Materials Sciences and Applicatio ns, 2011, 2, 1392-1398
doi:10.4236/msa.2011.210188 Published Online October 2011 (
Copyright © 2011 SciRes. MSA
Lime-Barium and Lime-Zinc Raw Glazes with
Raw Materials from Tanzania and Cameroon
U. Chinje Melo1,2, Ndigui Billong1,2*, E. C. Kimaro3
1Materials Analysis Laboratory, MIPROMALO, Yaoundé, Cameroon; 2Physico-Chemistry of Mineral Materials Laboratory, Univer-
sity of Yaoundé I, Yaoundé, Cameroon; 3Industrial Minerals Laboratory, SEAMIC, Dar-es-Salaam, Tanzania.
Email: *
Received March 1st, 2011; revised April 1st, 2011; accepted April 27th, 2011.
In the paper, the lime-barium and lime-zinc glazes used in Southern and Eastern Africa Mineral Center (SEAMIC) ce-
ramic unit in Tanzania were investigated for the production of high temperature raw glazes in Cameroon. The base
Seger formula used was 0.3KNaO, 0.3CaO, 0.4ZnO or BaO; xAl2O3 and ySiO2. Different Al2O3/SiO2 formula ratios for
glossy and matt glazes were tested with raw materials from Cameroon and Tanzania. In the Cameroonian glazes, a
local pegmatite (quartz, microcline, albite) with low coloured oxides replaced an albitic feldspar in the Tanzanian
glazes. The other materials were kaolin, limestone, silica and barium carbonate or zinc oxide. Th e glazes were tested at
Seger Cone 7 (1240˚C) on stoneware slabs (made up of the same feldspathic glaze material: 18%, silica sand or quartz
27%, ball clay 40% and kaolin 15%) from both Tanzania and Cameroon materials. From results obtained, zinc oxide
acted as a more vigorous flux than barium carbonate at cone 7, which is obvious in the glazes with pegmatite (with high
quartz content). At a Seger formula of Al2O3/SiO2 1:10, transparent glazes were obtained. In these glazes the alumina
content was lower and when increased to higher Seger ratios 1:4.7 - 8.7, the gloss was lowered to matt-luster due to
fine crystal formation.
Keywords: Lime-Zinc Gl azes, Lime-Bari um Glazes, Raw Glazes, Seger Cone 7
1. Introduction
Glazes constitute a fundamental material to the success
of the ceramic industry. During recent years, the de-
mands of shorter firing cycles and easy control of surface
microstructure have called for the use of fritted composi-
tions. Raw glazes have to a large extend been replaced by
fritted glazes. However, in the case of high temperature
ceramic products like tiles, stonewares and sanitarywares
fired at temperatures above 1200˚C, raw glazes are still a
competitive alternative to fritted formulations [1]. Glaze
is similar to glass but with a more complex system be-
cause it deals with more components; 7 or 8 oxides in
significant amounts are common [1-6]. These compo-
nents are divided into the glass network former (SiO2 or
B2O3), stabilizer (generally Al2O3) and flux or glass net-
work modifier or melter (R2O or RO, where R is an alka-
line, alkaline earth or fluxing metal). The complication
with glazes is accentuated by the fact that a thin layer is
applied to a ceramic body and the glaze body interaction
may result in faults. Such faults like cracks (at times
voluntary) can affect the aesthetics and technical proper-
ties of the product. A glaze is a mixture of materials,
powder or suspension in water which when applied as a
coating on a body and fired to an appropriate temperature,
vitrifies and develops specific properties. For every given
body composition, it is important to test the ideal glaze
based on aesthetics and properties envisaged.
This research was carried out to produce stoneware
glazes using Cameroonian raw materials, to replace
costly imported stoneware and porcelain glazes com-
monly used in Cameroon. The studies were carried out at
SEAMIC in Tanzania, investigating the base glaze com-
positions used on its decorative stoneware products.
These high temperature glazes are of lime-barium (bar-
ium glazes) and lime-zinc systems and are raw, not using
fritted materials or lead. Barium glazes are recommended
for their ability to produce brilliancy and hardness [7].
The base Seger formula for the glazes was 0.3KNaO,
0.3CaO, 0.4ZnO or BaO; x Al2O3 and y SiO2 at cone 7
(1240˚C). The x/y (Al2O3/SiO2) ratio has been reported
to have a great influence on the glaze surface as indicated
n igure 1 [2,3,8,9]. i
Lime-Barium and Lime-Zinc Raw Glazes with Raw Materials from Tanzania and Cameroon1393
Figure 1. Variation of x (Al2O3) and y (SiO2) molecular formula for glaze preparation.
2. Experimental
2.1. Raw Materials
Raw materials used in Tanzanian glazes (TG) were
available in SEAMIC and are given in Table 1. Included
are those for Cameroonian (CG) glazes, indicating their
sources. Chemical compositions were analysed by x-ray
florescence, fused bead method with a SIEMENS SRS
3000 spectrometer. Mineralogical analyses were carried
out with a SIEMENS D 5005 diffractometer.
2.2. Experimental Procedure
2.2.1. Glaze Form ulation
Molecular ratios of Al2O3 and SiO2 were varied to pro-
duce glossy glazes with single glass phase glossy (zone
A) and matt glazes with fine crystals (zone B) using Fig-
ure 1. Molecular Al2O3/SiO2 ratios 1:10 for A zone and
1:4.6 - 8.7 for the B zone were tested. Tables 2 and 3
indicates different ratios of x and y tested for Tanzanian
glazes and Cameroonian glazes. The glaze recipes were
calculated with these assumptions: that limestone, kaolin,
and silica sand (flint) were pure, of respective composi-
tion CaCO3, Al2O3·2SiO2·2H2O and SiO2; and that feld-
spathic materials (feldspar and pegmatite) were mixed
feldspar of composition KNaO·Al2O3·6SiO2.
2.2.2. Glaze Prepa ration a nd Applic ation
As received materials (flint, quartz, feldspar, limestone
and pegmatite) were crushed in a DIETZ jaw crusher to
<400 µm and dried at 105˚C for 24 hours. The glaze
recipes were composed using a Sartorius V3600S (max
load 3640 g and sensibility 0.1 g) scale. 45% water was
added to the raw glaze mixture which was then ball
milled until all passed a 75 µm sieve. Densities were
adjusted to about 1550 g/L using a Baume hydrometer.
Table 1. Raw materials and their source.
Materials Source
Pegmatite Mont Nlonako, Littoral Region, Cameroon
Kaolin C Mayoum, Foumban, West Region, Cameroon
Quartz Mvan, Yaoundé, Center region, Cameroon
Clay C1 Mvan, Yaoundé, Center Region, Cameroon
Clay C2 Bambili, North-West Region, Cameroon
Feldspar SEAMIC, Tanzania
Kaolin T SEAMIC, Tanzania
Flint T SEAMIC, Tanzania
Limestone T SEAMIC, Tanzania
Clay T SEAMIC, Tanzania
Zinc oxide SEAMIC, Tanzania
Barium carbonateSEAMIC, Tanzania
Copyright © 2011 SciRes. MSA
Lime-Barium and Lime-Zinc Raw Glazes with Raw Materials from Tanzania and Cameroon
Table 2. Tanzanian glaze (TG) recipes and Al2O3/SiO2 molecular ratios.
Recipe (wt%)*
Glaze x y Al2O3/SiO2 (Seger) F K (T) L Fl Zn Ba
Glaze system
TG1 0.30 3.0 1:10 54.6 n.u. 10.1 24.3 11.0 n.u. Ca-Zn
TG2 0.30 3.0 1:10 47.2 n.u 8.8 21.0 n.u. 23.0 Ca-Ba
TG3 0.35 3.5 1:10 42.4 3.4 7.9 25.3 20.0 20.8 Ca-Ba
TG4 0.40 4.0 1:10 38.9 6.2 7.2 28.8 n.u. 18.9 Ca-Ba
TG5 0.65 3.0 1:4.6 47.0 26.2 8.7 8.7 9.4 n.u Ca-Zn
TG6 0.65 3.0 1:4.6 41.4 23.1 7.7 7.7 n.u. 20.2 Ca-Ba
TG7 0.45 3.0 1:6.7 44.6 10.6 8.3 14.9 n.u. 21.7 Ca-Ba
TG8 0.65 5.0 1:7.7 31.7 17.7 5.9 29.4 n.u. 15.4 Ca-Ba
*F = Feldspar; K(T) = Kaolin; L = Limestone; Fl = Flint (silica sand); Zn = ZnO, Ba = BaCO3; n.u. = not used in recipe.
Table 3. Cameroonian glaze (CG) recipes and Al2O3/SiO2 molecular ratios.
Recipe (wt%)*
Glaze x y Al2O3/SiO2 (Seger) P K (C) L Q Zn Ba
Glaze system
CG1 0.30 3.0 1:10 54.7 n.u. 10.1 24.3 10.9 n.u. Ca-Zn
CG2 0.40 4.0 1:10 38.9 6.2 7.2 28.8 n.u. 18.9 Ca-Ba
CG3 0.40 4.0 1:10 44.0 7.0 8.0 32.0 9.0 n.u. Ca-Zn
CG4 0.35 3.0 1:8.6 46.3 3.7 8.6 18.9 n.u. 22.5 Ca Ba
CG5 0.45 3.0 1:6.7 44.5 10.6 8.3 14.9 n.u. 21.7 Ca Ba
CG6 0.50 3.0 1:6.0 43.7 13.9 8.1 13.0 n.u. 21.3 Ca Ba
CG7 0.50 3.0 1:6.0 48.0 15.0 9.0 18.0 10.0 n.u. Ca Zn
CG8 0.35 3.5 1:10 56.7** n.u. 8.7 24.9 9.6 n.u. Ca - Zn
*P = Pegmatite; K = Kaolin; L = Limestone; Q = Quartz; Zn = ZnO; Ba = BaCO3; n.u. = not used; **Pegmatite was replaced by the fusible clay C2.
Slips were applied by dipping method on unfired stone-
ware body.
2.2.3. Stonewa re Body C om p osi tion
The Tanzanian glazes were applied on a body using raw
materials from Tanzania. Similarly, Cameroonian glazes
were applied on a body made up of local materials from
Cameroon. The Tanzanian ceramic body was made up of
18% feldspar T, 27% flint T, 40% clay T and 15% kaolin
T and that of Cameroonian 18% pegmatite C, 27% quartz
C, 40% clay C1 and 15% kaolin C. The stoneware body
test pieces were produced by crushing the raw materials
with water in a ball mill and filter pressed to remove ex-
cess water. The paste obtained was flattened on a roller,
and slabs cut for glaze trials.
2.2.4. Firing Sc hedules
The glazed test pieces were single fired on Seger cone 7:
1240˚C, in a Linn Electrotherm electric kiln. From am-
bient temperature to 650˚C, the firing rate was 2.71˚C/
min, and from 650˚C to 1240˚C, 2.46˚C/min. The test
pieces were soaked at 1240˚C for 2 hours and were al-
lowed to cool in the kiln to ambient temperature before
3. Results and Discussion
3.1. Chemical and Mineralogical Compositions
of Raw Materials
The results of chemical composition determinations on
Tanzanian and Cameroonian raw materials are given in
Tables 4 and 5 respectively. The feldspar is more sodic
and the pegmatite more potassic. The percentage of
non-feldspathic components in these materials is quite
low. The high SiO2 content of pegmatite is expected to
alter the melting behaviour as compared to feldspar,
when the recipe contains the same amount of feldspathic
Copyright © 2011 SciRes. MSA
Lime-Barium and Lime-Zinc Raw Glazes with Raw Materials from Tanzania and Cameroon1395
Table 4. Chemical composition (wt%) of Tanzanian raw materials.
Oxide Silica sand Feldspar T Kaolin T Limestone T Clay T
SiO2 97.58 65.18 49.60 5.25 51.28
Al2O3 1.41 20.70 35.76 2.03 30.14
Fe2O3 0.19 0.10 1.07 0.99 1.69
MnO <0.01 <0.01 <0.01 <0.01 <0.01
TiO2 0.31 <0.01 0.40 <0.01 0.31
MgO 0.05 0.12 0.11 0.41 0.41
CaO 0.18 1.40 0.09 49.70 0..41
Na2O 0.06 8.70 0.17 0.40 0.61
K2O <0.01 2.61 0.19 <0.01 2.00
P2O5 <0.01 <0.01 <0.01 <0.01 <0.01
L. O. I. 0.20 0.41 12.58 40.70 13.04
Total 99.98 99.22 99.97 99.48 99.89
Table 5. Chemical composition (wt%) of Cameroonian raw materials.
Oxide Quartz Pegmatite Kaolin C Clay C1 Clay C2
SiO2 99.02 82.55 45.12 60.02 71.71
Al2O3 0.40 8.93 34.59 27.83 12.86
Fe2O3 0.22 0.52 1.25 2.17 2.86
MnO <0.01 0.03 <0.01 <0.01 0.03
TiO2 <0.01 0.03 4.76 1.15 0.19
MgO 0.15 0.28 0.35 0.56 0.26
CaO <0.01 0.38 <0.01 <0.01 0.35
Na2O 0.17 2.05 0.15 0.22 0.74
K2O <0.01 4.85 0.13 0.39 3.90
P2O5 <0.01 <0.01 0.33 0.39 <0.01
L. O. I. 0.03 0.08 13.15 7.08 6.83
Total 99.99 99.70 99.83 99.81 99.73
material. Both kaolin T and C contain about 35% Al2O3
and iron content within the stoneware composition. The
presence of 4.76% TiO2 in kaolin C resulted in stoneware
with a darker colour. The limestone contain 89% CaCO3,
with main impurities SiO2 and Al2O3 and low MgO. The
quartz stones are of very high purity.
From x-ray analyses, pegmatite contained quartz, al-
bite and microcline; clay C1 quartz and kaolinite; and
clay C2 a very fusible clay (fusion point 1200˚C): kao-
linite, montmorillonite, quartz and microcline. Kaolin C,
from previous analysis [10] is made up essentially of
kaolinite and quartz with traces of illite, anatase and an-
orthite. Clays T and C1 are ball clays for stoneware bod-
3.2. Glazed Test Pieces
The visual appearance of the glazed test pieces are de-
scribed in Tables 6 to 8 in which the glaze systems, SiO2
and Al2O3 contents, are included.
TG1-TG4 gave glossy transparent glazes (Table 6).
The Al2O3/SiO2 molecular ratio is 1:10. TG1 is a Bristol
glaze and is colourless, whereas the barium glazes TG2-
Copyright © 2011 SciRes. MSA
Lime-Barium and Lime-Zinc Raw Glazes with Raw Materials from Tanzania and Cameroon
Table 6. Visual appearance of TG and their silica and alumina contents.
Surface structure
Glaze SiO2 wt% A l2O3 wt% Glaze system Colour Appearance
TG1 59.9 11.3 Ca-Zn colourless high gloss, transparent, smooth feel, no craze
TG2 51.8 9.8 Ca-Ba light khaki high gloss, coloured, smooth, crazed
TG3 54.8 5.5 Ca-Ba light khaki high gloss, coloured, smooth, crazed
TG4 45.9 10.5 Ca-Ba light khaki glossy, coloured, no crazing, fine bubbles, rough feel
TG5 52.3 19.1 Ca-Zn white high gloss, opaque, no crazing
TG6 46.1 16.8 Ca-Ba pastel beige opaque, matt, no crazing
Table 7. Effect of colorants on Tanzanian glazes.
Surface structure
Glaze Description Glaze system Colour Appearance
TG7 TG7 + 5% MnO2 Ca-Ba purple + whitish specks low gloss, opaque, no crazing, smooth feel
TG8 TG8 + 5% MnO2 Ca-Ba yellow-brown glossy ,opaque, no crazing
TG9 TG6 + 5% MnO2 Ca-Ba dark brown opaque, matt, no crazing
TG10 TG6 + 5% Fe2O3 Ca-Ba chocolate brown opaque, matt, no crazing
TG11 TG6 + 1% CoO Ca-Ba blue opaque, matt, no crazing
TG12 TG6 + 2% NiO Ca-Ba pale pink opaque, matt, no crazing
TG13 TG6 + 2% Cr2O3 Ca-Ba dark brown opaque, matt, no crazing
TG14 TG1 + 2% Fe2O3 Ca-Zn brown glossy, opaque, no crazing
TG15 TG1 +1% CoO Ca-Zn deep marine blue high gloss, opaque, no crazing
Table 8. Visual appearance of Cameroonian glazes and their silica + alumina contents.
Surface structure
Glaze SiO2 wt% Al2O3 wt% Glaze system Colour* Appearance
CG1 74.0 8.4 Ca-Zn whitish incomplete fusion
CG2 65.7 5.7 Ca-Ba colourless Glossy, transparent, no crazing
CG3 71.4 6.3 Ca-Zn colourless Glossy, transparent, no crazing
CG4 58.7 5.4 Ca-Ba colourless Opaque, matt, no crazing
CG5 56.4 7.7 Ca-Ba whitish Opaque, matt, fine crazes
CG6 55.3 8.71 Ca-Ba colourless Glossy, transparent, no crazing
CG7 64.4 9.5 Ca-Zn colourless Glossy, transparent, no crazing
CG8 65.5 7.3 Ca-Zn yellow Opaque, matt, no crazing
*The stoneware body fired to beige, consequently mild coloration could not be perceived.
TG4 possess a light khaki tint. Decreasing BaCO3, a
fluxing agent, reduces the glossiness. In the BaO-CaO-
SiO2 ternary phase diagram, a liquid phase exists at as
low as 1268˚C [10]. This eutectic contributes to the low
temperature phases obtained at cone 7. Omar et al. [11],
observed that to introduce glass-ceramics into industrial
Copyright © 2011 SciRes. MSA
Lime-Barium and Lime-Zinc Raw Glazes with Raw Materials from Tanzania and Cameroon1397
applications, crystallizing agents had to be added to Ba-
aluminosilicate glasses. With high BaCO3 contents: 23%
in TG2 and 21% in TG3, the glazes showed mild crazing,
probably due to the high dilatation coefficient of BaO, in
addition to the high amount of alkaline feldspar. The
dilatation coefficient of BaO is 3.0 × 107/˚C in fritted
glazes but can attain 6.0 × 107/˚C in raw glazes [12].
The thermal expansion in these glazes could be lowered
by reducing the alkali content, that is, the feldspar con-
tent [2,8,13]. ZnO showed a vigorous fluxing action
(TG1, 11.0% ZnO) when compared to BaO (TG2, 16%
BaO) for the same Al2O3/SiO2 molecular ratio, 1:10. In
the ZnO-Al2O3-SiO2 ternary phase diagram, low tem-
perature compounds were located at the poor Al2O3 and
high (above 50%) SiO2 ends [10]. In TG5 Bristol glaze, a
white opacification occurred. ZnO which forms the basis
of Bristol glazes has been noted to increase whiteness,
especially in zircon glazes. At low Al2O3/SiO2 molecular
ratio 1:4.6 (high amount of Al2O3), opacity is probably
due to very fine crystals of zinc compound(s), such as
gahnite, ZnO·Al2O3.
The barium glaze TG6, with Al2O3/SiO2 molecular ra-
tio 1:4.6, is the base matt glaze in SEAMIC’s ceramic
unit. It has been reported that low barium release (non
toxic) could be obtained with up to 20% barium, when
the Al2O3 + SiO2 content is high, i.e. above 75% [14]. In
this matt lime-barium glaze, the barium content was 14%
and the Al2O3 + SiO2 63%. For the other barium glazes
tested, the barium contents were between 10.7% and
16.0% with Al2O3 + SiO2 content between 60% and 56%.
It is important to study barium release for these glazes
when use in dinner ware industry is envisaged.
Comparing the Al2O3/SiO2 molecular ratios of the
glazes, for Bristol glazes, 1:10 ratio (TG1) gave trans-
parent glaze and opacity developed when varied to 1:4.6
(TG5). This confirms the widely accepted Figure 1 and
observations that the x-y ratio has a great influence on
glaze surface. In Table 6, it is noted that this is directly
related to the Al2O3 content; 11.3% Al2O3 in TG1 and
19.1% in TG5. The same is true for the barium glazes;
TG2-TG4 of 1:10 molecular ratio are transparent with <
10.5% Al2O3 and a matt glaze is obtained with formula
1:4.6, containing 16.8% Al2O3. Transition metal oxides,
when added raw, give colour upon firing of ceramic
glazes. The colours obtained with MnO2, Fe2O3, CoO,
NiO and Cr2O3 are indicated in Table 7. TG7, TG8 and
TG9 (base TG6) all barium glazes contain 5% MnO2.
TG7 is matt and purple; TG8 is of low gloss, fine bub-
bles and brown, while TG9 is of low gloss, and dark
brown. MnO2 generally gives brown but in high alkali
glazes, fires to purple [8]. TG7 with the highest feldspar
content (45%) fired to purple, while TG9 (base TG6) and
TG8 containing 42% and 32% feldspar respectively,
gave brown hues. 5% Fe2O3 gave a matt chocolate-brown
on barium base glaze TG6, i.e. TG10; while 2% Fe2O3 on
Bristol base glaze TG1 (i.e. TG14) gave a high gloss pale
brown glaze.
With 2% NiO, the barium base TG6 glaze gave pale
pink (TG12). 2% Cr2O3 produced a brown coloration on
barium base TG6 glaze. The refractoriness of Cr2O3 was
obvious as this glaze TG13 did not fuse completely. 1%
CoO on both barium and Bristol base TG6 and TG1
glazes gave respectively a matt blue TG11 and high gloss
and strong ultramarine coloration, TG15. Cobalt is a very
powerful colorant which easily dissolves in glaze matrix
(with fluxing action) and enters the structure [8]. Cobalt
colours tend to be brighter in Bristol glazes whereas iron
colours are duller [4].
Cameroonian glazes CG2 and CG3, Table 8, respect-
tively for barium and Bristol glazes with Al2O3/SiO2
molecular ratio 1:10 were uncoloured, transparent and of
high gloss. These surface structures were similar to Tan-
zanian glazes with same ratio. These glazes can serve as
base glazes for transparent stoneware using Cameroonian
local materials. Barium glaze CG6 and Bristol CG7 of
ratio 1:6 were of mild gloss and showed some opacifica-
tion. This was probably due to the formation of very fine
barium or zinc aluminosilicate compounds. CG1 of Bris-
tol glaze series presented an incomplete fusion at cone 7.
The insufficient fluxing action of the 11% ZnO was
probably due to the high quartz content: 82% SiO2 in
pegmatite (54.7% in the recipe). An increase in tempera-
ture to a higher cone, >1240˚C is required. It is worth
observing that with 54.6% feldspar in the Tanzanian
glaze TG1, a single glass phase was obtained at cone 7.
This implies that ZnO had a higher fluxing action on the
feldspar (Tanzanian) than on the pegmatite (Cameroo-
nian). At lower pegmatite content, 44% in the recipe, the
fluxing action of 9.0% ZnO is evident in CG3. Some
opacification gave a mid matt glaze in CG4 barium glaze
(Al2O3/SiO2 molecular ratio 1:8.6). A matt glaze was
obtained with CG5 of ratio 1:6.7. Both CG4 and CG5 are
recommended as base glazes for coloured Cameroonian
matt glazes. Clay C2 of high fusibility (melts at 1200˚C)
replaced pegmatite in glaze CG8. A yellow opacification
was obtained on a matt surface. This clay could be ex-
plored in the production of glazes in Cameroon. On the
recommended base glazes for transparent (CG2 and CG3)
and matt glazes (CG4 and CG5), detail investigation of
the effect of colorant transition metal oxides should be
carried out when required by exploring the results on the
Tanzanian glazes.
4. Conclusions
Cameroonian materials (pegmatite, kaolin, quartz and
limestone) can be used in raw lime-barium and lime-zinc
Copyright © 2011 SciRes. MSA
Lime-Barium and Lime-Zinc Raw Glazes with Raw Materials from Tanzania and Cameroon
Copyright © 2011 SciRes. MSA
(Bristol) glazes at cone 7 on stoneware when fritting is
not possible. The base Seger or molecular formula was
0.3KNaO, 0.3CaO and 0.4ZnO or BaO; x Al2O3 and y
SiO2. The influence of Al2O3/SiO2 molecular formula
that is x-y variation on the glaze surface structure was
evident on both the Tanzanian and Cameroonian glazes.
Ratio 1:10 gave transparent glazes and 1:4.6 - 1:8.7 mid
matt to matt glazes. The latter, a result of very fine crys-
tal formation. Transparent glazes contained lower amounts
of Al2O3 than matt glazes, alumina therefore influenced
Different hues were obtained upon addition of metal
oxides MnO2, Fe2O3, Cr2O3, NiO and CoO on the base
glazes for both transparent and matt glazes. The glaze
recipe and system (lime-barium or lime zinc) influenced
the colour and the surface structure for the same colorant
metal oxide.
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