Сraig. Dental Materials

462 Chapter 15 NOBLE DENTAL ALLOYS AND SOLDERS

Ag-Pd alloys. The Au-Cu-Ag-Pd-111alloys typically have a gold content of 40 wt%. The reduced gold is compensated primarily with silver, thus the copper and palladium contents are not changed much from the Au-Cu-Ag-Pd-11alloys. The Au-Ag-Pd-In alloys have a gold content of only 20 wt%, and have about 40 wt% silver, 20 wt% palladium, and 15 wt% indium. The Pd-Cu-Ga alloys have little or no gold, with about 75 wt% palladium and roughly equal amounts of copper and gallium. Finally, the Ag-Pd alloys have no gold, but have 70 wt% silver and 25 wt% palladium. By the ADA specification, these alloys are considered noble because of their palladium content.

As Table 15-5 shows, the wt% and at% of dental casting alloys can differ considerably. For example, by weight, the Au-Cu-Ag-Pd-I alloys have 76% gold. However, only 57% of the atoms in these alloys are gold. Other elements that have less mass than gold increase in atomic percentage. For these same alloys, the copper content by weight is lo%, but by atoms is 24%. For other alloys whose elements have similar mass, the differences between wt% and at% are less pronounced. For example, in the Ag-Pd alloys the weight and atomic percentages are similar. Weight percentages of the alloys are most cominonly used by manufacturers in the production and sales of the alloys. However, the physical, chemical, and biological properties are best understood in terms of atomic percentages.

The compositions of casting alloys determine their color. In general, if the palladium content is >10 wt%, the alloy will be white. Thus, the Pd-Cu-Ga and Ag-Pd alloys in Table 15-5 are white, whereas the other alloys are yellow. The Au-Ag-Pd-In alloys are an exception because they have a palladium content of >20% and retain a light yellow color. The color of this alloy results from interactions of the indium with the palladium in the alloy. Among the yellow alloys, the composition will modify the shade of yellow. Generally, copper adds a reddish color and silver lightens either the red or yellow color of the alloys.

GRAIN SIZE

Recent studies have described the influence of minute quantities of various elements on the grain size of dental casting alloys. In the past, many alloys had relatively coarse grain structures. Now, by the addition of small amounts (0.005% or 50 ppm) of elements such as iridium and ruthenium, fine-grained castings are produced (see Fig. 6-12). Adding one of these elements to the alloy is believed to develop centers for nucleating grains throughout the alloy. Most alloy manufacturers use grain refinement in present-day products. The mechanical properties of tensile strength and elongation are improved significantly (30%) by the fine grain structure in castings, which contributes to uniformity of properties from one casting to another. Other properties, however, such as hardness and yield strength, show less effect from the grain refinement.

PROPERTIES

Melting Range Dental casting alloys do not have melting points, but rather melting ranges, because they are combinations of elements rather than pure elements. The magnitude of the solidus-liquidus melting range is important to the manipulation of the alloys (see Fig. 15-1, Table 15-6). The solidus-liquidus range should be narrow to avoid having the alloy in a molten state for extended times during casting. If the alloy spends a long time in the partially molten state during casting, there is increased opportunity for the formation of oxides and contamination. Most of the alloys in Table 15-6 have solidus-liquidus ranges of 70" C or less. The Au-Ag-Pt, Pd-Cu-Ga, and Ag-Pd alloys have wider ranges, which makes them more difficult to cast without problems.

The liquidus temperature of the alloys determines the burnout temperature, type of investment, and type of heat-source that must be used during casting. In general, the burnout temperature must be about 500" C below the liquidus temperature (for details see Chapter 17 on cast-

ing). For the Au-Cu-Ag-Pd-I alloys, therefore, a burnout temperature of about 450" to 475" C should be used. If the burnout temperature approaches 700" C, a gypsum-bonded investment cannot be used because the calcium sulfate will decompose and embrittle the alloys. At temperatures near 700" C or greater, a phosphate-bonded investment is used. As shown in Table 15-6, a gypsum-bonded investment may be used with the Au-Cu-Ag-Pd-I, II, and 111and the Au-Ag-Pd- In alloys, but a phosphate-bonded investment is advisable for the other alloys. The gas-air torch will adequately heat alloys with liquidus temperatures below 1100" C. Above this temperature, a gas-oxygen torch or electrical induction method must be used. Again from Table 15-6, a gas-air torch would be acceptable only for the Au-Cu-Ag-Pd-I, IT, and 111and the Au-Ag- Pd-In alloys.

The composition of the alloys determines the liquidus temperatures. If the alloy contains a significant amount of an element that has a high melting point, it is likely to have a high liquidus. Thus alloys that contain significant amounts of palladium or platinum, both of which have high

melting points (see Table 15-I), will have high liquidus temperatures. In Table 15-6, these include the Pd-Cu-Ga, Ag-Pd, and Au-Ag-Pt alloys.

The solidus temperature is important to soldering and formation of ordered phases, because during both of these operations, the shape of the alloys is to be retained. Therefore during soldering or hardening-softening, the alloy may be heated only to the solidus before melting occurs. In practice, it is desirable to limit heating to 50" C below the solidus to avoid local melting or distortion of the casting.

Density Density is important during the acceleration of the molten alloy into the mold during casting. Alloys with high densities will generally accelerate faster and tend to form complete castings more easily. Among the alloys shown in Table 15-6, all have densities sufficient for convenient casting. Lower densities (7 to 8 g/cm3) seen in the predominately base-metal alloys sometimes present problems in this regard. Alloys in Table 15-6 with high densities generally contain higher amounts of denser elements such as gold or platinum. Thus the Au-Ag-

Pt alloys and Au-Cu-Ag-Pd-Ialloys are among the most dense of the casting alloys.

Strength Strength of alloys can be measured by either the yield strength or tensile strength. Although tensile strength represents the maximum strength of the alloy, the yield strength is more u s e f ~in~ldental applications because it is the stress at which periwanent deformation of the alloys occurs (see Chapter 4). Because permanent deforlnation of dental castings is generally undesirable, the yield strength is a reasonable practical maximum strength for dental applications. The yield strengths for the different classes of alloys are shown in Table 15-6. Where applicable, the hard and soft conditions, resulting from the formation of ordered solutions, are shown. For several alloys, such as Au-Cu-Ag-Pd- I, 11,and III, the formation of the ordered phase increases the yield strength significantly. For example, the yield strength of the Au-Cu-Ag-Pd- 11alloys increases from 350 to 600 MPa with the formation of an ordered phase. For other alloys, such as the Au-Ag-Pt and Ag-Pd alloys, the increase in yield strength is more modest in the hardened condition. The Pd-Cu-Ga alloys d o not support the formation of ordered phase because the ratio of palladium and copper are not in the correct range for ordered phase formation (see Table 15-5 and Fig. 15-1, B).

The yield strengths of these alloys range from 320 to 1145 MPa (hard condition). The strongest alloy is Pd-Cu-Ga, with a yield strength of 1145 MPa. The other alloys range in strength from 320 to 600 MPa. These latter yield strengths are adequate for dental applications and are generally in the same range as those for the base-metal alloys, which range from 495 to 600. The effect of solid-solution hardening by the addition of copper and silver to the gold or palladium base is significant for these alloys. Pure cast gold has a tensile strength of 105 MPa (see Table 15-2). With the addition of 10 wt% copper (coin gold), solid-solution hardening increases the tensile strength to 395 MPa. With the further addition of 10 wt% silver and 3 wt% palladium (Au-Cu-Ag-Pd-4, the tensile strength

increases to about 450 MPa and 550 MPa in the hard condition.

Hardness Hardness is a good indicator of the ability of an alloy to resist local permanent deformation under occlusal load. Although the relationships are complex, hardness is related to yield strength and gives some indication of the difficulty in polishing the alloy. Alloys with high hardness will usually have high yield strengths and are more difficult to polish. As Table 15-6 shows, the values for hardness generally parallel those for yield strength. In the hard condition, the hardness of these alloys ranges from 155 kg/mm2 for the Ag-Pd alloys to 425 kg/mm2 for the Pd-Cu-Ga alloys. More typically, the hardness of the noble casting alloys is around 200 kg/mm2. The Ag-Pd alloys are particularly soft because of the high concentration of silver, which is a soft metal. The Pd-Cu-Ga alloys are particularly hard because of the high concentration of Pd, which is a hard metal. The hardness of most noble casting alloys is less than that of enamel (343 kg/mm2), and typically less than that of base-metal alloys. If the hardness of an alloy is greater than enamel, it may wear the enamel of the teeth opposing the restoration.

Elongation Elongation is a measure of the ductility of the alloy. For crown and bridge applications, a low value of elongation for an alloy is generally not a big concern, because permanent deformation of the alloys is generally not desirable. However, the elongation will indicate if the alloy can be burnished. Alloys with high elongation can be burnished without fracture. Elongation is sensitive to the presence or absence of an ordered phase, as shown in Table 15-6. In the hardened condition, the elongation will drop significantly. For example, for the Au-Cu- Ag-Pd-11alloys, the elongation is 30% in the soft condition versus only 10% in the hard condition. In the soft condition, the elongation of noble dental casting alloys ranges from 8% to 30%. These alloys are substantially more ductile than the base-metal alloys, which have elongation on the order of 1% to 2% (see Chapter 16).

Biocompatibility The biocompatibility of noble dental alloys is equally important as other physical or chemical properties. A detailed discussion about the principles of biocompatibility can be found in Chapter 5 , but a few general principles will be mentioned here. The biocompatibility of noble dental alloys is primarily related to elemental release from these alloys (i.e., their corrosion). Thus any toxic, allergic, or other adverse biological response is primarily influenced by elements released from these alloys into the oral cavity. The biological response is also influenced significantly by exactly which elements are released, their concentrations, and duration of exposure to oral tissues. For example, the short-term (more than 1 to 2 days) release of zinc may not be significant biologically, but longer-term (more than 2 to 3 years) might have more-significant effects. Similarly, equivalent amounts (in moles) of zinc, copper, or silver will have quite different biological effects, because each of the elements is unique in its interactions with tissues.

Unfortunately, there is currently no way of completely assessing the biocompatibility of noble alloys (or any other material), because the effects of elemental release on tissues are not completely understood. However, in general, several principles apply to alloy biocompatibility. The elemental release from noble alloys is not proportional to alloy composition, but rather is influenced by the numbers and types of phases in the alloy microstructure and the composition of the phases. In general, multiple-phase alloys release more mass than single-phase alloys. Some elements, such as copper, zinc, silver, cadmium, and nickel, are inherently more prone to be released from dental alloys than others, such as gold, palladium, platinum, and indium. Alloys with high-noble metal content generally release less mass than alloys with little or no noble-metal content. However, the only reliable way to assess elemental release is by direct measurement, because there are exceptions to each of the generalizations just mentioned. Similarly, it is difficult to predict, even knowing the elemental release from an alloy, what the biolog-

ical response to the alloy will be. Thus the only reliable way is to measure the biological response directly, either in vitro, in animals, or in humans (see Chapter 5). It is important to also remember that combinations of alloys used in the mouth may alter their corrosion and biocompatibility.

The Identalloy program was developed in an effort to make dentists and patients more aware of the composition of dental alloys that are used. Under this program, each alloy has a certificate (Fig. 15-3) that lists the complete composition of the alloy, its manufacturer, name, and the ADA compositional classification (highnoble, noble, or predominately base metal). When the dental prosthesis is delivered by the laboratory to the dental office, a certificate is placed in the patient's chart. In this manner, all parties know the exact composition of the material used. This information can be invaluable later if there are problems with the restoration;

Fig. 15-3An example of an Identalloy8 certificate showing the alloy name, manufacturer, composi-

tion, and ADA classification.This section of the certificate is for the dentist's records. A duplicate retained by the laboratory is not shown here. Many dentists will give this information to the patient upon delivery of the crown.

GOLD-BASED ALLOYS FOR PORCELAIN-METAL RESTORATIONS

Mechanical work

Alloys used for porcelain-to-metal restorations are discussed in detail in Chapter 19. The application of porcelain imposes several additional requirements on alloys to be used for porcelainmetal restorations. However, the principles in the current chapter concerning composition, metallurgy, and physical properties all apply to porcelain-metal alloys. Several sintered (rather than cast) alloy systems have been introduced for porcelain-metal restorations. These systems use gold-based high-noble metals and follow the principles outlined in the current chapter.

WROUGHT ALLOYS

Alloys that are worked and adapted into prefabricated forms for use in dental restorations are described as wrought alloys. A wrought form is one that has been worked or shaped and fashioned into a serviceable form for an appliance (Fig. 15-4).The work done to the alloy is usually at a temperature far below the solidus, and is therefore referred to as cold work.Wrought forms may include precision attachments, backings for artificial teeth, and wire in various cross-sectional shapes. Wrought alloys are used in two ways in dental prostheses. First, they can be soldered to a previously cast restoration. An example is a wrought wire clasp on a removable partial denture framework. Second, they can be embedded into a cast framework by "casting to" the alloy, as a precision attachment is "cast-to" the retainer of a crown, bridge, or partial denture. The physical properties required of the wrought alloy will depend on the technique used and the composition of the alloy in the existing appliance.

Fig. 15-4 Diagram of the process of mechanical work that transforms cast structures into wrought structures. The microstructure and mechanical properties of cast and wrought forms are fundamentally different (see Chapter 6).

MICROSTRUCTURE

As indicated in Chapter 6, the microstructure of wrought alloys is fibrous. This fibrous structure results from the cold work applied during the operations that shape the alloy into its final form. Wires or other wrought forms normally have a measurable increase in tensile strength and hardness when compared with corresponding cast structures. The increase in these properties results from the entangled, fibrous internal structure created by the cold work.

Wrought forms will recrystallize during heating operations unless caution is exercised (see Figs 6-18, 6-19). During recrystallization, the fibrous microstructure is converted to a grained structure similar to the structure of a cast form. In general, the amount of recrystallization increases as both the heating time and temperature become excessive. For example, in most noble dental wires, a short heating cycle during the soldering operation is not sufficient to appreciably recrystallize the wire, even though the temperature approaches the fusion temperature. However, a prolonged heating period of 30 to

60 seconds or longer may cause recrystallization, depending on the time, temperature, alloy composition, and manner in which the wire was fabricated. Recrystallization results in a reduction in mechanical properties in proportion to the amount of recrystallization. Severe recrystallization can cause wrought forms to become brittle in the area of recrystallization. Therefore heating operations must be minimized when working with wrought forms.

COMPOSITION

By the current ADA definitions, all alloys used for wrought forms are high-noble alloys except one, which is a noble alloy (Table 15-7). As with the casting alloys, several strategies have been used to formulate alloys with appropriate properties. The compositions in Table 15-7 are not inclusive of all available wrought alloys, but are intended to demonstrate typical alloys. These compositions are designed to provide a range of melting ranges and mechanical properties that are appropriate for wrought alloy applications. The Pt- Au-Pd alloys contain primarily platinum with equal amounts (27 wt%) of palladium and gold. These "PGP" alloys have been commonly used as clasping wires on removable partial dentures. The Au-Pt-Pd alloys are primarily gold with plat-

inum and palladium. The Au-Pt-Cu-Ag, ALI-Pt- Ag-Cu, and Au-Ag-CLI-Pdalloys contain approximately 60 wt% gold, but have adopted different strategies for the remaining 40% of the mass. The first two of these alloys contain about 15 wt% platinum with the balance in silver, copper, and palladium, whereas the third of these alloys contains no platinum and higher amounts of silver. The last alloy shown in Table 15-7 contains no appreciable gold or platinum, but consists of palladium and silver in approximately equal amounts with about 16 wt% copper. The Au-Ag- Cu-Pd wrought alloy (see Table 15-71is similar to the Au-Cu-Ag-Pd-IIcasting alloy (see Table 15-5). These alloys differ only slightly in the gold/silver ratio. Other wrought alloys differ from the casting alloys primarily in their higher platinum contents and absence of iridium or ruthenium grain refiners. Platinum is added to increase the melting temperature of the alloys. The grain refinement is not necessary because these alloys are coldworked into their final forms.

PROPERTIES

The properties of alloys used for wrought applications are shown in Table 15-8. The solidus of these alloys ranges from 875" C for Au-Ag-Cu- Pd to 1500' C for Pt-Au-Pd. If the wrought form

is to be cast-to or soldered-to, the solidus must be sufficientlyhigh so the form does not melt or lose its fibrous structure during burnout or casting operations. The solidus required will depend on the metals to be joined, the solder, and the burnout and casting temperatures to be used. In general, alloys with high solidus temperatures also have higher recrystallization temperatures. These alloys are mostly white because of the high platinum and palladium contents. Exceptions are the Au-Pt-Ag-Cu and Au-Ag-Cu-Pd alloys, which are light yellow and yellow, respectively. Yield strength, elongation, and hardness are properties relevant to wrought alloys (see Table 15-8). The wrought form must generally have a yield strength low enough to allow for adjustment (of a clasp or attachment), but be high enough that permanent distortion does not occur in service. Furthermore, the elongation must be sufficient to allow for adjustment without fracture. Three of the wrought alloys shown in Table 15-8 can be hardened by formation of ordered phases. The Au-Pt-Ag-Cu and Au-Ag-Cu-Pd alloys are hardened by an Au-Cu ordered phase, whereas the Pd-Ag-Cu alloys are hardened by a Pd-Cu ordered phase. As with the casting alloys, the or-

dered phase imparts significantly more strength and hardness to the alloy and lower elongation.

It is often necessary to construct a dental appliance in two or more parts and then join them together by either a soldering or welding process. The terms soldering, welding, and brazing have specific meanings in industry. The term welding is used if two pieces of metal are joined together directly (generally, but not always, without adding a third metal); that is, the metal pieces are heated to a high enough temperature so they attach to each other. The words soldering and brazing are used if two pieces of metal are joined by adding a third metal. If the temperature used in the process is below 425" C, the operation is soldering; however, if the temperature is above 425" C, the operation is brazing. In dentistry, the parts are joined at temperature above 425" C, so the operation should be called brazing. However, because it is most commonly called soldering, that is term used in this chapter.

TYPES OF SOLDERS

In general, solders may be divided into two major groups: soft and hard. The soft solders include the lead-tin alloys of eutectic type with a low melting point, sometimes known as "plumber's" solder. The soft solders have several interesting properties, including a low fusion range of about 260" C or less, which permits them to be applied by simple means, such as by a hot soldering iron. Many also possess good working or mechanical properties, making them favorable for use in industry. However, the soft solders lack corrosion resistance, which makes them impractical for dental applications.

Hard solders have a much higher melting temperature than soft solders and possess greater hardness and strength. The high melting range of these solders precludes the use of soldering irons for melting. In industry, special melting methods are used, such as heating with a gas torch, in a furnace, or with other special heating devices.

Two types of hard solders are used in dentistry. Gold-based solders that have good tarnish and corrosion resistance are extensively used in crown and bridge applications. Silver-based solders are commonly used in orthodontic appliances. For dental applications, goldor silverbased solders are normally melted with a specifically designed dental type of gas blowtorch. Occasionally a method involving an electric furnace or other heating equipment is used, but this is the exception rather than common practice.

Two techniques of dental soldering are used to assemble dental appliances. One is know as free-hand soldering, commonly used in assembling orthodontic and other appliances, and the other is investmentsoldering, customarily used in assembling bridges and similar restorations. In freehand soldering, the parts to be assembled are manually held in contact while the heat and solder are applied. As soon as the solder has flowed to position, the heating is discontinued and the appliance is cooled. In investment soldering, the parts to be assembled are mounted in a soldering investment (similar to a casting investment) and held in intimate contact by the hardened investment while the heat and solder

are applied. These two techniques are described in detail in appropriate textbooks or manuals on orthodontics, crown and bridge prostheses, and operative dentistry.

Basis of Selecting Solders Certain principles must be observed in selecting solders, regardless of the application. The ideal solder includes qualities such as (1) ease of flow at relatively low temperature, (2) sufficient fluidity to freely flow when melted, (3) strength compatible with that of the structure being soldered,

(4) acceptable color to give an inconspicuous joint, (5) resistance to tarnish and corrosion , and

(6) resistance to pitting during heating and application. No single, dental, gold-based solder has all of these qualities; therefore manufacturers provide solders that cover a range of fineness and a number of solders that have special qualities. Manufacturers will generally provide detailed information about which solder should be used with each of their alloys. The properties of solders are significantly influenced by the method used during the soldering operation. Thus a recommended procedure must be faithfully followed to obtain the maximum quality from a product.

Composition Gold-based solders for dental use are primarily alloys of gold, silver, and copper, with small quantities of tin, zinc, and sometimes phosphorus included to modify the fusion temperature and flow qualities. The typical composition and resulting fusion temperature values of a variety of gold solders are given in Table 15-9 The compositions of different solders vary considerably from one another. For example, the gold content may vary from 45 to 81 wt%, silver from 8 to 30 wt%, and copper from 7 to 20 wt%, with little variation in the tin or zinc content. Most solders have a copper/gold ratio to support the formation of a Au-Cu ordered phase. The fusion temperature is lower for alloys that have reduced gold content, but the reduction is not as great as sometimes believed. For example, the difference in fusion temperature between solders 1 and 4 is only 69' C when the gold content is reduced by approximately 16%.

In the past, solders were commonly referred to by a carat number. The number did not describe the actual carat of the solder, but rather the carat of the gold alloy on which the solder was to be used. This permitted a wide range of compositions, or actual carat values, to be used for solders described as a specific carat, because the solder was intended for use with an alloy of a definite carat. For example, solders that varied in

gold content from 58.5 to 65 wt% might be described as 18k because they were to be used on an casting made of an 18k alloy. The gold content of an 18k alloy should be 75 wt% gold, but the solders contained only 58.5 to 65 wt% gold. This system led to much conf~~sionIn. recent years, the degree of fineness has been used to describe the various solders, such as those values designated in Tables 15-9 and 15-10.This

designation is appropriate because it is specific, and when the fineness is changed, the actual carat change is indicated.

Easy-Flowing and Free-Flowing Qualities Dental gold-based solders are frequently described by terms such as easy-flowing or free-flowing. Although these terms are sometimes interchanged, they refer to two different qualities in the solder alloys. An easy-flowing solder is one that has a relatively low fusion temperature; the lower the temperature of fusion, the easier to melt and form the joint. However, the difference in fusion temperatures of highand low-fineness gold solder is approximately 56" C, so differences in flow caused by a lower fusion temperature are small among these solders (see Table 15-9).

The fusion temperature of the solder must be below that of the alloys being soldered, or the joined pieces will melt during the operation. Previous discussion of casting alloys indicated that the lowest solidus values for the casting alloys are for the Au-Cu-Ag-Pd-11and -III alloys and the Au-Ag-Pd-In alloys, ranging from 865" to 875" C (see Table 15-6). For wrought alloys, the lowest solidus values ranged from 875" to 935" C, although most of the wrought alloys have substantially higher solidus temperatures (see Table 15-81,As Table 15-9indicates, most of the solders have fusion temperatures below the solidus temperatures for the casting or wrought alloys. Thus solders are available for even the lowest fusing alloys. In general, the fusion temperature of the solder should be at least 56" C below that of the parts being joined to prevent distortion.

The free-flowing quality refers specifically to the ability of the solder to spread and flow freely over the surfaces of the parts begin joined. This quality is closely related to the surface tension of the melted solder. The surface tension (and freeflowing qualities) control the capillary action, which causes the melted solder to penetrate into fine openings between the parts being assembled. In general, lower-fineness solders are more fluid in the molten state than higher-fineness solders because of their lower gold content and the presence of small quantities of alloying met-

als such as zinc and tin. For this reason, the lower-fineness solders are preferred when joining parts because the solder flows promptly and freely into the available spaces. Solders that spread slowly are often described as being sticky, and they resist spreading even when properly heated. As a result, they tend to penetrate or "burn through" the part being soldered if the solder is forced to spread by overheating. Obviously, the easyand free-flowing qualities work together, because the lower-fineness gold solders have the lowest melting range and greatest freedom of flow.

Mechanical Properties The values in Table 15-10 illustrate typical mechanical properties for dental solders. There are a wide range of strengths and hardness available. As stated previously, it is most desirable to use a solder with a strength similar to that of the parts being joined. All solders in Table 15-10 are subject to hardening by ordered-phase formation except solder 1. Solder 1 does not form an ordered phase because the gold/copper ratio is not appropriate (see Table 15-9). A considerable increase in strength and hardness can be obtained by slowcooling the solder and allowing the ordered phase to form.

The percentages of elongation in noble solders are notably less than those of many noble casting alloys (see Tables 15-10 and 15-61, Even in the soft condition, the elongation ranges only from 3%to 9% for all of the solders except solder 1. In the hard condition the elongation is most often 1%, which is quite brittle. In general, it is more desirable in most appliances to obtain an improved hardness and proportional limit that accompany the ordered-phase formation than to sacrifice these properties for a small increase in elongation.

Color and Tarnish Resistance The color of dental noble solders varies from deep yellow to light yellow and white in much the same manner as casting alloys. Although the ability to match the color of the appliances is an important quality of a solder, producing an inconspicuous solder joint is not difficult in prac-