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Original Article

Evaluating the effect of re-glazing on dental

porcelain surfaces e An invitro study

Q3 R. Himabindua,*, Vinod Krishnan b

a Professor, Dept of Prosthodontics, Gitam Dental College & Hospital, GITAM Campus, Rushikonda, Visakhapatnam

530045, Indiab

Professor, Dept of Prosthodontics, Amrita School of Dentistry, AIMS Campus, Ponekkara, Cochin 682030, India

a r t i c l e i n f o

Article history:

Received 31 May 2013

Accepted 29 July 2013

Keywords:

Glazing

Polishing

Re-glazing

Electron microscopy

Micro cracks

Wear of natural dentitionQ1

a b s t r a c t

Aim: The aim is to study the surface finish of ceramic restorations by glazing and polishing

procedures and compare the surface morphology to determine the presence or absence of

surface cracks on porcelain specimens observed under optical (light) microscope and a

scanning electron microscope (SEM).

Materials and methods: Thirty six specimens of vita VMK 68 No. 559 incisal porcelain were

prepared and divided into eight groups (n 6) (Group IeGroup VI) and then subjected to

various polishing and glazing procedures. The specimens were then observed under optical

(light) microscope and scanning electron microscope (SEM).

Results: Groups I, II, III, V, did not exhibit any surface cracks whereas Groups IV & VI which

were ground, polished and then re-glazed exhibited micro cracks on the surface of the

specimens.

Conclusion: Dental porcelains which are extensively used in dentistry have been constantly

evolving to maximize strength. Within the limitations of this study it was observed that

the optical (light) microscope and SEM analysis correlate with each other. The specimen

surfaces that were polished and then self-glazed exhibited formation of fine surface

cracks.

2013 Indian Journal of Dentistry. All rights reserved.

1. Introduction

Ceramics in modern dentistry is being used since the 18th

century1 and has rapidly emerged as one of the major dental

biomaterials in prosthodontics due to exceptional aesthetics

and outstanding biocompatibility. But a major drawback is

the abrasiveness to opposing natural dentition. Occlusal

contacts between unglazed porcelain and enamel are un-

desirable because of the high rate of wear of enamel and

porcelain. When unglazed porcelain opposes natural teeth,the results of occlusal wear can be broadening of the

occlusal table and loss of tooth structure.2 Several methods,

from re-glazing to polishing, are available to achieve a

smooth porcelain surface. Glazing of ceramic surfaces is

recommended in order to improve the physical properties of

dental ceramics. Plaque cannot accumulate on the glazed

surface, and the brightness and surface features of the

restoration can be retained long term.3 A smooth surface

* Corresponding author. Tel.: 91 9440448333 (mobile).E-mail addresses: [email protected] (R. Himabindu), [email protected] (V. Krishnan).

Available online at www.sciencedirect.com

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / i j d

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Please cite this article in press as: Himabindu R, Krishnan V, Evaluating the effect of re-glazing on dental porcelain surfaces eAn invitro study, Indian Journal of Dentistry (2013), http://dx.doi.org/10.1016/j.ijd.2013.07.016

0975-962X/$ e see front matter 2013 Indian Journal of Dentistry. All rights reserved.

http://dx.doi.org/10.1016/j.ijd.2013.07.016
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improves the flexural strength of the restoration and de-

creases abrasion against opposite teeth.4,5 However, the

dentist often removes part of the glazed surface after

cementation of the ceramic restoration because of the need

for occlusal adjustment, correction of inadequate contour,

or improvement of esthetics.6 By using various intraoral and

extra oral porcelain polishing kits smoothness levels equal

or better than those attained through glazing procedurescan be obtained.

2. Materials and methods

Thirty six porcelain test specimens are fabricated in the form

of discs in the dimensions of 10 mm diameter and 2 mm

thickness. Vita VMK 68 No. 559 incisal porcelain was the ma-

terial of choice as it is commonly used in fabrication of

ceramo-metal restorations. 600 mg of Vita VMK 68 No. 559

incisal porcelain pre weighed in an electronic balance and

0.4 ml of modeling fluid supplied by the manufacturer

measured in a micropipette are mixed together. The mix is

placed in the metallic mold and the mold is placed in the

hydraulic press and compacted at 22,000 N/Sq. inch and fired

in a commercially available porcelain vacuum furnace (Vita

Vacumat 50). Then the specimens are divided into six groups

of six specimens each and then subjected to grinding and

polishing procedures as done in the chair side of a dental

clinic or dental lab.Q2

The samples are examined under an optical microscope at

a resolution of 100 magnification and the final image wasthen viewed on a fluorescent screen and recorded photo-

graphically. The porcelain samples viewed under the light

microscope are cleaned in acetone solution to remove any dirt

sticking to the surface and then air dried. The specimens are

then mounted on brass stubs and placed in Ion sputtering

device (JFC 1100E) where they are gold coated. The gold

coating increases the surface contrast and enables identifi-

cation of areas of apparent smoothness and roughness. The

specimens arethen re-examined at higher magnifications e.g.:

500 or more in SEM (JSM 5300) where secondary electrons are

collected, amplified and used to form an image on the fluo-

rescent screen of a cathode tube in a display unit which is

recorded on a camera attached to it.

3. Results

It was observed that, the self-glazed (Group II); wet and

polished specimens (Group III & V) surfaces appear to be

smooth without any evidence of surface cracks. The polished

and then re-glazed groups (IV & VI) showed significant evi-

dence of surface micro cracks which occurred at varying

lengths and very fine in nature. Large variation in levels of

cracking was observed. On comparison the average level of

cracking observed in Group IV is 4744 mm/mm2 and in Group

VI it is 3578 mm/mm2. The relative levels of cracking were

determined by measuring crack length/unit area on SEM.

Statistical analysis was not done because of the observational

nature of the study.

4. Discussion

Dental ceramics are known for their exhibit excellent tissue

compatibility, extreme chemical durability and low thermal

conductivity.7 A smooth surface is important in three terms:

function, esthetics, and biologic compatibility. In fact, a sur-

face compression layer was found to occur on a wide range of

ceramic materials following different treatment processes

that acts to strengthen ceramic material. It can be achieved by

thermal tempering, machining and polishing and the appli-

cation of a glazing layer with a lower coefficient of thermal

expansion than the adjacent ceramic material. There are

numerous instances in clinical practice whenit is necessary toadjust a glazed porcelain surface by grinding. Such adjust-

ments break the glaze, resulting in a rougher surface and

inferior surface properties of the restoration.8 Early re-

searchers agreed that re-glazing was necessary after porcelain

adjustment in the clinical setting.9 Many dentists therefore,

prefer the porcelain surface of a restoration to be re-glazed

prior to cementation.10

The aim of glazing is to seal the open pores on the surface

of a fired porcelain. The firing of the porcelain must be carried

out exactly according to the manufacturers instructions. If

the crown should remain in the furnace for too long it will lose

form, due to pyroplastic flow (flow of the molten glass), and

will become highly glazed. A lower temperature and longer

Group I d As fired;

Group IId

Self-glazed;Group IIId Self-glazed samples grinded with 220, 500, 600 and 800

grit silicon carbide paper followed by 0e2m diamond polishwith a

felt wheel.

Group IV d The specimens are grinded and polished as above and

then subjected to re-glazing at 940 C for 4 min.

Group Vd The self-glazed specimens are chair side polished using

a commercially available porcelain adjustment kit (Shofu).

Group VI d The specimens are polished as above and then re-

glazed at 940 C for 4 min.

Group I (As fired) d Appears to be pitted and rough.

Group II (Self-Glazed) d Surface appears to be smooth.

Group III (Ground and Polished)d

Surface appears to be smoother(Fig. 1).

Group IV (Ground, polished and then re-glazed) d Surface less

smooth, few voids and irregularities seen. Micro cracks are

visible on the surface. The surface shows fragments which seem

to be completely fractured leaving away a ragged chip on the

surface (Figs. 2 and 3).

Group V (Ground and polished using Shofu polishing kit)d Surface

appears to be smoother (Fig. 4).

Group VI (Polished and then re-glazed) d Surface appears to be

chipped of in certain areas and rough. Voids are seen. Micro

cracks are visible which are very fine in nature (Figs. 5 and 6).

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residual stresses produced created a compressive layer that

can close an existing surface crack. It is thereby assumed that

when the samples are subjected to self-glazing again after

polishing the compressive residual stresses may be released

thereby, exhibiting the micro cracks on the surface.

5. Review of literature

Glazing versus polishing techniques received recent attention

in a review of a number of studies comparing prepared sur-

faces using visual, microscopic, and profilometry measures.

12

The literature clearly shows that grinding, polishing, and

glazing increase a restorations flexural strength. Actually,

many ceramists advocate polishing, rather than glazing, to

control the surface luster, glossiness and the esthetic result of

the ceramic restoration.

Nasser Barghi & King (1975)13 who reported that smooth

surface is obtained whenpolishing is followed by glazing. This

study also shows evidence of polished surfaces to be as

smooth as that of self-glazed surfaces.

Sulik and Plekavich (1981)14 reported that, with a well-

condensed porcelain surface, the surface achieved by polish-

ing can be as smooth as that of a glazed surface and they

found no differences clinically or by means of scanning elec-

tron microscopy, between the polished and naturally glazed

surfaces of vacuum fired porcelain. Some voids were presenton the polished surface which were not evident on the glazed

surface.

Klausner et al (1982)15 who reported that no significant

differences were found between the final polished surfaces

and auto-glazed surfaces.

Kazuyuki and Tomozawa (1987)16 who declared that the

thermal effect of laser would melt a thin superficial layer of

ceramic surface and this layer would fill in surface flaws,

reducing their depth and blunting the flaw tips. This should

provide an increase in hardness because, for a given ceramic

material, strength and hardness would increase with

decreasing flaw depth and sharpness.

Wiley (1989)17 suggested that a polished surface may be asacceptable as a glazed surface.

Rosenstiel et al (1989)18 found that the fracture toughness

of polished porcelain was greater than that of glazed

porcelain.

Edge and Wagner (1994)19 whose study coincides with the

present study reported that surface cracking was identified in

polished and then self-glazed specimens of dental porcelain.

Giordano et al (1994)20 and Williamson et al (1996)21 found

that mechanical polishing increases the surface resistance of

a ceramic restoration more than the natural glaze and over

glaze processes do.

Few other studies 1988,22 1990,23 200624 have shown that

polished ceramics produced surfaces that were as smooth asglazed ceramics, or provided smoother surfaces than glazing.

6. Conclusion

It is a common clinical practice to adjust ceramic restorations

to correct occlusal interferences, improve esthetic appear-

ance, finish the margins of porcelain e bonded restorations,

and improve surface smoothness. The dental practitioner can

choose between several different polishing methods to ach-

ieve a surface equal to or better than over-glazing. An

advantage of polishing is that it affords greater control of thesurface luster and distribution than does glazing.25 Using

various intraoral and extra oral porcelain polishing kits,

smoothness levels equal or better than those attained through

glazing procedures can be obtained.26 Within the limitations

of this study it wasobserved that the optical (light)microscope

and SEM analysis correlate with each other. The specimen

surfaces that were polished and then self-glazed exhibited

formation of fine surface cracks. This observation is of clinical

significance as it might lead to chipping of porcelain surfaces

leading to wear of natural dentition. Therefore, it can be

concluded from this study is that the restorations which need

adjustments should be subjected to polishing methods

instead of re-glazing it again.

Fig. 5 e Chair side polished and re-glazed sample surface

of incisal porcelain 1003 magnification (optical

microscope).

Fig. 6 e Chair side polished and re-glazed sample surface

of incisal porcelain exhibiting micro cracks under SEM at

5003 magnification.

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Conflicts of interest

All authors have none to declare.

Acknowledgments

1. Dr. KSGA Nasser MDS, TNGDC, Chennai.

2. Prof. FD Gnanam, Anna University, Chennai.

3. Prof. Gokularatnam, IIT, Chennai.

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