Precious metal alloys/ orthodontic seminars

779 views
643 views

Published on



Indian Dental Academy: will be one of the most relevant and exciting training center with best faculty and flexible training programs for dental professionals who wish to advance in their dental practice,Offers certified courses in Dental implants,Orthodontics,Endodontics,Cosmetic Dentistry, Prosthetic Dentistry, Periodontics and General Dentistry.

Published in: Education
0 Comments
4 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
779
On SlideShare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
1
Comments
0
Likes
4
Embeds 0
No embeds

No notes for slide

Precious metal alloys/ orthodontic seminars

  1. 1. PRECIOUS METAL ALLOYS INDIAN DENTAL ACADEMY Leader in continuing dental education www.indiandentalacademy.com www.indiandentalacademy.com
  2. 2.  “ALL NOBLE METALS ARE PRECIOUS BUT ALL PRECIOUS ARE NOT NOBLE METALS” www.indiandentalacademy.com
  3. 3. INTRODUCTION  Noble metals: • gold , • platinum, • palladium, • rhodium, • ruthenium, • iridium, • osmium, • and silver www.indiandentalacademy.com
  4. 4. www.indiandentalacademy.com
  5. 5. www.indiandentalacademy.com
  6. 6.  These are called noble metals as they do not form oxides.  Gold and palladium do not oxidise at any tempratures www.indiandentalacademy.com
  7. 7.  Rhodium has a very good resistance to oxidation at all temps.  Osmium and ruthenium forms volatile oxides, and palladium and iridium form oxides in temperatures ranges from 400 to 600 to 1000 degrees centigrade respectively www.indiandentalacademy.com
  8. 8. www.indiandentalacademy.com
  9. 9.  In 1971 the gold came into market and due to the increasing price of that gold made it to be replaced with palladium after that base metal alloys had replaced these noble metal alloys completely.  The use of palladium in cars as a catalyst increased its cost and demand. www.indiandentalacademy.com
  10. 10. ALLOYS  . The alloys are compounded to produce properties most acceptable for their intended dental applications, such as simple inlays, bridges, removable cast restorations, solders, (or) wrought wire forms www.indiandentalacademy.com
  11. 11. HISTORICAL PERSPECTIVE ON DENTAL CASTING ALLOYS  The history of dental casting alloys has been influenced by 3 major factors:  1.The technologic changes of dental prosthesis.  2.Metallurgic advancements; and  3.Price changes of the noble metals since 1968. www.indiandentalacademy.com
  12. 12.  Taggart’s presentation to the New York odontological group in 1907 on the fabrication of cast inlay restorations often has been acknowledged as the first reported application of the lost wax technique in dentistry  The inlay technique described by Taggart was an instant success. www.indiandentalacademy.com
  13. 13.  It soon led to the casting of complex inlays such as onlays, crowns, FPD, and RPD frame works.  Because pure gold did not have the physical properties required of these dental restorations, existing jewellery alloys were quickly adopted. These gold alloys were further strengthened with copper, silver (or) palladium www.indiandentalacademy.com
  14. 14.  In 1932, the dental materials group at the National Bureau of Standards surveyed the alloys being used and roughly classified them as Type 1, 2, 3 and 4. www.indiandentalacademy.com
  15. 15.  By 1948, the composition of Dental Noble metal alloys for cast metal restorations had become rather diverse, with these formulations, the tarnishing tendency of the original alloys apparently had disappeared. It is now known that in gold alloys, palladium is added to counter act the tarnish potential of silver. www.indiandentalacademy.com
  16. 16.  The base metal removable partial denture alloys were introduced in the 1930s. Since that time, both Nickel-chromium and cobalt-chromium formulations have become increasingly popular compared with conventional Type IV gold alloys www.indiandentalacademy.com
  17. 17.  by 1978, the price of gold was climbing so rapidly the attention focused on the noble metal alloys. To reduce the precious metal content yet retain the advantages of the noble metals for dental www.indiandentalacademy.com
  18. 18. PROPERTIES OF NOBLE METALALLOYS  GOLD :  Pure gold is a soft, malleable, ductile metal that does not oxidize under atmospheric conditions and is attacked by only a few of the most powerful oxidizing agents.  It has a rich yellow colour with a strong metallic luster.  Although it is the most ductile and malleable of all metals, it ranks much lower in strength. www.indiandentalacademy.com
  19. 19.  The pure metal fuses at 1063°C, which is only 20° below the melting point of copper (1083°C).  Small amounts of impurities have a pronounced effect on the mechanical properties of gold and its alloys. www.indiandentalacademy.com
  20. 20.  The presence of less than 0.2% lead will cause gold to be extremely brittle.  Mercury in small quantities also has a harmful effect on its properties. www.indiandentalacademy.com
  21. 21.  Gold is nearly as soft as lead, with the result that in dental alloys, coins, and articles of jewellery it must be alloyed with copper, silver, platinum and other metals to develop the necessary hardness, durability and elasticity.  The specific gravity of pure gold is between 19.30 and 19.33, making it one of the heavy metal. www.indiandentalacademy.com
  22. 22.  Air (or) water at any temperature does not affect (or) tarnish gold.  Gold is not soluble in sulfuric, nitric (or) hydrochloric acids www.indiandentalacademy.com
  23. 23. PALLADIUM  Palladium is not used in the pure state in dentistry, but it is used in many dental alloys, combined with either gold (or) silver.  It is cheaper than platinum and since it imparts many of the properties of platinum to dental alloys, it is often used as a replacement for platinum. www.indiandentalacademy.com
  24. 24.  Palladium is a white metal some what darker than platinum.  Its specific gravity is 11.4 (or) about half that of platinum. www.indiandentalacademy.com
  25. 25.  It is a malleable and ductile metal with a melting point of 1555°C, which is the lowest of the platinum group of metals.  The metals has the quality of absorbing (or) occluding large quantities of hydrogen gar when heated. This can be an undesirable quality when alloys containing palladium are heated with an improperly adjusted gas air torch. www.indiandentalacademy.com
  26. 26. IRIDIUM, RUTHENIUM, AND RHODIUM  Small amounts of iridium are some times present in dental alloys, either as impurities combined with platinum (or) as additions to modify the properties.  As little as 0.005% (50 ppm) is effective in refining the grain size of cast gold alloys. www.indiandentalacademy.com
  27. 27.  Ruthenium produces a similar effect.  Iridium is a hard metal that is quite brittle, white with a high sp. Gravity of 22.42 and an exceptionally high melting point estimated to be about 2440'C www.indiandentalacademy.com
  28. 28. SILVER  Silver is malleable and ductile, white, the best- known conductor of heat and electricity, and stronger and harder than gold but softer than copper.  It melts at 960.5'C, which is below the melting point of both gold and copper. www.indiandentalacademy.com
  29. 29.  It is unaltered in clean, dry air at any temperature but combines with sulfur, chlorine and phosphorous, (or) vapors containing these elements (or) their compounds.  Foods containing sulfur compounds cause severe tarnish on silver. www.indiandentalacademy.com
  30. 30.  Pure silver is seldom employed in dental restorations because of the black sulfide formation on the metal in the mouth, although it is used extensively for small additions to many gold alloys. www.indiandentalacademy.com
  31. 31.  Addition of small amounts of palladium to silver containing alloys prevents the rapid corrosion of such alloys in the oral environment. www.indiandentalacademy.com
  32. 32. KARAT AND FINENESS OF GOLD  For many years the gold content of gold alloys has been described on the basis of karat, (or) in terms of fineness, rather than by weight %.  The term karat refers to the parts of pure gold in 24 parts of an alloy. www.indiandentalacademy.com
  33. 33.  For many years the gold content of gold alloys has been described on the basis of karat, (or) in terms of fineness, rather than by weight %.  The term karat refers to the parts of pure gold in 24 parts of an alloy. www.indiandentalacademy.com
  34. 34.  Fineness describes gold alloys by the number of parts per 1000 of gold. For example, pure gold has a fineness of 1000, and 650 fine alloy has a gold content of 65%. Thus the fineness rating is to timer the gold % in an alloy. www.indiandentalacademy.com
  35. 35. CLASSIFICATION OF DENTAL CASTING ALLOYS (OR) DENTAL GOLD ALLOYS  Type I  Type II  Type III and  Type IV www.indiandentalacademy.com
  36. 36.  Type I (Soft): These alloys are limited to use in inlays that are subject only to slight stress during mastication www.indiandentalacademy.com
  37. 37.  Type II (Medium): These medium alloys can be used for all types of cast inlays and onlays. www.indiandentalacademy.com
  38. 38.  Type III (Hard): These alloys are most acceptable for crowns, thin 3/4th crowns, and anterior and posterior bridge abutments, which should not be cast from the softer and weaker Type I and Type II alloys. www.indiandentalacademy.com
  39. 39.  Type IV (Extra hard): These alloys are designed to have sufficient strength and adequate properties for cast removable partial dentures with clasps, precision cast fixed bridges and ¾ crowns, which are not subjected to hard working (or) burnishing operations. www.indiandentalacademy.com
  40. 40.  Composition:  The composition of the gold casting alloys that meet the requirements of ADA Sp. No. 5 are given in the Table below: www.indiandentalacademy.com
  41. 41. TYPE GOLD PLATINUM PALLADIUM I 81-83% - 0.2-4.5% II 76-78% - 1-3% III 73-77% - 2-4% IV 71-74% 0-1% 2-5% www.indiandentalacademy.com
  42. 42. www.indiandentalacademy.com
  43. 43.  It is apparent from the Table that there is some reduction in gold content when a comparison is made between Type I and IV alloys.  An increase in copper content is observed as the gold content is decreased.  An increase in the zinc content also occurs in Type IV alloys. www.indiandentalacademy.com
  44. 44.  Platinum is rarely added to Type 1 gold alloys, but a small amount of palladium is always added to all 4 types. www.indiandentalacademy.com
  45. 45. PROPERTIES OF GOLD CASTING ALLOYS Type Vickers hardness number (Kg/mm2) Softened Yield strength, (0.1% Offset) MPa Elongation Minimum Maximum Softened minimum Hardened minimum Softened minimum Hardened minimum (%) I 50 90 None None 18 None II 90 120 140 None 12 None III 120 150 200 None 12 None IV 150 None 340 500 10 2 www.indiandentalacademy.com
  46. 46.  From the above table it may be seen that as the hardness increases from Type I to Type IV, the yield strength and tensile strength values are also increased, and the elongation generally decreased. www.indiandentalacademy.com
  47. 47.  Soft alloys have a higher degree of elongation and a relatively greater quality of ductility than the alloys of higher hardness values www.indiandentalacademy.com
  48. 48.  Melting range:  its between 920-960  The melting range of the alloy is important for selecting the type of investment and type of heating source needed  Density:  It indicates the number of dental castings made from an unit weight of the metal.  Gold alloys are lighter than pure gold www.indiandentalacademy.com
  49. 49.  The Castability of the alloy is also effected by the density  Hardness:  this indicates the ease of with which these alloys can be cut , ground , and polished  Gold alloys are generally more user friendly www.indiandentalacademy.com
  50. 50.  Elongation:  it indicates the ductility of the material  Alloys with low elongation are brittle www.indiandentalacademy.com
  51. 51.  Modulus of elasticity:  It indicates the stiffness/ flexibility of the metal  Tarnish and corrosion resistance:  gold alloys are resistant to tarnish and corrosion  Casting shrinkage:  It is less than 1.25 and 1.65% when compared to base metal alloys www.indiandentalacademy.com
  52. 52.  This shrinkage occurs in 3 stages 1. Thermal contraction of the liquid material 2. Contraction of the metal while changing from liquid to solid state. 3. Thermal contraction of the solid metal as it reaches room temperature www.indiandentalacademy.com
  53. 53.  Biocompatibility: def :ability of a material to elicit an appropriate biological response in an given application in a body  gold alloys are relatively compatible  Casting investment: gypsum bonded investments are used for gold alloys because of their low fusion temperature www.indiandentalacademy.com
  54. 54. FUSION TEMPERATURES OF DENTAL GOLD CASTING ALLOYS METAL (or) ALLOY TYPICAL FUSION TEMPERATURE (°C) Type I 1005-1070 Type II 900-970 Type III 875-1000 Type IV 875-1000 www.indiandentalacademy.com
  55. 55.  The fusion temperatures are important factors in choosing the type of investment to be used.  Alloys having fusion temperatures higher than about 1100°C should not be cast into calcium sulfate bonded investment www.indiandentalacademy.com
  56. 56.  In 1984 the ADA proposed a simple classification of dental casting alloys. ALLOY TYPE TOTAL NOBLE METAL CONTENT High noble metal Contains  40 wt% AV and  60% wt of the noble metal elements (Au+Ir+OS+Pt+Rh+Ru). Noble metal Contains  25% of the noble metal elements. Predominantly base metal Contains < 25 wt% of noble metal elements. www.indiandentalacademy.com
  57. 57.  Many manufacturers have adopted this classification to simplify the communication between dentists and dental laboratory technologists. www.indiandentalacademy.com
  58. 58.  CLASSIFICATION OF ALLOYS FOR ALL METAL RESTORATIONS METAL, CERAMIC RESTORATIONS AND FRAME WORKS FOR REMOVABLE PARTIAL DENTURES www.indiandentalacademy.com
  59. 59. Alloy Type All-Metal Metal-Ceramic Removable Partial Dentures High noble Au-Ag-Cu-Pd metal ceramic alloys Au-Pt-Pd Au-Pd-Ag (5-12 wt% Ag) Au-Pd-Ag (>wt 12% Ag) Au-Pd (no Ag) Au-Ag-Cu-Pd Noble Ag-Pd-Au-Cu Ag-Pd Metal ceramic alloys Pd-Au (no Ag) Pd-Au-Ag Pd-Ag Pd-Cu Pd-Co Pd-Ga-Ag Ag-Pd-Au-Cu Ag-Pd www.indiandentalacademy.com
  60. 60. HEAT TREATMENT OF HIGH NOBLE AND NOBLE METALALLOYS:  Gold alloys can be significantly hardened if the alloy contains a sufficient amount of copper.  Types I and II alloys usually do not harden, (or) harden to a lesser degree than do the types III and IV alloys. www.indiandentalacademy.com
  61. 61.  The actual mechanism of hardening is probably the result of several different solid - solid transformations www.indiandentalacademy.com
  62. 62.  SOLUTION HEAT TREATMENT: Alloys that can be hardened can of course, also be softened. In metallurgical terminology the softening heat treatment is referred as  AGE HARDENING: The hardening heat treatment is termed so www.indiandentalacademy.com
  63. 63. SOFTENING HEAT TREATMENT  The casting is placed in an electric furnace for 10 minutes at a temperature of 700°C (129°F) and then it is quenched in water  The tensile strength, hardness and proportional limit are reduced by such a treatment but the ductility is increased www.indiandentalacademy.com
  64. 64.  The softening heat treatment is indicated for structures that are to be ground, shaped (or) otherwise cold worked, either in or out of the mouth.  www.indiandentalacademy.com
  65. 65. HARDENING HEAT TREATMENT:  The age hardening of the dental alloys can be accomplished 'in several ways. One of the most practical hardening treatments is by "Soaking" (or) aging the casting at a specific temperature for a definite time, usually 15 to 30 minutes, before it is water quenched. www.indiandentalacademy.com
  66. 66.  The aging temperature depends on the alloy composition but is generally between 200°C and 450°C.  Ideally, before the alloy is given an age-hardening treatment, it should be subjected to a softening heat treatment to relieve all strain hardening www.indiandentalacademy.com
  67. 67.  The hardening heat treatment is indicated for metallic partial dentures, bridges, and other similar structures. www.indiandentalacademy.com
  68. 68. CASTING SHRINKAGE:  Most metals and alloys, including gold and noble metal alloys, shrink when they change from the liquid to the solid state  The values for the casting shrinkage differ for the various alloys presumably because of differences in their composition www.indiandentalacademy.com
  69. 69. Alloy Casting shrinkage (%) Type I, Gold base 1.56% Type II, Gold base 1.37% Type III, Gold base 1.42% Ni-Cr-Mo 2.3% www.indiandentalacademy.com
  70. 70. SILVER PALLADIUM ALLOYS:  These alloys are white and predominantly silver 'm composition but have substantial amounts of palladium (at least 25/o) that provide nobility and promote the silver tarnish resistance. They may (or) may not contain copper and a small amount of gold. www.indiandentalacademy.com
  71. 71.  The copper (Cu) free Ag-Pd alloys may contain 70% to 72% silver and 25% palladium and may have physical properties of a Type III gold alloy.  Other silver-based might contain roughly 60% silver 25% palladium and as much as 15% or more of copper and may have properties more like a Type IV gold alloy. www.indiandentalacademy.com
  72. 72.  Because of the increasing interest in aesthetics by dental patients, a decreased use of all metal restorations has occurred during the past decade  The use of metal ceramic restorations in posterior sites has increased relative to the use of all metal crowns and onlays. www.indiandentalacademy.com
  73. 73. HIGH NOBLE ALLOYS FOR METAL CERAMIC RESTORATIONS  The original metal ceramic alloys contained 88% gold and were much to soft for stress bearing restorations such as FPD. Because there was no evidence of a chemical bond between these alloys and dental porcelain, mechanical retention and undercuts were used to prevent detachment of the ceramic veneer www.indiandentalacademy.com
  74. 74.  Using the stress bond test, it was found that the bond strength of the porcelain to this type of alloy was less than the cohesive strength of the porcelain itself  This mean that if the failure occurred in the metal-ceramic restoration, it would most probably arise at the porcelain metal interface. www.indiandentalacademy.com
  75. 75.  By adding less than 1% of oxide forming elements such as iron, indium, and tin to this high gold content alloy, the porcelain metal bond strength was improved by a factor of 3. Iron also increase the proportional limit and strength of the alloy. www.indiandentalacademy.com
  76. 76.  This 1% addition of base metals to the gold, palladium and platinum alloy was all that was necessary to produce a slight oxide film on the surface of the substructure to achieve a porcelain- metal bond strength level that surpassed the cohesive strength of porcelain itself. www.indiandentalacademy.com
  77. 77. THE HIGH NOBLE ALLOYS FOR METAL CERAMIC RESTORATIONS ARE: www.indiandentalacademy.com
  78. 78. GOLD-PLATINUM - PALLADIUM ALLOYS  These9alloys have a gold content ranging up to 88% with varying amounts of palladium, platinum and small amounts of base metals.  Some of these alloys are yellow in colour. Alloys of this type are susceptible to sag deformation and FPD's should be restricted to 3-unit spans, anterior cantilevers (or) crowns www.indiandentalacademy.com
  79. 79. Gold-Palladium-Silver Alloys:  These gold based alloys contain between 39% and 77% upto 35% palladium and silver levels as high as 22%.  The silver increases thermal contraction coefficient but it also has a tendency to discolour some porcelains www.indiandentalacademy.com
  80. 80. Gold-Palladium Alloys:  A gold content ranging from 44% to 55% and a palladium level of 35% to 45% is present in these metal ceramic alloys, which have remained popular despite their relatively high cost. www.indiandentalacademy.com
  81. 81.  The lack of silver results in a decreased thermal contraction coefficient and the freedom from silver discolouration of porcelain. www.indiandentalacademy.com
  82. 82.  Alloys of this type must be used with porcelains that have low coefficients on to avoid the development of axial and circumferential of thermal contraction to avoid the development of axial and circumferential tensile stresses in Porcelain during the cooling part of the porcelain firing cycle. www.indiandentalacademy.com
  83. 83. NOBLE ALLOYS FOR METAL CERAMIC RESTORATIONS www.indiandentalacademy.com
  84. 84. PALLADIUM BASED ALLOYS  Palladium - Silver alloys.  Palladium Copper alloys:  Palladium - Cobalt alloys:  Palladium- gold alloys:  Palladium - Gallium - Silver and Palladium - Gallium – Silver – Gold alloys www.indiandentalacademy.com
  85. 85. PALLADIUM BASED ALLOYS  Noble palladium based alloys offer a compromise between the high noble gold alloys and the predominantly base metal alloys  The price per ounce of a palladium alloy is generally one half to one third that of a gold alloys. www.indiandentalacademy.com
  86. 86.  The density of a palladium based alloy is midway between that of base metal and of high noble alloys www.indiandentalacademy.com
  87. 87. Palladium - Silver Alloys:  Pd-Ag alloys were introduced widely in the late 1970s. This alloy type was introduced to the U.S. market in 1974 as the first gold free noble metal available for metal ceramic restorations www.indiandentalacademy.com
  88. 88.  Pd-Ag alloys enjoyed wide spread popularity for a few years after they were introduced, but their popularity has declined some what in recent years because of their tendency to discolour porcelain during firing. www.indiandentalacademy.com
  89. 89.  The compositions of Pd-Ag alloys fall within a narrow range: 53% to 61% palladium and 28% to 40% silver  Tin (or) Indium or both are usually added to increase alloy hardness and to promote oxide formation for adequate bonding of porcelain. In some of these alloys, the formation of an internal oxide rather than an external oxide has been reported. www.indiandentalacademy.com
  90. 90.  Other palladium alloys contain 75% to 90% palladium and no silver and were developed to eliminate the greening problem some of the high palladium alloys develop a layer of dark oxide on their surface during cooling from the degassing cycle, and this oxide layer has proven difficult to mask by the opaque porcelain. www.indiandentalacademy.com
  91. 91.  Other high palladium alloys such as the Pd-Ga- Ag-Au type seem not to be plagued by this problem.  The replacement of gold by palladium raises the melting range but lower the contraction coefficient of an alloy. Increases the silver content tends to lower the melting range and raises the contraction coefficient www.indiandentalacademy.com
  92. 92.  Because of their high silver contents compared with the gold based alloys, the silver discoloration effect is most severe for these alloys. Gold metal conditioners or ceramic coating agents may minimize this effect. www.indiandentalacademy.com
  93. 93.  The low specific gravity of these alloys (10.7- 11.1) combined with their low intrinsic cost makes these alloys attractive as economical alternatives to the gold based alloys. www.indiandentalacademy.com
  94. 94. PALLADIUM-COPPER ALLOYS:  This alloy is comparable in cost to the Pd-Ag alloys  Because of their low melting range of approximately 1170°C to 1190°C, these alloys are expected to be susceptible to creep deformation (Sag) at elevated firing temperatures www.indiandentalacademy.com
  95. 95.  These alloys contain between 74-80% palladium and 9-15% copper  One should be aware of the potential effect on aesthetics of the dark brown (or) black oxide formed during oxidation and subsequent porcelain firing cycles www.indiandentalacademy.com
  96. 96.  Care should be taken, to mask this oxide completely with opaque porcelain and to eliminate the un-aesthetic dark band that develops at metals porcelain junctions.  The Pd-Cu alloy have yield strengths upto 1145MPa. www.indiandentalacademy.com
  97. 97.  Elongation values of 5% to 11% and hardness values as high as some base metal alloys.  Thus, these alloys would appear to have a poor potential for burnishing except when the marginal areas are relatively thin. www.indiandentalacademy.com
  98. 98. PALLADIUM-COBALT ALLOYS:  This alloy group is comparable in cost to the Pd- Ag and Pd-Cu alloys. They are often advertised as gold free, nickel-free, beryllium-free, and silver-free alloys.  . The reference to nickel and beryllium indicates that these alloys, as is true with the other noble metals, are generally considered biocompatible. www.indiandentalacademy.com
  99. 99.  This Pd-Co group is the most sag resistant of all noble metal alloys.  The noble metal content (based on palladium) ranges from 78% to 88%.  The cobalt content ranges between 4 and 10wt% over commercial alloy contains 8% gallium. www.indiandentalacademy.com
  100. 100.  An example of typical properties of a Pd-Co alloy is as follows:  Hardness 250DPH  Yield strength 586MPa  Elongation 20% and  Modulus of elasticity 85.2Gpa www.indiandentalacademy.com
  101. 101.  Although these alloys are silver-free, discolouration of porcelain can still result because of the presence of cobalt www.indiandentalacademy.com
  102. 102. PALLADIUM-GALLIUM-SILVER AND PALLADIUM-GALLIUM-SILVER-GOLD ALLOYS  These alloys are the most recent of the noble metals. This group of alloys was introduced because they tend to have a slightly lighter coloured oxide than Pd-Cu (or) Pd-Co alloys and they are thermally compatible with lower expansion porcelains. www.indiandentalacademy.com
  103. 103.  The oxide that is required for bonding to porcelain is relatively dark, but it is somewhat lighter than those of the Pd-Cu and Pd-Co alloys  The silver content is relatively low (508 wt%) and is usually inadequate to cause porcelain “greening”. www.indiandentalacademy.com
  104. 104. GREENING:  It is believed that the colloidal dispersion of the silver atoms entering body and incisal porcelain or the glazed surface from vapour transport or surface diffusion may cause colour change including green, yellow-green, yellow-orange, orange, and brown hues www.indiandentalacademy.com
  105. 105.  . One theory that has been proposed for this greenish yellow discoloration, popularly termed "Selling" is that the silver vapour escapes from the surface of these alloys during firing of the porcelain, diffuses as ionic silver into the porcelain and is reduced to form colloidal metallic silver in the surface layer of porcelain. www.indiandentalacademy.com
  106. 106.  Porcelains with higher sodium contents are believed to exhibit a more intense discoloration because of more rapid silver diffusion in sodium containing glass  This hypothesis is based on observations of greater discoloration in lighter shades of porcelain and in porcelain with lower opacifier contents and higher sodium content. www.indiandentalacademy.com
  107. 107.  The intensity of discolouration ( chroma ) usually increases in the cervical region because surface diffusion of silver from marginal metal provides a higher localised silver concentration. www.indiandentalacademy.com
  108. 108.  This greening is more in:  1.high silver content alloys  2.lighter shades multiple firing procedures  3. higher temperatures  4. body porcelain in direct contact with the alloys vacuum firing cycles  5. and certain brands of porcelain www.indiandentalacademy.com
  109. 109.  Claire Manaranchea, Helga Hornbergerb. “A proposal for the classification of dental alloys according to their resistance to corrosion”.  The purpose of this study was to establish a method to compare and classify dental alloys in relation to their resistance to corrosion www.indiandentalacademy.com
  110. 110.  Results.  High gold alloys had a similar polarization curve than gold. The same effect was observed for Pd– base alloys, their curves were similar to the one of palladium.  The ions released during chemical corrosion were non-precious metallic ions.  Thereby Ni–Cr alloys were found to release the most ions. www.indiandentalacademy.com
  111. 111.  Au–Pt alloys showed the highest release of ions compared with other precious alloys but low compared with Ni–Cr.  Electrochemical corrosion was more aggressive than chemical corrosion and every type of elements was etched,  the higher the precious metal content, the higher the resistance to corrosion of the alloy. www.indiandentalacademy.com
  112. 112.  N. Silikas a, P.L. Wincott b, D. Vaughanb, D.C. Wattsa, G. Eliadesc,∗ “Surface characterization of precious alloys treated with thione metal primers”  Objectives. To characterize the effect of two thione metal primers with phosphate groups on the surface morphology and composition of two noble Prosthodontic alloys. www.indiandentalacademy.com
  113. 113.  Results.  After Alloy Primer treatment, Polarised Light Microscopy revealed a crystalline phase dispersed in an amorphous phase on both the alloys tested.  MP demonstrated a fibrial arrangement with the most dense structure found on the Hi–Pd alloy. Fourier-transform infrared micro spectroscopy failed to clearly resolve the presence of S H peaks on alloy surfaces. www.indiandentalacademy.com
  114. 114.  Moreover, NH and P S peaks were identified denoting the presence of original thione tautomers.  In both primers, phosphates were detected in a dissociative state ( PO3 2−). www.indiandentalacademy.com
  115. 115. CONCLUSION  Though these noble metals are costlier than base metal alloys their properties like biocompatibility gained them a lot of importance in dentistry  So one should have a through knowledge of these precious metal alloys www.indiandentalacademy.com
  116. 116.  REFERENCES: 1. ANUSAVICE- PHILLIPS SCIENCES OF DENTAL; MATERIALS- 19TH EDITION 2.DENTAL MATERIALS PROPERTIES AND MANIPULATION –CRAIG OBREIN POWERS 5TH EDITION 3. E.C. COMBE NOTES ON DENTAL MATERIALS, 6TH EDITION www.indiandentalacademy.com
  117. 117. 4. Claire Manaranchea, Helga Hornbergerb. “A proposal for the classification of dental alloys according to their resistance to corrosion”. 5. N. Silikas a, P.L. Wincott b, D. Vaughanb, D.C. Wattsa, G. Eliadesc,∗ “Surface characterization of precious alloys treated with thione metal primers” www.indiandentalacademy.com
  118. 118. www.indiandentalacademy.com

×