Low Gold Content Research Articles

Corrosion and tarnishing characteristics of low gold content dental casting alloys

Quantitative measurements of tarnishing and corrosion resistance in low gold content dental casting alloys in a variety of metallurgical states have shown that the desirable features of such alloys are a large grain size, a minimum of segregation, and the absence of precipitates and ordered phases.

Age-hardening and the associated phase transformation in an Au-55.2 at% Cu-17.4 at% Ag ternary alloy

To develop a low gold content dental alloy, age-hardening characteristics in an Au-55.2 at% Cu-17.4 at% Ag alloy were studied by means of hardness, electron microscopy and X-ray diffraction examination. Three distinct age-hardening behaviours depending on temperature were found in the alloy, i.e. (i) a dual mechanism of spinodal decomposition and Cu3Au ordering below 673 K, (ii) a single mechanism of spinodal decomposition at 693 K, and (iii) a single mechanism of nucleation and growth of silver-rich precipitate at 773 K. A marked over-ageing was observed by lengthy ageing over the whole range of temperature. The long-period superstructure of Cu3Au(II) was found only in the grain boundary product at temperatures between 623 and 633 K.

The 8-year clinical performance of 15 low-gold casting alloys

The objectives of this study were to (1) evaluate the clinical performance of low-gold alloys over time, with special attention to tarnish resistance, and (2) determine the influence of compositional factors on performance. A total of 264 restorations were inserted in 50 patients using Modulay (control), Forticast, Midas, Maxigold, Midigold, Minigold, Mowrey 120, Mowrey 46, and 8 experimental alloys (UNC 1–8); 161 (61%) of the original restorations were recalled after an average of 8 years and rated for tarnish, plaque, type of material at contacts, retention of polish, surface porosity, and secondary caries. The incidence of tarnish at ranked locations was relatively low and ranged from 0.004-0.2 on a 0–2 tarnish scale. Low-gold alloys as a group were not significantly more tarnished than the control but there were differences between alloys. Palladium content appears especially important to improved tarnish resistance in alloys with very low gold content. Incidence of plaque accumulation and tarnish was highest in the proximal and cervical one thirds. There was no secondary caries and no surface porosity.

Precambrian and Phanerozoic history of gold mineralization in the Bohemian Massif

A similarity has been found between processes of gold concentration in the volcano-sedimentary formations of Proterozoic and Devonian age of the Bohemian massif, Czechoslovakia, and of the gold concentration in the greenstone belts of the Archean shields. Based on this, the earlier concept of gold associated with Variscan granitoids has been revised and gold concentration explained in terms of mobilization of gold from the volcano-sedimentary units by metamorphic processes and granitization. Attention has been focused on the gold content of individual rocks and rock units in the central part of the Bohemian massif. Very low gold contents have been found in the unmetamorphosed Paleozoic rock units and in the high-grade crystalline rocks of the Moldanubicum. Relatively higher gold contents occur in all types of Proterozoic sedimentary and volcanic rocks and even the granitoids intruding them. The spatial relationships between gold and scheelite mineralization have been established. The new concept has led to the discovery of economically important gold and scheelite deposits in Czechoslovakia described in this paper.

Electrochemical Charge and Protein Adsorption

AbstractThe amount of albumin adsorbed to three dental crown and bridge alloys (high- and low-gold content and silver-palladium), platinum, silver, and stainless steel was quantitated for the as-immersed conditions as well as with anodic and cathodic charge transfers in artifical saliva at a pH = 6.8. A methanolchloroform technique was utilized prior to protein assay which precipitated protein by phase separation. Amido black staining of protein on nitrocellulose filter paper was evaluated for optical density and compared to a standard curve of known quantities of protein. Cathodic charges decreased the quantity of adsorbed protein while anodic charges both increased and decreased adsorbed protein depending upon metal or alloy. Quantities adsorbed ranged from 0.5 to 8 μg/cm2. Interaction of positive oxidized species with the excess negative charges of the albumin molecules is thought to have increased protein adsorption. Formation of corrosion products with inorganic conponents is thought to have masked positive charges from protein thereby decreasing adsorption.

Aging characteristics of a dental Ag-Pd-Cu alloy with low gold content

Aging of a dental Ag-Pd-Cu alloy with low gold content was investigated by optical microscopy, electron probe microanalysis, and x-ray diffraction. Following quenching, the alloy was aged in the temperature range from 200° to 500°C. A marked increase in hardness was ascribed to the ordering of the silver-rich f.c.c. solid solution into an ordered f.c. tetragonal phase of AuCu I type. A decrease in hardness which was observed at the higher aging temperatures was the result of disordering of the tetragonal phase.

Quantitative study of soldered connector tensile strength using low-gold content alloys

Quantitative study of soldered connector tensile strength using low-gold content alloys

The role of microstructure in the tarnish of low-gold alloys

The recent surge in the price of gold has led to the widespread use of low-gold alloys. In a critical application such as for cast dental restorations, the consequence of a low gold content is more rapid chemical attack, typically in the form of tarnish. This study shows that microstructure as well as gold content influences an alloy's tarnish response in simulated oral solutions. For the copper-rich segment of the AgAuCu ternary, quantitative sulfide tarnish data demonstrate that a single-phase microstructure and at least 40 wt.% Au are necessary to minimize tarnishing. For more complex six-component alloys, it is shown that nonuniform microstructures enhance tarnish reactions through microscopic galvanic cells.

Gold plating in the electronics industry

Predictably enough at a meeting devoted to plating in the electronics industry, gold plating figured prominently at the Fourth Symposium on this theme held by the American Electroplaters' Society in Indianapolis, no less than 11 out of a total of 24 papers dealing with various aspects of this subject. Of these, four were concerned with applications in the semiconductor and microelectronics field, two with the requirements of gold plating for use on sealed reed relay contacts, three with processes and coating properties in the more general context of electrical contact applications, and two with analytical methods for the determination of metallic impurities or organic contaminants in plating baths. A common factor was the preoccupation with coating properties in relation to particular applications as affected by operational parameters, compositional variations and plating solution contaminants, reflecting the increasing specificity and control of coating characteristics demanded by today's electronics industry. The concern felt over the recent rise in the price of gold was also evident; of the various approaches to the problem of off-setting the resulting cost increases, selective plating which limits the gold coating to operational areas only, and the general improvement in deposit characteristics such as porosity, hardness and wear resistance to permit the use of thinner gold deposits, were accorded little more than passing general reference. Two papers were, however, concerned with a third possibility, namely the replacement of pure gold by alloy gold deposits of relatively low gold content. Such coatings have been used for many years in the plating of watch cases, where the requirements in terms of wear resistance and corrosion protection over long periods are not dissimilar to those of engineering applications, but though successful use in the electrical and electronics field has been documented in a number of cases in Europe, this type of coating has been more generally viewed with some suspicion on the general grounds of anticipated difficulty in compositional control and the possible effect of the presence of significant amounts of base metals on oxidation and corrosion resistance, particularly at elevated temperature. The fourth approach to the problem, the replacement of gold by an alternative metal which could match its unique combination of properties in terms of corrosion and oxidation resistance, low and stable electrical contact resistance, high conductivity, good solderability and weldability, seems very distant of realisation. Indeed, E. F. Duffek, of Chemline Industries, Santa Clara, in a general review of the semiconductor and microelectronics field, reported increased usage of gold, notwithstanding the price rise, and attributed this to a market trend toward consumer products as opposed to military and strategic items, again emphasising the need for low cost production.

Neutron activation analysis of gold in Archean igneous and metamorphic rocks of the Rice lake-Beresford lake area, southeastern Manitoba

Gold abundances determined by the method of neutron activation are reported for 26 replicate analyses of a variety of igneous and metamorphic rocks. Samples analyzed come from scattered locations in and adjacent to an Archean greenstone belt containing numerous low-grade gold-quartz veins. Most of the richer veins are enclosed by mafic intrusions. The average gold content of each of the rock types is as follows:Rock Type No. of analyses Au (ppb)Northern Granites; quartz diorite: 4 1.9 + or - 1.2Manigotagan Gneissic Belt; paragneiss: 2 0.6 + or - 0.4associated quartz monzonite: 2 0.3 + or - 0.1Rice Lake Group; metavolcanics: 3 0.3 + or - 0.2Mafic intrusives; gabbro, diabase: 2 18.2 + or - 1.5Central plutons; quartz diorite: 5 1.1 + or - 0.6related felsic porphyry dykes: 2 6.1, 1.1Altered diabase wall-rock and vein zone: 6 0.5-325Gold abundances for the quartz diorite, quartz monzonite and metavolcanic groups are fairly typical of younger rocks of similar composition. Explanations are put forward to account for the unexpectedly low gold content of the paragneiss samples and relative gold enrichment in one felsic dyke sample.There is evidence to indicate that the mafic intrusions may be the source rocks of the gold. Thus, in addition to the spatial relationship of the richer veins to the mafic intrusions the latter also contain an order of magnitude more background gold than other rocks in the area. Furthermore, gold is apparently depleted in altered diabase wall-rock adjacent to a vein in the San Antonio gold mine.Non-repeatable replicate analyses observed in some mafic samples may be due to inhomogeneous distribution of gold between powder aliquots resulting from an erratic distribution of gold-rich host minerals.

Observations of onset of sulfide tarnish on gold-base alloys

lh e various gold alloys used in dentistry differ in composition for the purpose of meeting the different mechanical requirements of the different types of applications. However, they must all have sufficient noble metal content to be essentially tarnish resistant in the oral environment. In spite of this, clinical dental examples ot: tarnish occur occasionally even in the alloys of high gold c0ntent.l We have received in our laboratory over the last ten years about 80 tarnished restorations submitted by dentists for our examination. On analyzing the tarnished areas with an electron probe, we found the tarnish to be essentially a sulfide intermixed with a carbonaceous material. It has been observed that the tarnish favors regions of subsurface porosity, or regions where a copper redeposit has apparently occurred, probably due to old pickling solution. Tarnish also favors the low gold content alloys, but surprisingly, this is not always true. Some 22-karat gold alloys tarnish in the mouth rather consistently. It is, therefore, of interest to investigate the nature of tarnish when deliberately forced to occur on a series of gold-base alloys which arc generally considered tarnish resistant for dental purposes. “Tarnish,” in this article, will be defined as the visibly detectable discoloration of a metal caused by a thin adherent layer of a reaction product, such as an oxide or a sulfide, induced by a chemical reaction between the metal and the environnlent. In general, such discoloration is the basis for tarnish when caused by mouth fluids.‘, 3 The criterion of tarnish onset is not limited in this article to the unaided “eye” type of detection but will include the detection of discoloration as first appearing at microscopic examination. ‘rhe alloys reported on in this article lvere tested for possible microscopic 9:nrnish produced by a synthetic saliva solution.*-’ After ten days of room-temperature immersion at one minute intervals (the method described below), no microscopic tarnish could be detected. Therefore, synthetic saliva was discarded as a means for _Presented at the Annual Meeting of the International Association of Dental Research in Washington, I). C., 1967. *Manager, Research and Development, J. F. Jelenko & Company, Inc. **Director, Research and Development, J. F. Jelenko & Company, Inc.

Current Distribution During the Electrodeposition of Gold

The phenomenon of macro‐ and microcurrent distribution in plating systems is studied. At high current densities where mass transport of the reacting species is rate controlling, the plating process will yield relatively uniform macro‐ but nonuniform microcurrent distribution. At low current densities where depletion of the reacting species is negligible, relatively uniform micro‐ but nonuniform macrocurrent distribution will be observed. The effect of surface roughness and current density on the microcurrent distribution of gold plated from a citrate bath is studied experimentally. The microcurrent distribution is found to improve considerably by reducing the surface roughness and by performing the deposition process at a lower current density. Due to the low gold content of most gold baths, practical gold plating is performed under conditions where mass transport limitations of the reacting species are significant.

The Control and Characteristics of Acid Alloy Gold Used as a Decorative Plate

SummaryBrief comparison has been made between the alkaline cyanide and acid citrate gold plating baths. Operating conditions of a proprietary acid bath and their effect on cathode efficiency have been investigated. The low gold and nickel metal content generally adopted in these baths and the marked effect of slight variation in these metals on the characteristics of the deposit and trouble-free running of the process make regular analytical control important, as this affects economy of process and quality of product. Brief mention has been made of the application of atomic absorption spectrophotometry as a convenient means of analytical control. Plating distribution can be greatly improved by careful consideration of anode configuration and cathode jig design. The most suitable anode material was found to be gold strip, or as an alternative, platinized expanded titanium mesh. In the proprietary acid bath under consideration, carbon or stainless steel anodes were found to be unsuitable.

Aboriginal Gilding in Panama

AbstractLothrop places on record the original study of the “mise en coleur” gilding of Coclé objects which Bergsøe had completed in 1940. The Coclé specimens were made of a copper-gold alloy with low gold content, and were cast in molds. Because such an alloy corrodes easily, the exact composition of the Coclé alloy could not be determined. The Coclé goldsmiths did not use the hammered overlay, mercury gilding, or leaf gilding processes, but rather the “mise en coleur” process in which the copper is removed from the surface of the alloy leaving a blackish-brown, spongy gold deposit as an overlay. This spongy overlay is compressed and returned to a golden color by hammering and burnishing. The surface copper was probably dissolved by ammonium carbonate derived from urine.

Chemicals from corn cobs

The objectives of this study were to (1) evaluate the clinical performance of low-gold alloys over time, with special attention to tarnish resistance, and (2) determine the influence of compositional factors on performance. A total of 264 restorations were inserted in 50 patients using Modulay (control), Forticast, Midas, Maxigold, Midigold, Minigold, Mowrey 120, Mowrey 46, and 8 experimental alloys (UNC 1–8); 161 (61%) of the original restorations were recalled after an average of 8 years and rated for tarnish, plaque, type of material at contacts, retention of polish, surface porosity, and secondary caries. The incidence of tarnish at ranked locations was relatively low and ranged from 0.004-0.2 on a 0–2 tarnish scale. Low-gold alloys as a group were not significantly more tarnished than the control but there were differences between alloys. Palladium content appears especially important to improved tarnish resistance in alloys with very low gold content. Incidence of plaque accumulation and tarnish was highest in the proximal and cervical one thirds. There was no secondary caries and no surface porosity.