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The effect of recasting on the oxidation layer of a palladium-silver porcelain alloy
RHODES
SUMMARY After preliminary scientific data, the dentist commonly evaluates the success of a recommended clinical technique. However, the accuracy of a restoration constructed in the laboratory depends directly upon the abilities of the laboratory technician. The communication between the dentist and the laboratory technician is crucial for success. This study examined the practicality of the porcelain/ butt margin. When combined with the appropriate tissue management, sulcular placement, and impressions, this consistent procedure was easily evaluated, financially sound, and time-conserving.
8. 9. 10.
11.
12. 13.
14.
REFERENCES 1.
2.
3. 4.
5.
6.
7.
Shillingburg HT, Hobo S, Fischer DW. Preparation design and margin distortion in porcelain-fused-to-metal restorations. J PROSTHETDENT 1973;29:276-84. Shillingburg HT, Hobo S, Whitsett LD, Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1981;79-128. Vryonis P. A simplified approach to the complete porcelain margin. J PROSTHETDENT 1979;42:592-3. Vryonis P. A manual for the fabrication of the complete porcelain margin. Melbourne, Australia: Stock Journal Publishers, 1982; 7-27. Goodacre CJ, Van Roekel NB, Dykema RW, TJllmann RB. The collarless metal-ceramic crown. J PROSTHETDENT 197’7;38:61522. Faucher RR, Nicholls JI. Distortion related to margin design in porcelain-fused-to-metal restorations. J PROSTHET DENT 1980;43:149-55. Pascoe DF. Analysis of the geometry of finishing lines for full crown restorations. J PROSTHETDENT 1978;40:157-62.
15. 16.
17.
18.
19.
Donovan T, Prince J. An analysis of margin configurations for metal-ceramic crowns. J PROSTHETDENT 1985;53:153-7. Sozio RB, Riley EJ. A precision ceramic-metal restoration with a facial butted margin. J PR~~THET DENT 1977;37:517-21. Belser UC, MacEntee MI, Richter WA. Fit of three porcelainfused-to-metal margipal designs in vivo: a scanning electron microscope study. J PROSTHETDENT 1985;53:24-9. Faucher RR, Nicholls JI. Distortion related to margin design in porcelain-fused-to-metal restorations. J PROSTHETDENT 1980; 43:149-55. Waerhaug J. Effect of rough surfaces upon gingival tissue. J Dent Res 1956;35:323-5. Waerhaug J. Histologic considerations which govern where the margins of restorations should be located in relation to the gingiva. Dent Clin North Am 1960;Mar:161-76. Newcomb GM. The relationship between the location of subgingival crown margins and gingival inflammation. J Periodontol 1974;45:151-4. Marcum JS. The effect of crown marginal depth upon gingival tissue. J PROSTHETDENT 1967;17:479-87. Hollinger JO, Locon L, Krantz WA. Connelly M. A clinical and laboratory comparison of irreversible hydiocolloid impression techniques. J EROSTHETDENT 1984;51:3@-9. Johnston JF, Phillips RW, Dykema RW. Modern practice in crown and bridge prosthodontics. 3rd ed. Philadelphia: WB Saunders Co, 1971;206-18. Appleby DC, Pameijer CH, Moffa J. The combined reversible/ irreversible hydrocolloid impression system. J PROSTHETDENT 1?80;44:27-35. Fusayama T, Kurosaki N, Node H, Nakamura M. A laminated hydrocplloid impression for indirect inlays. J PROSTHETDENT 1982;47:171-6.
Reprmt requests to: DR STEPHENK. RHODES 425 SOUTH MAIN ST. MADISONVILLE, KY 42431
The effect of recasting on the oxidation layer of a palladium-silver porcelain alloy Jau-Min Hong, p.D.S.,* Michael E. Razzoog, D.D.S., M.P.H., M.S.,** and Brien R. Lang, D.D.S., M.S.*** Kaohsiung Medical College, School of Dentistry, Taiwan, Republic of China, and University of Michigan, School of Dentistry, Ann Arbor, Mich.
lhe search for casting metals to replace the high gold content alloys has been motivated by economics. Dental alloys with palladium and silver as the main components
*Assistant Professor, Department of Removable Prosthodontics, Kaohsiung Medical College, School of Dentistry. **Associate Professor of Dentistry, University of Michigan, School of Dentistry. ***Chairperson and Professor, Complete Denture Department, University of Michigan, School of Dentistry. 420
were introduced in the 193Os,’ whereas their use as porcelain bonding alloys began in 1974.2 Investigators have confirmed that the physical properties of these alloys are acceptable for dental treatment.3-5 ,In the dental ladoratory, surplus alloy is commonly reused from the initial casting, and with the addition of new alloy, produces other restorations. Although ‘there have been various reports on the repeated use of noble and base metal alloys,6-* the structure and composition of palladium-silver alloy is unique. Whereas studies have APRIL 1988
VOLUME 59
NUMBER 4
EFFECT
OF RECASTING
ON
OXIDATION
<3 Al
LAYER
Surplus
only
surplus
+ 50%
Surplus
only
Surplus
Surplus
only
Surplus + 50% new alloy
Surplus
only
Surplus + 50% new alloy
new alloy + 50%
new alloy
Fig. 1. Specimens preparation. Group A (Al through A4) was cast with surplus metal from previous casting. Group I3 (Bl through B4) was cast with 50% by weight new alloy and 50% by weight surplus metal from previous casting. revealed that the metalic oxides are a factor in the bonding of porcelain to metal,‘, lo there is sparse mention of the effects of repeated casting on the oxide layer of palladium-silver alloys. This investigation determined the effect of repeated casting on the’ oxidation layer of a commercial palladium-silirer porcelain alloy (Esteticor Actual, Cendres & Metaux SA, Biel-Bienne, Switzerland). MATERIAL Preparation
AND METHODS of specimens
Ten 6 mm diameter X 1 mm thickness Pd-Ag porcelain alloy sp&imenS were prepared conventionally. Wax patterns were attache< to IO separate crucible formers with sprues 15 mm in length and 1 mm in diameter. The wax patterns were placed in IO casting rings and invested with a phosphate-bonded refractory material (Ceramicor, Cendres & Metaux SA). Each casting ring was placed in the electric furnace, heated slowly to . 1500” F, and held for 45 mmutes. The casting was performed in an electromatic casting machine (Howmet Corp., Chicago, Ill.) at 2420” F and allowed to cool to room temperature. The methods of recasting were divided into two groups (Fig. 111. Gr&p A. The initial casting was completed with a totally new alloy. Surplus (sprue and button) alloy from the previous cas,ting was used without the addition of any new alloy for the next casting. The sequence was repeated through four generations. Group B. The initial casting was accomplished with a totally oew alloy. Part of the metal assembly was removed from the previous casting, weighed, and comTHE
JOURNAL
OF PROSTHETIC
DENTISTRY
bined with appropriate new alloy to achieve 50% weight ratio for the next casting. This sequence was repeated through four generations. The sprues and buttons sectioned from each casting were sandblasted with aluminum oxide and ultrasonically cleaned with distilled water for 30 minutes. Ten specimens, including two initial casting specimens, were made after repeated casting through four generations in two groups. Preparation of specimens for oxidation and micro. probe analysis of oxidation layer. One initial casting specimen and the eight recasting specimens were placed in the furnace simultaneously and preheated at 1200” F for 5 minutes, heated from 1200” td 1900” F, and held at 1900” F for IO minutes. The air pressure in the furnace was reduced to 0.1 atmosphere while the temperature was elevated at a rate of 100” F per minute from 1200” to 1900” F and sustained for 10 minutes. The chamber was then vented and allowed to air cool to simulate degassing and oxidation. To examine the oxidized surface in cross section, specimens were mounted in epoxy resin and sectioned perpendicular to the oxides formed on the surface. Sectioned surfaces were ground and polished to 0.05 pm. The chemical composition of the alloy was determined qualitatively with an energy dispersive x-ray analyzer (EDX) (Kevex 7000, Fester, Calif.). The quantitative analysis was accomplished by a wavelength dispersive x-ray analyzer (WDX) (JEOL, Tokyo, Japan) that can also be operated as a scanning electron microscope (SEM). The alloy was used as the unknown and compared with metals with purity of 99.9%. Distribution of chemical elements in the oxidized 421
HONG, RAZZOOG, AND LANG
Fig. 2. Secondary electron image (SEX) micrographs with element analysis by XMA on oxidation zone of initial casting specimen. (a), Pd L-cr and Ag L-a! radiation line analysis; (b) Sn L-o and In L-a radiation line analysis. (Original magnificaiton X3000.)
specimens was determined by using an x-ray microprobe analyzer (XMA) (JSM 35, JEOL) with the same accelerating current (12 Kv, 0.4 X lo-’ amp; speed of scanning: 8 pm/min). Secondary electron image (SEI) photographs were procured of the regions analyzed for comparing the distribution of the chemical elements between the matrix of the alloy and the oxidation layer.
RESULTS The chemical composition of the alloy as analyzed by EDX was determined as palladium, silver, tin, indium, gold, platinum, and copper. Results from the quantitative analyses are presented in Table I. Except for the listing of palladium (54%) and silver (38%), additional information regarding the chemical composition of the 422
alloy had not been offered by the manufacturer. Results of x-ray microprobe analysis for element distribution at the oxidation zone in the initial casting specimen are available in Fig. 2. The intensity curves of each element are tracked from the first to fourth generation of group A and group B castings as illustrated in Fig. 3. The analysis curve reveals the distribution of four kinds of elements in the specimens from the matrix of the alloy to the oxidation zone. The highest intensity peaks of tin occurred at the oxidation zone, and the intensity peaks of tin increased progressively from the first to fourth generation in both groups. Tin intensity peaks for group A occurred higher in the oxidation zone than in group B. Indium intensity curve rises slightly at the oxidation zone but at a lower level than tin. However, there were no significant changes in the intensity peaks of indium along the casting generations or between group A and group B. Intensity curves for silver inclined moderately in the oxidation zone of groups A and B and increased progressively from the first to fourth generation. The intensity curves for palladium declined in the oxidation zone of each generation for groups A and B. Palladium curves did not differentiate between groups A and B among the casting generations. To determine the effects of repeated casting on oxidation the following procedures were performed. The XMA with the same current condition (12 Kv, 0.4 x lo-’ amp, speed of scanning: 8 pm/min) for 0 K-alpha was used from the alloy matrix to the external oxidation zone. The 0 K-alpha intensity curves with SEM micrograph of each generation of groups A and B were recorded. The SEM micrograph with the 0 K-alpha XMA of initial casting specimen is illustrated in Fig. 4. 0 K-alpha intensity curves of the four recasting generations of groups A and B were tracked and are displayed in Fig. 5. Intensity peaks of 0 K-alpha rise sharply at the oxidation zone and the intensity increases progressively from the first through third generations and then decreases slightly in the fourth, both in groups A and B. Before reaching the oxidation zone, small peaks of 0 K-alpha were discovered in the third and fourth generation of groups A and B. The thickness of the oxidation zone illustrates a substantial increase in the first generation and then a minimal increase with each succeeding generation in groups A and B (Table II). The thickness of group A’s oxidation zone increased at a greater rate than group B’s The microporosities distributed in the internal oxidation zone of group A castings became larger through each generation of recasting. This phenomenon did not occur in the castings from group B (Fig. 6).
DISCUSSION The x-ray microprobe analyzed at the oxidation zone exhibited significant differences between the four generAPRIL
1988
VOLUME
59
NUMBER
4
EFFECT OF RECASTING
Al
ON OXIDATION
ox.
Alloy Moth
LAYER
82
A2
ZOO0
::::, \
--
:,‘,..j&..$G
A4
B3
Electron Beom
Electron Beom
04
Electron Beam
:, ::’ i . ..., .:
,...,,.. 5 ,.... :‘-..
.‘? --* Electron
Beom
Electron
Electron Beom
0A
. .. K
p#j g
---
In
Electron Beam
Beam
pd -.-
0B
---
A9 Sn In
Fig. 3. Element analysis by XMA at alloy matrix and oxidation zone of four generations casting. Numbers 1 through 4 = number of recasting generation. A, Group A; B, group B. Table I. The chemical composition
of the alloy in this study obtained
by WDX quantitative
analysis
Element
Percent weztght
Pd
AR
Sn
In
AU
Pt
CU
53.76 (54Y
37.87 (3W
5.74
0.05
0.53
0.06
0.05
*Compositions reported by manufacturer.
Table II. Average thickness (pm) of the oxidation
layer of the initial
casting and four generations
of group A and B Generation 1
2
3
4
3.7 Tk 0.6
4.2 f 0.4
4.4 5 0.4
4.7 + 0.8
3.9 2 0.4
4.1 ir 0.4
4.0 * 0.3
4.2 + 0.4
EO Group A 2.7 + 0.3
Group B Readingstaken at 10 sites on each specimen.
EO = Initial
casting of all new alloy.
ations of castings in groups A and B. Although palladium and indium at the oxidation zone did not display significant differences among generations, the intensity of silver and tin increased through eadh generation in groups A and B. Tuccillo and Cascone” have suggested that silver can be absorbed by the porcelain at elevated temperatures and the precipitation of the particles responsible for discoloration occurred during the cooling cycle. The present study suggested that the concentration THE JOURNAL
OF PROSTHETIC
DENTISTRY
of silver on the oxidation zone was higher in the recasting specimens, even when 50% by weight of new alloy was added with the reusing of palladium-silver porcelain alloy. The potential for silver to revaporize and contaminate the surface and restoration is higher during the porcelain firing cycle. As expected, at the oxidation zone the intensity peaks of tin were much higher than indium because the concentration of tin in the alloy was higher and tin can 423
HONG,
RAZZOOG,
AND
LANG
Fig. 4. Secondary electron image (SEI) micrograph with 0 K-cux-ray microprobe analysis on oxidation zone of initial casting specimen. (Original magnification x3000.)
Electron Beam
Fig. 6. SE1 micrograph, XMA element analysis of fourth generation casting. (a), A-4; (b), B-4. (Original magnification X3000.)
Electron Beam
Fig. 5. 0 K-curadiation line analysis by XMA on oxidation zone of four generations of groups A and B. (In A and B groups; 1 through 4 = number of recasting generation; 12 Kv, 0.4 X lo+ amp, accelerating current; 8 pm/min, speed of scanning.)
be oxidized preferentially. The growth of the oxidation layer of this alloy was assumed to result mainly from the diffusion of interstitial Sn ions (Sn4+) through the oxide matrix forming Sn02.12 When findings related to the intensity of 0 K-alpha in the oxidation zone were reviewed, it was demonstrated that the oxidized condition of the specimens increased 424
through each generation of groups A and B castings. The activity of oxygen was higher in the various generations of group A than group B. The activity of oxygen in the oxidation zone and in the metal matrix near the oxidation zone became larger with each succeeding generation, even in the specimens where new alloy was added. The precipitation of oxides can be increased by recasting of the alloy. Diffusion of oxygen appeared more aggressive through the grain boundary and/or at the interface between the alloy matrix and oxide particles during the degassing of the recast alloy. Although the buttons and sprues may be sandblasted and cleaned, the contents of the metal oxidized cannot be completely eliminated under normal laboratory conditions. Therefore, for recasting a used alloy the addition of the new alloy is necessary, and a detailed decontamination procedure and a disciplined vacuum-casting process must be carefully followed. APRIL
1988
VOLUME
59
NUMBER
4
EFFECT OF RECASTING
ON OXIDATION
LAYER
The microporosities occurring at the internal oxidation zone and increasing in size through each succeeding generation of group A compromises the conditions for porcelain bonding. The oxidation layer formed in each generation of group B castings appeared more constant than group A, but the silver and metallic oxides that increased progressively by recasting group B could affect the porcelain bonding and discolor it. In this study, there were significant differences in the oxidation zone among the generations of groups A and B. Thus, the oxidation zone of the palladium-silver porcelain alloy can be affected by repeated casting, even when new alloy, 50% by weight, is added. This indicated that the recommendations by the manufacturers that at least one third new alloy be added when reusing metal is questionable and capricious. Additionally, the remelting of the palladium-silver porcelain alloys introduces an alteration of the chemical composit,ion and/or microstructure that might affect the oxidation layer. Before a definitive recommendation for repeated castings of palladium-silver porcelain alloys is made, further investigation is required to determine the alteration of chemical composition, microstructure, physical properties, color of bonding porcelain, and bonding strength of porcelain to metal. SUMMARY
REFERENCES 1.
2.
3.
4.
5.
6. 7.
8. 9. 10.
The oxidation zone of a commercial palladium-silver porcelain alloy was compared after repeated casting with and without the addition of new alloy. The intensity of palladium, silver, tin, indium, and 0 K-alpha near the oxidation zone was analyzed with XMA. The intensity curves of tin, &er, and oxygen increased progressively through each generation despite the addition of new alloy. The thickness of the oxidation zone and the microporosities at the internal oxidation zone increased through each generation without the addition of new alloy. Although the findings indicated that the oxidation zone was favorably formed by adding new alloy, 50% by weight, for four generations, the silver and metallic oxides of the oxidation zone increased through each generation. The reuse of the palladium-silver porcelain alloy remains questionable.
THE JOURNAL
Appreciation is expressed to Mr. J. W. Liu and Mr. F. C. Wu, Cheng-Kong University, Tainan, Taiwan, Republic of China for their technical assistance in this experiment.
OF PROSTHETIC
DENTISTRY
11.
12.
Craig RG, Peyton FA. Restorative dental materials. 5th ed. St Louis: The CV Mosby Co, 1975;318. Tuccillo JJ. Composition and functional characteristics of precious metal alloys for dental restorations. In: Valega TM, ed. Alternatives to gold alloys in dentistry. Bethesda Md: U.S. Department of Health, Education, and Welfare, Publication NO. (NIH) 77-1227, 1977;40-67. Bartkowiak H. The use, indications and considerations concerning the technology of Spa&WT-52 silver-palladium alloy. Protet Stomatol 1978;28:31-5. Gourley JM. Current status of semi-precious and conventional gold alloys in restorative dentistry. J Can Dent Assoc 1975;41:453-5. Huget EF, Dvivedi N, Cosner HE. Characterization of goldpalladium-silver and palladium-silver for ceramic metal restorations. J PROSTHETDENT 1976;36:58-65. Phillips RW. Skinner’s science of dental materials. 7th ed. Philadelphia: WB Saunders Co, 1973;397. Hesby DA, Kobes P, Garver DG, Pelleu GB. Physical properties of a repeatedly used nonprecious metal alloy. J PROSTHETDENT 1980;44:291-3. Nelson DR, Palik JF, Morris HF, Comella MC. Recasting a nickel-chromium alloy. J PROSTHETDENT 1986;55:122-7. McLean JW. The science and art of dental ceramics, vol. 1. Chicago: Quintessence Publishing Co, 1979;85. Dent RJ, Preston JD, Moffa JP, Caputo A. Effect of oxidation on ceramometal bond strength. J PROSTHETDENT 1982;47:5962. Tuccillo JJ, Cascone PJ. The evolution of porcelain-fusedto-metal (PFM) alloy systems. In McLean JW, ed. Dental ceramics: proceedings of the First IQS symposium. Chicago: Quintessence Publishing Co, 1983;362. Ohno H, Miyakawa 0, Watanabe K, Shiokawa N. The structure of oxide formed by high-temperature oxidation of commercial gold alloys for porcelain-metal bonding. J Dent Res 1982;61:1255-61.
Reprint request to: DR JAW-MING HONG KAOHSUJNCMEDICAL COLLEGE, SCHOLLOF DENTISTRY TAIWAN REPUBLICOF CHINA
425
SUMMARY After preliminary scientific data, the dentist commonly evaluates the success of a recommended clinical technique. However, the accuracy of a restoration constructed in the laboratory depends directly upon the abilities of the laboratory technician. The communication between the dentist and the laboratory technician is crucial for success. This study examined the practicality of the porcelain/ butt margin. When combined with the appropriate tissue management, sulcular placement, and impressions, this consistent procedure was easily evaluated, financially sound, and time-conserving.
8. 9. 10.
11.
12. 13.
14.
REFERENCES 1.
2.
3. 4.
5.
6.
7.
Shillingburg HT, Hobo S, Fischer DW. Preparation design and margin distortion in porcelain-fused-to-metal restorations. J PROSTHETDENT 1973;29:276-84. Shillingburg HT, Hobo S, Whitsett LD, Fundamentals of fixed prosthodontics. 2nd ed. Chicago: Quintessence Publishing Co, 1981;79-128. Vryonis P. A simplified approach to the complete porcelain margin. J PROSTHETDENT 1979;42:592-3. Vryonis P. A manual for the fabrication of the complete porcelain margin. Melbourne, Australia: Stock Journal Publishers, 1982; 7-27. Goodacre CJ, Van Roekel NB, Dykema RW, TJllmann RB. The collarless metal-ceramic crown. J PROSTHETDENT 197’7;38:61522. Faucher RR, Nicholls JI. Distortion related to margin design in porcelain-fused-to-metal restorations. J PROSTHET DENT 1980;43:149-55. Pascoe DF. Analysis of the geometry of finishing lines for full crown restorations. J PROSTHETDENT 1978;40:157-62.
15. 16.
17.
18.
19.
Donovan T, Prince J. An analysis of margin configurations for metal-ceramic crowns. J PROSTHETDENT 1985;53:153-7. Sozio RB, Riley EJ. A precision ceramic-metal restoration with a facial butted margin. J PR~~THET DENT 1977;37:517-21. Belser UC, MacEntee MI, Richter WA. Fit of three porcelainfused-to-metal margipal designs in vivo: a scanning electron microscope study. J PROSTHETDENT 1985;53:24-9. Faucher RR, Nicholls JI. Distortion related to margin design in porcelain-fused-to-metal restorations. J PROSTHETDENT 1980; 43:149-55. Waerhaug J. Effect of rough surfaces upon gingival tissue. J Dent Res 1956;35:323-5. Waerhaug J. Histologic considerations which govern where the margins of restorations should be located in relation to the gingiva. Dent Clin North Am 1960;Mar:161-76. Newcomb GM. The relationship between the location of subgingival crown margins and gingival inflammation. J Periodontol 1974;45:151-4. Marcum JS. The effect of crown marginal depth upon gingival tissue. J PROSTHETDENT 1967;17:479-87. Hollinger JO, Locon L, Krantz WA. Connelly M. A clinical and laboratory comparison of irreversible hydiocolloid impression techniques. J EROSTHETDENT 1984;51:3@-9. Johnston JF, Phillips RW, Dykema RW. Modern practice in crown and bridge prosthodontics. 3rd ed. Philadelphia: WB Saunders Co, 1971;206-18. Appleby DC, Pameijer CH, Moffa J. The combined reversible/ irreversible hydrocolloid impression system. J PROSTHETDENT 1?80;44:27-35. Fusayama T, Kurosaki N, Node H, Nakamura M. A laminated hydrocplloid impression for indirect inlays. J PROSTHETDENT 1982;47:171-6.
Reprmt requests to: DR STEPHENK. RHODES 425 SOUTH MAIN ST. MADISONVILLE, KY 42431
The effect of recasting on the oxidation layer of a palladium-silver porcelain alloy Jau-Min Hong, p.D.S.,* Michael E. Razzoog, D.D.S., M.P.H., M.S.,** and Brien R. Lang, D.D.S., M.S.*** Kaohsiung Medical College, School of Dentistry, Taiwan, Republic of China, and University of Michigan, School of Dentistry, Ann Arbor, Mich.
lhe search for casting metals to replace the high gold content alloys has been motivated by economics. Dental alloys with palladium and silver as the main components
*Assistant Professor, Department of Removable Prosthodontics, Kaohsiung Medical College, School of Dentistry. **Associate Professor of Dentistry, University of Michigan, School of Dentistry. ***Chairperson and Professor, Complete Denture Department, University of Michigan, School of Dentistry. 420
were introduced in the 193Os,’ whereas their use as porcelain bonding alloys began in 1974.2 Investigators have confirmed that the physical properties of these alloys are acceptable for dental treatment.3-5 ,In the dental ladoratory, surplus alloy is commonly reused from the initial casting, and with the addition of new alloy, produces other restorations. Although ‘there have been various reports on the repeated use of noble and base metal alloys,6-* the structure and composition of palladium-silver alloy is unique. Whereas studies have APRIL 1988
VOLUME 59
NUMBER 4
EFFECT
OF RECASTING
ON
OXIDATION
<3 Al
LAYER
Surplus
only
surplus
+ 50%
Surplus
only
Surplus
Surplus
only
Surplus + 50% new alloy
Surplus
only
Surplus + 50% new alloy
new alloy + 50%
new alloy
Fig. 1. Specimens preparation. Group A (Al through A4) was cast with surplus metal from previous casting. Group I3 (Bl through B4) was cast with 50% by weight new alloy and 50% by weight surplus metal from previous casting. revealed that the metalic oxides are a factor in the bonding of porcelain to metal,‘, lo there is sparse mention of the effects of repeated casting on the oxide layer of palladium-silver alloys. This investigation determined the effect of repeated casting on the’ oxidation layer of a commercial palladium-silirer porcelain alloy (Esteticor Actual, Cendres & Metaux SA, Biel-Bienne, Switzerland). MATERIAL Preparation
AND METHODS of specimens
Ten 6 mm diameter X 1 mm thickness Pd-Ag porcelain alloy sp&imenS were prepared conventionally. Wax patterns were attache< to IO separate crucible formers with sprues 15 mm in length and 1 mm in diameter. The wax patterns were placed in IO casting rings and invested with a phosphate-bonded refractory material (Ceramicor, Cendres & Metaux SA). Each casting ring was placed in the electric furnace, heated slowly to . 1500” F, and held for 45 mmutes. The casting was performed in an electromatic casting machine (Howmet Corp., Chicago, Ill.) at 2420” F and allowed to cool to room temperature. The methods of recasting were divided into two groups (Fig. 111. Gr&p A. The initial casting was completed with a totally new alloy. Surplus (sprue and button) alloy from the previous cas,ting was used without the addition of any new alloy for the next casting. The sequence was repeated through four generations. Group B. The initial casting was accomplished with a totally oew alloy. Part of the metal assembly was removed from the previous casting, weighed, and comTHE
JOURNAL
OF PROSTHETIC
DENTISTRY
bined with appropriate new alloy to achieve 50% weight ratio for the next casting. This sequence was repeated through four generations. The sprues and buttons sectioned from each casting were sandblasted with aluminum oxide and ultrasonically cleaned with distilled water for 30 minutes. Ten specimens, including two initial casting specimens, were made after repeated casting through four generations in two groups. Preparation of specimens for oxidation and micro. probe analysis of oxidation layer. One initial casting specimen and the eight recasting specimens were placed in the furnace simultaneously and preheated at 1200” F for 5 minutes, heated from 1200” td 1900” F, and held at 1900” F for IO minutes. The air pressure in the furnace was reduced to 0.1 atmosphere while the temperature was elevated at a rate of 100” F per minute from 1200” to 1900” F and sustained for 10 minutes. The chamber was then vented and allowed to air cool to simulate degassing and oxidation. To examine the oxidized surface in cross section, specimens were mounted in epoxy resin and sectioned perpendicular to the oxides formed on the surface. Sectioned surfaces were ground and polished to 0.05 pm. The chemical composition of the alloy was determined qualitatively with an energy dispersive x-ray analyzer (EDX) (Kevex 7000, Fester, Calif.). The quantitative analysis was accomplished by a wavelength dispersive x-ray analyzer (WDX) (JEOL, Tokyo, Japan) that can also be operated as a scanning electron microscope (SEM). The alloy was used as the unknown and compared with metals with purity of 99.9%. Distribution of chemical elements in the oxidized 421
HONG, RAZZOOG, AND LANG
Fig. 2. Secondary electron image (SEX) micrographs with element analysis by XMA on oxidation zone of initial casting specimen. (a), Pd L-cr and Ag L-a! radiation line analysis; (b) Sn L-o and In L-a radiation line analysis. (Original magnificaiton X3000.)
specimens was determined by using an x-ray microprobe analyzer (XMA) (JSM 35, JEOL) with the same accelerating current (12 Kv, 0.4 X lo-’ amp; speed of scanning: 8 pm/min). Secondary electron image (SEI) photographs were procured of the regions analyzed for comparing the distribution of the chemical elements between the matrix of the alloy and the oxidation layer.
RESULTS The chemical composition of the alloy as analyzed by EDX was determined as palladium, silver, tin, indium, gold, platinum, and copper. Results from the quantitative analyses are presented in Table I. Except for the listing of palladium (54%) and silver (38%), additional information regarding the chemical composition of the 422
alloy had not been offered by the manufacturer. Results of x-ray microprobe analysis for element distribution at the oxidation zone in the initial casting specimen are available in Fig. 2. The intensity curves of each element are tracked from the first to fourth generation of group A and group B castings as illustrated in Fig. 3. The analysis curve reveals the distribution of four kinds of elements in the specimens from the matrix of the alloy to the oxidation zone. The highest intensity peaks of tin occurred at the oxidation zone, and the intensity peaks of tin increased progressively from the first to fourth generation in both groups. Tin intensity peaks for group A occurred higher in the oxidation zone than in group B. Indium intensity curve rises slightly at the oxidation zone but at a lower level than tin. However, there were no significant changes in the intensity peaks of indium along the casting generations or between group A and group B. Intensity curves for silver inclined moderately in the oxidation zone of groups A and B and increased progressively from the first to fourth generation. The intensity curves for palladium declined in the oxidation zone of each generation for groups A and B. Palladium curves did not differentiate between groups A and B among the casting generations. To determine the effects of repeated casting on oxidation the following procedures were performed. The XMA with the same current condition (12 Kv, 0.4 x lo-’ amp, speed of scanning: 8 pm/min) for 0 K-alpha was used from the alloy matrix to the external oxidation zone. The 0 K-alpha intensity curves with SEM micrograph of each generation of groups A and B were recorded. The SEM micrograph with the 0 K-alpha XMA of initial casting specimen is illustrated in Fig. 4. 0 K-alpha intensity curves of the four recasting generations of groups A and B were tracked and are displayed in Fig. 5. Intensity peaks of 0 K-alpha rise sharply at the oxidation zone and the intensity increases progressively from the first through third generations and then decreases slightly in the fourth, both in groups A and B. Before reaching the oxidation zone, small peaks of 0 K-alpha were discovered in the third and fourth generation of groups A and B. The thickness of the oxidation zone illustrates a substantial increase in the first generation and then a minimal increase with each succeeding generation in groups A and B (Table II). The thickness of group A’s oxidation zone increased at a greater rate than group B’s The microporosities distributed in the internal oxidation zone of group A castings became larger through each generation of recasting. This phenomenon did not occur in the castings from group B (Fig. 6).
DISCUSSION The x-ray microprobe analyzed at the oxidation zone exhibited significant differences between the four generAPRIL
1988
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EFFECT OF RECASTING
Al
ON OXIDATION
ox.
Alloy Moth
LAYER
82
A2
ZOO0
::::, \
--
:,‘,..j&..$G
A4
B3
Electron Beom
Electron Beom
04
Electron Beam
:, ::’ i . ..., .:
,...,,.. 5 ,.... :‘-..
.‘? --* Electron
Beom
Electron
Electron Beom
0A
. .. K
p#j g
---
In
Electron Beam
Beam
pd -.-
0B
---
A9 Sn In
Fig. 3. Element analysis by XMA at alloy matrix and oxidation zone of four generations casting. Numbers 1 through 4 = number of recasting generation. A, Group A; B, group B. Table I. The chemical composition
of the alloy in this study obtained
by WDX quantitative
analysis
Element
Percent weztght
Pd
AR
Sn
In
AU
Pt
CU
53.76 (54Y
37.87 (3W
5.74
0.05
0.53
0.06
0.05
*Compositions reported by manufacturer.
Table II. Average thickness (pm) of the oxidation
layer of the initial
casting and four generations
of group A and B Generation 1
2
3
4
3.7 Tk 0.6
4.2 f 0.4
4.4 5 0.4
4.7 + 0.8
3.9 2 0.4
4.1 ir 0.4
4.0 * 0.3
4.2 + 0.4
EO Group A 2.7 + 0.3
Group B Readingstaken at 10 sites on each specimen.
EO = Initial
casting of all new alloy.
ations of castings in groups A and B. Although palladium and indium at the oxidation zone did not display significant differences among generations, the intensity of silver and tin increased through eadh generation in groups A and B. Tuccillo and Cascone” have suggested that silver can be absorbed by the porcelain at elevated temperatures and the precipitation of the particles responsible for discoloration occurred during the cooling cycle. The present study suggested that the concentration THE JOURNAL
OF PROSTHETIC
DENTISTRY
of silver on the oxidation zone was higher in the recasting specimens, even when 50% by weight of new alloy was added with the reusing of palladium-silver porcelain alloy. The potential for silver to revaporize and contaminate the surface and restoration is higher during the porcelain firing cycle. As expected, at the oxidation zone the intensity peaks of tin were much higher than indium because the concentration of tin in the alloy was higher and tin can 423
HONG,
RAZZOOG,
AND
LANG
Fig. 4. Secondary electron image (SEI) micrograph with 0 K-cux-ray microprobe analysis on oxidation zone of initial casting specimen. (Original magnification x3000.)
Electron Beam
Fig. 6. SE1 micrograph, XMA element analysis of fourth generation casting. (a), A-4; (b), B-4. (Original magnification X3000.)
Electron Beam
Fig. 5. 0 K-curadiation line analysis by XMA on oxidation zone of four generations of groups A and B. (In A and B groups; 1 through 4 = number of recasting generation; 12 Kv, 0.4 X lo+ amp, accelerating current; 8 pm/min, speed of scanning.)
be oxidized preferentially. The growth of the oxidation layer of this alloy was assumed to result mainly from the diffusion of interstitial Sn ions (Sn4+) through the oxide matrix forming Sn02.12 When findings related to the intensity of 0 K-alpha in the oxidation zone were reviewed, it was demonstrated that the oxidized condition of the specimens increased 424
through each generation of groups A and B castings. The activity of oxygen was higher in the various generations of group A than group B. The activity of oxygen in the oxidation zone and in the metal matrix near the oxidation zone became larger with each succeeding generation, even in the specimens where new alloy was added. The precipitation of oxides can be increased by recasting of the alloy. Diffusion of oxygen appeared more aggressive through the grain boundary and/or at the interface between the alloy matrix and oxide particles during the degassing of the recast alloy. Although the buttons and sprues may be sandblasted and cleaned, the contents of the metal oxidized cannot be completely eliminated under normal laboratory conditions. Therefore, for recasting a used alloy the addition of the new alloy is necessary, and a detailed decontamination procedure and a disciplined vacuum-casting process must be carefully followed. APRIL
1988
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EFFECT OF RECASTING
ON OXIDATION
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The microporosities occurring at the internal oxidation zone and increasing in size through each succeeding generation of group A compromises the conditions for porcelain bonding. The oxidation layer formed in each generation of group B castings appeared more constant than group A, but the silver and metallic oxides that increased progressively by recasting group B could affect the porcelain bonding and discolor it. In this study, there were significant differences in the oxidation zone among the generations of groups A and B. Thus, the oxidation zone of the palladium-silver porcelain alloy can be affected by repeated casting, even when new alloy, 50% by weight, is added. This indicated that the recommendations by the manufacturers that at least one third new alloy be added when reusing metal is questionable and capricious. Additionally, the remelting of the palladium-silver porcelain alloys introduces an alteration of the chemical composit,ion and/or microstructure that might affect the oxidation layer. Before a definitive recommendation for repeated castings of palladium-silver porcelain alloys is made, further investigation is required to determine the alteration of chemical composition, microstructure, physical properties, color of bonding porcelain, and bonding strength of porcelain to metal. SUMMARY
REFERENCES 1.
2.
3.
4.
5.
6. 7.
8. 9. 10.
The oxidation zone of a commercial palladium-silver porcelain alloy was compared after repeated casting with and without the addition of new alloy. The intensity of palladium, silver, tin, indium, and 0 K-alpha near the oxidation zone was analyzed with XMA. The intensity curves of tin, &er, and oxygen increased progressively through each generation despite the addition of new alloy. The thickness of the oxidation zone and the microporosities at the internal oxidation zone increased through each generation without the addition of new alloy. Although the findings indicated that the oxidation zone was favorably formed by adding new alloy, 50% by weight, for four generations, the silver and metallic oxides of the oxidation zone increased through each generation. The reuse of the palladium-silver porcelain alloy remains questionable.
THE JOURNAL
Appreciation is expressed to Mr. J. W. Liu and Mr. F. C. Wu, Cheng-Kong University, Tainan, Taiwan, Republic of China for their technical assistance in this experiment.
OF PROSTHETIC
DENTISTRY
11.
12.
Craig RG, Peyton FA. Restorative dental materials. 5th ed. St Louis: The CV Mosby Co, 1975;318. Tuccillo JJ. Composition and functional characteristics of precious metal alloys for dental restorations. In: Valega TM, ed. Alternatives to gold alloys in dentistry. Bethesda Md: U.S. Department of Health, Education, and Welfare, Publication NO. (NIH) 77-1227, 1977;40-67. Bartkowiak H. The use, indications and considerations concerning the technology of Spa&WT-52 silver-palladium alloy. Protet Stomatol 1978;28:31-5. Gourley JM. Current status of semi-precious and conventional gold alloys in restorative dentistry. J Can Dent Assoc 1975;41:453-5. Huget EF, Dvivedi N, Cosner HE. Characterization of goldpalladium-silver and palladium-silver for ceramic metal restorations. J PROSTHETDENT 1976;36:58-65. Phillips RW. Skinner’s science of dental materials. 7th ed. Philadelphia: WB Saunders Co, 1973;397. Hesby DA, Kobes P, Garver DG, Pelleu GB. Physical properties of a repeatedly used nonprecious metal alloy. J PROSTHETDENT 1980;44:291-3. Nelson DR, Palik JF, Morris HF, Comella MC. Recasting a nickel-chromium alloy. J PROSTHETDENT 1986;55:122-7. McLean JW. The science and art of dental ceramics, vol. 1. Chicago: Quintessence Publishing Co, 1979;85. Dent RJ, Preston JD, Moffa JP, Caputo A. Effect of oxidation on ceramometal bond strength. J PROSTHETDENT 1982;47:5962. Tuccillo JJ, Cascone PJ. The evolution of porcelain-fusedto-metal (PFM) alloy systems. In McLean JW, ed. Dental ceramics: proceedings of the First IQS symposium. Chicago: Quintessence Publishing Co, 1983;362. Ohno H, Miyakawa 0, Watanabe K, Shiokawa N. The structure of oxide formed by high-temperature oxidation of commercial gold alloys for porcelain-metal bonding. J Dent Res 1982;61:1255-61.
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