Walid Sadig, BDS, MS ,
Hamdi Mohammed-AI Tahawi, DDS, MScD, PhD
 King Saud University, College of Dentistry, P.O. Box 60169, Riyadh 11545,
Kingdom of Saudi Arabia.

Various materials and techniques are used for refinishing ground dental porcelain. Most of the final polishing materials are diamond pastes and are quite expensive. Industrial glass manufacturers use inexpensive polishing materials to attain essentially the same degree of polish as porcelain. One such industrial polish was compared to auto glazing, liquid glazing and two polishing techniques as to rendering ground dental porcelain smooth. Twenty-five specimens were made from Vita porcelain and auto glazed. One group of five specimens was left as is. The other twenty specimens were ground to remove the glaze layer then were divided into four groups of five specimens each. One group was liquid glazed. Two groups were refinished and polished using a dental porcelain adjustment system with and without polishing paste. The last group was refinished but polished with an industrial glass polishing paste. Roughness of the specimen surfaces were determined by using a surface roughness microanalyzer. Specimens from each group were examined under the SEM. Statistical analysis showed that the industrial glass polish produced a surface as smooth as the auto glazed surface and as two of the porcelain polishing techniques. Liquid glazing produced a surface significantly more rough than the other four treatments.

Porcelain has been available as a restorative material for over 150 years.¹ The material is widely used mainly because of its aesthetic qualities. There are numerous instances in laboratory and clinical practice when it is necessary to adjust an autoglazed porcelain surface by grinding.² The resulting break in the glazed surface need not be ignored. Monasky and Taylor³ demonstrated an increased potential for wear of occlusal surfaces opposed by a ground porcelain surface. Podshadley and Harrison⁴ reported that an unglazed porcelain surface in contact with soft tissues can elicit an unfavorable response. Hence, ground porcelain surfaces, if not glazed, should be polished thoroughly to eliminate the rough surface layer.⁵ Various techniques have been used for refinishing ground porcelain⁶ and several porcelain refinishing diamond pastes are now available commercially. These diamond pastes are capable of restoring adjusted porcelain to a degree of surface smoothness comparable to that of glazed porcelain.⁷⁹ Actually, many ceramists advocate polishing, rather than glazing, to control the surface luster, glossiness and the aesthetic result of the ceramic restoration.¹⁰ However, these commercial pastes are expensive.

The mean roughness, standard error of mean and 95% confidence interval for each of the autoglazed (AG), the liquid glazed (LG), the Shofu polished (SH), the Vident polished (VV), and the industrial glass polishing paste (GS) specimens are shown in Table 1. The table shows that the lowest mean roughness was that of the autoglazed porcelain. The mean roughness obtained as a result of using the Shofu and Vident pastes were equal to each other and quite close to that of the industrial glass polish. The latter three polishing materials resulted in a surface that has a higher mean roughness than that obtained by autoglazing but much smoother than that obtained from a liquid glazed surface.
One way analysis of a variance {ANOVA} (Table 2) showed that significant differences exist between the five means. Multiple range analysis (Table 3) showed that there was no difference in surface roughness among four treatments and all of them were significantly different (P < .05) from the liquid glazed surface.
SEM examination revealed that the liquid glazed surface [Fig. 1] is much rougher than surfaces treated by the other four methods.

Different polishing techniques for porcelain have been studied using both methods of this study.¹⁶'⁸¹¹ The polishing techniques were also evaluated by SEM alone²⁹¹² or by a magnifying glass.² In many previous studies¹⁸¹² no differences were found between the autoglazed and polished porcelain surfaces. These reports are in agreement with our study where two porcelain polishing techniques and one industrial glass polishing paste produced smooth surfaces which was statistically not different from an autoglazed surface. Obviously, not all post-grinding porcelain polishing methods are capable of restoring the smoothness of a ground porcelain surface to its autoglazed condition since the liquid glazing technique tested by us failed to do so. However, many ceramists believe that liquid glazing creates comparable smoothness to autoglazing, when it is not. Nevertheless, a more recent study showed that liquid glazing significantly increased the flexural strength of feldspathic porcelain.¹³ It is quite interesting to note that the inexpensive industrial glass polishing agent was as effective as professional dental porcelain polishing agents in polishing ground dental porcelain.
One must keep the issue of surface smoothness in perspective, however. While a ground then polished porcelain may be as smooth as an autoglazed porcelain, there are clinical functional differences.¹⁴   McLean   stated   that   a   ground porcelain surface that is not reglazed will cause impairment of fracture resistance.¹⁵¹⁶ However, McLean's methods of specimen preparation differed from the clinical situation. His specimens were hydraulically pressed from powder, the level of glaze used was higher than that used when matching the luster of natural teeth, and a medium- fusing porcelain rather than low-fusing metal ceramic porcelain was tested. In 1959, Hodson¹⁷ reported that grinding glazed high- and medium- fusing porcelains reduced the modules of rapture but grinding low-fusing specimens did not. More recently in 1989, Rosensteil¹⁰ found that the fracture toughness increased by using polishing as an alternative to autoglazing. Another study showed that fracture toughness increased by 22% when polishing was applied. In addition, the stainability was unaffected.'³ The author considered the results unexpected. Porcelain polishing is practiced more routinely especially after the introduction of porcelain veneers, inlays and onlays.¹⁸
It was further pointed out that an unglazed occlusal surface will cause more occlusal wear of opposing teeth than will a glazed surface.³ It appears that good polishing ameliorates all negative effects of unglazing.

Based on the findings of this study, the authors conclude that:

1. Klausner LH, Cartwright CB, Charbaneau GT. Polished versus autoglazed porcelain surfaces. J Prosthet Dent 1982;47:157-61.
2. Sulik WD, Plekavich EJ. Surface finishing of dental porcelain. J Prosthet Dent 1981 ;46:21 7-21.
3. Monasky GE, TaylorDF. Studies on the wear of porcelain, enamel and gold. J Prosthet Dent 1971 ;25:299-306.
4. Podshadley AG, Harrison )D. Rat connective tissue response to pontic materials. ) Prosthet Dent 1966;16:110.
5. Henry PJ, Johnston JF, Mitchell DF. Tissue changes beneath fixed partial dentures. J Prosthet Dent 1966;16:937-47.
6. Smith GA, Wilson NH. The surface finish of trimmed porcelain. Br Dent J 1981;151:222-24.
7. Newitter DA, Schlissel ER, Wolff MS. An evaluation of adjustment and postadjustment finishing techniques on the surface of porcelain-bonded-to-metal crowns. J Prosthet Dent 1982:48:388-95.
8. Patterson CJ, McLundie AC, Stirrups DR, Taylor WG. Refinishing of porcelain by using a refinishing kit. J Prosthet Dent 1991, 65:383-88.
9. Raimondo RL Jr, Richardson JT, Wiedner B. Polished versus autoglazed dental porcelain. J Prosthet Dent 1990;64:553-57.
10. Rosensteil SF, Baiker MA, Johnston WM. A comparison of glazed and polished dental porcelain. Int J Prosthodont 1989:2:524-29.
11. Goldstein GR, Barnhard BB, Penugonda B. Profilometer, SEM, and visual assessment of porcelain polishing methods. J Prosthet Dent 1991,'65:627-34.
12. Schlissel ER, Newitter DA, Renner RR, Gwinnet Aj. An evaluation of post-adjustment polishing techniques for porcelain denture teeth.) Prosthet Dent 1980;43:258-65.
13. Giordano RA, Campbell S, Pober R. Flexural strength of feldspathic porcelain treated with ion exchange, overglaze and polishing. J Prosthet Dent 1994;71:468- 72.
14. Morrow RM, Brown CE, Larkin JD, Bernut R, Rudd KD. Evaluation of methods for polishing porcelain denture teeth. J Prosthet Dent 1973;30:222-26.
15. McLean JW. Science and art of dental ceramics. Volume I. The nature of dental ceramics and their clinical use. 2nd ed. BerlimQuintessencePubICo, 1979:263.
16. McLean JW, Hughes TH. The reinforcement of dental porcelain with ceramic oxides. Br Dent J 1965;119:251- 67.
17. Hodson JT. Some physical properties of three dental porcelain with ceramic oxides. Br Dent J 1965; 251-67.
18. Hubbard JR. Natural texture and luster in ceramics. In: Preston JD ed. Perspectives in dental ceramics. Chicago: Quintessence Publ Co, 1988:263-66.