relaxation in
the liquid helium Mossbauer spectra (Fig. 5 ( a ) ) hinders a quan-
November 1983
779
Effect of Reducing Atmosphere on Minerals and Iron Oxides Developed in Fired Clays
titative analysis of the iron phases in this clay. Both room
temperature and liquid helium spectra, however, show a distinct
increase in the iron oxide phases, mainly at the expense of the
Fez' component of the clay fired at 700°C (Table 11). The roomtemperature spectra can be fitted with two magnetic components.
The parameters of this fit indicate nonstoichiometric magnetite
with poor crystallizationz6(Table VI). The magnetic measurements
further support the presence of magnetite since they show an increase in both the spontaneous and remanence magnetization.
The increase in the iron oxide phases at the expense of iron in
other phases, particularly Fe2+,and the appearance of phases with
intermediate relaxation at this temperature indicate major alterations in the crystalline phases and agree with the XRD data, which
show formation of gehlenite. Gehlenite can take some ferric iron
in replacement of aluminum sites, thus explaining the presence of
ferric iron under these reducing conditions (note that in Karfi there
is no ferric iron remaining at 900°C). A small amount of ferrous
iron probably still exists in the glass or other unidentified semiamorphous phases which cannot be picked up by XRD. The sample
becomes a very light gray.
Firing Corinth clay at 900°C in stronger reducing conditions
(sample placed in sawdust) results in the reduction of the paramagnetic ferric iron and a large increase in the ferrous component,
together with the appearance of metallic iron, indicating strongly
reducing conditions inside the clay body. The XRD again shows
only gehlenite (Table IV), which must be responsible for the ferric
iron remaining even under these strongly reducing conditions.
Metallic iron must be in fine aggregates since it does not appear in
the XRD spectra. Also, the paramagnetic ferrous iron is probably
either in the glass matrix or in some poorly crystallized ferrous
iron-bearing mineral. The formation of metallic iron leads to strong
ferromagnetic behavior with large spontaneous and remanence
magnetization values as well as a large coercive force (Table 111).
These values are the strongest encountered in the present work and
they are probably due to the existence of magnetite together with
the metallic iron shown in the Mossbauer spectra. The usefulness
of the Mossbauer measurements in studying the changes in the iron
phases and in explaining the bulk magnetic properties is emphasized since the XRD seems insensitive to the iron-bearing phases
in the fircd clay, presumably because they are in the form of very
small particles and/or because they are not well crystallized.
Whcn fired at 1080°C the microstructure remains essentially
the same as at 900°C (Fig. 4(C)), or slightly coarser due probably
to the formation of more glass. What is much more evident is the
diffusion of Ca into the quartz grains, increasing the wollastonite
layer on the surface to a thickness of about 1 to 1.5 pm. The small
quartz grains have almost completely reacted with CaO. To further
illustrate the diffusion of Ca into quartz grains we obtained a Ca
profile on a quartz grain at high magnification (Fig. 6). This picture shows a peak in the amount of Ca as the beam sweeps over the
bright surface area of the grain and decreases continuously toward
the center. Considering that the beam diameter is = 1 p m (see
Section II), Ca can be seen diffused to a depth of about 2 pm.
The XRD results (Table IV) show quite clearly the formation of
substantial amounts of wollastonite formed at the expense of S O z ,
which decreases in quantity. This is the result of reaction of quartz
grains with CaO. Gehlenite appears to be better crystallized but not
much different in amount at this temperature. The possibility of
small amounts of hercynite cannot be ruled out since its strongest
lines, which coincide with gehlenite, appear stronger in intensity
than would be expected for gehlenite.
The Mossbauer results (Table 11, Fig. 5).show a strong decrease
in the amount of iron oxide particles and their diameters relative to
the 900°C sample. Since magnetic ordering is not seen at 50 K, but
only at 4.2 K, we must be dealing with particle diameters of the
order of 0.005 pm.2"-2* This form in fact accounts for 38% of the
total amount of iron. The balance is 26% Fez+and 36% Fe", both
in paramagnetic form. The ferric ion is probably in the gehlenite
phase, which is well crystallized at this temperature, but the ferrous
formation cannot be easily understood. Certainly it is not hercynite
because this mineral is ordered magnetically at 4.2 K. It is worth
pointing out the persistence of ferric iron in reducing conditions
..*. .
970 I
I
~~
--t
.
_,-_-8
t----++
-.+
-4----C
-6
-4
.z
uELNCITY
u
L
4
6
0
10
!MM/S)
Fig. 5. Mossbauer spectra of Corinth clay taken at liquid helium
temperature for clay fired at (a) 900°C and ( b ) 1080°C.
Fig. 6 . Calcium profile across quartz particle
at high magnification (bar=l pm).
even at these high firing temperatures, which contrasts with the
behavior of Karfi clay under the same conditions. The fired clay
body becomes a pale gray, lighter than the 900°C sample, which
is consistent with the observed general decrease in amount and size
of the iron oxide particles.
The magnetization of the sample fired at 1080°C drops drastically to low values of M , and M,. (Table 111), which is consistent
with the dissociation of magnetic iron oxides and the diffusion of
iron into paramagnetic mineral phases observed in the Mossbauer
results. Gehlenite and wollastonite were observed to form in this
clay. According to equilibrium phase diagram^,'^ however, anorthite, hercynite, and fayalite should also be present in a system
Ca0-Fe0-Al2O3-SiOZsuch as the present one. The lack of formation of fayalite and hercynite is probably due to the low iron
content of the Corinth clay. These phases, however, were observed
in a fired German clay16with the same iron content but with much
lower amount of Ca. Peters and Jenni8 suggested the formation of
anorthite and wollastonite at the expense of gehlenite according to
the following scheme:
(2)
CazAlzSiO7+SiOZ-CaSiO3+CaAl,Si,O,
The lack of anorthite in the Corinth clay and the stability or increase instead of a decrease of gehlenite on firing indicate that,
instead of the above reaction, wollastonite is formed in this clay via
the reaction:
CaO +Si02+CaSi03
(3)
Journal of the American Ceramic Society-Maniatis
Fig. 7. Mossbauer spectra of treated clays taken at liquid
hclium temperature. ( a ) Corinth clay without CaCO,, (b)Karfi
clay+l3% CaCO?, and ( c ) Karfi clay+33% CaC03. Arrows
indicate possible hercynite peaks.
Similar to our observation is that of B r o ~ n e l l , ~who
" also did not
observe anorthite formation in a Ca-rich clay.
The CaO and Si02participating in the above reaction come from
the remaining unreacted CaO and the quartz grains. Indeed, from
the cvidcnce obtained from the micrographs, it is clear that the
amount of SiO, initially participating in the reaction is much lower
than the total of 36% found by the analysis. This is due to SiO,
present as large quartz particles which do not react initially.
(3) The Role of Calcium
So far we have seen that the two clays fired in reducing conditions are characterized by the following behavior: Both clays at
700°C contain very similar distributions of iron between the various phases. However, at higher firing temperatures, where CaO is
being activated, there are marked differences between the two
clays. In the noncalcareous Karfi clay only one mineral is formed
(the ferrous mineral hercynite) from 900°C upward, together with
large quantities of glass. In the calcareous Corinth clay the mineral
gehlenite is formed from 900" and wollastonite at 1O8O0C,whereas
the same minerals are formed under oxidizing conditions," indicating that their formation is independent of the firing atmosphere.
Ferric ions, presumably incorporated in gehlenite, survive in this
clay even under the most severly reducing conditions. There is also
a marked difference in magnetization between the two clays, especially at 900°C.
To clarify the role of Ca for the mineral formation in these two
clays, we prepared their "mirror images" with respect to Ca by
adding calcite in the Karfi clay (13% and 33% by weight) and
removing the calcite from the Corinth clay, as described in
Section 11.
The appcarance of these two clays after firing at 1080°C was
opposite to that of the untreated clays. The Karfi clay with the
added CaC03 is light in color without deformation, like the
untreated Corinth clay, whereas the decalcified Corinth clay is
dark-gray, deformed, bloated, and double in size, similar to the
et al.
Vol. 66, No. 11
untreated Karfi clay. The role of Ca therefore seems to be very
important even in the macroscopic properties of the ceramics (i.e.
color, bloating, and deformation).
The XRD results showed that the treated Corinth clay exhibited
hercynite peaks together with the quartz peaks. The hercynite is
well crystallized since practically all the lines are present. Note that
in the untreated Karfi clay only the three stronger hercynite lines
were observed. The Mossbauer spectra of the treated Corinth clay
showed that most of the iron now is in ferrous form. At liquid
helium temperature (Fig. 7 ( a ) )part of the ferrous iron is magnetically ordered, exhibiting the hercynite spectrum. However, the
amount of iron present as hercynite is much less than in the untreated Karfi clay. These results, combined with the XRD data,
indicate that the hercynite which is formed in the Corinth clay after
the dissolution of CaC03 is much better crystallized than in the
untreated Karfi clay but there is less of it. This may be due to
different distributions of Fe and different particle sizes of the
oxides in the original raw clay.
The XRD results of the Karfi+l3% CaCO? (7.3% CaO) show
clearly the formation of anorthite, with no other calcium aluminosilicate present. The presence of a small amount of hercynite
cannot be ruled out. According to Mossbauer results, at room and
liquid helium temperature iron is distributed between large and
small iron oxides and paramagnetic ferrous ions from which a
small amount is ordered at liquid helium temperature in the hercynite arrangement (Fig. 7 ( b ) ) .In this sample, therefore, we have
anorthite, iron oxides, and hercynite.
The XRD results of the Kar€i+33% CaC03 (18.5% CaO) indicate the formation of gehlenite in addition to anorthite, which is
also present. The Mossbauer spectra (Fig. 7 ( c ) ) indicate that part
of the iron is in the form of oxides and the rest is divided equally
between paramagnetic ferrous and ferric ions. No hercynite was
detected at liquid helium temperature. Thus, as in the case of the
untreated Corinth clay, the ferric iron is again associated with the
presence of gehlenite. These results produce further convincing
evidence that gehlenite accommodates ferric iron in its structure,
probably in A1 sites, and that the formation of this Fe-gehlenite
phase is not inhibited by the reducing conditions.
The experiments with the treated samples prove the essential
role of Ca in the mineralogy, vitrification, and refractory properties
of fired clays in reducing conditions. The elimination of Ca from
the Corinth clay led to the formation of hercynite, together with a
low-viscosity glass and bloating. The addition of Ca to the Karfi
clay produced crystalline Ca-A1 silicate phases and hindered the
bloating and formation of the hercynite. In addition, the formation
of iron oxides in the naturally calcareous Corinth clay and in the
artificially calcareous Karfi clay, as opposed to the total absence of
iron oxides from the naturally noncalcareous Karfi clay and (partial
absence) in the artificially noncalcareous Corinth, strongly indicate
the effect of Ca on the iron phases.
Summarizing, the foregoing results in conjunction with our previous work on clays fired in an oxidizing atmosphere," lead to the
following tentative picture of the mechanism of the key role of
calcium in the formation of iron oxide phases. In the noncalcareous clays, either in reducing or oxidizing conditions, there
is lack of crystallization during firing and the destruction of the
original clay minerals (which contain an amount of iron) is complete above 800" to 850°C. These minerals turn into an amorphous
state, as evidenced by the glass which is produced in large quantities. '531 Under these conditions iron is released from the minerals
and, if the atmosphere is oxidizing, it forms iron oxides and,
specifically, a-Fe201. If it is reducing, iron is reduced to divalent
form and the initially existing oxides are dissociated. The resulting
ferrous iron is partly dissolved into the glass matrix and partly
forms hercynite. In the calcareous clays the destruction of the
initial clay minerals is followed immediately by the formation of
calcium aluminosilicates which trap ferrous and/or ferric ions under either reducing or oxidizing conditions (ferrous ions are absent
in the latter conditions). Thus, in calcareous clays iron passes
directly into the new phases and can remain ferric even in reducing
conditions. The openness and the low density of the microstructure
of these clays probably let the reducing agents (i.e. organic re-
November 1983
78 I
Effect of Reducing Atmosphere on Minerals and Iron Oxides Developed in Fired Clays
mains, CO, ctc.) burn away and so the semiferrous iron oxides,
like magnetite, which are formed at low temperatures in reducing
conditions, remain undissociated up to higher temperatures. In
oxidizing conditions the calcium aluminosilicate minerals are
probably better crystallized and greater in amount and therefore
they can accommodate larger amounts of ferric iron supplied by the
iron oxides which in turn become smaller in size and amount.”
In the light of the above results, the presence of hercynite,
observed in ancient pottery (Grey Minyan) and in black slip decorations,” is directly related to Ca-free low-refractory clays fired in
reducing atmospheres at or above 900°C. This may suggest the
deliberate use of such clays to obtain dark-gray, high-contrast
colors. On the other hand, the consistent light-gray coloration of
certain types of ancient pottery is due to the use of calcareous clay
fired under the same conditions. It is worth noting that calcareous
clays have not been applied for black slip decorations.
Finally, an interesting result, relevant to archaeomagnetic
studies, is the indirect effect of the presence of Ca on the magnetic
properties, through the control of iron oxides. In the noncalcareous
Karfi clay, the saturation magnetization (Ms)
and remanence (M,.)
attain their highest value at 700°C and decrease by factors of =20
and = 100 at firing temperatures of 900” and 1080”C, respectively.
In the calcareous Corinth clay, on the other hand, the peak of M ,
and M , occurs at 900” and the decrease at 1080°C is smaller than
It.
is therefore possible
in noncalcareous clays (a factor of 4)
that, if ancient pottery fired under reducing conditions is used for
archaeomagnetic studies, calcareous clays would be more sensitive
as recorders of the magnetic field of the earth since firing temperatures are more commonly in the region of 900°C.
IV. Conclusions
The principal conclusions of this study are the following:
( a ) A clear relation was verified between Ca content and the
iron phases developed during firing. It was found that the presence
of Ca in increasing amounts (Corinth clay and Karfi clay with
added CaC03) leads to the formation of progressively higher
amounts of iron oxides in reducing conditions, whereas the opposite is true in oxidizing conditions. The absence of Ca on the
other hand (Karfi clay and decalcified Corinth clay) leads to
the dissociation of iron oxide phases,
( h ) In the calcareous clays (Corinth clay and Karfi clay with
added CaCO?), ferric iron persists in reducing atmospheres and
is associated with the mineral gehlenite. It persists even under
strongly reducing atmospheres where metallic iron appears.
Wollastonite is also observed in these clays and is attributed
to the reaction of CaO with quartz grains. The combination of the
XRD and the microprobe analyzer data indicate that gehlenite
forms at lower temperatures (900°C) by the reaction of CaO with
the breakdown products of clay minerals, whereas at higher
temperatures (1080°C) the unreacted CaO attacks the large quartz
grains and forms wollastonite zones around them.
( c ) Extensive vitrification is observed in the noncalcareous
clays (Karfi clay and decalcitkd Corinth clay). Part of the ferrous
iron, produced after dissociation of the iron oxides, is dissolved in
the vitreous matrix and the remaining is incorporated in the spinel
mineral hercynite. The formation of hercynite indicated that
strongly reducing conditions were obtained inside the clay body
although the samples were fired in a flow of inert gas.
( d ) The saturation and isothermal remanence magnetization
exhibit a peak at firing temperatures of 700°C for noncalcareous
clays, whereas in calcareous clays the peak occurs at 900°C. This
information may be significant for archaeomagnetic studies of
ancient pottery.
Acknowledgments: The authors thank V. Tsoukala and N. Dris for assistance
during the first stages of this work and C. Leonis of I. G. M. E. for chemical analysis
of the samples.
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