UNIVERSIDADE ESTADUAL PAULISTA
“JÚLIO DE MESQUITA FILHO”
Instituto de Ciência e Tecnologia
Campus de São José dos Campos
ORIGINAL ARTICLE DOI: https://doi.org/10.4322/bds.2024.e4356
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Braz Dent Sci 2024 Apr/June;27 (2): e4356
This is an Open Access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
The optical behavior of pressable lithia-based glass-ceramics under
two different heat treatment protocols
O comportamento óptico de vitrocerâmicas prensadas à base de lítio sob dois protocolos diferentes de tratamento térmico
Iman Haggag Elnagar AHMED1 , Amr EL-ETREBY2,3 , Sara Mehanna FOUDAH2
1 - October 6 University, Faculty of Dentistry, Fixed Prosthodontics. Cairo, Egypt.
2 - Ain Shams University, Faculty of Dentistry, Fixed Prosthodontics. Cairo, Egypt.
3 - Tallinn Health Care College. Tallinn, Estonia.
How to cite: AHMED IHE, El-Etreby A, Foudah SM. The optical behavior of pressable lithia-based glass-ceramics under two different heat
treatment protocols. Braz Dent Sci. 2024;27(2):e4356. https://doi.org/10.4322/bds.2024.e4356
ABSTRACT
Objective: This study aimed to evaluate the optical behavior of pressable lithia-silicate and lithia-zirconia-
silicate glass ceramics toward additional heat treatment protocols. Material and Methods: 40 lithia-silicate discs
(15mm x 1mm) were heat pressed following the manufacturers’ instructions. Discs were divided into four groups
(n=10) according to type as follows: two groups of lithia-silicate-glass ceramics; Gp(E) (IPS e.max Press; Ivoclar
Vivadent AG), Gp(L) (GC Initial LiSi Press, GC), two lithia-zirconia-silicate pressable glass ceramics; Gp(C) (Celtra
Press, Dentsply Sirona) and Gp(A) (VITA Ambria, VITA Zahnfabrik). Each group was subdivided into (n=5):
Subgroup(T1): the thermal tempering temperature was set 9% below the pressing temperature, Subgroup(T2):
the temperature was set 5% below the pressing temperature. Optical properties: color, translucency parameter
(TP), and contrast ratio (CR) were evaluated by spectrophotometer (Aglient Cary 5000 UV-Vis–NIR) after
pressing and after thermal tempering. Results: Thermal tempering regardless of temperature resulted in a color
shift within the acceptability level as ΔE for Gp(E) (3.18±2) followed by ΔE for Gp(L) (2.47±0.19) by ΔE for
Gp(C) (2.26±0.14) and the last ΔE for Gp(A) (1.62±0.13). Subgroup(T2) showed a signicantly higher color
shift with mean ΔE(2.55±0.63) compared to Subgroup(T1) ΔE(2.35±0.59). There was a statistically signicant
increase in TP after tempering for all tested groups parallelled with a decrease in CR values. Conclusion: Heat
tempering of the tested lithia-silicate pressable ceramics had a signicant effect on the optical outcome of these
materials, being lithia-zirconia-silicate ceramics more stable and less affected optically than other lithia-silicate-
glass ceramics.
KEYWORDS
Ceramics; Glass ceramics; Hot temperature; Silicates; Zirconium oxide.
RESUMO
Objetivo: Avaliar o comportamento óptico de cerâmicas pressionáveis de vidro de litia-silicato e litia-zircônia-
silicato sob protocolos adicionais de tratamento térmico. Materiais e métodos: 40 discos de litia-silicato (15mm
x 1mm) foram prensados a quente conforme instruções dos fabricantes. Material e Métodos: 40 discos de litia-
silicato (15mm x 1mm) foram prensados e divididos em quatro grupos (n=10): dois de lithia-silicato-vidro,
Gp(E) (IPS e.max Imprensa) e Gp(L) (GC inicial LiSi Press), e dois de vidro prensado de litia-zircônia-silicato,
Gp(C) (Celtra Press) e Gp(A) (VITA Ambria). Cada grupo foi subdividido em (n=5): Subgrupo(T1): amostras
temperadas a 9% abaixo da temperatura de prensagem, e Subgrupo(T2): a temperatura foi ajustada 5% abaixo
da temperatura de prensagem. As propriedades ópticas, incluindo cor, translucidez (TP) e contraste (CR), foram
avaliadas com um espectrofotômetro (Aglient Cary 5000 UV-Vis-NIR) após prensagem e temperagem térmica.
Resultados: O tratamento térmico resultou em mudança de cor dentro do nível aceitável, com ΔE mais alto para
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Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
INTRODUCTION
Lithium silicate-based ceramics (LSCs) stand
out among the many all-ceramic materials that
have entered the dental market in recent years
and have the potential to establish themselves
as the preferred choice, particularly for single
restorations [1,2]. Dental manufacturers have
frequently attempted to combine mechanical
efciency and aesthetic quality, making lithium
silicate materials an exemplar that meets both
requirements [3]. The introduction of such
materials that are effectively used in monolithic
mode rendered the necessity for bilayer restoration
redundant [4]. Nevertheless, these materials
were produced in a versatile manner using the
well-known earlier heat pressing technology or
the following digital production using CAD/CAM,
which made them tempting and easy to use [5].
A material with visual qualities comparable
to glass ceramics and superior mechanical
performance was a revolutionary breakthrough
when it was rst introduced to the dental market
in the 1990s as IPS Empress II (Ivoclar Vivadent,
Schaan, Liechtenstein) utilizing the hot-press
technology [6,7]. The next generation was
around 2001 with the release and patenting
of IPS e.max Press (Ivoclar Vivadent, Schaan,
Liechtenstein) [8]. This moldable version utilized
the lost wax technology based on the viscous ow
of glass-ceramics with the advantages of net-
shape processing, decreased porosity, increased
Weibull modulus, increased exural strength, and
providing excellent marginal t [9]. Early around
the year 2005 the introduction of IPS e.max CAD®
(Ivoclar Vivadent, Schaan, Liechtenstein) [10],
took place as the machinable CAD/CAM version
for the chair side delivery of such restorations
utilizing the digital workow. These moldable
and machinable glass-ceramics that precipitate
lithium disilicate have been utilized to create more
than one hundred eight million dental restoration
pieces over 8 years of exclusive presence in
the dental market with recorded excellence of
clinical durability [11,12]. This encouraged
producers and scientists to continue developing
this kind of material to increase its potential
applications, especially once the material’s patent
expired. In 2013 just after the launching of a
second lithium silicate-based ceramic material by
Glidewell Laboratories [13], reinforcements of
these materials by adding zirconia to its chemical
structure was pioneered by lithia-zirconia silicate
glass ceramics such as Celtra Duo (Dentsply
Sirona, York, PA, USA) and Suprenity PC (Vita
Zahnfabrik, Bad Sӓckingen, Germany) with
addition of around 8-12% zirconium oxide to
the structure. The trend of creating novel lithia
silicates with reducing crystal dimensions while
maintaining around 50 vol% crystallinity - to
aid in millability and subsequent processing
capability - is being driven by the inclusion of
ZrO2 as an auxiliary nucleate center [14]. This
resulted in lithium (mono)-silicate crystals and
the presence of biphasic variants with lithium
meta-silicate and lithium disilicate is evident [15].
Variations in the manufacturing techniques and
different microstructural forms of the material
were noticed to have consequences for the nal
material properties [15]. Heat application is
an essential process required during the whole
process of manufacturing. Starting with the early
preprocessing stages of ingot fabrication and
reaching the further treatments involved to create
the final tooth-shaped restoration [6,15,16].
An essential second round of heat application at the
laboratory level is needed as follows; for moldable
versions, these ingots are being subjected to high
pressing temperature dictated by the manufacturer
at the dental lab. Typically, machinable versions
are supplied in a pre-crystallized state that eases the
machinability but needs an essential crystallization
firing cycle. Additional third levels of firing
cycles may be recommended for some esthetic
characterizations, corrections, glazing, or healing
and reinforcements. These tuning procedures with
variable temperatures, heating rates, holding times,
Gp(E) (3,18±2), seguido por Gp(L) (2,47±0,19), Gp(C) (2,26±0,14), e Gp(A) (1,62±0,13). No subgrupo (T2),
houve uma mudança de cor mais signicativa, com ΔE médio de (2,55±0,63), comparado ao subgrupo (T1) com
ΔE médio de (2,35±0,59). Houve aumento signicativo na TP e redução nos valores de RC após o tratamento
térmico em todos os grupos testados. Conclusão: O tratamento térmico das cerâmicas prensadas de litia-silicato
teve um efeito signicativo na sua qualidade ótica, com as cerâmicas de litia-zircônia-silicato mostrando-se mais
estáveis e menos afetadas visualmente em comparação com outras cerâmicas de litia-silicato-vidro.
PALAVRAS-CHAVE
Cerâmica; Cerâmica de vidro; Temperatura quente; Silicatos; Óxido de zircónio.
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Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
or cooling rates have been evidenced to affect the
material structure, crystallization formation, and
growth [17]. Consequently, the mechanical, and
physical impacts of such treatments were reported
in the literature with controversial conclusions.
While some discussed how applying heat can
elevate internal stresses [18,19], affecting the
strength negatively [18,20], and increasing the
chairside time with post-processing additional
firings required, others have noted that it can
also have a strengthening impact [21,22]. While
speed firing has been attempted to facilitate
crystallization [23], prefacing of such lithia-
silicate based restoration was introduced so
that restorations could be milled, polished, and
delivered with the elimination of the post-mill
crystallization firing needed and substitution
by a minor post-mill glaze ring that inherently
heals any machining aws like in Cerec Tessera
(Dentsply Sirona, York, PA, USA). Moreover
grinding fully crystallized materials without any
post-grind heat treatment was nally introduced
with the launching of materials such as GC Initial
LiSi Block (GC, Tokyo, Japan) [24], and N!CE
(Straumann, Basel, Switzerland) [25]. On the
other hand, manufacturers reported an increase
of about 160 MPa in exural strength for the LSCs
subjected to an additional ring cycle over that of
the machined one [26]. Not only for mechanical
benet, optical optimization for the translucency
level of a material like Amber mill (HASSBio,
Kangneung, Korea) [27], was recommended
by the manufacturer through changing the
temperature of the crystallization cycle. For a
recently introduced lithia-zirconia silicate ceramic
in 2021, VITA Ambria (VITA Zahnfabrik, Bad
Säckingen, Germany) [28], an additional third
step of heat tempering at 800 °C is recommended
by the manufacturer to increase the strength of
the pressed restorations [29]. The effect of such
heat treatments on optical properties such as
translucency and color duplication does not have
a cutting end in the literature up to the authors’
knowledge [6,30,31]. This study simulates two
of the third-level thermal tempering protocols
in the laboratory and explores their impact
on aesthetic parameters such as the color and
translucency of four lithia-silicate pressed glass
ceramics, providing recommendations for the
clinical implications of such tempering protocols
regarding esthetics. The null hypothesis was that
no significant difference in optical properties
between two lithia-silicate and two lithia-zirconia
silicate pressable glass ceramics tested and that the
thermal tempering procedures had no signicant
impact on these properties.
MATERIAL AND METHODS
Sample preparation
Out of 40 pressable lithia-silicate ceramic
ingots, 40 rounded discs measuring 15 mm in
diameter and 1 mm in thickness were created.
These discs were then categorized into 4 groups
(n=10) according to the composition of the
material used;
Two lithia-silicate glass ceramics (LSGCs)
used in this study were:
• Gp (E) (IPS e.max Press; Ivoclar Vivadent AG),
• Gp (L) (GC Initial LiSi Press, GC).
While the two Lithia-zirconia silicate (LZSCs)
glass ceramics used in this study were:
• Gp (C) (Celtra Press, Dentsply Sirona) and
• Gp (A) (VITA Ambria, VITA Zahnfabrik).
For construction of these samples, Wax
patterns (Elastiwax; Keram & Keramik) were
made in a special Teon mold with the desired
geometry, sprued and invested (IPS e.max Special
Investment Material; Ivoclar Vivadent AG), the
wax eliminated at 850 °C for 1 hour, and the
ingots were pressed in a furnace (EP600; Ivoclar
Vivadent AG) as per the protocol recommended
by each manufacturer as shown in Table I.
The specimens were divested by sandblasting
with 50 μm alumina particles at 4 bars of
atmospheric pressure. Then smooth divesting was
used at 2 bars till complete removal of investment
material from discs. After sprues were separated by
separating diamond discs, each attachment point
was smoothened by a diamond disc. The samples
were nally nished and polished using (Optrane
Ceramic Polishing System) sequentially, using the
rst tip for 60 seconds with a speed of 10000 rpm.
While the next tip was used for the same duration
Table I - Recommended pressing temperatures of ceramics
Group (T0)
Gp (A) 880 °C
Gp (C) 870 °C
Gp (E) 917 °C
Gp (L) 910 °C
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Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
of 60 seconds, but with a slower speed of 6000 rpm
to simulate the clinical situation.
First phase measurements
Optical properties measurements
The color was measured on both a black
and white background using Aglient Cary
5000 UV-Vis-NIR spectrophotometer (Agilent
Technologies, Santa Clara, USA) where the discs
were aligned in the device that is considered
a PC and a spectrophotometer unit. The light
beam passes from the tungsten lamp through
a double monochromator to allow being
monochromatic. Then passes through the
sample to the detector. The passing light beam is
detected by a photomultiplier that is sensitive to
ultraviolet regions. The wavelength ranged from
380 to 780 nm. CIE L*a*b* color values for each
sample were then calculated from the diffuse
reectance using color software which is available
through the Cary WinUV instrument and supports
extensive color standards and calculations.
L
* Is a measure of the lightness-darkness of
material (perfect black has an
L
* = 0, and perfect
white has an
L*
= 100).
The
a
* coordinate is a measure of the redness
(positive value) or the greenness (negative
value), while the
b
* coordinate is a measure of
the yellowness (positive value), or the blueness
(negative value).
The translucency parameter (TP) was
obtained by calculating the color difference
between specimens over black and white
backgrounds.
( ) ( ) ( )
2 2 2 ½TP Lb Lw ab aw bb bw= − + +−


−
(1)
where: (b) refers to color coordinated over a
black background; (w) refers to color coordinated
over a white background.
While Contrast ratio (CR) was calculated
through;
/CR YB YW=
(2)
[ ]
( )
3
½ 16 116 100YL= +− ×
(3)
And the CR range was defined from
0 (transparent) to 1 (totally opaque).
Scanning electron microscope (SEM)
A Field Emission Scanning Electron
Microscope (FESEM, Carl Zeiss Ultra, Oberkochen,
Germany) was used to analyze the microstructure.
The glass-ceramics microstructure was examined
using the specimens. Using an etched specimen,
the crystallite microstructure was investigated.
The specimens were cleaned with distilled water
and then etched for 20 seconds using a 9.5%
hydrouoric acid gel. The operating distance was
8 mm, and the acceleration voltage was 5 kV.
Before SEM examination, all specimens were gold
sputter-coated to avoid charging effects during
imaging.
X-ray diffraction analysis
Bruker’s AXS D8 Advance X-ray
diffractometer was used to examine the samples’
mineralogy using monochromatic Cu K α x-ray
angle (= 1.5406 Å). With a counting time of two
seconds per step and a step size of 0.006◦(2),
X-ray diffractograms were obtained between 10◦
and 60◦(2). ICSD reference data served as the
basis for phase identication.
Energy dispersive X-ray analysis (EDX)
A non-destructive X-ray technology termed
energy dispersive X-ray analysis (EDX) was
implemented to gure out the elemental makeup
of materials. The specimen of interest was
identied by the microscope imaging on Electron
Microscopy instruments, which have been
equipped with EDX systems. The obtained EDX
ndings are provided as spectra that display the
peaks of the analysis’s compositional constituents.
Thermal tempering
Following that, each major group of the
four LSCs was split into two groups (n = 5)
in accordance with the thermal tempering
procedure, as follows:
• Sub gp T1 (n = 10), where specimens were
red at a temperature 9% below the pressing
temperature;
• Sub gp T2 (n = 10) the specimens underwent
a ring cycle at a temperature 5% below the
pressing temperature based on the following
re charts for each material.Recommended
tempering temperatures are shown in
Table II.
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Braz Dent Sci 2024 Apr/June;27 (2): e4356
Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
Second phase measurements
Optical properties measurements, Scanning
Electron Microscopic analysis (SEM), X-ray
Diffraction analysis (XRD), and Energy Dispersive
X-ray Analysis (EDX) were repeated for all
samples after thermal tempering the same way
discussed earlier.
Statistical analysis
Using tests of normality (Kolmogorov-
Smirnov and Shapiro-Wilk tests) and examining
the data distribution, numerical data were
examined in preparation for statistical analysis.
The data were all parametrically distributed.
The mean and standard deviation (SD) values of
the data were displayed. Two-way ANOVA testing
and repeated measures ANOVA were employed.
When the ANOVA test was signicant, pairwise
comparisons were performed using Bonferroni’s
post-hoc test.
RESULTS
Translucency: at T0 Pairwise comparisons
between LSCs types revealed there was a
statistically signicant difference between them
(
P
-value <0.001, Effect size = 0.998). that
Gp (E) showed the statistically significantly
highest mean TP. Gp (L) showed a statistically
signicantly lower mean TP followed by Gp (A)
with a statistically signicant difference between
the two ceramics. Gp (C) showed the statistically
signicantly lowest mean as shown in Table III.
After heat treatment whether with T1 or
T2, in all groups and subgroups, there was a
statistically signicant increase in mean TP after
thermal tempering. Comparing the two tempering
protocols, T1 showed statistically signicantly
lower TP mean values than T2. The difference
was signicantly highest recorded for Gp (E).
In terms of CR mean values, the results
showed that LSCs type had a statistically
signicant effect on mean CR (
P
-value <0.001,
Effect size = 0.995). recorded the signicantly
highest CR mean values for (LZSC) in Gp (C)
and Gp (A), followed by Gp (L) and Gp (E)
respectively. The selected thermal tempering
whether T1 or T2 had a statistically signicant
decrease in mean CR (
P
-value <0.001, Effect
size = 0.96). while the effect of changing the
temperature of the thermal tempering procedure
had no statistically signicant effect on mean CR
(
P
-value = 0.694, Effect size = 0.005).
Color changes
Table IV shown a signicant shift in color in
all groups witnessed after the selected thermal
tempering procedures, being highest evidenced
in Gp (E) with ΔE values of (3.18± 0.2) followed
by Gp (L) ΔE of (2.47± 0.19) then Gp (C) ΔE of
(2.26 ±0.14) and the last Gp (A) ΔE (1.62 ±0.13).
The T2 thermal tempering procedure
provided a noticeably greater color shift ΔE mean
value of (2.55 ± 0.63) against ΔE of (2.35 ±
0.59) with the T1 protocol.
With thermal tempering, the shift was
caused by an increase in L* parameter values and
moving positively toward redness in a* parameter
consistently with moving toward the yellow
direction in b* as shown in Table V.
Scanning electron microscopy
Typically, Gp (ET0) showed multilayered
crystals in the form of rods lying parallel to the
direction of pressing with the average crystal’s
length × width diameters of 3 μm × 680 nm
(Figure 1A). For Gp (ET1), the length and width
grew to 4.5 μm and 700 nm, respectively
(Figure 1B). Further growth to 4 μm length and
870 nm width with Gp (ET2) (Figure 1C).
Table II - Recommended tempering temperatures of ceramics
Group (T0) (T1) (T2)
Gp (A) 880 °C 800 °C 836 °C
Gp (C) 870 °C 790 °C 826 °C
Gp (E) 917 °C 835 °C 872 °C
Gp (L) 910 °C 828 °C 865 °C
Table III - The mean, standard deviation (SD) values and results of repeated measures ANOVA test for comparison between TP of ceramic types
Gp (E) Gp (L) Gp (A) Gp (C)
P
-value Effect size
(Partial eta squared)
Mean SD Mean SD Mean SD Mean SD
17.53A0.89 15.5B0.7 13.69C0.54 12.24D0.4 <0.001* 0.998
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Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
Comparing the Gp (LT0) group to the Gp
(ET0) group, there is a noticeable difference in the
microstructure; An interlocking microstructure
was formed by layering platelet-shaped crystals
embedded in the glass matrix. The crystals were
not oriented parallel to the direction of pressing
as compared to Gp (ET0). The crystals in the Gp
(LT0) featured an average length of 2 um and a
diameter of 600 nm (Figure 2A). In contrast, Gp
(LT1) (Figure 2B) displayed 2.5 um in length
Table IV - The mean, standard deviation (SD) values and results of the two-way ANOVA test for comparison between ΔE of ceramic types
regardless of temperature
Gp (E) Gp (L) Gp (C) Gp (A)
P
-value Effect size
(Partial
eta squared)
Mean SD Mean SD Mean SD Mean SD
3.18 A0.2 2.74 B0.19 2.26 C0.14 1.62 D0.13 <0.001* 0.957
*: Significant at P ≤ 0.05
Table V - Average change values of L* a* b* after heat treatments
ΔL Δa Δb ΔE
Gp E T1 1.47 1.17 2.40 3.05
T2 1.56 1.40 2.55 3.30
Gp L T1 1.26 1.10 2.02 2.63
T2 1.40 1.09 2.23 2.85
Gp C T1 1.20 0.67 1.69 2.18
T2 1.28 0.84 1.76 2.34
Gp A T1 0.92 0.67 1.04 1.55
T2 1.02 0.70 1.14 1.69
Figure 1 - (A) SEM IPS e.max Press without tempering; (B) SEM showing IPS e.max Press after the first heat tempering at 9%; (C) SEM showing
IPS e.max Press after the second heat tempering at 5%.
A B
C
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Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
and 630 nm in width. Meanwhile, reaching a
length of 2.4 um and 985 nm width with Gp
(LT2) (Figure 2C).
The microstructure of Gp (CT0) was found
to be interlocking, with an average crystal length
of 1.8 um and a width of 500 nm (Figure 3A).
The crystal’s length shrank to 1.3 um and its width
expanded to 600 nm in Gp (CT1) (Figure 3B).
The length increased to 2 um as well as its width
signicantly increased to 770 nm in the (GpC)
T2 (Figure 3C).
Figure 4A of Gp (AT0) revealed needle-like
crystals with an average length of 3 um and a
width of 380 nm. Figure 4B of Gp (AT1) revealed
an additional increase in the length and width
of the crystals, measuring 4.5 um and 530 nm,
respectively. The lithium disilicate crystals in Gp
(AT2) solidied and shrank in size (Figure 4C),
which made it challenging to estimate grain sizes
with any degree of accuracy.
X-ray diffraction
Major peaks of lithium disilicate (LisSi2O5)
were observed at 2Ɵ values of 24.7°, and
40 degrees. Traces of lithium phosphate (Li3PO4)
were detected in all groups while traces of
Lithium metasilicate (Li2SiO3) traces were
detected with Gp (C) and Gp (A). The dominant
peak (highest peak) was at 24.7 degrees. The XRD
data showed the highest peak intensity in Gp (E)
equally in T0, T1, and T2, while for Gp (L) highest
peak intensity was at T1 with equal intensities
in T0 and T2. For Gp (C) the highest peak is
with T2 and the least was with T0. For Gp (A)
the highest intensity was at T0 and the lowest
intensity at T1 (Figures 5, 6, 7 and 8).
Energy dispersive X-ray analysis (EDAX)
Minor differences in compositions
between materials and thermal tempering
protocols were analyzed as shown in the
Figures 9, 10, 11 and 12 below.
DISCUSSION
The study aimed to assess how specific
thermal tempering techniques affect the optical
characteristics of pressable LSCs whether
with LSGCs or LZSCs. The selection of those
specific two thermal protocols was based on
the ’manufacturer’s recommendation of one
Figure 2. (A) SEM GC Initial LiSi Press without tempering); (B) SEM showing GC Initial LiSi Press after the second heat tempering at 9%; (C)
SEM showing GC Initial LiSi Press after the second heat tempering at 5%.
A B
C
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Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
Figure 3 - (A) SEM Celtra Press without tempering; (B) SEM showing Celtra Press after the second heat tempering at 9%; (C) SEM showing
Celtra Press after the second heat tempering at 5%.
A B
C
Figure 4 - (A) SEM VITA Ambria Press without tempering; (B) SEM showing VITA Ambria Press after the second heat tempering at 9%; (C) SEM
showing VITA Ambria Press after the second heat tempering at 5%.
A B
C
9
Braz Dent Sci 2024 Apr/June;27 (2): e4356
Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
of the selected materials for an additional
firing cycle at a temperature of 9% below
the pressing temperature [29]. The second
protocol was selected with 5% below the pressing
temperature which is the maximum limit for
temperature deviations below the pressing and
should not be exceeded by any means [32,33].
The obtained results demonstrated that the
chosen thermal tempering processes could have a
considerable impact on the optical result of these
chosen materials, rejecting the null hypothesis.
Translucency is a vital key optical property that
can be employed to replicate the appearance
of natural teeth and precisely match with the
surrounding structure, particularly in the anterior
and esthetic zones [34]. The translucency
of lithium disilicate glass-ceramics depends
on several factors such as: thermal history,
pressure, time (for heat-pressed), amount and
type of nucleating agent, crystal morphology,
microstructure, glass matrix, volume ratio
crystal/glass and difference in the refractive index
between these phases, phase composition and
chemical composition including grain boundaries,
pores, second-phase component, additives, and
light scattering from the surface [35].
Effect on translucency
Before any thermal process: at T0
The translucency was greatly impacted
by the type of LSCs utilized, as seen by the TP
mean values. The LSCs recorded signicantly
higher translucency than the LZSCs as; Gp
(E) recorded the significantly highest mean
TP value (TP=17.52), according to published
literature [36], this is within the same translucency
range of 1 mm for human enamel and dentin,
which are 18.7 and 16.4, respectively. This
implies this material is best suited as an aesthetic
material for anterior cases. Gp(L), with a mean
Figure 5 - XRD analysis of IPS E.max Press, (T0) control group, (T1)
tempering at 9% below pressing temperature, and (T2) tempering at
5% below pressing temperature.
Figure 6 - XRD analysis of LiSi Press (T0) control group, (T1)
tempering at 9% below pressing temperature and (T2) tempering at
5% below pressing temperature
T0
T1
T2
10 20 30 40 50 60 70
Counts
0
20
40
60
80 X-2022-679 (12)
10 20 30 40 50 60 70
Counts
0
100
200
X-2022-678 (11)
10 20 30 40 50 60 70
Counts
0
20
40
X-2022-677 (10)
Intensity
°2θ
Intensity
°2θ
Intensity
°2θ
(○) Li2Si2O3 () Li3PO4 (▲) Li2SiO3
T0
T1
T2
P i i [°2θ] (C (C ))
10 20 30 40 50 60 70
Counts
0
50
100 X-2022-676 (9)
P i i [°2θ] (C (C ))
10 20 30 40 50 60 70
Counts
0
50
100
X-2022-675 (8)
10 20 30 40 50 60 70
Counts
0
50
100
X-2022-674 (7)
Intensity
°2θ
Intensity
°2θ
Intensity
°2θ
(○) Li2Si2O3 () Li3PO4 (▲) Li2SiO3
10
Braz Dent Sci 2024 Apr/June;27 (2): e4356
Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
TP value of (TP=15.5), came next. Even though
both materials have considerable crystallinity,
the matching of refractive indices between their
crystalline phase 1.55 and glassy phase 1.50,
which permits greater light transmission, was
considered one of the main causes for their
unique translucency [37].
Since it has been shown that lithium disilicate
crystals obstruct light transmission, varying the
size of the crystals in these materials can result
in varying levels of translucency. An earlier
investigation comparing the translucencies of IPS
e.max CAD LT, MT, and HT concluded that the
larger crystal sizes of the IPS e.max HT reduced
its dispersion and lowered its crystal density,
which in turn led to its higher translucency [38].
SEM analysis showed that GP (E) has larger rod-
shaped crystals with an average measured length
of 2.762-3.293 μm and an average measured width
of 533.5-687.9 nm, while GP (L) has more rounded
smaller platelet-shaped crystals with an average
measured length of 2.2μm and an average width
of 580nm. Similar crystal sizes for both materials
were reported by Ohashi et al. [39]. Further
differences in spatial crystal arrangement were
revealed by the SEM analysis. For example, the
crystals in Gp (E) aligned parallel to the sample
surface in an interlocking arrangement, indicating
the pressing direction. Conversely, the scanning
image of GP (L) revealed platelet-shaped crystals
that were randomly oriented, densely fused and
had a highly interlocking microstructure with
irregular edges. In contrast to (Gp E), the crystals’
orientation was not parallel to the pressing plane.
Higher transmittance values may be explained
by the linear, more ordered crystalline structure,
whereas the noticeably lower transmittance values
for the Gp (L) may be explained by the irregular
crystal arrangement, which increases scattering
and reflectance in addition to increasing the
incidence of porosities, which negatively affects
translucency due to the mismatch between the
Figure 7 - XRD analysis of Celtra Press, (T0) control group, (T1)
tempering at 9% below pressing temperature, and (T2) tempering
at 5% below pressing temperature.
Figure 8 - XRD analysis of VITA ambria, (T0) Control group, (T1)
tempering at 9% below pressing temperature, and (T2) Tempering
at 5% below pressing temperature
T0
T1
T2
P i i [°2θ] (C (C ))
10 20 30 40 50 60 70
Counts
0
10
20
X-2022-673 (6)
10 20 30 40 50 60 70
Counts
0
20
40
60
X-2022-672 (5)
10 20 30 40 50 60 70
Counts
0
50
100
X-2022-671 (4)
Intensity
°2θ
Intensity
°2θ
Intensity
°2θ
(○) Li2Si2O3 () Li3PO4 (▲) Li2SiO3
T0
T1
T2
10 20 30 40 50 60 70
Counts
0
100
200
X-2022-670 (3)
10 20 30 40 50 60 70
Counts
0
10
20
30
X-2022-669 (2)
P i i [°2θ] (C (C ))
10 20 30 40 50 60 70
Counts
0
20
40
60
80
X-2022-668 (1)
Intensity
°2θ
Intensity
°2θ
Intensity
°2θ
(○) Li2Si2O3 () Li3PO4 (▲) Li2SiO3
11
Braz Dent Sci 2024 Apr/June;27 (2): e4356
Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
air refractive index and the other phases. Similar
microstructural observations were found when
Hallman et al. [35] investigated the microstructure
of the same materials following pressing.
The pressing temperature for both ceramics
was above 900 °C where a complete transformation
to lithium disilicate Li2Si2O5 occurred with only
minor traces of lithium orthophosphate Li3PO4.
This was evident with the XRD analysis at T0.
In terms of the lowest TP mean values for
the LSCs; the two LZSCs were ordered as follows;
GP (A) and Gp (C) (TP = 13.69 and 12.24,
respectively). As mentioned by Yan et al. [40],
the 8-12% zirconia content in these materials may
seem to be the cause of the increased dispersion
and decreased transmission. This reason could
not be fully depended upon in our investigation
because hardly any phases containing zirconia
were found, either in XRD or in EDAX analysis,
which outweighed the dissolution during the
crystallization procedure. The Zr role was evident
in modifying the crystallization process and
increasing the viscosity of the glassy matrix.
It proved that the presence of ZrO and AlO allowed
the precipitation and domination of the lithium
metasilicate Li2SiO3 phase at a temperature of
around 700 °C as stated by Zhang et al. [15], which
is then reacted again, at higher temperatures of
pressing at 880-870 °C, with the silica in the glass
matrix to form the lithium disilicate crystal phase.
This nal reaction was not complete with both
LZSCs tested materials as traces of the lithium
metasilicate phase were evident in XRD analysis at
T0. This is logically one of the reasons for hindering
the translucency of these materials. Besides the
smaller crystal sizes were evident through SEM
analysis as the crystals were narrower and more
irregularly arranged.
Figure 9 - Microanalysis by EDAX of Gp(E), (T0) Control group, (T1) tempering 9% below pressing temperature and (T2) tempering 5% below
pressing temperature.
12
Braz Dent Sci 2024 Apr/June;27 (2): e4356
Ahmed IHE et al.
The optical behavior of pressable lithia-based glass-ceramics under two different heat treatment protocols
Ahmed IHE et al. The optical behavior of pressable lithia-based glass-ceramics
under two different heat treatment protocols
Consistently the signicantly highest CR
mean value was for LZSCs: GP (C) followed by
GP (A) with no signicant difference between
them, and then the two other groups of LSCs;
Gp (L) and GP (E) the significantly lowest.
The non-significance between both LZSCs
groups may be attributed to that the CR is
mainly for measuring opacity and small light
transmittance differences can’t be detected
through such measurement, while TP is
calculated through the color parameters that
give a more accurate indication about the light
transmittance.
Effect of thermal tempering on translucency
Regardless of the ceramic type and
temperature of the tempering process there was
a signicant increase in translucency with thermal
tempering expressed in a considerable increase
in TP mean value paralleled with a signicant
decrease in CR values. This is consistent with
Farahnaz Nejatidanesh et al. [41] study results
that a signicant increase in translucency with
repeated firing was reported. This might be
explained by the fact that heat treatment causes
the crystals to grow larger while also changing
Figure 10 - Microanalysis by EDAX of Gp(L), (A) Control group, (B) tempering 9% below pressing temperature, and (C) tempering 9% below
pressing temperature.