Archaeology Magazine - April 1971 - 16

OFTHESILICA
INTHEBODIES
ICALNATURE
TABLE I - THEMINERALOG
ANDGLAZED
SURFACES
OFANCIENT
OBJECTS
REFIRED
ATVARIABLE
FRAGMENTS
ANDOFANCIENT
TEMPERATURES
INA CYCLE
OF100°C.ANHOUR.
ANCIENT
OBJECTS
STUDIED
Convextile of Body
the Illrd
Surface
dynasty
glazed
Vase (ca. 30
cm. high) of
the New
Empire

Wedjat eye
Late Period

Body
Exterior
glazed
surface
Interior
semi-glazed
surface
Body

Glazed
surface
Cylindrical Glazedsurbead
face,fine
body
Cylindrical Glazedsurbead
face,fine
body
(Qtz = Quartz Cri = Cristobalite

Qtz

I
X-tracesor smallquantity

The chemical analysis of the vitreous flux is
very difficult. We have never completely isolated
it from the body. Nevertheless, M. I. Voinovitch
and co-workers of the Department of Chemistry
of the Laboratoire Central des Ponts et Chausses
made several analyses of surface fragments of
ancient objects from a vase of the New Empire
and, for comparison, of fragments of pieces made
by us. Chemical analysis showed, outside of the
presence of copper, that there are, in decreasing
order for the alkali ions, sodium then potassium;
and for the alkaline earth ions, calcium then
magnesium. The calculation, taking into account
the probable quantity of silica in the quartz of
the body samples analyzed, has shown us that the
composition of the flux is analogous to that of
the glazes on the siliceous body fired between
900° and 1050°C.
All the technical analyses presented here permit us to conclude that the glaze layer of the
blue pharaonic ceramics does not result from the
usual process: that is a glaze covering a paste
body. Everything suggests that the glaze forms
by itself on the surface of the body.
As for the temperature during firing, x-ray
diffraction has allowed us to define precisely the
firing temperature of pharaonic ceramics. In
fact the siliceous bodies have been fired in a
range of temperatures near to that of the transformation from quartz to cristobalite. The most
important factors which accelerate the process of
transforming the quartz phase to the cristobalite
112

MINERALOGICAL
NATURE
OFTHESILICA
REFIRED REFIRED REFIRED REFIRED
ORIGINALAT
87Ò°C. AT900°C. AT950°C. AT1000°C.
Qtz
Qtz
Qtz
Qtz
Otz
^
^

COLOR
AND
APPEARANCE
White,fine
grains
Clear greenblue, glaze
& semi-glaze
Grey-White,
rather large
grains
Blueturquoise,
veryvitreous,
high glaze
Violetfrom
Manganese,
semi-glazed
Clear yellow,
rather large
grains
Green-yellow
semi-glaze
Green-blue
glazeand
semi-glaze
Blue-grey
glaze

Qtz

Qtz-Cri
XX

Qtz-Cri
XXX

Qtz

Qtz-Cri
X

Qtz

Qtz-Cri
X

Qtz-Cri
XX

Qtz

Qtz-Cri
X

Qtz-Cri
XX

Qtz

Qtz-Cri
X

Qtz-Cri
XX

Qtz-Cri
XXX

Qtz-Cri
Qtz-Cri
Qtz-Cri
Qtz-Cri
Qtz-Cri
X
X
XX
XX
I X
XX-moderatequantities XXX-largequantities)
phase are: the firing temperature, the fineness of
the quartz particles and the presence of catalysts.
The bodies and, above all, the glazed surfaces of
pharaonic objects contain very fine grained
quartz and the most active mineralizers : alkali
and alkaline earth ions.
T
lhe studies of earthenware have shown that
cristobalite is formed above 920°C. in the vitreous earthenware rich in alkali ions. Our research into the conditions of reproducing pharaonic ceramics has confirmed that cristobalite
can in the presence of certain salts be formed at
900°C. In reality, refiring of fragments of ancient
objects can supply some interesting elements for
understanding the maximum temperature at which
they were fired. The bodies and glazed surfaces
of ancient objects contain only quartz (see Table
I). Refiring at 900° zb 10°C. with thermal cycle
of 100°C. an hour causes the formation of cristobalite in certain samples. A refiring at 950°C.
under the same conditions causes its formation in
all samples; at 1000°C., more than half of the
quartz is transformed into cristobalite.
Thus we can determine that the pharaonic
blue ceramics were fired at a temperature between 870° and 920°C. and that the temperature
of 920°C. was practically never exceeded. This
temperature is substantially below the 1000°C.
which other investigators have proposed.
The thermal expansion-contraction curves of

Archaeology Magazine - April 1971

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